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 2Πi state populated the υ″=4 and υ″=5 levels of the X state. The observed X state υ″ level population inversions, viz., Nυ″=0?Nυ″=1∼ Nυ″=2≲Nυ″=3<Nυ″=4≳ Nυ″=5 (0.38:0.28:0.25:0.32:1.00:0.17) confirm the presence of CN (A 2Πi) 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 measured X state vibrational ratio Nυ″=1/Nυ″=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.