Lyman‐α absorption photometry at high pressure and atom density kinetic results for H recombination

Atomic absorption and fluorescence spectrophotometry have been routinely used in kinetic investigations as probes of relative, rather than absolute, atom concentration. The calibration of a Lyman‐α photometer for measurement of absolute hydrogen atom concentrations at levels [H] ι ≤ 1.8 × 10 14 atoms/cm 2 and total pressure of 1.5 torr He is described. The photometer is characterized in terms of a two‐level emission source and an absorption region in which only Doppler broadening of the transition is considered. The modifications due to pressure broadening by high pressures (500 ≤ P ≤ 1500 torr) in the absorption region are discussed in detail. Application of the technique is reported for the recombination of hydrogen atoms in the presence of six nonreactive heat bath gases. Experiments were performed in a static reaction cell at pressures of 500–1500 torr of heat bath gas, and hydrogen atoms were produced by Hg ( 3 P 1 ) photosensitization of H 2 . The technique is critically evaluated and the mechanistic implications of the hydrogen atom recombination results are examined. The measured room temperature recombination rate constants in H 2 , He, Ne, Ar, Kr, and N 2 are 8.5 ± 1.2, 6.9 ± 1.5, 5.9 ± 1.5, 8.0 ± 0.8, 10.2 ± 0.9, and 9.6 ± 1.4, respectively, where the units are 10 33 cm 6 /molec 2 · sec.