Momentum Imaging Technique Research Articles

Contribution of different electron-capture mechanisms in the fragmentation of CO23+ upon slow ion-molecule collision

Abstract We present recoil ion - projectile ion coincidence study on the three body fragmentation of CO23+ ion into C+ + O+ + O+ fragments upon impact with
Ar3+, Ar6+ and Ar8+ projectile ions having the same velocity of vp = 0.49 a.u. (atomic units) using recoil ion momentum imaging technique. The kinetic energy
release (KER) distributions are presented and effect of different electron-capture associated processes such as pure capture, electron capture followed by target ionization or projectile auto-ionization is discussed based on the analysis of KER corresponding to different final projectile charge state post collision. As the capture stabilization increased the relative yields in the KER distribution are found to decrease in the high KER side for low charged Ar3+ impact, while an increase is observed for Ar6+ and Ar8+ impacts. We discussed the interplay between electron-capture, projectile auto-ionization and target ionization for the dominance of particular process. We also observed a low-KER feature around 15 eV which was observed for electron
and proton impacts, which was not yet observed for highly charged ion impacts

Three-dimensional (3D) moment imaging with a USB3 oscilloscope.

We report a new implementation of a recently developed 3D momentum imaging technique [Lee etal. J. Chem. Phys. 141, 221101 (2014)]. The previously employed high-speed digitizer in the setup is replaced by a portable USB3 oscilloscope. A new triggering scheme was developed to suppress trigger jitters and to synchronize the signals from a camera and the oscilloscope. The performance of the setup was characterized in the study of laser desorption/ionization of 2,5-dihydroxybenzoic acid on a velocity map imaging apparatus. A ∼60 picosecond time resolution in measuring time-of-flight is achieved with a count rate of ∼1kHz, which is comparable to the system using high-speed digitizers. The new setup affords great portability and wider accessibility to the high-performing 3D momentum imaging technique.

Three-body fragmentation dynamics of BrCNq+ (q=3–6) induced by 1-keV electron impact

The three-body fragmentation dynamics of $\mathrm{BrC}{\mathrm{N}}^{q+} (q=3--6)$ produced by 1-keV electron impact has been investigated using the ion momentum imaging technique. Up to 11 three-body Coulomb explosion channels were identified and analyzed. The corresponding kinetic energy release (KER) distributions were obtained and compared with the predictions of the Coulomb explosion model. By means of the Dalitz plot, Newton diagram, and native frame method, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{Br}}^{l+}+{\mathrm{C}}^{+}+{\mathrm{N}}^{+} (l=1--3)$. The KER for the intermediate dications $\mathrm{Br}{\mathrm{C}}^{2+}$ and $\mathrm{C}{\mathrm{N}}^{2+}$ from the sequential mechanism were determined and the electronic states of the intermediate molecular ion $\mathrm{C}{\mathrm{N}}^{2+}$ were discussed. For the channels leading to higher charge states of the carbon and nitrogen ions, only the concerted fragmentation mechanism was observed.

Picosecond vs Femtosecond: Are All Laser Desorption Ionizations Created Equal?

With a three-dimensional (3D) momentum imaging technique, we investigated the laser desorption ionization dynamics initiated by both chirped picosecond and femtosecond pulses. 3D momentum images of desorbed ions from 2,5-dihydroxybenzoic acid (DHB), a common laser desorption matrix, were obtained for the first time. A striking difference was observed between the processes initiated by femtosecond and picosecond pulses. The lack of initial momentum in ions produced by femtosecond pulses suggests a suppression of plume formation, which can be exploited to increase the sensitivity of matrix-assisted laser desorption ionization.

Multichannel dissociative ionization of ethanol in intense ultraviolet laser fields: Energy correlation between photoelectron emission and fragment recoil

We investigate multichannel dissociative ionization of ethanol in intense ultraviolet laser fields using a photoelectron-photoion coincidence momentum imaging technique. Product channels are clearly separated with detected photoions, and channel-specific photoelectron spectra exhibit multiple spectral components unambiguously assigned to four- and five-photon ionization to the electronic ground state and the electronically excited state. We measure kinetic energy distributions of fragment ions as a function of the energy of a correlated photoelectron to reveal how much energy is provided to the fragment recoil of fragment ions in the course of photoelectron emission and subsequent electronic excitation. Subsequent electronic excitation rather than photoelectron emission governs the internal energy of ethanol cations. The role of subsequent electronic excitation becomes more decisive as the laser intensity increases.

Electron-impact-like feature in triple fragmentation of CO23+ under slow proton impact

We have studied the three-body breakup of ${\mathrm{CO}}_{2}^{3+}$ created by the impact of slow ($v<1$ a.u.) protons on ${\mathrm{CO}}_{2}$. Employing the coincidence momentum imaging technique, we have obtained the correlated momenta of the three fragments, ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{O}}^{+}$, and thence the kinetic energy release (KER) distribution. By examining the Dalitz plots for the three-body breakup, we separate the sequential and concerted breakup events and analyze the corresponding KER distributions. Based on the potential energy curves reported in the literature, we identify the possible transient states of ${\mathrm{CO}}_{2}^{3+}$ that lead to the different features in the KER distribution. We find that under ${\mathrm{H}}^{+}$ impact, the KER distribution shows a low-energy feature, which is seen in the case of electron impact, but not under the impact of slow or fast highly charged ions. This feature is identified as arising via the low-lying ${}^{2}\mathrm{\ensuremath{\Pi}}$ and ${}^{4}\mathrm{\ensuremath{\Pi}}$ states of the transient molecular ion. We further note that slow protons, which are the simplest of projectiles, are able to excite the ${\mathrm{CO}}_{2}$ molecule to a broad range of triply ionized transient states, spanning those accessed by the impact of slow highly charged ions and also those accessed by electron impact, but with varying probabilities.

Momentum-Resolved Observation of Exciton Formation Dynamics in Monolayer WS2.

The dynamics of momentum-dark exciton formation in transition metal dichalcogenides is difficult to measure experimentally, as many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tunable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS2. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast time scale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic time scale and observe the formation of a momentum-forbidden dark KΣ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy, we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2.

Triple ionisation of OCS: orientation dependence in the case of ion impact

The dependence of the cross-section for multiple ionisation on the orientation of free molecules with respect to the incident projectile has long been demonstrated in the case of diatomic molecules. The dependence has been studied via the coincidence momentum imaging technique, where, under the axial recoil approximation, the orientation of the molecule with respect to the incident projectile is determined from the measured momentum vectors of fragments, which, for diatomic molecules, are necessarily emitted back-to-back. The case of triatomic molecules is complicated owing to the possibility of simultaneous or step-wise breaking of the two bonds, precluding a simple mapping of the fragment momentum vectors to the initial orientation. We report a measurement of triple ionisation of an asymmetric triatomic molecule, OCS, which clearly shows, that the process indeed has an orientation dependence similar to that in diatomic molecules. As expected from the studies on diatomic molecules, the dependence is stronger for small values of q/v and negligible for large values of q/v, where q and v are the projectile charge and velocity, respectively.

Orientation dependence of multiple ionisation of a diatomic molecule under proton impact

Fragmentation of multiply ionised molecular ions is investigated using the coincidence momentum imaging technique in an experiment involving proton–CO collisions over the energy range 25–200 keV. From the measured momentum vectors of the fragment ions, the orientation angle—that is the angle between the axis of the molecule and the direction of the projectile at the instant of collision—is determined under the axial recoil approximation. The multiple ionisation yield is found to strongly depend on the orientation angle, giving rise to an anisotropy as well as a forward–backward asymmetry, reflecting the heteronuclear nature of the CO target. A simple model is used to calculate the probability of multiple ionisation, involving Rutherford-like trajectories with an orientation dependent distance of closest approach and an impact parameter dependent single ionisation probability. The computed probabilities show an orientation dependence which agrees closely with experimental observations.

Anisotropy in multiple ionisation of CO by ion collisions at intermediate interaction strengths

Multiple ionisation of the CO molecule following the impact of 100–700 keV He2+ ions was studied. Fragments arising from the dissociation of were recorded using the coincidence momentum imaging technique. By measuring the angular distribution of the fragment ions in coincidence, and applying the axial recoil approximation, the angle between the axis of the molecule and the direction of the projectile was determined. Strong anisotropy was observed for high degree of ionisation. The dependence of the anisotropy on the interaction strength and the projectile energy was studied and the results were compared with the predicted values in the literature for diatomic molecules. Furthermore, an asymmetry in the angular distribution owing to the heteronuclear nature of the molecule was observed and quantified, showing that the anisotropy is not entirely a geometric effect as has been presented in earlier works.

Absolute ion detection efficiencies of microchannel plates and funnel microchannel plates for multi-coincidence detection.

Modern momentum imaging techniques allow for the investigation of complex molecules in the gas phase by detection of several fragment ions in coincidence. For these studies, it is of great importance that the single-particle detection efficiency ε is as high as possible, as the overall efficiency scales with εn, i.e., the power of the number of detected particles. Here we present measured absolute detection efficiencies for protons of several micro-channel plates (MCPs), including efficiency enhanced "funnel MCPs." Furthermore, the relative detection efficiency for two-, three-, four-, and five-body fragmentation of CHBrClF has been examined. The "funnel" MCPs exhibit an efficiency of approximately 90%, gaining a factor of 24 (as compared to "normal" MCPs) in the case of a five-fold ion coincidence detection.

Soft-x-ray-induced ionization and fragmentation dynamics of Sc3N@C80 investigated using an ion-ion-coincidence momentum-imaging technique

The fragmentation dynamics of an endohedral fullerene, $\mathrm{S}{\mathrm{c}}_{3}\mathrm{N}@{\mathrm{C}}_{80}$, after absorption of a soft-x-ray photon, has been studied with an ion-ion-coincidence momentum-imaging technique. Molecular inner-shell ionization at 406.5 eV, targeting the Sc ($2p$) shell of the encapsulated $\mathrm{S}{\mathrm{c}}_{3}\mathrm{N}$ moiety and the C ($1s$) shell of the ${\mathrm{C}}_{80}$ cage, leads to the cage fragmentation through evaporation of ${\mathrm{C}}_{2}$, emission of small molecular carbon ions (${\mathrm{C}}_{n}{}^{+}$, $n\ensuremath{\le}24$), and release of Sc and Sc-containing ions associated with the carbon cage opening or fragmentation. The predominant charge states of Sc and Sc-containing ionic fragments are $+1$ despite an effective Sc valence of 2.4, indicating that charge transfer or redistribution plays an important role in the fragmentation of the encaged $\mathrm{S}{\mathrm{c}}_{3}\mathrm{N}$. Sequential emission of two out of the three Sc atoms of the encaged moiety, via Coulomb explosion in the form of $\mathrm{S}{\mathrm{c}}^{+}$ or Sc-containing ions, is significant. We also find that the resonant excitation of the Sc ($2p$) shell electrons significantly increased the yield of the parent $\mathrm{S}{\mathrm{c}}_{3}\mathrm{N}@{\mathrm{C}}_{80}$ and its fragment ions, partially attributed to the collision of the energetic Auger electrons from the Sc site with the carbon cage.

Enhanced ionisation of polyatomic molecules in intense laser pulses is due to energy upshift and field coupling of multiple orbitals

We present the results of a combined experimental and numerical study on strong-field ionisation of acetylene performed with the aim of identifying the mechanism behind the previously reported surprisingly large multi-electron ionisation probabilities of polyatomic molecules. Using coincidence momentum imaging techniques and time-dependent density functional simulations, we show that the reported efficient ionisation is due to the combined action of a significant geometrically induced energy upshift of the most relevant valence orbitals as the C–H distance stretches beyond about two times the equilibrium distance, and a strong increase in the coupling between multiple molecular orbitals concomitant with this stretch motion. The identified enhanced ionisation mechanism, which we refer to as EIC-MOUSE, is only effective for molecules aligned close to parallel to the laser polarisation direction, and is inhibited for perpendicularly aligned molecules because of a suppression of the C–H stretch motion during the onset of ionisation.

Fragmentation dynamics of carbonyl sulfide in collision with 500 eV electron.

The fragmentation dynamics of OCSq+ (q = 2, 3, 4) induced by electron collision at an impact energy of 500 eV is studied. By using the momentum imaging technique, the three dimensional momentum vectors of all the fragments are obtained, which enables us to analyse both the kinetic energy release and the momentum correlations for a certain fragmentation channel. Up to fifteen dissociation channels are analyzed including six, five, and four channels for two-body, and incomplete and complete three-body Coulomb fragmentations. For three-body dissociation, the fragmentation mechanisms are investigated with the help of Dalitz plot and Newton diagram. It is found that the sequential fragmentation involves in OCS2+→O+C++S+ with S+ emitted first and in OCS3+→O++C++S+ with O-C and C-S bonds breaking first. The remaining channels, however, always dissociate through a concerted mechanism. The relative intensities of the channels are also presented in this work.

Fragmentation dynamics in dissociative electron attachment to CO probed by velocity slice imaging.

Complete dissociation dynamics in electron attachment to carbon monoxide (CO) have been studied using the newly developed velocity slice imaging (VSI) technique. Both kinetic energy and angular distributions of O(-) ions formed by dissociative electron attachment (DEA) to CO molecules have been measured for 9, 9.5, 10, 10.5, 11, and 11.5 eV incident electron energies around the resonance. Detailed observations conclusively show that two separate DEA reactions lead to the formation of O(-) ions in the ground (2)P state along with the neutral C atoms in the ground (3)P state and the first excited (1)D state, respectively. Within the axial recoil approximation and involving four partial waves, our angular distribution results clearly indicate that the two reactions leading to O(-) formation proceed through the specific resonant state(s). For the first process, more than one intermediate state is involved. On the other hand, for the second process, only one state is involved. The observed forward-backward asymmetry is explained in terms of the interference between the different partial waves that are involved in the processes.

Hydrogen and fluorine migration in photo-double-ionization of 1,1-difluoroethylene (1,1-C2H2F2) near and above threshold

We have studied the nondissociative and dissociative photo-double-ionization of 1,1-difluoroethylene using single photons of energies ranging from 40 to 70 eV. Applying a coincident electron-ion three-dimensional momentum imaging technique, kinematically complete measurements have been achieved. We present the branching ratios of the six reaction channels identified in the experiment. Electron-ion energy maps and relative electron emission angles are used to distinguish between direct and indirect photo-double-ionization mechanisms at a few different photon energies. The influence of selection and propensity rules is discussed. Threshold energies of double ionization are extracted from the sum of the kinetic energies of the electrons, which hint to the involvement of different manifolds of states. The dissociative ionization channels with two ionic fragments are explored in detail by measuring the kinetic energy release of the fragment ions, sum of the kinetic energies, as well as the energy sharing of the two emitted electrons. We investigate the migration of hydrogen and fluorine atoms and compare the experimental results to the photo-double-ionization of centrosymmetric linear and planar hydrocarbons (${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$ and ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$) whenever possible.

Attosecond evolution of energy- and angle-resolved photoemission spectra in two-color (XUV+IR) ionization of rare gases

We have analyzed the angular distributions of the photoelectrons emitted upon photoionization of rare gases by a comb of harmonics in the extreme ultraviolet range, in the presence of a “dressing” infrared (IR) field with controlled delay ?, stabilized down to about ±60 as. The measurements have been performed with the help of the coincidence momentum imaging technique. We evidence marked differences in the measured angular distributions of the photoelectrons, depending on the number of IR photons exchanged. Joined to a theoretical interpretation, these observations bring new insights into the dynamics of this class of two-color photoionization processes that are a key step towards studying photoionization in the time domain, with attosecond time resolution.

Carrier-Envelope Phase Control over Pathway Interference in Strong-Field Dissociation ofH2+

The dissociation of an H2+ molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10(14) W/cm2 with a central wavelength of 730 nm was studied using a coincidence 3D momentum imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission direction of H+ fragments relative to the laser polarization were observed. These asymmetries are caused by interference of odd and even photon number pathways, where net zero-photon and one-photon interference predominantly contributes at H+ + H kinetic energy releases of 0.2-0.45 eV, and net two-photon and one-photon interference contributes at 1.65-1.9 eV. These measurements of the benchmark H2+ molecule offer the distinct advantage that they can be quantitatively compared with ab initio theory to confirm our understanding of strong-field coherent control via the carrier-envelope phase.

Dissociative-electron-attachment dynamics near the 8-eV Feshbach resonance of CO2

We present experimental results for dissociative electron attachment to carbon dioxide near the 8-eV Feshbach resonance. In particular, the dissociation channel leading to O${}^{\ensuremath{-}}$ production has been investigated with a momentum imaging technique that utilizes a supersonic gas jet to form a low-temperature, confined molecular target. Angular fragmentation and kinetic energy release distributions are compared to recent results by Slaughter et al. [J. Phys. B 44, 205203 (2012)] and Wu et al. [Phys. Rev. A 85, 052709 (2012)] using similar techniques. We show that careful attention to weighting of the O${}^{\ensuremath{-}}$ fragmentation momentum space is required to interpret the kinetic energy release observations and that there is no appreciable change in the angular distribution of O${}^{\ensuremath{-}}$ fragments at different energies around the resonance peak, as previously reported by Wu et al. The present O${}^{\ensuremath{-}}$ momentum distribution and kinetic energy release differ from previous results and provide alternative guidance for the theoretical consideration of potential energy surface dynamics that takes place during attachment.

Strong field dissociation dynamics of NO2+: A multiphoton electronic or vibrational excitation?

A coincidence 3-D momentum imaging technique is employed to study the dissociation of a metastable NO2+ beam in an intense ultrafast laser field. We find that dissociation into N+ + O+ is aligned perpendicular to the laser polarization for intensities below 1015 W/cm2 while higher intensities (~1016 W/cm2) yield a prominent contribution from molecules breaking parallel to the field. The latter feature may be due to direct 2-photon transition to the vibrational continuum of the X2Σ+ ground state, a surprising multiphoton vibrational excitation.