Simultaneous tracking of two coherently launched wave packets in real time using short intense laser pulses

A. Alnaser¹, B. Ulrich^1,2, X.-M. Tong¹, P. Ranitovic¹, C. Marhajan¹, I. Litvinyuk¹, A. Hupach¹, T. Osipov¹, R. Ali³, C.D. Lin¹ and C.L. Cocke¹

1) Kansas State University, Manhattan, Kansas 66503
2) Institut für Kernphysik, Univ. Frankfurt, Frankfurt, Germany
3) Dept. of Physics, Hashemite Univ., Zarqua 13115, Jordan

The availability of extremely short intense laser pulses makes it now possible to follow in real time the heavy particle dynamics of even the simplest and lightest molecules. A non-stationary wave packet can be launched onto a potential curve of the molecule through ionization of the neutral, and the subsequent motion of this time-dependent wave packet can be followed as it coherently couples to other potential curves in the ionized system.

We have used intense laser pulses in a pump-probe configuration to track the vibrational motion of the wave packet launched onto the ground state potential curve of the H₂⁺ molecule from neutral H₂. A short "weak" (<1x10¹⁴ W/cm², 8-12 fs) ionizing pulse launches the packet onto the 1sσ_g potential curve of H₂⁺. After a time delay ranging from 0 to 100 fs, a second "strong" pulse removes the remaining electron, causing the system to "Coulomb explode" into two protons. These protons are detected in time coincidence, and their momenta are measured, using well established COLTRIMS techniques.

Figure 1 shows the kinetic energy release of the two protons (KER) plotted as a function of the time delay between the pump and probe pulses. Two "trajectories" of the wave packet are seen. The first is an oscillatory one producing a KER near 10 eV, and is due to that part of the vibrational wave packet which remains bound on the 1sσ_g potential curve. The period of oscillation is characteristic of motion in that well, approximately 15 fs. The second trajectory shows a monotonically descending KER as a function of time and represents the wave packet which follows the 2pσ_u dissociating potential curve. The population of this trajectory occurs primarily near the crossing between the single-photon-promoted 1sσ_g potential curve and the 2pσ_u curve, which occurs near an internuclear distance of 4.6 a.u., and gives rise to the well known bond-softening KER in dissociation of H₂⁺.

A quantum-mechanical model of this process has been constructed by launching a wave packet characteristic of the H₂ ground state onto the 1sσ_g potential curve and propagating this wave packet coherently on the (coupled) 1sσ_g and 2pσ_u curves of the H₂⁺. Excellent agreement between the experiment and model is found. In particular, nonclassical behavior of the shapes of the wave packets is seen in several spectra (not presented here) and is in agreement with model calculations.

Figure 1: Plot of KER from H₂⁺ as a function of pump-probe time separation.

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 ICPEAC, July 2005 in Rosario, Argentina.

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