Photofragment spectrum of ICN

Abstract

We have used photofragment spectroscopy to study the near ultraviolet photodissociation of ICN at 37 550 cm−1 (266.2 nm) by crossing an ICN molecular beam with a pulsed fourth harmonic neodymium laser. A mass spectrometer is used to measure the densities of both the recoiling CN and I fragments as a function of recoil direction with respect to the polarization direction of the laser light and as a function of flight time. From these measurements, the fragment center-of-mass distributions with respect to angle, total translational energy, and total internal energy are calculated. Two peaks are observed in the internal energy distribution, both with angular distributions peaking parallel to the electric vector of the light, indicating that the dissociation process or processes must be predominantly derived from parallel transitions. This conclusion appears to rule out both of the previous spectral assignments for this region. The angular distributions show that the ICN excited state lifetimes are short compared with rotation. The internal energy distribution is such that the fragments are produced with population inversion. The lower internal energy peak, corresponding to 40% of the fragments, has insufficient energy to electronically excite either fragment, and can be matched by the assumed production of a mixture of v = 2, Trot = 1100°K and v = 3, Trot = 400°K CN fragments. This match is not unique, and while appreciable v = 4 and higher contributions may be ruled out, contributions from v = 0 and 1 with correspondingly higher rotational energies are possible. The higher internal energy peak, corresponding to 60% of the dissociation, can be matched by the assumed production of ground state I and ? 2Πi electronically excited CN fragments with no vibrational excitation and little or no rotational excitation. Other possibilities for this peak which cannot be ruled out by these experiments are the formation of both CN and I fragments in their ground electronic states with a very narrow combination of CN vibrational (maximum v = 4) and rotational excitation, or alternatively the production of I* 2P1/2 with no vibrational but appreciable rotational excitation of the CN. The latter possibility would, however, contradict experimental conclusions by Donovan et al. Possible models for the two peaks seen in the dissociation process include (i) the excitation of two separate ICN upper states, and (ii) the interaction between two upper states, which may be viewed as the initial exclusive production of excited ? 2Πi CN fragments followed by electronic quenching of a portion of them by a ’’half-collision’’ with the recoiling I atom, leaving the quenched CN fragments vibrationally and rotationally excited.