Collisions between Metastable Hydrogen Atoms at Thermal Energies

Abstract

The complex interaction potentials arising in the approach of two metastable hydrogen 2s atoms are calculated and the cross sections for ionization, excitation transfer, and elastic scattering are predicted. The measured cross section for associative ionization at E = 4.1 meV equals 2x10(-15) cm (2). We calculate a total ionization cross section of 2x10(-13) cm (2), varying as E(-2/3) at higher energies. Thus it appears that dissociative ionization is the major ionization channel. We find also that double excitation transfer into two excited H(2p) atoms is still more probable with the large cross section of 9x10(-12) cm (2) at E = 4.1 meV varying as E(-1/2) at higher energies. The detection of the resulting Lyman alpha photons would provide a diagnostic test of our predictions.

Ionization in collisions between metastable hydrogen atoms

“…Previous calculations [48,50] have shown that the total ionization cross sections for the 1 + g and 3 + u states are 23E −2/3 and 15E −2/3 , respectively.…”

“…The full set of doubly excited molecular states converging to the H(n = 2) + H(n = 2) limit, the so-called Q(2) states, has been calculated over the entire range of internuclear distance R, and the associated energies and autoionization widths have been used to construct complex potential curves [49,51]. Multichannel scattering formulations have been used to calculate the collision cross sections at low translational energies [48,50]. At thermal energies, the tiny energy splitting between the 2s and 2p states may be ignored, and the contribution from the Coriolis force is also expected to be weak.…”

“…At thermal energies, the tiny energy splitting between the 2s and 2p states may be ignored, and the contribution from the Coriolis force is also expected to be weak. Therefore, it is possible to simplify the analysis by considering only single-channel scattering on the complex potentials V ± (R) which have the asymptotic behavior [48] …”

“…The differential cross sections for elastic scattering may be obtained from the coefficient of the spherical wave part of the wave function (7) by expressing the molecular eigenfunctions in terms of their asymptotic atomic states. The result is [48] …”

“…The competitive balance between the collisional quenching processes (1)-(4) should be understood at the highest-possible level of detail. A series of calculations [48][49][50][51][52] have addressed this issue and provided theoretical benchmarks for this fundamental system. However, the calculated trap loss rate coefficients are about 4 times larger than the experimental values and show little temperature variation in the 100 to 200 μK region where the experiment suggests there may be a significant decrease [39].…”