Laser-driven quantum dynamics in half and full collisions: fragmentation of H₂⁺ and laser-assisted charge transfer in p+H collisions

Uwe Thumm¹, Thomas Niederhausen¹, Bernold Feuerstein^1,2

1) J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA
2) Max-Planck-Institut für Kernphysik, Heidelberg, Germany

The fragmentation of H₂⁺ molecular ions in 25 fs, 800 nm laser pulses with 0.05 to 0.5 PW/cm² peak intensity was investigated by means of wave packet propagation calculations. Our collinear reduced-dimensionality model represents the nuclear and electronic motion by one degree of freedom and includes their (non-Born-Oppenheimer) coupling. We introduced a modified ‘soft-core’ Coulomb potential, using a softening function that depends on the internuclear distance, in order to reproduce some properties of the ‘real’ 3D molecule. By analyzing the calculated flux of the outgoing nuclear wave packets, we obtained fragmentation probabilities and kinetic-energy spectra. We found that the relative probabilities for dissociation and Coulomb explosion depend critically on the initial vibrational state of the molecular ion.

Fast ionization of H2 leads to the coherent population of many vibrational states of H₂⁺. However, insufficient experimental time resolution usually averages out all coherence effects between stationary vibrational states of H₂⁺. We propose a Coulomb explosion imaging method to monitor the coherent motion and decoherence of bound nuclear wave packets using ultrashort laser pulses. This method would make decoherence times in the fs to ps range observable [1].

We also investigated the effects of a strong laser field on the dynamics of electron capture and emission in ion-atom collisions using the same numerical technique. Within a reduced dimensionality model in which the motion of the active electron and the laser electric field vector are confined to the scattering plane, we examined the probabilities for electron capture and ionization as a function of laser intensity, projectile impact parameter b, and laser phase Φ. (Φ determines the orientation of the laser electric field with respect to the internuclear axis at the time of closest approach between target and projectile). Our results for the b-dependent ionization and capture probabilities show a strong dependence on both Φ and the helicity of the circularly polarized laser light. For intensities above 2x1012 W/cm² our model predicts noticeable circular dichroism in the capture probability for slow proton-hydrogen collisions that persists after averaging over Φ. Capture and electron emission probabilities defer significantly from results for laser-unassisted collisions. We found evidence for a charge resonance enhanced ionization mechanism that may enable the measurement of the absolute phase Φ[2].

References:

[1] B. Feuerstein and U. Thumm, J. Phys. B 36, 707 (2003); Phys. Rev. A 67, 043405 (2003); Phys. Rev. A 67, 063408 (2003).
[2] T. Niederhausen, B. Feuerstein, and U. Thumm, Phys. Rev. A (to appear).

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to the Heraeus Seminar: Manipulation of Few-Body Quantum Dynamics, June 2004 in Bad Honnef, Germany.

 

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