Collisions at thermal energy between metastable hydrogen atoms and hydrogen molecules: total and differential cross sections

Abstract

A metastable hydrogen (deuterium) atom source in which groundstate atoms produced by a RF discharge dissociator are bombarded by electrons, provides a relatively large amount of slow metastable atoms (velocity 3-5 kin/s). Total integral cross sections for H*(D*)(2s) +H2(X1S~ +, v=O) collisions have been measured in a wide range of relative velocity (2,5-30 kin/s), by using the attenuation method. A significant improvement of accuracy is obtained, with respect to previous measurements, at low relative velocities. Total cross sections for H* and D*, as functions of the relative velocity, are different, especially in the low velocity range. H* + H2 total differential cross sections have also been measured, with an angular spread of 3.6 ° , for two different collision energy distributions, centered respectively at 100 meV and 390 meV. A first attempt of theoretical analysis of the cross sections, by means of an optical potential, is presented.PACS: 34.40.+n; 34.50.-s the H f potential energy presents also a well, the depth of which is comparable to that of H*, the entrance channel H* + H2 is coupled to the electronic continuum, leading to the associative ionization (Hf + e-). For all these reasons, the H~ system may be regarded as a model system, the study of which can provide information about various atom-diatom collisional processes.H*(2s) on H2 collisions at relatively low energies (E ~ 10-1 eV) have been intensively studied since many years. Nevertheless only integral total cross sections [1-4], quenching total cross sections [5] and reaction rates for the associative ionization [6] have been measured so far. Theoretical studies of H* and H f are of two different types (with no overlapping up to now): (i) molecular calculations [7]; (ii) treatment of the collisional quenching [4,8]. The purpose of the present work is to extend integral cross section measurements towards lower energies (down to 0.06 eV), to measure total differential cross sections, as well as to propose an interpretation in term of long range potentials.

H Lymanαline in Jovian aurorae: electron transport and radiative transfer coupled modelling

“…So if the atmosphere is dense enough, this collisional quenching makes H(2s) deexcite via H(2p) and a Lyman α photon. For particles with thermal velocity (v th ∼ a few km s −1 in the atmosphere of Jupiter), the cross section for collisional quenching of H(2s) by collisions with H 2 is σ quench ∼ 100 Å 2 (Vassilev et al 1990). The collisional lifetime of H(2s) can thus be evaluated by τ coll = 1 nσ quench v th , where n is the density of H 2 .…”

Collisions between Metastable Hydrogen Atoms at Thermal Energies

“…We find also that double excitation transfer into two excited H͑2p͒ atoms is still more probable with the large cross section of 9 3 10 212 cm 2 at E 4.1 meV varying as E 21͞2 at higher energies. The detection of the resulting Lyman alpha photons would provide a diagnostic test of our predictions.Several experiments involving interactions of metastable hydrogen atoms with various targets have been performed [1][2][3][4][5][6][7][8]. The theoretical simplicity of atomic hydrogen makes it an ideal system for studying fundamental physics.…”

Slow metastable H(22S1/2) from dissociation of cold H2 induced by electrons

“…The good resolution obtained was due in part to the use of a supersonic jet that produces a beam of H 2 molecules with cold internal degrees of freedom. For a thermal jet or gas cell, the kinematical effects explained hereafter would not be visible because the thermal movement of the molecules broadens the velocity distribution [14]. In our case, with Mach number of the order of 10 and with low internal temperature as mentioned before, the initial velocity vector and the levels involved (v = 0; J = 0, 1) were well defined.…”