VERY SLOW H⁺ + D(1s) HALF COLLISIONS

E. Wells^*, B.D. Esry, K.D. Carnes, and I. Ben-Itzhak
J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506 USA

^*Current Address: Department of Physics, University of Virginia, Charlottesville, VA 22904-4714 USA

Sub-eV collisions between protons and deuterium atoms have long been a proving ground for theorists for two primary reasons: first, the simplicity of the system makes it ideally suited for studying electron translation factors; and second, HD⁺ structure calculations are challenging since there is no symmetry under exchange of nuclei, and as a result, the calculations must extend beyond the Born-Oppenheimer approximation. Despite the theoretical interest, there have been very few experiments below 10 eV, due to difficulties in controlling the collision energy. A new experimental approach allows the measurement of H⁺ + D(1s) collisions down to a few meV, lower by more than an order of magnitude and with better energy resolution than previous measurements.

This new experimental approach uses the ground state dissociation (GSD) of HD⁺ to produce H⁺ + D(1s) "half" collisions^1,2. Fast proton impact with neutral HD initiates vertical transitions that populate the vibrational continuum of the HD⁺(1sσ) state about 1% of the time, resulting in a very slow dissociation. The half collision energy is determined a posteriori from the measured momentum vector of each charged fragment using a COLTRIMS-style apparatus³.

For the half collision system, the coupled channels problem is solved using a variational R-matrix approach with incoming-wave boundary conditions. The Franck-Condon transition from the neutral molecule is taken into account by projecting the outgoing waves onto the ground state of the HD molecule. The coupled channels calculations are solved in a similar manner for the "full" collision system⁴. The experimental results, shown in Figures 1 and 2, provide a direct test of the half collision calculations. The experimental and theoretical results are in good agreement, except in the region very near the charge transfer threshold, where systematic problems with the subtraction of contaminant H₂⁺ molecular ions plague the experiment. Since similar computational techniques are used for both the full and half collision systems, these experimental results can also probe calculations of full collisions at much lower energies than previously accessible. Improvements in the experiment should allow measurements of the threshold behavior in charge transfer and resonances in the elastic channel.

Figures:

Figure 1: Experimental and theoretical results for
charge transfer in the H⁺ + D(1s) half collision system.
V_pusher is a measure of the strength of the extraction field.

Figure 2: Experimental and theoretical results for elastic
scattering in the H⁺ + D(1s) half collision system. The
structure in the calculations below E_k = 3.7 meV
is due to two J = 0 Feshbach resonances^2,4.

References:

1) I. Ben-Itzhak et al., Phys. Rev. Lett. 85, 58 (2000).
2) E. Wells et al., Phys. Rev. A 62, 062707 (2000).
3) E. Wells et al., submitted to Phys. Rev. Lett. (2001).
4) B.D. Esry et al., J. Phys. B 33, 5329 (2000).

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 2001, July 2001 in Santa Fe, NM.

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