Target Recoil Ion Momentum Spectroscopy Research Articles

CO Dissociation Induced by 1 keV/u Ar2+ Ion

CO is one of the important molecules in dense molecular clouds, and its dissociation induced by cosmic ray heavy ions is a fundamental process for molecular breaking up and rearrangement in astronomical networks. Extensive laboratory simulations are required to understand molecular evolution in astrophysical contexts. Here, we investigate the CO dissociation induced by 1 keV/u Ar2+ using cold target recoil ion momentum spectroscopy. Kinetic energy release for double electron capture Ar2++CO→Ar0+C++O+ and transfer ionization Ar2++CO→Ar++C++O++e− was obtained. The dissociation mechanisms are attributed to different KER distributions. The autoionization process is identified below the CO2+ double ionization threshold.

Impact of Nuclear Motion on Light-Induced Bimolecular Interaction Dynamics

In chemical reactions, the nuclear motion of the molecules plays a crucial role in determining the reaction rates and outcomes. Employing the cold target recoil ion momentum spectroscopy and femtosecond pump-probe techniques, we perform a molecular-level study into the influence of nuclear vibrations on light-induced bimolecular reactions within H2−D2 dimers. The study focuses on the formation dynamics of D2H+ and H2D+ cations, shedding light on the interplay between translational and vibrational motions of the nuclei steering the bimolecular reactions. Our observations reveal a notable yield ratio of 1:1.6 between H2D+ and D2H+ channels, accompanied with a faster formation of D2H+ compared to H2D+. Molecular dynamics simulations unveil that the faster vibrational motion of H2+ than that of D2+ upon single ionization within the dimer accounts for these differences. Our findings provide new insight into the time-resolved kinetic isotope effect on the bimolecular reactions, highlighting the critical relationship between nuclear vibrational motions and reaction dynamics. Published by the American Physical Society 2024

Demonstration of full polarization control of soft X-ray pulses with Apple X undulators at SwissFEL using recoil ion momentum spectroscopy.

The ability to freely control the polarization of X-rays enables measurement techniques relying on circular or linear dichroism, which have become indispensable tools for characterizing the properties of chiral molecules or magnetic structures. Therefore, the demand for polarization control in X-ray free-electron lasers is increasing to enable polarization-sensitive dynamical studies on ultrafast time scales. The soft X-ray branch Athos of SwissFEL was designed with the aim of providing freely adjustable and arbitrary polarization by building its undulator solely from modules of the novel Apple X type. In this paper, the magnetic model of the linear inclined and circular Apple X polarization schemes are studied. The polarization is characterized by measuring the angular electron emission distributions of helium for various polarizations using cold target recoil ion momentum spectroscopy. The generation of fully linear polarized light of arbitrary angle, as well as elliptical polarizations of varying degree, are demonstrated.

Nondipolar photoelectron angular distributions from fixed-in-space N2 molecules

We investigate experimentally and theoretically the N 1s photoionization of fixed-in-space N2 molecules at a photon energy of 880 eV. In our experiment, we employed circularly polarized synchrotron radiation for the photoionization and coincident electron and fragment-ion detection using cold target recoil ion momentum spectroscopy. The accompanying angle-resolved calculations were carried out by the multichannel single-center method and code within the frozen-core Hartree–Fock approximation. The computed emission distributions exhibit two distinct features along the molecular axis, which are the results of a superposition of the direct and nearest-neighbor scattering amplitudes for the photoemission from two nitrogen atoms. In the electric-dipole approximation, these peaks are symmetric with respect to both nitrogen atoms. Including nondipole (retardation) effects in the calculations results in a simultaneous increase and decrease of the scattering peaks towards the nitrogen atoms pointing in the forward and backward directions along the light propagation, respectively. These theoretical findings are in agreement with our experimental findings.

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.

Experimental measurement of state selective double electron capture in 1.4-20 keV/u Ar<sup>8+</sup> collision with He

Electron(s) capture in highly charged ions collision with atoms and molecules is a fundamental process related to the electron transition between bound states belongs to two atomic-centers. The X-ray emission following electron capture is important for X-ray astrophysical modeling, fusion plasma diagnosing, and ion irradiated biophysics. Over past decades, momentum-imaging cold-target recoil ion momentum spectroscopy is one of significantly developed techniques and has been widely applied to measure the quantum state-selective population in electron(s) capture processes. Based on the cold target recoil ion momentum spectroscopy mounted at the 150 kV highly charged ion platform in Fudan University, the state-selectivity of double electron capture in 1.4-20 keV/u Ar<sup>8+</sup> on He collision was measured, and the relative cross sections of the 3<i>l</i>3<i>l</i>’to 3<i>l</i>7<i>l</i>’ double excited states were obtained. It is found that with the increase of collision energy, more quantum state-selectivity channels are open in the double electron capture (DEC) of Ar<sup>8+</sup>-He collision. It is also found that the relative cross section of the quantum state population is strongly dependent on the collision energy of the projectile ion. This present measurement not only enriches the state-selective cross-section repository and collision dynamics for highly charged ion charge exchange processes, but also provides experimental benchmarks for the available theoretical calculations.

Addressing three-body fragmentation of methane dication using "native frames": Evidence of internal excitation in fragments.

The three body fragmentation of methane dication has been studied using the technique of cold target recoil ion momentum spectroscopy. The process is initiated by impact of energetic Ar9+ ions on neutral methane and the data is subsequently collected in coincidence with Ar8+ projectile. By analysing the dissociation channels leading to (H + H+ + CH2+) and (H + H2+ + CH+) fragments, it is concluded that these fragments are formed in a sequential manner via formation of molecular intermediates CH3+ and CH2+ respectively. It is shown that these molecular intermediates carry a few eVs as their internal energies, part of which is released when they emit an H-atom with the open possibility that the final detected fragments may still be internally excited. This was accomplished by analysing the two-steps of the sequential process in their own native frames. For a molecular system having three-dimensional structure, our results prove to be an ideal example to highlight the importance of using native frames for correct interpretation of the obtained results. Our results indicate that the dissociation of methane dication can be a major source of production of H-atoms in addition to H+ fragments with the probability of the two being of similar order.

State-selective charge exchange cross sections in collisions of highly-charged sulfur ions with helium and molecular hydrogen

The state-selective cross section data are useful for understanding and modeling the x-ray emission in celestial observations. In the present work, using the cold target recoil ion momentum spectroscopy, for the first time we investigated the state-selective single electron capture processes for S q+–He and H2 (q = 11–15) collision systems at an impact energy of q × 20 keV and obtained the relative state-selective cross sections. The results indicate that only a few principal quantum states of the projectile energy level are populated in a single electron capture process. In particular, the increase of the projectile charge state leads to the population of the states with higher principal quantum numbers. It is also shown that the experimental averaged n-shell populations are reproduced well by the over-barrier model. The database is openly available in Science Data Bank at 10.57760/sciencedb.j00113.00091.

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.

Atomic autoionization in the photo-dissociation of super-excited deuterated water molecules fragmenting into D+ + O+ + D.

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

Measurement of n- and l-resolved State-selective Charge Exchange in Ar8+ Collision with He

The state-resolved capture cross sections for principal n and orbital angular momentum l play an important role in modeling soft X-ray emissions induced by charge exchange for many astrophysical environments. However, the empirical and semiclassical theories used to produce these data of n- and l-resolved state-selective capture have not been well tested. Using the cold target recoil ion momentum spectroscopy apparatus at Fudan University, we perform a series of measurements of Ar8+ ion charge exchange with He in the collision energy range from 1.4 to 20 keV u−1. We find that electrons are mainly captured in the n = 4 state of Ar7+ ions. This agrees with the prediction of the scaling law for n capture. Moreover, the relative cross sections are reported for 4s-, 4p-, 4d-, and 4f-resolved state-selective capture. The often used analytical l distributions in the astrophysical literature are evaluated by comparing to the measurements.

Unexplained dissociation pathways of two-body fragmentation of methane dication.

The ion-induced fragmentation of CH4 2+ into H+ and CH3 + is studied using a cold target recoil ion momentum spectroscopy in coincidence with the charge state of the post-collision projectile. Using constant velocity Ar9+ and N3+, results from four different datasets are presented, with a selection on the final charge state of the projectile (Ar8+ or Ar7+ and N2+ or N+). Three distinct dissociation pathways (I, II, and III) are observed for each dataset, with the mean kinetic energy release values of around 4.7, 5.8, and 7.9eV, respectively. The electronic states that are populated correspond to electronic configurations (1t2)-2 and (2a1)-1(1t2)-1 of the methane dication, CH4 2+. The relative branching ratios between the three pathways are discussed as a function of the charge state of the post-collision projectile, and a strong correlation with the specific nature of the ion-molecule interaction is found. The existing ab initio calculations have provided an explanation only for pathway II. In this article, we propose an explanation for pathway III, but pathway I still remains unexplained and requires further theoretical efforts. A discussion of the dependence of dissociation on the mode of excitation is presented.

Investigation of the carbon monoxide dication lifetime using (CO) 2 dimer fragmentation

The fragmentation of carbon monoxide dimers induced by collisions with low-energy ${\mathrm{Ar}}^{9+}$ ions is investigated using the cold target recoil ion momentum spectroscopy technique. The presence of a neighbor molecule in the dimer serves here as a diagnostic tool to probe the lifetimes of the $\mathrm{C}{\mathrm{O}}^{2+}$ molecular dications resulting from the collision. The existence of metastable states with lifetimes ranging from 2 ps to 200 ns is clearly evidenced experimentally through a sequential three-body fragmentation of the dimer, whereas fast dissociation channels are observed in a so-called concerted three-body fragmentation process. The fast fragmentation process leads to a kinetic energy release distribution also observed in collisions with monomer CO targets. This is found in contradiction with the conclusions of a previous study attributing this fast process to the perturbation induced by the neighbor molecular ion.

Orbital effects in strong-field Rydberg state excitation of N2, Ar, O2 and Xe

Rather than being freed to the continuum, the strong-field tunneled electrons can make a trajectory driven by the remaining laser fields and have certain probability to be captured by the high lying Rydberg states of the parent atoms or molecules. To explore the effect of molecular orbital on Rydberg state excitation, the ellipticity dependence of Rydberg state yields of N2 and O2 molecules are experimentally investigated using cold target recoil ion momentum spectroscopy and are compared with their counterpart atoms Ar and Xe with comparable ionization potentials. We found the generation probability of the neutral Rydberg fragment O2* was orders of magnitude higher than that of Xe* due to the butterfly-shaped highest occupied molecular orbital of O2. Meanwhile, our experimental and simulation results reveal that it is the initial momentum distribution (determined by the detailed characteristics of orbitals) that finally leads to the tendency that the Rydberg state yield of O2 (Ar) decreased slower than that obtained for Xe (N2) when the ellipticity of the excitation laser field is increased.

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.

Delayed fragmentation of ethylene and allene induced by electron impact

We experimentally investigated the delayed fragmentation processes of ethylene and allene dications induced by 300-eV electrons with the cold target recoil ion momentum spectroscopy in an observation window up to \ensuremath{\sim}2 \ensuremath{\mu}s. For each molecule, several distinct pathways involving prompt or delayed fragmentations were observed and relative branching ratios among them were analyzed. According to the time-of-flight coincidence measurements of corresponding recoil ionic fragments, the survival times and thus lifetimes from nanoseconds to microseconds of different delayed-dissociative precursor dications were retrieved. Besides the two-body channels, i.e., ${\mathrm{H}}^{+}+{\mathrm{C}}_{2}{{\mathrm{H}}_{3}}^{+}$ and ${\mathrm{H}}^{+}+{\mathrm{C}}_{3}{{\mathrm{H}}_{3}}^{+}$, the delayed deprotonation was also observed in many-body channels, i.e., $\mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{C}}_{n}{{\mathrm{H}}_{2}}^{+}$ ($n=2$, 3) and $2\mathrm{H}/{\mathrm{H}}_{2}+{\mathrm{H}}^{+}+{\mathrm{C}}_{3}{\mathrm{H}}^{+}$. The lifetimes of the metastable intermediate diacations ${\mathrm{C}}_{n}{{\mathrm{H}}_{3}}^{2+}$ and ${\mathrm{C}}_{3}{{\mathrm{H}}_{2}}^{2+}$ were estimated.

Photoelectron circular dichroism of O 1s-photoelectrons of uniaxially oriented trifluoromethyloxirane: energy dependence and sensitivity to molecular configuration.

The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of trifluoromethyloxirane (TFMOx) is studied experimentally and theoretically for different photoelectron kinetic energies. The experiments were performed employing circularly polarized synchrotron radiation and coincident electron and fragment ion detection using cold target recoil ion momentum spectroscopy. The corresponding calculations were performed by means of the single center method within the relaxed-core Hartree-Fock approximation. We concentrate on the energy dependence of the differential PECD of uniaxially oriented TFMOx molecules, which is accessible through the employed coincident detection. We also compare the results for the differential PECD of TFMOx to those obtained for the equivalent fragmentation channel and similar photoelectron kinetic energy of methyloxirane (MOx), studied in our previous work. Thereby, we investigate the influence of the substitution of the methyl group by the trifluoromethyl group at the chiral center on the molecular chiral response. Finally, the presently obtained angular distribution parameters are compared to those available in the literature.

Experimental measurement of state-selective charge exchange and test of astrophysics soft X-ray emission model

Charge exchange, or electron capture, between highly charged ions and atoms and molecules has been considered as one of important mechanisms controlling soft X-ray emissions in many astrophysical objects and environments. However, to model charge exchange soft X-ray emission, astrophysicists commonly use principal quantum number <i>n</i> and angular momentum quantum number<i>l</i> resolved state-selective capture cross section data, which are usually obtained by empirical and semi-classical theory calculations. The accuracy of the theoretical model is the key to constructing an accurate X-ray spectrum. With a newly-built cold target recoil ion momentum spectroscopy apparatus, we perform a series of precise state-selective cross section measurements on Ne<sup>8+</sup> ions’ single electron capture with He targets, with the projectile energy ranging from 1.4 to 20 keV/u. The experimentally measured Q value spectrum shows that the process of electron captured to state of Ne<sup>7+</sup> with <i>n</i> = 4 is the main reaction channel, and that with <i>n</i> = 3 and 5 are the small reaction channels. Using Gaussian curve to fit the area of each channel on the Q value spectrum and normalizing the area of all channels, we obtain the <i>n</i>-resolved relative state-selective cross section. By comparing the measured relative cross sections with the results calculated by the multichannel Landau-Zener method and molecular Coulomb over-barrier model, significant difference among the strengths of small reaction channels is found. Specifically, the multichannel Landau-Zener method overestimates the contribution of <i>n</i> = 2 channel and <i>n</i> = 3 channel, and underestimates the contribution of <i>n</i> = 5 channel. The molecular Coulomb over-barrier model overestimates the contribution of <i>n</i> = 5 channel and underestimates the contribution of <i>n</i> = 3 channel. The significant difference between the theoretical model calculation and experimental measurement is due to the limitations of semiclassical theoretical method and classical theoretical method. Furthermore, with <i>l</i> distribution models commonly used in the astrophysical literature, including the statistical model, separable model, Landau-Zener-I model, Landau-Zener-II model and even model, we calculate the soft X-ray emissions in the charge exchange between 1.6 and 2.4 keV/u Ne<sup>8+</sup> and He. It is found that the calculated intensities of X-ray spectra significantly deviate from the existing measurements, and only the separable model can partly match the laboratory simulated solar wind charge exchange X-ray measurement. Furthermore, we find that the intensity of the charge exchange X-ray emission spectrum measured experimentally is dependent on the collision energy, while the emission spectrum calculated based on the model seems to be unchanged with the increase of the collision energy. These results indicate that if the classical and semi-classical models are applied to the astrophysical plasma for studying diffusive soft X-ray background, the obtained parameters of the astrophysical plasma will be inaccurate.

Angular emission distribution of O 1s photoelectrons of uniaxially oriented methanol

The angular distribution of O 1s photoelectrons emitted from uniaxially oriented methanol is studied experimentally and theoretically. We employed circularly polarized photons of an energy of hν = 550 eV for our investigations. We measured the three-dimensional photoelectron angular distributions of methanol, with the CH3–OH axis oriented in the polarization plane, by means of cold target recoil ion momentum spectroscopy. The experimental results are interpreted by single active electron calculations performed with the single center method. A comparative theoretical study of the respective molecular-frame angular distributions of O 1s photoelectrons of CO, performed for the same photoelectron kinetic energy and for a set of different internuclear distances, allows for disentangling the role of internuclear distance and the hydrogen atoms of methanol as compared to carbon monoxide.

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.