Evidence for three interacting potential energy surfaces in the photodissociation of ICN at 249 nm

Abstract

Cyanogen iodide (ICN) is photodissociated at 249 nm. The CN X 2Σ+ photofragment is probed by sub-Doppler laser-induced fluorescence (LIF), allowing the extraction of recoil velocity anisotropies and branching ratios to the two iodine atom spin–orbit states I(2P1/2) and I(2P3/2) as a function of the CN (v=0) rotational state. The quantum yield for I(2P1/2) production ΦI* is found to be 43%±3%, in excellent agreement with the recent diode laser spectroscopic measurement of Hess and Leone. The population of the F1 and F2 spin–rotation doublet components shows nonstatistical behavior over a wide range of N for both I atom spin–orbit state exit channels. The results suggest that trajectories leading to I(2P1/2) evolve on an essentially collinear surface; the CN fragments being found in low rotational levels with almost limiting values of the system anisotropy parameter (β=1.85 to 1.9). This value of β yields an estimate for the dissociative lifetime [CN X 2Σ+v=0, N=0; I(2P1/2)] of 90±15 fs at this photolysis wavelength. There is evidence that trajectories leading to I(2P1/2)+CN(v=0) in intermediate N levels have sampled a bent surface, indicating that multiple curve crossings occur in this channel. The nature of trajectories correlated to I(2P3/2) is very complicated, with clear evidence for a mixed parallel and perpendicular initial transition and subsequent curve crossings. The CN (v=0) fragments formed in conjunction with this channel are found predominantly in medium to high rotational quantum states. The system anisotropy parameters vary as a function of N, being negative at low N and becoming positive at high N. An analysis of the correlation between fragment velocity and rotation yields results inconsistent with a pure parallel or perpendicular excitation. We present a model involving three interacting surfaces, by which we can reconcile all major experimental observations at this photolysis wavelength.