Journal of Physics B: Atomic, Molecular and Optical Physics

We experimentally study the heating of trapped atomic ions during measurement of their internal qubit states. During measurement, ions are projected into one of two basis states and discriminated by their state-dependent fluorescence. We observe that ions in the fluorescing state rapidly scatter photons and heat at a rate of $\dot{\bar{n}}\gtrsim 2\times 10^4$ quanta s⁻¹, which is orders of magnitude faster than typical anomalous ion heating rates. We introduce a quantum trajectory-based framework that accurately reproduces the experimental results and provides a unified description of ion heating for both continuous and discrete sources.

High-intensity laser fields can ionize atoms and molecules and also initiate molecular dissociation. This review is on the recent progress made using experiments that harness the potential of cold-target recoil-ion momentum spectroscopy and femtosecond laser pulses with tailored intense fields. The possibility to image the molecular structure and the orientation of small molecules via the detection of the momenta of the ions is illustrated. The process of non-adiabatic tunnel ionization is analyzed in detail focusing on the properties of the electronic wave packet at the tunnel exit. It is reviewed how the electron gains angular momentum and energy during tunneling in circularly polarized light. The electron is a quantum object with an amplitude and a phase. Most experiments in strong field ionization focus on the absolute square of the electronic wave function. The technique of holographic angular streaking of electrons enables the retrieval of Wigner time delays in strong field ionization, which is a property of the electronic wave function's phase in momentum space. The relationship between the phase in momentum space and the amplitudes in position space enables access to information about the electron's position at the tunnel exit. Finally, recent experiments studying entanglement in strong field ionization are discussed.

We employ the Markov approximation and the well-known Fresnel-integral to derive in 'one-line' the familiar expression for the Landau–Zener transition probability. Moreover, we provide numerical as well as analytical justifications for our approach, and identify three characteristic motions of the probability amplitude in the complex plane.

Realistic fault-tolerant quantum computing at reasonable overhead requires two-qubit gates with the highest possible fidelity. Typically, an infidelity of $\lesssim 10^{-4}$ is recommended in the literature. Focusing on the phase-sensitive architecture used in laboratories and by commercial companies to implement quantum computers, we show that even under noise-free, ideal conditions, neglecting the carrier term and linearizing the Lamb–Dicke term in the Hamiltonian used for control-pulse construction for generating Mølmer–Sørensen XX gates based on the Raman scheme are not justified if the goal is an infidelity target of ${\lt}10^{-4}$. We obtain these results with a gate simulator code that, in addition to the computational space, explicitly takes the most relevant part of the phonon space into account. With the help of a Magnus expansion carried to the third order, keeping terms up to the fourth order in the Lamb–Dicke parameters, we identify the leading sources of coherent errors, which we show can be eliminated by adding a single linear equation to the phase-space closure conditions and subsequently adjusting the amplitude of the control pulse (calibration). This way, we obtain XX gates with infidelities ${\lt}10^{-4}$.

The photodissociation dynamics of UV excited CS₂ are investigated using time-resolved Auger–Meitner (AM) spectroscopy. AM decay is initiated by inner-shell ionisation with a femtosecond duration x-ray (179.9 eV) probe generated by the FERMI free electron laser. The time-delayed x-ray probe removes an electron from the S(2p) orbital leading to secondary emission of a high energy electron through AM decay. We monitor the electron kinetic energy of the AM emission as a function of pump-probe delay and observe time-dependent changes in the spectrum that correlate with the formation of bound, excited-state CS₂ molecules at early times, and CS + S fragments on the picosecond timescale. The results are analysed based on a simplified kinetic scheme that provides a time constant for dissociation of approximately 1.2 ps, in agreement with previous time-resolved x-ray photoelectron spectroscopy measurements (Gabalski, et al 2023 J. Phys. Chem. Lett. 14 7126–7133).

Ultralong-range Rydberg molecules (ULRMs) comprise a Rydberg atom in whose electron cloud are embedded one (or more) ground-state atoms that are weakly-bound through their scattering of the Rydberg electron. The existence of such novel molecular species was first predicted theoretically in 2000 but they were not observed in the laboratory until 2009. Since that time, interest in their chemical properties, physical characteristics, and applications has increased dramatically. We discuss here recent advances in the study of ULRMs. These have yielded a wealth of information regarding low-energy electron scattering in an energy regime difficult to access using alternate techniques, and have provided a valuable probe of non-local spatial correlations in quantum gases elucidating the effects of quantum statistics. Studies in dense environments, where the Rydberg electron cloud can enclose hundreds, or even thousands, of ground-state atoms, have revealed many-body effects such as the creation of Rydberg polarons. The production of overlapping clouds of different cold atoms has enabled the creation of heteronuclear ULRMs. Indeed, the wide variety of atomic and molecular species that can now be cooled promises, through the careful choice of atomic (or molecular) species, to enable the production of ULRMs with properties tailored to meet a variety of different needs and applications.

Short-lived core-ionized neon atoms were investigated by measuring under resonant Raman conditions the Auger decay following the excitation of a second core electron into a Rydberg state. Making use of intense and narrow bandwidth x-ray free-electron laser pulses, the photoexcitation spectrum of the femtosecond-lived Ne$^{+}1s^{0}2s^{2}2p^{6}np$ series was characterized. Energy position and lifetimes of the lower-lying Rydberg states were determined and the final state configurations following the decay of the Ne$^{+}1s^{0}2s^{2}2p^{6}3p$ double-core hole resonance were partially resolved.

Single center convergent close-coupling calculations have been completed for positron scattering from atomic fluorine. Total, electron-loss, positronium-formation, direct ionization, momentum transfer, elastic, bound-state excitation, and stopping power cross sections have been determined for energies between threshold and 5000 eV. Past calculations for this scattering system exist only for elastic and momentum-transfer cross sections. For high energies, good agreement is found between current and past results. At low energies, however, large differences are found between the current calculations and previous results. The atomic fluorine results are then used in a modified independent atom approach to calculate cross sections for positron scattering on F₂, HF, CF₄, C₂F₆, C₃F₆, C₃F₈, and C₆F₆. The current molecular results are typically higher than previous positron experiments across the calculated energy range, however, these experiments were not corrected for the forward angle scattering effect and likely underestimate the true result. Good agreement is found between the current positron results and previous electron experiments and calculations at high energies.

Projected variational wavefunctions such as the Gutzwiller, many-body correlator and Jastrow ansatzes have provided crucial insight into the nature of superfluid-Mott insulator transition in the Bose Hubbard model (BHM) in two or more spatial dimensions. However, these ansatzes have no obvious tractable and systematic way of being improved. A promising alternative is to use Neural-network quantum states (NQS) based on Restricted Boltzmann Machines (RBMs). With binary visible and hidden units NQS have proven to be a highly effective at describing quantum states of interacting spin-$\frac{1}{2}$ lattice systems. The application of NQS to bosonic systems has so far been based on one-hot encoding from machine learning where the multi-valued site occupation is distributed across several binary-valued visible units of an RBM. Compared to spin-$\frac{1}{2}$ systems one-hot encoding greatly increases the number of variational parameters whilst also making their physical interpretation opaque. Here we revisit the construction of NQS for bosonic systems by reformulating a one-hot encoded RBM into a correlation operator applied to a reference state, analogous to the structure of the projected variational ansatzes. In this form we then propose a number of specialisations of the RBM motivated by the physics of the BHM and the ability to capture exactly the projected variational ansatzes. We analyse in detail the variational performance of these new RBM variants for a $10 \times 10$ BHM, using both a standard Bose condensate state and a pre-optimised Jastrow + many-body correlator state as the reference state of the calculation. Several of our new ansatzes give robust results as nearly good as one-hot encoding across the regimes of the BHM, but at a substantially reduced cost. Such specialised NQS are thus primed tackle bosonic lattice problems beyond the accuracy of classic variational wavefunctions.

The full bound rovibrational spectra $E_{(\nu,L)}$ for the ground electronic state $X^{1}\Sigma^+$ of the hydrogen bromine HBr and hydrogen iodine HI is calculated in the Born-Oppenheimer approximation. As a starting point an analytical expression for the potential energy curve V(R), valid in the entire domain $R\in [0,\infty)$, is obtained in the form of a two-point Padé approximant that correctly reproduces the asymptotic behavior for small $R\rightarrow 0$ and large $R\rightarrow\infty$ internuclear distances. Remaining free parameters are fixed by making a fit with the experimental RKR points. In a second stage, the rovibrational spectra $E_{(\nu,L)}$ is obtained by solving the nuclear Schröodinger equation with nuclear interaction V(R). It was found that the potential energy curve for the ground electronic state $X^1\Sigma^+$ supports a total of 798/748 rovibrational states with maximal vibrational/rotational quantum numbers $v_{\max} = 19/17$ and $L_{\max} = 65/66$ for hydrogen halides HBr/HI, respectively. Through a simple modification in the nuclear Schröodinger equation, the bound rovibrational spectra $E_{(\nu,L)}$ is obtained for the isotopological species with deuterium (D) and tritium (T) content: DBr/TBr (1575/2329 states in total with $\nu_{\max} = 27/34$ and $L_{\max} = 92/112$) and DI/TI (1485/2207 states in total with $\nu_{\max} = 25/31$ and $L_{\max} = 93/114$).

X-ray free electron lasers (XFELs) have emerged as powerful sources of short and intense x-ray pulses. We propose a simple and robust procedure which takes advantage of the inherent stochasticity of self-amplified stimulated emission (SASE) pulses to enhance the time-resolution and signal strength of the recorded data. Notably, the proposed method is able to enhance the average signal without knowledge of the signal strength of individual shots. Simple metrics for the probe pulses are introduced, such as an effective pulse duration applicable to SASE pulses characterised in the time domain using e.g. an X-band transverse cavity. The approach is evaluated using simulated and real pulse data in the context of ultrafast electron dynamics in a molecule. Utilising H₂ as a model system, we demonstrate the efficacy of the method theoretically, successfully enhancing the predicted nonresonant ultrafast x-ray scattering signal associated with electron dynamics. The method presented is broadly applicable and offers a general strategy for enhancing time-dependent observables at XFELs.

Electron–molecule collisions drive many natural phenomena and are playing an increasing role in modern technologies. Over recent years, studies of the collision processes have become increasingly driven by quantum mechanical calculations rather than experiments. This tutorial surveys important issues underlying the physics and theoretical methods used to study electron–molecule collisions. It is aimed at nonspecialists with suitable references for further reading for those interested and pointers to software for those wanting to perform actual calculations.

We demonstrate real-time observations of nanoplasma formation and expansion using intense extreme-ultraviolet (XUV) and near-infrared (NIR) pump–probe electron spectroscopy. We identified the formation of a nanoplasma by the sudden enhancement of low-energy electron emission within a few tens of femtoseconds after XUV excitation, which indicates considerable heating of the clusters by the NIR field. We probed the subsequent expansion of the nanoplasma by monitoring the transient resonant enhancement of high-energy electron emission. The dependence of the resonance on the XUV intensity is explained by the expansion speed of the XUV-induced nanoplasma.

In the context of the recently reported experiment on photoionization in neon atom, we theoretically study the photoionization of neon atom at a comparatively intense laser field. The calculated photoelectron spectrum for a Gaussian laser pulse show an asymmetric double peak line shape at a pulse duration of 14.2 fs and peak intensity of 1 × 10¹⁵ W cm⁻². A systematic study clearly indicates that the ponderomotive potential of the photoelectron released during photoionization of neon is instrumental in causing the visible asymmetry. Interestingly, for similar laser parameters asymmetry in the photoelectron spectrum gets significantly reduced for a Sech² shaped laser pulse. Time resolved photoelectron spectrum reveals that even for a Sech² shaped laser pulse the two peak photoelectron spectrum is initially asymmetric and evolves into a symmetric line shape with increase in time. The results clearly indicate that irrespective of laser pulse shape asymmetry shows a non-linear decrease as a function of time. Our study also shows the possibility of controlling the asymmetry by varying the pulse duration. The calculations establishes a correlation between the effects of direct double ionization and ponderomotive potential on the asymmetry of the photoelectron spectrum at different pulse durations.

Attosecond chronoscopy typically utilises interfering two-photon transitions to access the phase information. Simulating these two-photon transitions is challenging due to the continuum–continuum transition term. The hydrogenic approximation within second-order perturbation theory has been widely used due to the existence of analytical expressions of the wave functions. So far, only (partially) asymptotic results have been derived, which fail to correctly describe the low-kinetic-energy behaviour, especially for high angular-momentum states. Here, we report an analytical expression that overcomes these limitations. It is based on the Appell's F₁ function and uses the confluent hypergeometric function of the second kind as the intermediate state. We show that the derived formula quantitatively agrees with the numerical simulations using the time-dependent Schrödinger equation for various angular-momentum states, which improves the accuracy compared to the other analytical approaches that were previously reported. Furthermore, we give an angular-momentum-dependent asymptotic form of the outgoing wavefunction and the corresponding continuum–continuum dipole transition amplitudes.

Electron–molecule collisions drive many natural phenomena and are playing an increasing role in modern technologies. Over recent years, studies of the collision processes have become increasingly driven by quantum mechanical calculations rather than experiments. This tutorial surveys important issues underlying the physics and theoretical methods used to study electron–molecule collisions. It is aimed at nonspecialists with suitable references for further reading for those interested and pointers to software for those wanting to perform actual calculations.

Ultralong-range Rydberg molecules (ULRMs) comprise a Rydberg atom in whose electron cloud are embedded one (or more) ground-state atoms that are weakly-bound through their scattering of the Rydberg electron. The existence of such novel molecular species was first predicted theoretically in 2000 but they were not observed in the laboratory until 2009. Since that time, interest in their chemical properties, physical characteristics, and applications has increased dramatically. We discuss here recent advances in the study of ULRMs. These have yielded a wealth of information regarding low-energy electron scattering in an energy regime difficult to access using alternate techniques, and have provided a valuable probe of non-local spatial correlations in quantum gases elucidating the effects of quantum statistics. Studies in dense environments, where the Rydberg electron cloud can enclose hundreds, or even thousands, of ground-state atoms, have revealed many-body effects such as the creation of Rydberg polarons. The production of overlapping clouds of different cold atoms has enabled the creation of heteronuclear ULRMs. Indeed, the wide variety of atomic and molecular species that can now be cooled promises, through the careful choice of atomic (or molecular) species, to enable the production of ULRMs with properties tailored to meet a variety of different needs and applications.

With the availability of modern laser and detection technologies, the investigation of ultrafast molecular dynamics induced by intense laser pulses has become a routine practice. In this Topical Review, we present a survey of recent progress in the timing and control of ultrafast molecular dynamics, encompassing processes initiated by both extreme ultraviolet and near infrared pulses. Prospects and perspectives of this field are given. This Review underscores the remarkable potential for further advances in understanding and harnessing ultrafast molecular processes.

High-intensity laser fields can ionize atoms and molecules and also initiate molecular dissociation. This review is on the recent progress made using experiments that harness the potential of cold-target recoil-ion momentum spectroscopy and femtosecond laser pulses with tailored intense fields. The possibility to image the molecular structure and the orientation of small molecules via the detection of the momenta of the ions is illustrated. The process of non-adiabatic tunnel ionization is analyzed in detail focusing on the properties of the electronic wave packet at the tunnel exit. It is reviewed how the electron gains angular momentum and energy during tunneling in circularly polarized light. The electron is a quantum object with an amplitude and a phase. Most experiments in strong field ionization focus on the absolute square of the electronic wave function. The technique of holographic angular streaking of electrons enables the retrieval of Wigner time delays in strong field ionization, which is a property of the electronic wave function's phase in momentum space. The relationship between the phase in momentum space and the amplitudes in position space enables access to information about the electron's position at the tunnel exit. Finally, recent experiments studying entanglement in strong field ionization are discussed.

Fast heavy-ion collisions with molecules that constitute a liquid are fundamental to the field of radiation chemistry and its application to biology. However, although collision-induced physical and chemical processes in liquids have been extensively studied, the initial stages of such processes remain not fully understood because of their complex behaviors. Accordingly, our group has studied the initial reactions occurring in the vicinity of fast-ion trajectories in liquids by mass spectrometric analysis of the secondary ions ejected from microdroplet surfaces upon fast heavy-ion impacts. In this topical review, we present our recent experimental advances in secondary-ion mass spectrometry using microdroplets of water, alcohols, and amino acid solutions. Our findings demonstrate the complex physicochemical behaviors of positive and negative product ions and highlight the role of secondary electrons in the mechanisms of biomolecular damage triggered by fast heavy ions.

We investigate fine structure changing collisions in ⁸⁷Rb vapour upon D₂ excitation in a thermal vapour at 75°C; the atoms are placed in a 0.6 T axial magnetic field in order to gain access to the hyperfine Pashen-Back regime. Following optical excitation on the D₂ line, the exothermic transfer ⁵P_3/2 →⁵P_1/2 occurs as a consequence of buffer-gas collisions; the ⁸⁷Rb subsequently emits a photon on the D₁ transition. We employ single-photon counting apparatus to monitor the D₁ fluorescence, with an etalon filter to provide high spectral resolution. By studying the D₁ fluorescence when the D₂ excitation laser is scanned, we see that during the collisional transfer process the m'_J quantum number of the atom changes, but the nuclear spin projection quantum number, m'_I, is conserved. A simple kinematic model incorporating a coefficient of restitution in the collision accounted for the change in velocity distribution of atoms undergoing collisions, and the resulting fluorescence lineshape. The experiment is conducted with a nominally "buffer-gas free" vapour cell; our results show that fine structure changing collisions are important with such media, and point out possible implications for quantum-optics experiments in thermal vapours producing entangled photon pairs with the double ladder configuration, and solar physics magneto-optical filters.

Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, as they feature both excellent control over the electronic state preparation and readout as well as a versatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single ¹³⁸Ba⁺ ion prepared in the 5d ²D_3/2,5/2 metastable states with a ground state ⁶Li gas near quantum degeneracy. We show that in contrast to previously reported atom-ion mixtures, several non-radiative processes, including charge exchange, excitation exchange and quenching, compete with each other due to the inherent complexity of the ion-atom molecular structure. We present a full quantum model based on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internal angular momenta and the mechanical rotation of the colliding pair, which is relevant in any other hybrid system composed of an alkali-metal atom and an alkaline-earth ion.

In our study, we conduct a comprehensive theoretical analysis on the propagation behavior of a Gaussian pulse through a four-level Λ-type Rubidium atomic medium under room temperature conditions. Our investigation uncovers the presence of two distinct wavepackets within the medium's transmission signal. The primary wavepacket, linked to electromagnetically induced transparency transmission, serves as the central signal in the study. Characterized by its optical beat signal utilized for fast microwave strength detection, this wavepacket demonstrates notable features such as pronounced normal dispersion and decreased group velocity. Additionally, the emergence of the Sommerfeld-Brillouin precursor as the second wavepacket further enriches our understanding of pulse dynamics in the medium. Our simulation findings reveal the potential for the optical precursor to play a dominant role in the transmission signal with the adopted methodology. Furthermore, we identify that experimental parameters like atomic density, vapor cell length, and control field intensity play crucial roles in modulating the time delay of the primary signal and the amplitude of the optical precursor.

New information demonstrating the importance of both sequential and simultaneous (or direct) multiphoton ionization of inner shell electrons from neon is discussed in this paper. Ne was irradiated with intense 93 eV free electron laser (FEL) pulses at FLASH and studied with the aid of photoelectron spectrometry. This resulted in two and three photon, single and double ionization of neon, removing electrons from 2s and 2p subshells of the neutral Ne atom in multiple different pathways. The spectral features of the photoelectrons were identified through comparison with the NIST database and field averaged time-dependent density matrix theory. The calculations show the direct multiphoton ionization processes to be extremely sensitive to the focused FEL intensity.

Molecular ions formed in cold hybrid ion-atom experiments may find interesting applications ranging from precision measurements to controlled chemical reactions. Here, we investigate electronic structure of the Sr₂⁺ molecular ion, which may be produced by photoassociation of laser-cooled Sr⁺ ions immersed into an ultracold gas of Sr atoms or by ionization of ultracold Sr₂ molecules. Using ab initio electronic structure methods, such as the coupled cluster and configuration interaction ones with small-core relativistic energy-consistent pseudopotentials and large Gaussian basis sets, we calculate potential energy curves for the ground and 41 excited electronic states, and electric dipole transition moments between them. We show that alkaline-earth molecular ions despite of their apparently simple structure with three valence electrons only are challenging for state-of-the-art quantum chemistry methods due to their multireference nature and high density of states. Finally, we calculate and analyze Franck-Condon factors governing the photoionization of ground-state Sr₂ molecules into ²Σ_u⁺ and ²Σ_g⁺ states of Sr₂⁺ molecular ions. The present results may be useful for studying and guiding formation and spectroscopy of cold Sr₂⁺ molecular ions.

X-ray free electron lasers (XFELs) have emerged as powerful sources of short and intense x-ray pulses. We propose a simple and robust procedure which takes advantage of the inherent stochasticity of self-amplified stimulated emission (SASE) pulses to enhance the time-resolution and signal strength of the recorded data. Notably, the proposed method is able to enhance the average signal without knowledge of the signal strength of individual shots. Simple metrics for the probe pulses are introduced, such as an effective pulse duration applicable to SASE pulses characterised in the time domain using e.g. an X-band transverse cavity. The approach is evaluated using simulated and real pulse data in the context of ultrafast electron dynamics in a molecule. Utilising H₂ as a model system, we demonstrate the efficacy of the method theoretically, successfully enhancing the predicted nonresonant ultrafast x-ray scattering signal associated with electron dynamics. The method presented is broadly applicable and offers a general strategy for enhancing time-dependent observables at XFELs.

Electron–molecule collisions drive many natural phenomena and are playing an increasing role in modern technologies. Over recent years, studies of the collision processes have become increasingly driven by quantum mechanical calculations rather than experiments. This tutorial surveys important issues underlying the physics and theoretical methods used to study electron–molecule collisions. It is aimed at nonspecialists with suitable references for further reading for those interested and pointers to software for those wanting to perform actual calculations.

Attosecond chronoscopy typically utilises interfering two-photon transitions to access the phase information. Simulating these two-photon transitions is challenging due to the continuum–continuum transition term. The hydrogenic approximation within second-order perturbation theory has been widely used due to the existence of analytical expressions of the wave functions. So far, only (partially) asymptotic results have been derived, which fail to correctly describe the low-kinetic-energy behaviour, especially for high angular-momentum states. Here, we report an analytical expression that overcomes these limitations. It is based on the Appell's F₁ function and uses the confluent hypergeometric function of the second kind as the intermediate state. We show that the derived formula quantitatively agrees with the numerical simulations using the time-dependent Schrödinger equation for various angular-momentum states, which improves the accuracy compared to the other analytical approaches that were previously reported. Furthermore, we give an angular-momentum-dependent asymptotic form of the outgoing wavefunction and the corresponding continuum–continuum dipole transition amplitudes.

Resonant formation of the muonic molecular ion $(\alpha \mu t)_{01}^{2 + }$ in the collision of the muonic tritium atom with the helium hydride ion, ${(t\mu )_{1s}} + {}^4{\text{He}}{{\text{H}}^ + } \to {[(\alpha \mu t)_{01}^{2 + } {\text{, }} p,ee]^ + }$, is proposed. The $(\alpha \mu t)_{01}^{2 + }$ ion is formed in the rotational-vibrational state (0,1) as one of the two nuclei of the final complex. Back-decay process $[(\alpha \mu t)_{01}^{2 + }{\text{, }} p,ee]_{Kn}^ + \to {(t\mu )_{1{\text{s}}}} + {}^4{\text{HeH}}_{K{^{^{\prime}}}n{^{^{\prime}}}}^ +$, where K and n ($K{^{^{\prime}}}$and $n{^{^{\prime}}}$) is the rotational and vibrational quantum number, respectively, is considered in detail and the corresponding decay widths for all possible rotational-vibrational $(K,n) \to (K{^{^{\prime}}},n{^{^{\prime}}})$ transitions are calculated. Numerous inelastic decay channels of $(\alpha \mu t)_{01}^{2 + }$ are also presented and discussed. The possible fast resonant formation of the $(\alpha \mu t)_{01}^{2 + }$ ion may provide an interesting tool for experimental studies of $t(\alpha ,\gamma ){}^7{\text{Li}}$ nuclear fusion, which is one of the most important reactions in Big-Bang nucleosynthesis models. Studying this reaction may prove important for solving the lithium problem.

To better understand plasma wall interactions involving tungsten, accurate atomic structure and electron-impact driven collisional processes for near-neutral ion stages of tungsten are required. Complementing existing work on neutral and singly ionised tungsten, atomic structure and collisional calculations for W²⁺ electron-impact excitation have been completed. These excitation calculations are an important component of S/XB coefficients for near-neutral charge states, which may be used to spectroscopically infer re-deposition of tungsten at the plasma-solid boundary of fusion relevant devices. With W²⁺ in particular having emission lines that can be observed at ultraviolet (UV) wavelengths, while higher charge states of tungsten are unlikely to have lines possible to observe outside of the vacuum UV range. The atomic structure was generated using the General-purpose Relativistic Atomic Structure Package (GRASP⁰), implementing the Multi-configuration Dirac Fock approach. This structure was the basis for a subsequent Dirac R-matrix electron-impact excitation calculation to provide Maxwellian averaged rate coefficients. A synthetic spectrum was generated from this data using a collisional-radiative model to predict the strongest W III spectral lines and these lines were compared to emission from the Compact Toroidal Hybrid (CTH) plasma device. Several of the strongest W III lines are observed in CTH and agree well with the modelled line wavelengths and intensities, a table of these lines is provided that could be observed in other devices.

The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I⁺ with phenyl or phenyl-like cations (C_nH$_x\,^+$ or C_nH$_x\,^{2+}$, with n  =  1 – 6 and x  =  1 – 5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of $\mathrm{C}_6\mathrm{H}_x\,^{2+}$ intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps.

We investigate fine structure changing collisions in ⁸⁷Rb vapour upon D₂ excitation in a thermal vapour at 75°C; the atoms are placed in a 0.6 T axial magnetic field in order to gain access to the hyperfine Pashen-Back regime. Following optical excitation on the D₂ line, the exothermic transfer ⁵P_3/2 →⁵P_1/2 occurs as a consequence of buffer-gas collisions; the ⁸⁷Rb subsequently emits a photon on the D₁ transition. We employ single-photon counting apparatus to monitor the D₁ fluorescence, with an etalon filter to provide high spectral resolution. By studying the D₁ fluorescence when the D₂ excitation laser is scanned, we see that during the collisional transfer process the m'_J quantum number of the atom changes, but the nuclear spin projection quantum number, m'_I, is conserved. A simple kinematic model incorporating a coefficient of restitution in the collision accounted for the change in velocity distribution of atoms undergoing collisions, and the resulting fluorescence lineshape. The experiment is conducted with a nominally "buffer-gas free" vapour cell; our results show that fine structure changing collisions are important with such media, and point out possible implications for quantum-optics experiments in thermal vapours producing entangled photon pairs with the double ladder configuration, and solar physics magneto-optical filters.

Single center convergent close-coupling calculations have been completed for positron scattering from atomic fluorine. Total, electron-loss, positronium-formation, direct ionization, momentum transfer, elastic, bound-state excitation, and stopping power cross sections have been determined for energies between threshold and 5000 eV. Past calculations for this scattering system exist only for elastic and momentum-transfer cross sections. For high energies, good agreement is found between current and past results. At low energies, however, large differences are found between the current calculations and previous results. The atomic fluorine results are then used in a modified independent atom approach to calculate cross sections for positron scattering on F₂, HF, CF₄, C₂F₆, C₃F₆, C₃F₈, and C₆F₆. The current molecular results are typically higher than previous positron experiments across the calculated energy range, however, these experiments were not corrected for the forward angle scattering effect and likely underestimate the true result. Good agreement is found between the current positron results and previous electron experiments and calculations at high energies.

Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, as they feature both excellent control over the electronic state preparation and readout as well as a versatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single ¹³⁸Ba⁺ ion prepared in the 5d ²D_3/2,5/2 metastable states with a ground state ⁶Li gas near quantum degeneracy. We show that in contrast to previously reported atom-ion mixtures, several non-radiative processes, including charge exchange, excitation exchange and quenching, compete with each other due to the inherent complexity of the ion-atom molecular structure. We present a full quantum model based on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internal angular momenta and the mechanical rotation of the colliding pair, which is relevant in any other hybrid system composed of an alkali-metal atom and an alkaline-earth ion.

We experimentally study the heating of trapped atomic ions during measurement of their internal qubit states. During measurement, ions are projected into one of two basis states and discriminated by their state-dependent fluorescence. We observe that ions in the fluorescing state rapidly scatter photons and heat at a rate of $\dot{\bar{n}}\gtrsim 2\times 10^4$ quanta s⁻¹, which is orders of magnitude faster than typical anomalous ion heating rates. We introduce a quantum trajectory-based framework that accurately reproduces the experimental results and provides a unified description of ion heating for both continuous and discrete sources.