Vector correlations in the 157 nm photodissociation of OCS and the 266 nm photodissociation of methyl iodide

Abstract

The photodissociations of OCS at 157 nm and of CH₃I(CD₃I) at 266 nm have been investigated by using tunable vacuum ultraviolet laser-induced fluorescence and multiphoton ionization to probe the CO or S and the CH₃(CD₃) or I photoproducts, respectively. In the OCS dissociation, sulphur is produced almost entirely in the S(¹S) state, while CO is produced in its ground electronic state and in vibrational levels ν= 0–3 in the approximate ratio (ν= 0) : (ν= 1) : (ν= 2) : (ν= 3)=(1.0) : (1.0) : (0.5) : (0.3). The rotational distribution for each vibrational level is found to be near-Boltzmann, with temperatures that decrease from 1350 K for ν= 0 to 770 K for ν= 3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character (β > 1.3) and that the CO velocity and angular momentum vectors are perpendicular to one another. In the CD₃I dissociation, the ratio of CD₃(ν= 0)/(ν= 2) was estimated to be ca. 1.1, with multiple determinations in the range 0.47–2.1. The quantum number ν here denotes the nascent excitation of the ν₂‘umbrella’ mode. A value for the CH₃(ν= 0)/(ν= 2) ratio from dissociation of CH₃l could not be estimated, although it was clearly larger than that for CD₃. The CH₃(ν= 0) and CD₃(ν= 0) products from this dissociation are fitted by 120 ± 30 K and 105 ± 30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame such that there is a strong preference for low values of K. Assuming that the relative velocity vector lies along the CH₃C₃ axis, then the velocity and rotation vectors tend to be perpendicular.