Quantitative spectral analysis of HCl and DCl in 120–220 nm: Effects of singlet–triplet mixing

Photoabsorption cross sections of HCl and DCl in the spectral region 120–220 nm were measured using synchrotron radiation as a light source. The absorption contour of the A 1Π←X 1Σ+ band of HCl in the 134–195 nm region is slightly asymmetric, whereas that of DCl is symmetric, with wavelength of absorption maximum shifted from ∼153.9 nm for HCl to ∼155.8 nm. The asymmetry in HCl and the atypical direction of shift in peak wavelengths upon deuteration are attributed to singlet–triplet mixing in its ground X 1Σ+ state of HCl; this triplet component contributes to the oscillator strengths of the A–X band of HCl via transition to the repulsive triplet state 1 3Σ+. The small triplet character in HCl is further supported by an observation that the oscillator strength of the (0,0) band for the b 3Π–X 1Σ+ transition of DCl is ∼17% smaller than that of HCl. This triplet component may account for greater values of spin–orbit branching ratios for products, Cl*(2P1/2)/Cl(2P3/2), observed upon photolysis of HCl in the 143–167 nm region. For both HCl and DCl, ratios of observed oscillator strengths to calculated Franck–Condon factors of the C 1Π state decrease with vibrational levels, whereas those of the b 3Π state increases with increasing vibrational levels. Such a variation was attributed to perturbation between the b and C states, for which quantitative data are established in this study.