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-a photometer for measurement of absolute hydrogen atom concentrations at levels [HI 1 5 1.8 X 1014 atoms/cmz and total pressure of 1.3 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 5 P 6 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 HgThe 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,, He, Ne, Ar, Kr, and N 1 are 8.3 f 1.2, 6.9 f 1.5, 5.9 =k 1.5, 8.0 f 0.8, 10.2 0.9, and 9.6 f 1.4, respectively, where the units are cm6/molecz . sec. photosensitization of HP.

No abstract

A previously developed model for active species concentration profiles in infinite cylindrical systems has been extended to include the spherical system. The model couples the processes of diffusion to and reaction a t the wall. Predictions of time buildup under conditions of homogeneous production by light, and time decay after extinguishing the light source, are made for H atoms. Such predictions require a knowledge of the wall recombination coefficient and the binary diffusion coefficient for H in heat bath gas. The model is experimentally tested by measuring the first-order decay constants of H at room temperature in various pressures (10-1500 tom) of six heat bath gases. The atomic concentration is monitored by Lyman-a absorption photometry. The results show good agreement with model predictions in the various heat bath gases up to -400 torr and depend only on one parameter, y, the recombination coefficient. This should be contrasted with the earlier work where slight variation in y was invoked. The rate constants at pressures higher than 400 torr are consistently higher than model predictions.