Cold Target Recoil Ion Momentum Spectroscopy Research Articles

Strong Field-Induced Quantum Dynamics in Atoms and Small Molecules

Abstract 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 (COLTRIMS) 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 (HASE) 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.

Laser-induced electron diffraction: Imaging of a single gas-phase molecular structure with one of its own electrons.

Among the many methods to image molecular structure, laser-induced electron diffraction (LIED) can image a single gas-phase molecule by locating all of a molecule's atoms in space and time. The method is based on attosecond electron recollision driven by a laser field and can reach attosecond temporal resolution. Implementation with a mid-IR laser and cold-target recoil ion-momentum spectroscopy, single molecules are measured with picometer resolution due to the keV electron impact energy without ensemble averaging or the need for molecular orientation. Nowadays, the method has evolved to detect single complex and chiral molecular structures in 3D. The review will touch on the various methods to discuss the implementations of LIED toward single-molecule imaging and complement the discussions with noteworthy experimental findings in the field.

Study of state-selective single-electron capture in slow Ne7+ colliding with He and H2 target

Using the cold target recoil-ion momentum spectroscopy, we measured the state-selective single electron capture in collisions of 3.5–6.3 keV u−1 Ne7+ ions and He and H2. The Q-value spectra and angular distributions as a function of the projectile incident energy were obtained. The results show that the single electron capture into n = 4 states is the dominant reaction channel for the Ne7+-He system, while the capture into n = 4 and n = 5 states both are dominant processes for the Ne7+-H2 system. The molecular coulombic barrier model and the potential energy curves can qualitatively explain the measured state populations for both systems.

H2 formation via non-Born-Oppenheimer hydrogen migration in photoionized ethane

Neutral H2 formation via intramolecular hydrogen migration in hydrocarbon molecules plays a vital role in many chemical and biological processes. Here, employing cold target recoil ion momentum spectroscopy (COLTRIMS) and pump-probe technique, we find that the non-adiabatic coupling between the ground and excited ionic states of ethane through conical intersection leads to a significantly high yield of neutral H2 fragment. Based on the analysis of fingerprints that are sensitive to orbital symmetry and electronic state energies in the photoelectron momentum distributions, we tag the initial electronic population of both the ground and excited ionic states and determine the branching ratios of H2 formation channel from those two states. Incorporating theoretical simulation, we established the timescale of the H2 formation to be ~1300 fs. We provide a comprehensive characterization of H2 formation in ionic states of ethane mediated by conical intersection and reveals the significance of non-adiabatic coupling dynamics in the intramolecular hydrogen migration.

State-selective Charge Exchange in 19.5–100 keV amu−1 O6+ Collision with He and H2

The cross sections of state-selective charge exchange (CX) between highly charged ions and neutrals are important for modeling extreme ultraviolet and soft X-ray emissions in many astrophysical objects with hot plasma impacting cold media. By using cold-target recoil-ion momentum spectroscopy, we measure O6+ CX collisions with He and H2 in the collision energy range of 19.5 to 100 keV amu−1. The relative cross sections of state-selective single CX are reported for electron capture into O5+(1s 2 nl) n = 2, 3, 4, 5, ≥6 and n = 3, 4, 5, ≥6 for He and H2, respectively. With the collision energy increasing to 100 keV amu−1, the main capture channel shifts to higher n for both target species as compared to available theoretical results. We also report state-selective cross sections for double CX with He, where doubly excited states of with being 2s 2 (1S), 2s2p (1P), 2p 2 (1D), and 2p 2 (1S) [symmetric configurations] and 2s3l, 2p3l−2s4l, and 2s5l−2p5l [asymmetric configurations] are distinguished. It is found that the contributions of doubly excited states with asymmetric configurations are dominant and symmetric configurations increasingly come into play with the increase of collision energy. The present results provide experimental benchmarks available for theoretical calculations.

Production of autoionizing states by double-electron capture in intermediate-energy C4+ + He collisions

We have performed a kinematically complete experiment for state-selective double-electron capture occurring in 15-keV/u ${\mathrm{C}}^{4+}$ $+$ He collisions by means of cold-target recoil-ion momentum spectroscopy. It was shown that capture into ground and low excited states of the ${\mathrm{C}}^{2+}$ ions is overwhelmingly dominant. Besides, a small fraction of autoionization decays from doubly excited states of symmetric as well as asymmetric electron configurations following endothermic double-electron capture were clearly observed. Emphasis was given to the population mechanisms of the different electron configurations. Our analysis indicates that electron-electron correlation effects may play a major role in the double-electron capture process. In addition, the large transverse recoil-ion momentum of symmetric configurations as compared to asymmetric configurations for the doubly excited autoionizing states may be attributed to the different number of steps involved in the primary capture process, with the former likely produced by a two-step mechanism and the latter by a one-step mechanism.

Benchmark n ℓ-resolved Cross Sections of Single and Double Charge Exchange Processes in 1.67–20 keV u−1 C4+ Collisions with He

State-resolved charge exchange (CX) cross sections are of the utmost importance for modeling related photon emissions existing in a broad range of astrophysical environments. With the cold-target recoil-ion momentum spectroscopy, we determined with high accuracy the state-resolved single and double CX cross sections at the quantum orbital angular momentum level for solar wind ion C4+ collisions with He in an energy range of 1.67–20 keV u−1, which allow one to benchmark the CX calculations in great detail, and to test the applicability of the analytical n- and ℓ-distribution models widely adopted by the astrophysical community. We found that the present measurements are well reproduced by the most recent state-of-the-art atomic–orbital close-coupling calculations. However, the CX models failed to give a consistent description on the measured ℓ distributions. The present work reveals that the velocity and collision partner species dependence effects as well as electronic correlations for multielectron processes should be included in an improved model. Alternatively, in future modeling to interpret high-resolution astrophysical observations the more elaborate quantum-mechanical calculations may be resorted to with confidence.

Sequential deprotonation of the allene trication produced by 30-keV/u He2+ impact

The three-body fragmentation channel ${\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_{3}{\mathrm{H}}_{2}{}^{+}$ of the allene trication produced by 30-keV/u ${\mathrm{He}}^{2+}$ ion impact is investigated using cold-target recoil-ion momentum spectroscopy. The two protons with different kinetic energies are found emitted sequentially from the parent trication. The presence of such a sequential deprotonation process and the absence of corresponding concerted deprotonation may result from a major molecular deformation process, e.g., ultrafast hydrogen migration, which occurs in the parent ion prior to fragmentation, and finally leads to the formation of two different protons.

Uncertainties in Atomic Data for Modeling Astrophysical Charge Exchange Plasmas.

Relevant uncertainties of theoretical atomic data are vital to determining the accuracy of plasma diagnostics in a number of areas, including, in particular, the astrophysical study. We present a new calculation of the uncertainties on the present theoretical ion-impact charge exchange atomic data and X-ray spectra, based on a set of comparisons with the existing laboratory data obtained in historical merged-beam, cold-target recoil-ion momentum spectroscopy, and electron beam ion traps experiments. The average systematic uncertainties are found to be 35–88% on the total cross sections, and 57–75% on the characteristic line ratios. The model deviation increases as the collision energy decreases. The errors on total cross sections further induce a significant uncertainty to the calculation of ionization balance for low-temperature collisional plasmas. Substantial improvements of the atomic database and dedicated laboratory measurements are needed to obtain the current models, ready for the X-ray spectra from the next X-ray spectroscopic mission.

Measurement of n-resolved State-selective Charge Exchange in Ne(8,9)+ Collision with He and H2

Abstract Charge exchange between highly charged ions and neutral atoms and molecules has been considered as one of the important mechanisms controlling soft X-ray emissions in many astrophysical objects and environments. However, for modeling charge exchange soft X-ray emission, the data of n- and l-resolved state-selective capture cross sections are often obtained by empirical and semiclassical theory calculations. With a newly built cold target recoil-ion momentum spectroscopy (COLTRIMS) apparatus, we perform a series of measurements of the charge exchange of Ne(8,9)+ ions with He and H2 for collision energy ranging from 1 to 24.75 keV/u. n-resolved state-selective capture cross sections are reported. By comparing the measured state-selective capture cross sections to those calculated by the multichannel Landau–Zener method (MCLZ), it is found that MCLZ calculations are in good agreement with the measurement for the dominant n capture for He target. Furthermore, by using nl-resolved cross sections calculated by MCLZ and applying l distributions commonly used in the astrophysical literature to experimentally derived n-resolved cross sections, we calculate the soft X-ray emissions in the charge exchange between 4 keV/u Ne8+ and He by considering the radiative cascade from the excited Ne7+ ions. Reasonable agreement is found in comparison to the measurement for even and separable models, and MCLZ calculations give results in a better agreement.

Dissociation of NH32+ induced by collision of 300 eV electrons with NH3

The dissociation of ammonia dications induced by 300 eV electron impact has been studied using a cold target recoil-ion momentum spectroscopy. Three regimes were found in the evolution of the produced NH32+ ions, i.e. stable (long-lived), slowly dissociating (metastable), and promptly dissociating. Multiple dissociation channels of NH32+ have been distinguished and identified by ion–ion coincidence technique. The kinetic energy release of the deprotonation dissociation channel NH32+ → H+ + NH2+ and the bond-rearrangement channel NH32+ → H2+ + NH+ have been obtained. The dynamic information and complex fragmentation pathways were analyzed by combining Dalitz plot and Newton diagram for the three-body dissociation channel NH32+ → H+ + NH+ + H. Strong correlation between the dynamic processes of H+, H loss, and the kinetic energy release was found.

State-selective single electron capture in slow O 6+-He collisions

SynopsisThe state-selective single-electron capture processes were measured by cold-target recoil-ion momentum spectroscopy in O 6+-He collisions between 18keV and 96keV. From the longitudinal momentum spectrum of recoil ions, the single electron captured into n=3 states of projectile ion is predominant and n=4 states have a small contribution. The oscillation structure of angular differential cross sections was observed. The present experimental results are compared with other experiments at low energy and the predictions of the molecular Columbic barrier model.

Tracing intermolecular Coulombic decay of carbon-dioxide dimers and oxygen dimers after valence photoionization

We have conducted an experimental study on the photo double ionization (PDI) of carbon-dioxide dimers at photon energies of 37 and 55 eV and oxygen dimers at photon energies of 38, 41.5, and 46 eV, while focusing on the dissociation dynamics upon single-photon absorption. The investigation was performed by applying the cold-target recoil-ion momentum spectroscopy method in order to collect and record the three-dimensional momenta of the ionic fragments and emitted electrons from the dissociating dimer in coincidence. The kinetic-energy release upon fragmentation and the electron angular distributions in the laboratory and body-fixed frames, as well as the relative electron-electron emission angle, show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon-dioxide dimers upon photoionization below and above the double-ionization threshold of ${\mathrm{CO}}_{2}$ monomers. The PDI of oxygen dimers is less conclusive and shows contributions from ICD and knock-off ionization mechanisms. As for atomic dimers, the present results reveal that ICD in ${\mathrm{CO}}_{2}$ dimers after valence PDI can also serve as a source for low-energy electrons, known to be very relevant in biological systems, cells, and tissues.

Fragmentation of C2H2 2+ in collisions with 100 eV electrons

The fragmentation of C2H2 2+ dications following 100 eV electron impact double ionization of acetylene was studied experimentally using a cold target recoil-ion momentum spectroscopy. A weak extraction field was adopted to increase the experimental resolution. Meanwhile, as a consequence of the large momentum generated by the Coulomb repulsive force, the particles which have similar charge to mass ratio such as C+ and CH+ cannot be distinguished directly from the time of flight. In order to obtain a comprehensive understanding of the experimental results, a Monte Carlo (M-C) method was used to simulate the time of flight under experimental conditions. With the help of the M-C method, four reaction channels, the Coulomb explosion of the doubly charged parent ions C2H2 2+ into the fragment ions CH+ + CH+,C+ + CH+2, C+ + CH+, C+ + C+, were simulated and the kinetic energy release (KER) distributions were deduced. In particular, the structure of fragmentation channel to C+ + CH+ represents a typical feature of the absence of hydrogen migration, which was not observed in previous studies. The KER in the fragmentation reaction of C2H2 2+ has been used to obtain precursor state energy for the formation of product ion pairs. In general, good agreement was achieved between our KER results and available experimental data in previous works.

Coulomb over-the-barrier Monte Carlo simulation to probe ion-dimer collision dynamics

We present a combined theoretical and experimental study of primary and post-collision mechanisms involved when colliding low energy multiply charged ions with van der Waals dimers. The collision dynamics is investigated using a classical calculation based on the Coulombic Over-the- Barrier Model adapted to rare-gas dimer targets. Despite its simplicity, the model predictions are found in very good agreement with experimental results obtained using COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS), both for the relative yields of the different relaxation processes and for the associated transverse momentum exchange distributions between the projectile and the target. This agreement shows to which extent van der Waals dimers can be assimilated to independent atoms.

Classical simulation of differential single charge transfer in fast proton-helium collisions

Three-body classical trajectory Monte Carlo method is employed to simulate the differential single electron capture process in fast proton-helium collisions. For the considered collisional system, by means of an independent particle model, both electron capture and electron excitation probabilities are evaluated in terms of the classical impact parameter and the related discussions are presented. The method is also applied to calculate the projectile-angular distribution of the cross sections in energy range of 50–630 keV. The obtained results are compared to the available precise data due to the cold-target recoil ion momentum spectroscopy and good overall agreement found with these experimental data. Also, within a classical-trajectory framework, the correlation between the impact parameter and the projectile scattering angle is examined through the simulation of the collision process.

Three-body fragmentation of methane dications produced by slow Ar8+ -ion impact

The three-body fragmentation dynamics of ${{\mathrm{CH}}_{4}}^{2+}$ dications induced by single-electron capture of slow (3-keV/u) $\mathrm{A}{\mathrm{r}}^{8+}$ ions is investigated. The experiment is performed on a newly built, highly charged ion collision platform which consists of an electron cyclotron resonance ion source and a cold target recoil ion momentum spectroscopy (COLTRIMS) setup. Using the COLTRIMS methodology, the complete kinematical information is determined for two three-body breakup channels, ${{\mathrm{CH}}_{4}}^{2+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{{\mathrm{CH}}_{2}}^{+}+\mathrm{H}$ and ${{\mathrm{CH}}_{4}}^{2+}\ensuremath{\rightarrow}{{\mathrm{H}}_{2}}^{+}+\mathrm{C}{\mathrm{H}}^{+}+\mathrm{H}$. Then analyzing the complete kinematics with the Dalitz plot, very different fragmentation mechanisms (e.g., sequential and/or concerted pathway) are clearly identified for the two channels. To confirm the existence of some possible fragmentation pathways, we also simulate corresponding Dalitz plots employing a simple classical mechanical model. For the ${\mathrm{H}}^{+}+{{\mathrm{CH}}_{2}}^{+}+\mathrm{H}$ channel, the dependence of the fragmentation pathway on its kinetic energy release is studied, which reflects the different nature of the corresponding states of ${{\mathrm{CH}}_{4}}^{2+}$ dications. Furthermore, the kinetic energy ratio of two ionic fragments is analyzed to infer the three-body fragmentation mechanism of ${{\mathrm{CH}}_{4}}^{2+}$ dications.

Dissociative ionization of N2 by fast electron impact: roles of molecular orbitals

Dissociative single, double and triple ionization of N2 induced by fast electron impact are studied using the cold target recoil-ion momentum spectroscopy technique. Kinetic energy releases (KERs) are determined and possible states of parent ions are analyzed for the dissociation channels and also Coulomb explosion channels As the electron-impact energy increases, the different dissociation channels behave quite differently. For example, the KER spectra of the channel → N+ + N show that the intensity of the low-KER part increases and that of the high-KER part decrease, while the KER spectra of the channel → N+ + N+ seem to remain unchanged. Comparing and analyzing the KER distributions (KERDs) of the different dissociation channels, we find that both single and double ionization from different molecular orbitals, i.e. outer or inner valence orbitals can give rise to different decay channels with different electron energy dependence. Therefore, from the perspective of KERD, the present work clearly reveals the important roles of different molecular orbital ionization in the dissociation of molecule.

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers.

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector. Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.

N2 dissociative ionization by electron impact

SynopsisDissociative single, double and triple ionization of N2 induced by 200, 400 and 1000 eV electron impact are studied using the cold target recoil-ion momentum spectroscopy (COLTRIMS) technique. Kinetic energy releases (KERs) are determined for dissociation channels (N2+ → N+ + N, N22+ → N2+ + N) and Coulomb explosion channels (N22+ → N+ + N+, N23+ → N2+ + N+). It is observed that as the electron-impact energy increasing, different channels behave quite differently. Through analyzing the electronic states of the precursor molecular ion in each channel, the present KER measurements show that ionization from different valance orbitals plays a very important role in the dissociation of molecules.