Strong fragment alignment variation with internal state from ICN dissociation at 157.6 nm: Linear regression modeling of CN B 2Σ+ products

Abstract

Dispersed CN B 2Σ+−X 2Σ+ photofragment fluorescence polarization anisotropies measured following ICN dissociation at 157.6 nm vary widely and apparently erratically with emission wavelength, and cannot be converted directly to CN B 2Σ+ rotational alignments because of spectral congestion. A novel linear regression technique is used to extract CN B 2Σ+ populations and rotational alignments from fluorescence emission and polarization anisotropy measurements. We present a flexible procedure which allows one to consider many models for the population and alignment distributions. Criteria are established to identify the best models. The CN B 2Σ+ vibrational branching ratios for v′=0:1:2:3:4 are determined by linear regression to be 0.46:0.25:0.13:0.09:0.07, with a distinct rotational population dependence within each vibrational level. Extracted CN B 2Σ+ alignments for v′=0, 1, 2, and 3 are presented, and these range from −0.31 to nearly 0.2. The alignments vary smoothly with nuclear rotation N′ for each v′, demonstrating that the scatter in the measured polarization anisotropies results from vibrational band overlap at different wavelengths. These results show the largest photofragment alignment variation with vibration and rotation that has been measured following a single-photon dissociation process. A model is presented to estimate partial channel CN B(v′=0) product populations, and a discontinuity in the experimental v′=0 alignment is considered.