The internal state distribution of CN free radicals produced in the photodissociation of ICN

Abstract

Laser‐induced fluorescence has been used to measure the vibrational–rotational (υ″, N ″) product state distributions of CN (X 2Σ+) radicals produced in the photodissociation of ICN in two spectral regions. Intermolecular energy transfer studies of CN (X) produced by photolysis at λ?145 nm indicate that near resonant collision‐induced energy transfer from the υ′=0 and υ′=1 vibrational levels of the A 2Π i state populated the υ″=4 and υ″=5 levels of the X state. The observed X state υ″ level population inversions, viz., N υ″=0?N υ″=1∼ N υ″=2≲N υ″=3<N υ″=4≳ N υ″=5 (0.38:0.28:0.25:0.32:1.00:0.17) confirm the presence of CN (A 2Π i ) in υ′=0 and υ′=1. Time resolved laser‐induced fluorescence studies indicate that A state CN is a primary product with a calculated lower limit to the vibrational temperature of ∼1450 K. Photolysis of ICN in the low energy à state continuum at λ?220 nm produces primarily CN (X 2Σ+) with little vibrational excitation. The measuredX state vibrational ratio N υ″=1/N υ″=0 yields an initial vibrational Boltzmann temperature of ∼750 K. Furthermore, the X state CN radicals are observed to be rotationally excited. The rotational level distributions can be fit by a theoretical function that is the sum of two Boltzmann functions with temperatures of ∼400±50 K and 2700±400 K. The observed rotational distributions are discussed with emphasis upon the symmetry and geometry of the excited ICN molecule. Furthermore, the observation that X state CN is produced with little vibrational excitation is discussed in reference to several conflicting theoretical and experimental investigations by others.