Energy transfer from excited mercury atoms to electrons

This work verifies experimentally the principle of detailed balancing pertaining to the interdependence of the forward and reverse cross-sections q12 and q21 for electron-atom collisions. Previous experiments on superelastic collisions in excited mercury vapour, namely Hg*(3P1 or 3P0) + eslow → Hg (1S0) + efast show that electrons can acquire energies less or equal to the sum of the atom’s excitation energy and the initial electron energy ϵ. However, the cross-sections q21 were found to be about two orders of magnitude smaller than those theoretically predicted. The major cause of this discrepancy was the use of estimated values of the u. v. resonance light flux. It is measured here in situ by chemical actinometry. In the experiment, electrons are accelerated into a field-free space, while the Hg vapour is irradiated with its resonance light. Some electrons, colliding with Hg*, become fast enough to reach a collector kept at a retarding potential. To find q21, besides collector current and light flux, the concentrations of atoms in the resonance (3P1) and the lower metastable state (3P0) must be known. These are calculated from the rate equations; 3P2 atoms are neglected. It is found that between 5 mTorr and 5 Torr pressure (1 Torr ≈ 133 Pa) the 3P1 concentra­tion is nearly independent of p whereas that of the 3P0 atoms rises with p until it reaches at ca. 1 Torr a high maximum. This results from collisions with ground state atoms (1S0): at low p they induce transitions downwards from the 3P1 to the 3P0 state; at high p the fast 1S0 atoms in the tail of the distribution collide with the numerous 3P0 atoms lifting more up to the 3P1 level than arrive. At pressures p > 30 mTorr collisions between 3P1 and 3P0 atoms contri­bute to the collector current through ‘associative ionization’ which dominates at large p. The corresponding cross-section is qa85 = 4.6 x 10-14 cm2, valid between 383 and 424 K. The superelastic cross-section q2l = f(ϵ) in the pressure range 20-40 mTorr for ϵ = ½ to 5 eV is found to decrease with increasing ϵ, showing branches corresponding to the fine struc­ture of the P state; the position of the two maxima is probably near ½ eV. At low p, when the concentrations of 3P1 and 3P0 are comparable, the curve for q21 corresponds to collisions with a mixture of the two states, whereas at high p, when 3P0 atoms predominate, a lower q21 curve is found. The agreement with various theories is reasonable. In addition, approximate values of the momentum transfer cross-section for ½ - 5 eV electrons in Hg are obtained by measuring the current of elastically scattered electrons as a function of ϵ. The result compares satisfactorily with earlier determinations.