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Title: Molecular photoelectron angular distribution rotations in multi-photon resonant ionization of H{sub 2}{sup +} by circularly polarized ultraviolet laser pulses

Abstract

We study effects of pulse durations on molecular photoelectron angular distributions (MPADs) in ultrafast circular polarization ultraviolet resonant ionization processes. Simulations performed on aligned H{sub 2}{sup +} by numerically solving time dependent Schrödinger equations show rotations of MPADs with respect to the molecular symmetry axes. It is found that in multi-photon resonant ionization processes, rotation angles are sensitive to pulse durations, which we attribute to the coherent resonant excitation between the ground state and the intermediate excited electronic state induced by Rabi oscillations. Multi-photon nonresonant and single photon ionization processes are simulated and compared which exhibit a constant rotation angle. An asymmetry parameter is introduced to describe the pulse duration sensitivity by perturbation theory models. Influence of pulse frequency detunings on MPADs is also investigated where oscillations of rotations are absent at long pulse durations due to nonresonance excitation.

Authors:
; ;  [1]
  1. Laboratoire de Chimie Théorique, Faculté des Sciences, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1 (Canada)
Publication Date:
OSTI Identifier:
22415636
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 142; Journal Issue: 14; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
74 ATOMIC AND MOLECULAR PHYSICS; ANGULAR DISTRIBUTION; ASYMMETRY; COMPARATIVE EVALUATIONS; EXCITATION; GROUND STATES; HYDROGEN IONS 2 PLUS; MULTI-PHOTON PROCESSES; OSCILLATIONS; PERTURBATION THEORY; PHOTOIONIZATION; POLARIZATION; SCHROEDINGER EQUATION; TIME DEPENDENCE; ULTRAVIOLET RADIATION

Citation Formats

Yuan, Kai-Jun, E-mail: kaijun.yuan@usherbrooke.ca, Chelkowski, Szczepan, and Bandrauk, André D., E-mail: andre.bandrauk@usherbrooke.ca. Molecular photoelectron angular distribution rotations in multi-photon resonant ionization of H{sub 2}{sup +} by circularly polarized ultraviolet laser pulses. United States: N. p., 2015. Web. doi:10.1063/1.4917419.
Yuan, Kai-Jun, E-mail: kaijun.yuan@usherbrooke.ca, Chelkowski, Szczepan, & Bandrauk, André D., E-mail: andre.bandrauk@usherbrooke.ca. Molecular photoelectron angular distribution rotations in multi-photon resonant ionization of H{sub 2}{sup +} by circularly polarized ultraviolet laser pulses. United States. doi:10.1063/1.4917419.
Yuan, Kai-Jun, E-mail: kaijun.yuan@usherbrooke.ca, Chelkowski, Szczepan, and Bandrauk, André D., E-mail: andre.bandrauk@usherbrooke.ca. Tue . "Molecular photoelectron angular distribution rotations in multi-photon resonant ionization of H{sub 2}{sup +} by circularly polarized ultraviolet laser pulses". United States. doi:10.1063/1.4917419.
@article{osti_22415636,
title = {Molecular photoelectron angular distribution rotations in multi-photon resonant ionization of H{sub 2}{sup +} by circularly polarized ultraviolet laser pulses},
author = {Yuan, Kai-Jun, E-mail: kaijun.yuan@usherbrooke.ca and Chelkowski, Szczepan and Bandrauk, André D., E-mail: andre.bandrauk@usherbrooke.ca},
abstractNote = {We study effects of pulse durations on molecular photoelectron angular distributions (MPADs) in ultrafast circular polarization ultraviolet resonant ionization processes. Simulations performed on aligned H{sub 2}{sup +} by numerically solving time dependent Schrödinger equations show rotations of MPADs with respect to the molecular symmetry axes. It is found that in multi-photon resonant ionization processes, rotation angles are sensitive to pulse durations, which we attribute to the coherent resonant excitation between the ground state and the intermediate excited electronic state induced by Rabi oscillations. Multi-photon nonresonant and single photon ionization processes are simulated and compared which exhibit a constant rotation angle. An asymmetry parameter is introduced to describe the pulse duration sensitivity by perturbation theory models. Influence of pulse frequency detunings on MPADs is also investigated where oscillations of rotations are absent at long pulse durations due to nonresonance excitation.},
doi = {10.1063/1.4917419},
journal = {Journal of Chemical Physics},
number = 14,
volume = 142,
place = {United States},
year = {Tue Apr 14 00:00:00 EDT 2015},
month = {Tue Apr 14 00:00:00 EDT 2015}
}
  • We present a theoretical study of the electron angular distribution produced in resonance enhanced two-photon ionization of the H{sub 2}{sup +} molecular ion using ultrashort laser pulses. The method consists in solving the time dependent Schroedinger equation and includes all electronic and vibrational degrees of freedom. Differential (in proton energy and electron emission solid angle) ionization probabilities have been evaluated for various photon energies, laser intensities, and pulse durations. We show that (1+1) resonance-enhanced multiphoton ionization (REMPI) leads to angular distributions significantly different from those produced in direct two-photon ionization. The REMPI process is observed even at photon energies notmore » matching the energy difference between two electronic states in a perfect vertical transition. Interestingly, there is no trace of REMPI effects in the electron angular distribution when the fully differential probabilities are integrated over proton energy.« less
  • Molecular high-order harmonic generation (MHOHG) by a combined intense circularly polarized laser pulse and static electric field has been studied from the appropriate time-dependent Schroedinger equation (TDSE) for the H{sub 2}{sup +} molecular ion. It is found that for a particular static field strength derived from a classical model, efficient MHOHG spectra are obtained with maximum energy I{sub p} + 9.05U{sub p}, where I{sub p} is the ionization potential and U{sub p}=E{sub 0}{sup 2}/4m{sub e{omega}0}{sup 2} is the ponderomotive energy at amplitude E{sub 0} and frequency {omega}{sub 0} of the circularly polarized laser pulse. The static field controls recollision ofmore » the electron with parent ions and is confirmed by numerical solutions of the H{sub 2}{sup +} TDSE at equilibrium. To produce circularly polarized MHOHG spectra, a combination of an elliptically polarized pulse and a static electric field is found to be most efficient. A time-frequency analysis obtained via Gabor transforms is employed to identify electron recollision times responsible for the generation of these high-order harmonics. It is found that only single recollision trajectories contribute to the circularly polarized harmonics, thus generating new sources for high-frequency circularly polarized attosecond pulses.« less
  • In the case of resonant two-photon autoionization of H/sub 2/, the influence of laser bandwidth on the photoelectron angular distribution is seen to be large at lower intensities (approx.10/sup 6/ W/cm/sup 2/), but gradually disappears towards higher intensities. At very high intensities (approx.10/sup 12/ W/cm/sup 2/), the ac Stark shift of the resonant intermediate state is found to affect the distribution significantly. It has also been found that the Stark-shifted line shape of the intermediate state tends to be symmetrical around zero detuning on increasing the laser bandwidth.
  • We have studied the effect of laser intensity on the photoelectron angular distribution (PEAD) in (1+1{sup {prime}})-photon resonance-enhanced multiphoton ionization (REMPI) of the H{sub 2} molecule via {ital B}{sup 1}{Sigma}{sub {ital u}} ({ital v}=4; {ital j}=1,2) levels. It has been found that both the total and the vibrationally resolved angular distributions [for the formation of H{sub 2}{sup +} ion in ground {ital X}{sup 2}{Sigma}{sub {ital g}}({ital v}{sup +}=0,1) levels] vary with laser intensity. Intensity dependence of corresponding asymmetry parameters has also been studied. We have shown that this variation with laser intensity is caused by two effects: (i) the effectmore » of interference of ionization channels via different vibrational levels of {ital B}{sup 1}{Sigma}{sub {ital u}} and {ital C}{sup 1}{Pi}{sub {ital u}} states, and (ii) the effect of coupling between these levels (Raman-like transitions) via continuum. Furthermore, the nature of variation of the PEAD and of the corresponding asymmetry parameters with laser intensity have also been found to depend on the choice of different rotational levels as resonances. This difference in variation is due to the variation in strength of interference and the Raman-like couplings effective in these REMPI processes.« less
  • We solve the three-dimensional time-dependent Schroedinger equation for a few-cycle circularly polarized femtosecond laser pulse that interacts with an oriented target exemplified by an argon atom, initially in a 3p{sub x} or 3p{sub y} state. The photoelectron momentum distributions show distinct signatures of the orbital structure of the initial state as well as the carrier-envelope phase of the applied pulse. Our ab initio results are compared with results obtained using the length-gauge strong-field approximation, which allows for a clear interpretation of the results in terms of classical physics. Furthermore, we show that ionization by a circularly polarized pulse completely mapsmore » out the angular nodal structure of the initial state, thus providing a potential tool for studying orbital symmetry in individual systems or during chemical reactions.« less