Vacuum ultraviolet photoabsorption spectroscopy of CH2Cl2 and CD2Cl2 in the energy region 50,000–95,000 cm−1

Abstract

A consolidated study of the VUV absorption spectra of CH2Cl2 and CD2Cl2 in the 50,000–95,000cm−1 region using synchrotron radiation is presented. Rydberg series and vibronic analysis are carried out and supported by quantum chemical calculations. The broad absorption band of CH2Cl2 in the region 50,000–60,000cm−1 is attributed to the valence states 11B2, 11B1 and 11A1. Most of the bands in the 60,000–95,000cm−1 region are fitted to Rydberg series of ns, np and nd type converging to the first four ionization potentials 11.320, 11.357, 12.152 and 12.271eV of CH2Cl2 arising from excitation of an electron from one of the four outermost Cl non-bonding orbitals (2b1, 3b2, 1a2 and 4a1). Vertical excited states of CH2Cl2 calculated using TDDFT are correlated with experimentally observed electronic states based on the symmetries of the initial and final MOs involved in a transition. A few Rydberg transitions viz. 2b1→5s, 4p, 5p, 6p; 3b2→4p, 5p; 1a2→4p are accompanied by vibronic features. Observed vibronic bands are assigned mainly to the CCl symmetric stretch (ν3′) mode with smaller contributions from the CH symmetric stretch (ν1′), CH2 bend (ν2′) and CH2 wag (ν8′) modes. Assignments are corroborated by comparison with the VUV absorption spectrum of the deuterated isotopologue CD2Cl2, reported here for the first time. The high underlying intensities seen in several sub-regions are explained by valence or valence–Rydberg mixed type transitions predicted with high oscillator strengths by the TDDFT calculations.