EP0112858A1

EP0112858A1 - Method and device producing molecular beams and use of this method.

Info

Publication number: EP0112858A1
Application number: EP83901909A
Authority: EP
Inventors: Edward William Schlag
Original assignee: WEYSSENHOFF HANNS VON
Current assignee: WEYSSENHOFF HANNS VON
Priority date: 1982-07-02
Filing date: 1983-06-30
Publication date: 1984-07-11
Also published as: JPS59501234A; JPS6318320B2; DE3224801A1; DE3374373D1; EP0112858B1; WO1984000276A1; DE3224801C2; US4570066A

Abstract

Procédé et dispositif pour la production de faisceaux moléculaires comportant de grandes molécules instables thermiquement, dans lequel les molécules passent de la phase non gazeuse à la phase gazeuse par apport d'énergie et sont mélangées à un gaz porteur et refroidies par expansion adiabatique avec le gaz porteur. Les grandes molécules passent de la phase non gazeuse à la phase gazeuse à une telle température à laquelle leur taux d'évaporation est plus grand que leur taux de décomposition. L'énergie pour faire passer les grosses molécules thermiquement instables de la phase non volatile à la phase gazeuse est apportée si rapidement que les grosses molécules thermiquement instables, à une température supérieure à la température de décomposition, passent de la phase non volatile à la phase gazeuse, dans laquelle leur taux d'évaporation est supérieur au taux de décomposition. Ces molécules sont amenées, à l'état gazeux, dans une région d'un courant de gaz porteur dans laquelle elles commencent à se dilater, la température de ce courant de gaz étant essentiellement inférieure à la température d'évaporation resp. de décomposition des grandes molécules instables thermiquement.Process and device for the production of molecular beams comprising large thermally unstable molecules, in which the molecules change from the non-gas phase to the gas phase by supply of energy and are mixed with a carrier gas and cooled by adiabatic expansion with the gas carrier. Large molecules change from non-gas phase to gas phase at such a temperature where their rate of evaporation is greater than their rate of decomposition. The energy to move the large thermally unstable molecules from the non-volatile phase to the gas phase is supplied so rapidly that the large thermally unstable molecules, at a temperature above the decomposition temperature, change from the non-volatile phase to the gas phase. gas, in which their rate of evaporation is greater than the rate of decomposition. These molecules are brought, in the gaseous state, into a region of a stream of carrier gas in which they begin to expand, the temperature of this stream of gas being essentially lower than the temperature of evaporation resp. decomposition of large thermally unstable molecules.

Description

Method and device for generating molecular beams

The invention is based on a method for generating molecular beams, in which a Prohensbstanz is converted (evaporated) by supplying energy from the non-gaseous phase into the gaseous phase, the free molecules resulting from the transfer are mixed with a carrier gas and the carrier gas the molecules of the proth substance are cooled adiabatically by expansion of a beam of the carrier gas.

The invention further relates to a device for carrying out the above-mentioned method, comprising a gas jet nozzle for generating a carrier gas jet, a carrier gas supply device for supplying the carrier gas to the gas jet nozzle, an evaporation and mixing chamber for transferring a sample substance from the non-gaseous to the gaseous phase (evaporation) and for admixing this phase to the carrier gas, and an energy supply device for supplying evaporation energy to the evaporation and mixing chamber.

Such a method and such a device are known, for example from US-Z-Chemical Physics Letters, Vol. 77, No. 3, pages 448 to 451. According to the current state of the art, any molecular beams can be generated which consist of a carrier gas, which is usually a noble gas, such as argon, and molecules added to the carrier gas, provided that these molecules are of a volatile substance that can be vaporized at a temperature is at which the molecules do not decompose at all, that is, if they are thermally stable molecules.

In particular, according to the prior art, molecular beams are produced by means of a method and a device of the type mentioned above, which are only known for thermally stable molecules, by evaporating or otherwise evaporating the molecules in the carrier gas atmosphere, for example in an argon atmosphere, and after the formation of a molecular beam by means of a gas jet nozzle, they are jointly cooled by expansion, so that they thereby obtain a very low temperature, which is particularly suitable for their examination by means of mass spectroscopy or other molecular or ion examination methods.

The evaporation of the molecules added to the carrier gas takes place in the upstream of the gas jet nozzle provided evaporation and mixing chamber by simple thermal heating in the latter directly in front of the gas jet nozzle, through which the mixture of the molecules to be examined and the carrier gas emerges and forms the gas jet, if provided the volatile substance, which consists of the molecules to be examined or contains these molecules, does not evaporate on its own due to its vapor pressure at room temperature.

Preferred examination methods, by means of which such molecular beams are examined, are examination by means of tunable laser radiation either in fluorescence or by multiphoton ionization (MPI) by mass spectroscopy. The aforementioned method and the device for carrying it out are not suitable for thermally unstable molecules, that is to say for non-volatile molecules which decompose when heated before they have reached a sufficient vapor pressure.

To date, it has not been possible to generate molecular beams for the investigation of thermally unstable molecules, but rather such thermally unstable molecules have been evaporated directly into a vacuum, i.e. without carrier gas, whereby these thermally unstable molecules were more or less decomposed. After the evaporation, an ionization and an examination by ion examination methods, such as, for example, mass spectrometric methods, took place.

For such evaporation and ionization of thermally unstable molecules, there are a number of methods, most of which have been developed in recent times, with which these thermally unstable molecules can be evaporated directly into vacuum under ionization and examined by mass spectrometry. The most important methods here are, for example

Called "flash" pyrolysis, the spraying of a solution ("electrospray") with immediate evaporation of the solvent, the enemy desorption and ionization, the chemical ionization, the bombardment with fast atoms and the laser desorption. All of these methods have so far been used with varying degrees of success for the direct ionization of thermally unstable molecules. They have the considerable disadvantage that ions are generated and that these ions are also partly fragments and partly add-on ions, with "cationized" mother molecules very often occurring, ie mother ions to which another ion, for example a proton or alkali ion, is additionally attached. is attached. In addition, the same fragments and attachment complexes do not always form, so that it is difficult and not certain to draw conclusions about the undecomposed molecules via which the test results are supposed to be obtained are partially impossible. It may be that with most of the methods mentioned, neutral molecules in the undecomposed state may also be evaporated, especially with laser evaporation, but it has not yet been possible to directly detect such thermally unstable molecules, especially since these molecules are not in the form of a molecular beam and be decomposed before the actual detection, for example by ionization.

From the journal for natural research, volume 36a, 1981, pages 1338 to 1339 and volume 35a, 1980, pages 1429 to 1430 and from the journal "Chemical Physics Letters", Vol. 77, No. 3, pages 448 to 451 respectively a method for generating pulsed molecular beams is known, but the molecular beams generated by this particular method do not contain thermally unstable molecules.

Specifically, the process described in the journal for natural research, vol. 36a, 1981, pages 1338 to 1339 is the evaporation of a volatile substance which has a high vapor pressure even at room temperature, namely the evaporation of benzene. This substance is evaporated into the carrier gas, which is argon in the present case, and then a molecular beam is formed from the mixture thus obtained, with no problems whatsoever, as already mentioned above. The same applies to the method described in the journal "Chemical Physics Letters", Vol. 77, No. 3, pages 448 to 451, only one other volatile substance, namely diacetyl, being used here; In addition, the pulsed molecular beam is generated by means of a solenoid valve in this method. In principle, the same applies to the method described in the journal for natural research, volume 35a, 1980, pages 1429 to 1430, but with the difference that the sub punch, by means of which a molecular beam is to be formed, is heated, with this substance, which is anthracene in the present case, being heated to a temperature in the gas phase which is far below the decomposition temperature.

When a molecular beam is formed from the substance vapor / carrier gas mixture, the processes described in the journal for natural research, volume 36a, 1981, pages 1338 to 1339 and volume 35a, 1980, pages 1429 to 1430 do not occur until the first time after the molecular beam has been generated, this molecular beam as by-products complexes of the mentioned substance with the carrier gas. These complexes are not present at all before the formation of the molecular beam, in particular not before cooling due to the expansion of the carrier gas jet, but they only form as a result of this strong cooling. These complexes, which are not large, thermally unstable molecules, do not exist under normal conditions and are only interesting in that very weak Van der Waals interaction forces can be investigated. By contrast, the present invention is intended to generate molecular beams with large, thermally unstable molecules which exist under normal conditions. Such molecules can never form in the molecular beam by mere addition, as is the case with the aforementioned complexes, but must be introduced as such into a molecular beam.

Furthermore, it is known from US Pat. No. 4,259,572 to investigate large, thermally sensitive molecules, but the substance which contains these molecules is not brought into the gas phase in the form of neutral molecules, but rather in the form of molecular ions. The process according to US Pat. No. 4,259,572 consequently has the considerable disadvantage that no intact, large, thermally unstable molecules in the gas phase can be investigated. With regard to this method, the above statements apply to the recently developed methods for the vaporization and ionization of thermally unstable molecules.

Finally, US Pat. No. 4,091,256 describes a method and a device in which so much energy is supplied to a substance that the substance decomposes down to the individual atoms and thus provides a beam of neutral atoms. Large, thermally unstable molecules cannot be converted into the gas phase at all without decomposition using this method and this device.

A non-volatile substance is to be understood in particular as a substance which is non-volatile under normal conditions (20 ° C. and 1 bar).

At the temperature at which the anthracene is evaporated according to the method described in the journal for natural research, volume 36a, 1981, pages 1338 to 1339, there is a balance of solid anthracene and anthracene vapor. The invention is based on the object of developing a method of the type mentioned in such a way that it is possible to bring large, thermally unstable molecules, that is to say molecules of substances which cannot be vaporized without decomposition, undamaged into a molecular beam which, for spectroscopic purposes, especially mass spectroscopy.

This object is achieved in accordance with the invention by the method mentioned at the outset that in order to generate molecular beams with large, thermally unstable molecules, the energy is supplied in pulses at such a height that the sample substance evaporates faster than decomposes that the temperature of the carrier gas jet is in a range of the jet in which it begins to expand is set significantly lower than the decomposition temperature of the sample substance and that the released molecules of the sample substance are brought directly into this area of the carrier gas jet.

The underlying object is further achieved according to the device mentioned at the outset in that, in order to generate a molecular beam which contains large, thermally unstable molecules, the carrier gas supply device supplies the carrier gas to the gas jet nozzle upstream of the latter at a temperature which is substantially lower than the evaporation or The decomposition temperature of the sample substance is that the evaporation and mixing chamber, at least with the part in which the molecules are mixed into the carrier gas, is arranged downstream of and adjacent to the outlet opening of the gas jet nozzle, and that the energy supply device emits high energy in pulsed operation. In the context of the present invention, thermally unstable molecules are to be understood to mean molecules of those substances whose molecules already disintegrate at temperatures which are far below the evaporation temperature. These substances therefore have a vapor pressure that is far below the vapor pressure that would be required for the molecules to go into the gas phase at room temperature. In the context of the invention, “large” molecules are to be understood in particular as molecules whose molecular weight is 100 or more.

In the process according to the invention, the molecules are evaporated off under conditions in which there are more undecomposed molecules in the gas phase than corresponds to the thermodynamic equilibrium at the prevailing temperature, while in the state of thermodynamic equilibrium no undecomposed molecules in the case of substances, consisting of large, thermally unstable molecules.

In the method according to the invention and in the device according to the invention, two essential processes take place in combination due to the conception of the invention, which enables the generation of molecular beams with undecomposed large, thermally unstable molecules:

(1) The substance, which consists of the large, thermally unstable molecules or contains these molecules, is evaporated very quickly, and the evaporation takes place so quickly that the majority of the molecules cannot decompose. Such a very rapid evaporation is known per se, as will be explained in more detail below with reference to FIG. 1; however, it alone does not open up the possibility of generating the desired molecular beams which contain the large, thermally unstable molecules without decomposition, because the evaporated molecules decay shortly after the evaporation in the gas phase as a result of the energy absorbed during the evaporation.

(2) The second process consists in exposing the evaporated, thermally unstable molecules to heat dissipation in the carrier gas jet immediately after they have evaporated, by exposing the expanding carrier gas jet to a temperature which is substantially lower than the evaporation or decomposition temperature of the large, thermally unstable molecules is admixed, namely they are introduced into the area of the "cool" carrier gas jet in which this begins to expand. This prevents these thermally unstable molecules from disintegrating after the transition to the gas phase. The "stabilization cooling" that occurs here is to be distinguished from the adiabatic cooling, which takes place later and has a completely different purpose, the molecules to be examined cool down to an examination temperature of a few degrees Kelvin.

It appears important at this point to point out that according to the prior art, in which, as already mentioned above, the molecules in the carrier gas atmosphere, for example in an argon atmosphere, are evaporated upstream from the gas jet nozzle, the carrier gas as a result of the The fact that during the process of thermal evaporation of the molecules to be examined is heated together with them, is "hot" and thus contributes to the decomposition of the thermally unstable molecules. Only after the carrier gas and the vaporized molecules have mixed in the evaporation and mixing chamber in the case of thermal evaporation at a relatively high temperature, at which mixing thermally unstable molecules disintegrate, even if a certain percentage should have evaporated without being decomposed, do the molecules to be examined become common expanded with the carrier gas through the gas jet nozzle. As a result, it is actually not possible in the prior art to generate molecular beams that contain undecomposed large, thermally unstable molecules.

In contrast to this, the invention makes it possible to generate molecular beams in which the large, thermally unstable molecules are available undamaged for a variety of tests, in particular for optical spectroscopy, for reaction kinetics, in which molecular beams are known to be widely used for test purposes be, as well as for mass spectrometry. In the context of the invention, “evaporation” or “evaporation” or “evaporation” should be understood to mean all types of converting molecules into the gas phase; this transfer can therefore take place both from the solid substance which contains the molecules to be examined or consists of these molecules, and from a surface on which the 2-molecules are attached or adsorbed. The large, thermally unstable molecules are preferably vaporized by means of laser radiation; This type of evaporation has the advantage that it is particularly possible to carry out the very rapid, relatively high temperature evaporation of the large, thermally unstable molecules.

Another preferred embodiment of the method according to the invention is characterized in that the large, thermally unstable molecules are each evaporated into a carrier gas jet pulse by means of evaporation pulses. In this way, the molecules can be well examined without the substance from which they are evaporated being continuously heated and thus largely decomposed; rather, only the top layer of the substance is always heated to the high evaporation temperature which serves for rapid evaporation.

So that the large, thermally unstable molecules get directly into the relatively cool carrier gas jet which is at the beginning of its expansion after they have passed into the gas phase, they are preferably vaporized by a sample surface which runs essentially parallel to the axis of the carrier gas jet and is adjacent to the outlet opening of the gas jet nozzle , this sample surface nevertheless not protruding into the carrier gas jet, so that an undisturbed expansion of the carrier gas jet is made possible.

The device for carrying out the method according to the invention, which has already been given above in its basic structure, is preferably designed such that the large, thermally unstable molecules are exposed to the stabilization cooling mentioned immediately after their entry into the gas phase, in such a way that the evaporation point in from the outlet opening of the gas jet nozzle a longitudinal distance that is less than or equal to 20 times the effective diameter of the Outlet opening is arranged laterally from the outlet opening, the longitudinal distance being that along the axis of the gas jet nozzle and the effective diameter being the diameter corresponding to a circular outlet opening. So if the outlet opening of the gas jet nozzle has an annular cross section, then the effective diameter in the above sense means the diameter of a circular outlet opening which has the same area of outflow as the annular outlet opening.

Furthermore, likewise for the purpose of ensuring the most immediate possible stabilization cooling, the evaporation point is preferably arranged at a transverse distance from the axis of the gas jet nozzle which is less than or equal to 20 times, preferably less than or equal to 10 times, the effective diameter of the outlet opening.

It is particularly preferred that the transverse distance be less than half the longitudinal distance.

The device can furthermore be designed such that the evaporation and mixing chamber has a preferably cylindrical expansion channel for the carrier gas jet, downstream of the outlet opening of the gas jet nozzle, on or in the wall of which the evaporation point is provided. These evaporation parts can in particular be provided in an obliquely, preferably perpendicularly, to the axis of the gas jet nozzle arranged sample channel, which is formed in the side wall of the expansion channel.

With this construction, if the evaporation takes place by means of laser radiation, a laser beam channel lying in the axial extension of the sample channel can be formed in the side wall of the expansion channel. The distance of the sample surface from the side wall of the expansion channel is preferably less than or equal to the diameter of the sample channel, the sample diameter preferably being equal to the diameter of the sample channel.

The invention will be explained in more detail below with reference to Figures 1 to 12 of the drawing using a particularly preferred embodiment and the test results achieved thereby; show it:

Figure 1 is a graph showing the relationship between evaporation and decomposition of large, thermally unstable molecules;

FIG. 2 shows a longitudinal section through a gas jet nozzle and part of an evaporation and mixing chamber, as are preferably used to carry out the method according to the invention;

FIG. 3 shows a preferred embodiment of a device according to the invention with the adjoining field plates of a mass spectrometer, but without the device with which the evaporation energy is supplied to the sample located in the evaporation and mixing chamber;

Figure 4 shows an embodiment of the device with which the evaporation energy is generated and supplied to the sample; and

Figures 5 to 12 test results in curve form, as they have been obtained by means of the method and the device according to the invention.

Reference is first made to FIG. 1, in which the natural logarithm of the reaction constant k for the decomposition (straight line I) and for the evaporation (straight line II) is shown schematically as a function of the reciprocal of the absolute temperature T are shown. After that, it's like in RJ's work "Mass Spectrometry of Nonvolatile Compounds"("Mass Spectrometry of Non-Volatile Compounds") in Anal. Chem. 52 (1980) 1589A was shown so that with large, thermally unstable molecules the evaporation has a higher activation energy than the decomposition, so that the rate of evaporation increases with temperature faster than that of the decomposition. Thus, from a certain temperature, the evaporation rate becomes greater than the decomposition rate, namely from the intersection of the two straight lines I and II to higher temperatures.

If you go very quickly to a very high temperature, most of the molecules will evaporate before enough energy has accumulated in the internal vibration modes that can lead to dissociation. In addition to these two neutral pathways, there are various ionization pathways that include lead to the energetically favorable "cationized" species already mentioned.

The evaporation or evaporation of thermally unstable molecules can be done by extremely rapid heating, as is caused by a very short laser pulse with a high power density. The distribution of energy over the three evaporation processes, evaporation, decomposition and ionization, depends mainly on the following factors when bombarded with laser radiation: laser energy density, pulse duration and nature of the sample. The influence of the laser wavelength on the evaporation process seems to be of minor importance; However, it cannot be ruled out that particularly high evaporation rates can be achieved with specific wavelengths, for example in the infrared (resonance desorption). From this point of view, the CO ₂ laser (wavelength 10.6 μm) should be preferred to the alternative neodymium YAG laser (wavelength 1.06 μm), because at 10.6 / um most of the large organic anic molecules vibrational bands, which is not the case at 1.06 μm. A molecular beam is a bundled stream of molecules that move in a preferred direction essentially without jolts. The freedom from bumps is also given for free expansion into a vacuum, but the preferred direction is generally missing here. In the method according to the invention, the vaporized molecules are admixed to a carrier gas jet immediately after it emerges from a pulsed gas jet nozzle. The still "hot" molecules initially experience collisions with the carrier gas atoms and are thus deactivated by "stabilization cooling", so that the likelihood of a subsequent unimolecular decay greatly decreases. Through further adiabatic expansion, the initial nozzle jet changes into a molecular jet after a short running distance.

A particularly preferred embodiment of a device according to the invention will now be explained in more detail with reference to FIGS. 2 to 4. This device for generating a pulsed, doped molecular beam has the following four main components:

(1) a gas jet nozzle 1, which in the present embodiment is designed as an electromagnetically pulsed nozzle valve and is used to generate a carrier gas jet, for example an argon jet;

(2) an evaporation and mixing chamber 2 for admixing the molecules evaporated from a sample to the carrier gas jet; and

(3) an energy supply device 3 for supplying vaporization energy to the evaporation and mixing chamber 2, the main component of which is a pulsed CO ₂ laser (TEA laser); such as

(4) a carrier gas supply device 4 for supplying the carrier gas to the gas jet nozzle 1; from this carrier Gas supply device 4 is shown in the drawing, namely in Figure 3, only the connecting piece through which the carrier gas is supplied to the gas jet nozzle 1.

By means of these four structural units, the preferred embodiments of which are illustrated in more detail below, a molecular beam is generated, of which only the axis 5 is shown, which at the same time represents the axis of the gas jet nozzle 1 and accordingly also the carrier gas jet emerging therefrom and the axis of the carrier gas jet and the mixed gas jet existing in these evaporated molecules, which after adiabatic expansion becomes the molecular jet.

To demonstrate the properties of the molecular beam, a time-of-flight mass spectrometer with laser multiphoton ionization is provided in the present case, of which some field plates and part of the drift tube are indicated in the lower part of FIG. The ionizing laser is a tunable dye laser that is pumped by a Q-switched neodymium-YAG laser.

The impulse operation of the nozzle jet (this is how the entire jet is referred to here, which becomes the carrier gas jet via the mixed gas jet to the molecular jet) is not an absolutely necessary feature of the method according to the invention, but it is of extremely important practical importance in order to maintain a sufficient vacuum with a reasonable pumping effort. A continuous jet of carrier gas, for example made of argon, would have no practical meaning anyway because of the pulsed evaporation. Important for the functioning of the method is an exact temporal correlation of the nozzle jet, the evaporation laser pulse and the ionization laser pulse, which is carried out by standard electronic circuits. The individual structural units are now explained in more detail below, and then, for example, structural dimensions and preferred optimized operating data are specified.

The gas jet nozzle is first described in detail with reference to FIGS. 2 and 3:

The gas jet nozzle 1 is designed as a nozzle valve, in the present case it is a commercial electromagnetically operated valve from Bosch, which was originally intended for the operation of fuel injection engines. This nozzle valve has an annular outlet opening 6 which is delimited on the inside by a cylindrical end of a valve tappet 7 and on the outside by a cylindrical opening of a valve seat cylinder 8. Towards the inside of the gas jet nozzle 1, a conical valve surface 9 connects to the cylindrical end of the valve tappet 7, which cooperates with a complementary conical valve seat surface 10, which adjoins the cylindrical opening of the valve seat cylinder 8. The nozzle valve is reworked so that the valve seat cylinder 8 is freely accessible and is provided with an external thread 11 for screwing on the evaporation and mixing chamber 2.

In the present embodiment, the annular outlet opening 6 has a radial ring width of approximately 0.1 mm and an outer ring diameter of approximately 1 mm, so that a corresponding ring-shaped carrier gas jet is generated. The distance between the valve surface 9 and the valve seat surface 10 when the nozzle valve is open is approximately 0.1 mm. The nozzle valve is operated electromagnetically so that a carrier gas pulse of approx. 1 msec duration with rising and falling edges of approximately 200 μsec is produced. This is achieved by electrical pulses of 500 μsec that are applied to the magnetic winding of the nozzle valve. Next, the evaporation and mixing chamber 2 will also be explained in more detail with reference to FIGS. 2 and 3, which, at least with regard to its essential part in which the evaporation and mixing takes place, is arranged downstream of the outlet opening 6 of the gas jet nozzle 1 and adjacent to this outlet opening:

The evaporation and mixing chamber 2 has a cylindrical expansion channel 12 for the carrier gas jet, the axis of which coincides with the axis 5 of the nozzle jet and which forms an enlarged, downstream extension of the outlet opening 6 of the gas jet nozzle 1 and connects one end directly to the outlet opening 6 . The other end of the expansion channel 12 merges into a vacuum for further expansion of the jet.

On or in the wall of the expansion channel 12, the evaporation point 13 is provided, which in the present case is formed by the surface of a sample 14 pressed into a pill. This evaporation point 13 is provided in a sample channel 15 which is arranged perpendicular to the axis 5 of the gas jet nozzle 1 and is formed in the lateral wall of the expansion channel 12.

Furthermore, the evaporation and mixing chamber 2 has a laser beam channel 16, which is also formed in the lateral wall of the expansion channel 12, specifically in the axial extension of the sample channel 15 on the side of the expansion channel 12 opposite the latter is explained below with reference to Figure 4, the evaporation energy supplied.

So that an undisturbed expansion of the carrier gas jet can take place in the expansion channel 12, the diameter of the latter is substantially larger than the outer diameter of the outlet opening 6. So that the molecules from the sample 14 to the The following conditions are preferably met for the purposes of stabilizing cooling as directly as possible in the expanding carrier gas jet and in an area of the latter which is as close as possible to the beginning of the expansion:

(1) The longitudinal distance a of the evaporation point 13 from the outlet opening 6 of the gas jet nozzle 1 is less than or equal to 20 times the effective diameter of the outlet opening 6. Here, the distance between the outlet opening 6 and the projection point P of the center M is below the longitudinal distance a Evaporation point 13 to understand the axis 5 of the jet. The concept of the effective diameter of the outlet opening 6 has already been explained above.

(2) The transverse distance b of the evaporation point 13 from the axis 5 of the nozzle jet is less than or equal to 20 times, preferably less than or equal to 10 times the effective diameter of the outlet opening 6.

(3) The transverse distance b is preferably less than half the longitudinal distance a.

(4) The distance c of the evaporation point 13 from the side wall of the expansion channel 12 is less than or equal to the diameter d of the sample channel 15, which is preferably filled in by the sample 13 in its entire cross section.

In the present embodiment, the evaporation and mixing chamber 2 consists of a cylindrical block made of stainless steel, which has a threaded bore 17 which is concentric with the expansion channel 12 and by means of which it is screwed onto the external thread 11 of the valve seat cylinder 8. Preferred Ab Measurements of this cylindrical block, of the expansion channel 12 and of the sample channel 15 and laser beam channel 16 passing through from the expansion channel on the outside of the cylindrical block are as follows:

Diameter e of the laser beam channel: 2.5 mm Diameter d of the sample channel: 2.5 mm

Diameter g of the expansion channel: 2.5 mm axial length h of the expansion channel: 5 mm minimum distance i of the inner wall of the sample and laser beam channel from the face of the cylindrical block facing away from the gas jet nozzle: 1.5 mm diameter m of the evaporation and mixing chamber: 30 mm thickness n of the evaporation and mixing chamber: 13 mm

It should be pointed out that the evaporation and mixing chamber 2 can be modified in such a way that at the location of the pressed sample 14 there is a strip coated with the sample substance, which can consist, for example, of copper or Teflon etc., on which the evaporation laser beam 18 acting on the laser beam channel 16 (see FIG. 4) is guided past, so that the surface exposed to the evaporation laser beam can thereby be constantly renewed by continuously or stepwise moving the belt; however, this embodiment is not shown in the drawing.

The energy supply device 3, with which the large, thermally unstable molecules are vaporized at a temperature at which the vaporization rate of these molecules is greater than their decomposition rate, will now be described in more detail: This energy supply device 3 comprises, as an energy source, a laser 19, which in the present case is a pulsed CO ₂ - TEA laser which, with a beam cross section of 2.3 x 2.5 cm, has evaporation laser beam pulses of 0.3 J / cm ² and 1 / usec duration returns. The repetition frequency is variable in the range from 0 to 10 pulses / second. The evaporation laser beam 18 is, as shown in the not to scale drawing of Figure 4, deflected immediately after its exit from the laser 19 by a first gold-coated, flat deflection mirror 20 by 90 and via a first iris diaphragm 21 to a second flat gold-coated Deflecting mirror 22 and via a second iris diaphragm 23 and a third flat gold-coated deflecting mirror 24 are directed to a likewise gold-coated concave mirror 25. The two variable iris diaphragms 21 and 23 are provided to attenuate the evaporation laser beam 18, namely, as shown in FIG. 4, an iris diaphragm 21 is attached directly to the output of the laser 19 and the other iris diaphragm 23 is located in the vicinity of the third deflection mirror 24.

By means of the concave mirror 25, the evaporation laser beam 18 is directed through a window 26 into the interior of the vacuum space 27 containing the evaporation and mixing chamber and through the laser beam channel 16 onto the evaporation point 13, i.e. concentrated on the surface of the sample 14. However, it should be noted that the sample 14 is not exactly in the focal point of the concave mirror 25. Rather, the distance of the concave mirror 25 from the evaporation point 13 can be changed, and this change allows the energy density of the evaporation laser beam 18 impinging on the surface of the sample 14 to be varied in a simple manner.

Preferred data of the energy supply device according to FIG. 4 are given below: Distance p between the two iris diaphragms: approx. 2.5 m range within which the distance q between the concave mirror and the evaporation point can be set: 30 - 45 cm focal length of the concave mirror: 28 cm

Window material: barium fluoride

In order to investigate the molecular beam obtained in the time-of-flight mass spectrometer already mentioned, it must be ionized, which is indicated by A in FIG

This is done by means of an ionization laser beam 28, which is indicated in FIG. 3 in the plane of the drawing, but actually runs perpendicular to the plane of the drawing. A neodymium-YAG dye laser system from Quanta Ray is used to generate this ionization laser beam 28. This laser system works optimally at a pulse repetition frequency of 10 Hertz and delivers pulses of approx. 10 nsec duration. In addition to the basic wavelength of the YAG laser, which is at 1064 nm and its harmonics, which go up to the fourth harmonic, which is at 266 nm, the entire range from approx. 800 to 240 nm can be covered by suitable choice of dye as well as doubling and frequency mixing . The ionization laser beam 28 is focused on the intersection A of the molecular beam with the ion-optical axis 29 of the time-of-flight mass spectrometer by means of a lens, not shown, which preferably has a focal length of 20 cm or 50 cm. In the present embodiment, this intersection point A is at a distance r of 2.7 cm from the outlet opening 6 for the carrier gas jet.

In FIG. 3, the field plates 30 to 33 and a part of the drift tube 34 of a time-of-flight mass spectrometer of conventional design are indicated, which form a pulling field for extracting the ions generated at the intersection point A, a single lens and a drift space, the latter being provided by aperture plates 35 36 and 37, which are provided in the field plates 31, 32 and 33, are separated from the molecular beam. The perforated screens 35, 36 and 37 have, for example, a diameter s of 5 mm each. At the end of the drift distance, which is, for example, 25 cm, there is a secondary electron multiplier (not shown). The ions are detected via a preamplifier either on a fast oscillograph or on a TRANSIENT DIGITIZER (Tectronix), which is a device that registers and digitizes very fast processes (nanoseconds to picoseconds).

The molecular beam space has a buffer volume of approx. 6 1, so that the chamber pressure does not increase so much with every gas pulse; it is kept at a mean pressure of approx. 1.3 / ubar with a Roots pump with a suction capacity of 1000 m ³ / h and with a suitable backing pump. The pressure in the drift chamber is kept below 0.013 / ubar by a diffusion pump.

As FIG. 3 also shows, the gas jet nozzle 1 is screwed as a structural unit into an essentially hollow cylindrical socket 38, which in turn is attached to a larger flange 43, which is attached to a larger flange 43 and a spacer ring 40 and seals 41, 42 tubular part 44 (see Figure 4) is provided. This tubular part, which is not shown in FIG. 3, is located laterally at a distance from the evaporation and mixing chamber 2 and supports the window 26 via a corresponding holder 45.

Finally, the following are given below with reference to FIGS. 5 to 12 test tests which were carried out with the method and the device according to the invention and the results thereof: The generation of a molecular beam has been demonstrated with three different substances:

(1) anthracene (2) retinal (vitamin A aldehyde) and (3) tryptophan (an amino acid)

The measurement curves in FIGS. 7 to 12 are proof that thermally unstable molecules could actually be converted into a molecular beam without being destroyed.

Anthracene was detected in the molecular beam both by fluorescence and by a mass spectrum.

Figure 5 shows a resonance fluorescence spectrum at 0-0

Transition to the 1st excited electron state of anthracene.

Typical test conditions were:

Delay between valve opening and YAG laser: 1.3 msec,

Delay between CO ₂ laser and YAG laser: 500 / usec,

Argon pressure 0.5 bar, photon multiplier voltage: 1800 V, power density of the CO ₂ laser: approx. 1 MW / cm ² . The narrow band - approx. 0.1 nm wide - is characteristic of a molecule cooled to a few K.

The following figures (Figures 6 to 11) show mass spectra that were recorded under the following conditions:

Delay between valve opening and YAG laser: 880 / usec, delay between CO ₂ laser and YAG laser: 350 / usec, power density of the CO ₂ laser on the sample: approx.1.2 MW / cm ² , argon pressure 0, 3 - 0.4 bar, electron multiplier: 3000 V. The substance was introduced as a compact in the sample channel of the evaporation chamber.

Figure 6 shows a mass spectrum of anthracene. It essentially contains the mother mass and a calibration peak of toluene, which has been mixed in trace amounts with the argon. The wavelength of the ionization laser was 266 nm. It should be pointed out that an anthracene molecule is not in itself a thermally unstable molecule, rather anthracene has only been chosen as an example for the functioning of adiabatic cooling; It was found that this adiabatic cooling works even when the molecules are introduced into the carrier gas downstream of the gas jet nozzle.

The mother masses of retinal, anthracene and toluene also appear in FIG. The latter two were still present from the previous attempt. A peak appears at mass 35, probably due to fragmentation of the retina. The wavelength of the ionization laser was 266 nm.

Figure 8 shows a spectrum in which almost only the mother ion of the retinais appears. Here, the evaporation chamber had been thoroughly cleaned and filled with a fresh retinal sample. The incident wavelength was again 266 nm, but with a lower intensity than in FIG. 7.

The retinal spectra of the following two figures were obtained with an ionizing wavelength of 355 nm, specifically in FIG. 9 at high energy density and in FIG. 10 at low energy density. It can be seen that the fragmentation depends strongly on the intensity and does not result from the evaporation process.

The last two pictures show mass spectra of tryptophan. The ionization wavelength is 266 nm The spectrum of FIG. 11 is recorded with a compact, that of FIG. 12 with a tryptophan coating on copper strips. The other conditions are the same.

The results clearly document that the specified method can be used to produce molecular beams of argon with admixed thermally unstable molecules, for example retinal and tryptophan.

In conclusion, it should be pointed out that molecular beams are widely used in chemistry to elucidate molecular structures and reaction mechanisms, in spectroscopy and mass spectroscopy, etc. The more chemically and biologically interesting molecules that can be brought into the beam, the more they can be used. This number has been expanded by the device according to the invention and the method according to the invention by a considerable, previously inaccessible class of molecules. This leads to new applications for molecular beams in scientific, analytical and technical fields, both in chemistry and in biology, medicine and related sciences.

Claims

1. A method of generating molecular beams in which

a) a sample substance is converted (evaporated) by supplying energy from the non-gaseous to the gaseous phase;

b) the free molecules of the sample substance formed during the transfer are mixed with a carrier gas;

c) the carrier gas with the molecules of the sample substance is cooled adiabatically by expansion of a beam of the carrier gas;

characterized in that for generating molecular beams with large, thermally unstable molecules

d) the energy is supplied in pulses at such a height that the sample substance evaporates faster than it decomposes;

e) the temperature of the carrier gas jet in a region of the jet in which it begins to expand is set significantly lower than the decomposition temperature of the sample substance;

f) the released molecules of the sample substance are brought directly into this area of the carrier gas jet.

2. The method according to claim 1, characterized in that the sample substance is evaporated by means of pulsed laser radiation.

3. The method according to claim 1 or 2, characterized in that the carrier gas jet is generated in pulse form.

4. The method according to any one of the preceding claims, characterized in that the molecules from a substantially parallel to the axis of the carrier gas jet, the outlet opening (6) of a gas jet nozzle (1) for generating the carrier gas jet adjacent sample surface are evaporated as an evaporation point (13).

5. Device for performing the method according to one of the preceding claims, comprising a gas jet nozzle (1) for generating a carrier gas jet, a carrier gas supply device (4) for supplying the carrier gas to the gas jet nozzle (1), an evaporation and mixing chamber (2) for transfer a Prohensubst from the non-gaseous into the gaseous phase (evaporation) and for admixing this phase to the carrier gas, and an energy supply device (3) for supplying evaporation energy to the evaporation and mixing chamber (2), characterized in that for generating a molecular beam, which contains large, thermally unstable molecules

a) the carrier gas supply device (4) the carrier gas of the gas jet nozzle (1) upstream from the last rer supplies at a temperature which is significantly lower than the evaporation or decomposition temperature of the sample substance;

b) the evaporation and mixing chamber (2) is arranged at least with the part in which the molecules are added to the carrier gas, downstream of the outlet opening (6) of the gas jet nozzle (1) and adjacent to it, and

c) the energy supply device (3) emits high energy in pulse mode.

6. Device according to claim 5, characterized in that the evaporation point (13) of the large, thermally unstable molecules from the outlet opening (6) of the gas jet nozzle (1) at a longitudinal distance (a) which is less than or equal to 20 times the effective diameter the outlet opening (6) is arranged laterally from the outlet opening (6), the longitudinal distance (a) being that along the axis (5) of the gas jet nozzle (1)

and the effective diameter is the diameter corresponding to a circular exit opening.

7. Device according to claim 5 or 6, characterized in that the evaporation point (13) from the axis (5) of the gas jet nozzle (1) is arranged at a transverse distance (b) which is less than or equal to 20 times, preferably is smaller than or equal to ten times the effective diameter of the outlet opening (6).

8. Device according to claim 6 and 7, characterized in that the transverse distance (b) is less than half the longitudinal distance (a).

9. Device according to one of claims 5 to 8, characterized in that the evaporation and mixing chamber (2) has a downstream, preferably cylindrical, expansion channel (12) for the carrier gas jet downstream of the outlet opening (6) of the gas jet nozzle (1) in the wall of which the evaporation point (13) is provided.

10. Device according to claim 9, characterized in that the evaporation point (13) in an oblique, preferably perpendicular to the axis (5) of the gas jet nozzle (1) arranged sample channel (15) is provided, which in the side wall of the expansion channel (12th ) is trained.

11. The device according to claim 9, characterized in that in the lateral wall of the expansion channel (12) in the axial extension of the sample channel (15) lying laser beam channel (16) is formed.

12. The device according to claim 10 or 11, characterized in that the distance (c) of the evaporation point (13) from the side wall of the expansion channel (12) is less than or equal to the diameter (d) of the sample channel (15) , wherein the sample diameter is preferably the same as the latter.

13. Device according to one of claims 5 to 12, characterized in that the gas jet nozzle (1) is designed as an electromagnetically actuated nozzle valve.