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 CH31(CD,I) 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 CH,(CD,) or I photoproducts, respectively. In the OCS dissociation, sulphur is produced almost entirely in the S('S) state, while CO is produced in its ground electronic state and in vibrational levels u = 0-3 in the approximate ratio3). The rotational distribution for each vibrational level is found to be near-Boltzmann, with temperatures that decrease from 1350 K for u = 0 to 770 K for v = 3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character ( p > 1.3) and that the CO velocity and angular momentum vectors are perpendicular to one another. In the CD31 dissociation, the ratio of CD3 ( u = O)/( u = 2) was estimated to be ca. 1.1, with multiple determinations in the range 0.47-2.1. The quantum number u here denotes the nascent excitation of the u2 'umbrella' mode. A value for the CH, ( u = O)/( u = 2) ratio from dissociation of CH31 could not be estimated, although it was clearly larger than that for CD,. The CH3 ( u = 0) and CD, ( u = 0) products from this dissociation are fitted by 120* 30 K and 105 f 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 CH, C3 axis, then the velocity and rotation vectors tend to be perpendicular.

A method for the determination of speed-dependent semi-classical vector correlations from sliced image anisotropies

“…Any technique which combines probing of specific photofragment ro-vibrational states with simultaneous resolution of its velocity v is sensitive to these correlations. These techniques include Doppler spectroscopy, 3,[12][13][14] ion time-of-flight, 15,16 and most recently ion imaging. 6,7,17 In ion imaging experiments, fragment vector correlations are determined from the angular distribution and total signal intensities of the product fragment signals using different pump/probe polarization geometries.…”

Photodissociation Dynamics

“…39,40 A notable success of the REMPI scheme has been the extraction of rotational and vibrational populations of the CH 3 and CD 3 fragments produced from the photodissociation of CH 3 I (and CD 3 I). [41][42][43] Among other things, these studies demonstrate the importance of reactant initial conditions on the photofragments. In ref 41, an effusive (thermal) source of CD 3 I was used, and the resulting CD 3 rotational distribution showed a propensity for levels with N ) K. In contrast, a supersonic beam was used in the experiments of ref 42, and the rotational population of the methyl fragment was concentrated in low K levels.…”

General discussion

“…In addition to the energy-disposal data, we also present preliminary 0, j correlations for the OH( v = 0) and OD( u = 0) fragments from HOOD [p: (22) = 0.2 (OH) and -0.06 (OD), N = 8 ] . These data allow an approximate resolution of the rotational energy We note in particular that EkoT is considerably larger for both OD and OH fragments from HOOD relative to that from HOOH.…”