Molecular reaction dynamics study using high resolution and high sensitivity spectroscopy

Abstract

$\emph{Absolute primary H atom quantum yield measurements in the 193.3 nm and 121.6 nm photodissociation of acetylene}$ Absolute quantum yields for primary H atom formation ($φ_H$) were measured under collision-free conditions for the room-temperature gas-phase dissociation of acetylene ($C_2H_2$) after photoexcitation at 193.3 nm and at the H-atom Lyman-α wavelength (121.6 nm) by a pulsed laser photolysis (LP)-laser-induced fluorescence (LIF) ‘pump-and-probe’ technique. Using HCl and $CH_4$ photolysis at 193.3 and 121.6 nm, respectively, as a reference, values of $φ_H$ (193:3 nm) = 0.94±0.12 and $φ_H$ (121.6 nm) = 1.04±0.16 were obtained which demonstrate that for both photolysis wavelengths the H + $C_2H$ product channel dominates the primary $C_2H_2$ photochemistry. $\emph{Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of $SO_2$ at 222.4 nm}$ The dynamics of formation of oxygen atoms after UV photoexcitation of $SO_2$ in the gas-phase was studied by pulsed laser photolysis (LP)-laser-induced fluorescence ‘pump-and-probe’ technique in a flow reactor. $SO_2$ at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and $O(^3P_j)$ photofragments were detected under collision-free conditions by vacuum ultraviolet (VUV) laser-induced fluorescence (LIF). The use of narrow-band probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent $O(^3P_{j=2,1,0})$ fine-structure state distribution: $n_{j=2}/n_{j=1}/n_{j=0}$ = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing $NO_2$ photolysis as a reference, a value of $φ_{o(^3p)}$=0.13 ± 0.05 for the absolute $O(^3P)$ atom quantum yield was determined. The measured $O(^3P)$ quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via in...