Kinetic studies of ground state arsenic atoms, As(44S), by time-resolved resonance fluorescence

Abstract

We present a kinetic study of ground state arsenic atoms, generated photochemically and monitored by fluorescence in the time-resolved mode following optical excitation. As[4p³(⁴S)] was derived from the repetitive pulsed irradiation of low pressures of either AsMe₃ or AsCl₃, in the presence of excess helium, in a flow system which is kinetically equivalent to a static system. The transient atoms were then monitored photoelectrically by summation of the optically excited fluorescence, principally at the wavelengths λ= 189.04, 193.76 and 197.76 and 197.23 nm [4p²5s(⁴P_{5/2, , ½})→ 4p³(⁴S), respectively] using a “pre-trigger” photomultiplier gating system coupled with singal averaging. The decay of the arsenic atoms in the presence of the added reactant gases O₂, Cl₂, NO and C₂H₄ is described, for which second-order reaction rate constants are reported. These data are compared with previous results derived from time-resolved resonance absorption studies at λ= 189.04 nm. The two sets of rate constants resulting from the resonance absorption fluorescence and absorption studies are then employed in order to discuss the modified Beer–Lambert Law, I_tr=I₀ exp [–ε(cl)^γ], that has been used in the resonance absorption measurements. Finally, we present a model radiation trapping calculation including the effects of nuclear hyperfine interaction using the diffusion theory of radiation for the three wavelengths monitored in fluorescence, and calculate the relationship between fluorescence intensity and particle density. This is compared with empirical measurements of the fluorescence intensity against particle density calibration.