skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Electron angular distribution in resonance-enhanced two-photon ionization of H{sub 2}{sup +} by ultrashort laser pulses

Abstract

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 not 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.

Authors:
; ; ;  [1]
  1. Departamento de Quimica C-9, Universidad Autonoma de Madrid, 28049 Madrid (Spain)
Publication Date:
OSTI Identifier:
20982367
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. A; Journal Volume: 75; Journal Issue: 3; Other Information: DOI: 10.1103/PhysRevA.75.033419; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
74 ATOMIC AND MOLECULAR PHYSICS; ANGULAR DISTRIBUTION; DEGREES OF FREEDOM; ELECTRON EMISSION; ELECTRONS; HYDROGEN IONS 2 PLUS; LASER RADIATION; MOLECULAR IONS; MULTI-PHOTON PROCESSES; PHOTOIONIZATION; PHOTON-MOLECULE COLLISIONS; PHOTONS; PROBABILITY; PROTONS; PULSES; RESONANCE; SCHROEDINGER EQUATION; SOLIDS; TIME DEPENDENCE; VIBRATIONAL STATES

Citation Formats

Selstoe, S., Palacios, A., Fernandez, J., and Martin, F. Electron angular distribution in resonance-enhanced two-photon ionization of H{sub 2}{sup +} by ultrashort laser pulses. United States: N. p., 2007. Web. doi:10.1103/PHYSREVA.75.033419.
Selstoe, S., Palacios, A., Fernandez, J., & Martin, F. Electron angular distribution in resonance-enhanced two-photon ionization of H{sub 2}{sup +} by ultrashort laser pulses. United States. doi:10.1103/PHYSREVA.75.033419.
Selstoe, S., Palacios, A., Fernandez, J., and Martin, F. Thu . "Electron angular distribution in resonance-enhanced two-photon ionization of H{sub 2}{sup +} by ultrashort laser pulses". United States. doi:10.1103/PHYSREVA.75.033419.
@article{osti_20982367,
title = {Electron angular distribution in resonance-enhanced two-photon ionization of H{sub 2}{sup +} by ultrashort laser pulses},
author = {Selstoe, S. and Palacios, A. and Fernandez, J. and Martin, F.},
abstractNote = {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 not 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.},
doi = {10.1103/PHYSREVA.75.033419},
journal = {Physical Review. A},
number = 3,
volume = 75,
place = {United States},
year = {Thu Mar 15 00:00:00 EDT 2007},
month = {Thu Mar 15 00:00:00 EDT 2007}
}
  • 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.more » 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.« less
  • 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 study the control of dissociation of the hydrogen molecular ion and its isotopes exposed to two ultrashort laser pulses by solving the time-dependent Schroedinger equation. While the first ultraviolet pulse is used to excite the electron wave packet on the dissociative 2p{sigma}{sub u} state, a second time-delayed near-infrared pulse steers the electron between the nuclei. Our results show that by adjusting the time delay between the pulses and the carrier-envelope phase of the near-infrared pulse, a high degree of control over the electron localization on one of the dissociating nuclei can be achieved (in about 85% of all fragmentationmore » events). The results demonstrate that current (sub-)femtosecond technology can provide a control over both electron excitation and localization in the fragmentation of molecules.« less
  • We report correlated two-electron ab initio calculations for the hydrogen molecule H{sub 2} in interaction with intense ultrashort laser pulses, via a solution of the full three-dimensional time-dependent Schroedinger equation. Our results for ionization and excitation probabilities (at 800 and 400 nm) as a function of internuclear distance R show strong evidence of enhanced ionization, in both single and double ionization, as well as enhanced excitation, in single and double excitation, as the internuclear distance R increases from the equilibrium value R{sub e}. The enhancement of all these molecular processes exhibits a maximum at a critical distance R{sub c}, whichmore » can be predicted from simple electrostatic and recollision models.« less
  • We present a theoretical study of dissociative multiphoton ionization of the H{sub 2}{sup +} molecular ion in perturbative and nonperturbative regimes including both electronic and nuclear degrees of freedom. Differential (in proton and electron energy) ionization cross sections have been evaluated for various photon energies, laser intensities, and pulse lengths. We have found that the proton energy distribution is modulated by vertical Franck-Condon transitions but also by vibrational resonances associated with intermediate electronic states. We have also found that, as expected, nonperturbative results tend to the time-independent perturbative ones when both the pulse length increases and the laser intensity decreases.more » No divergence near intermediate-state resonances is found in the perturbative results when the nuclear motion is properly taken into account in the calculations.« less