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Photoabsorption and photodissociation studies of dimethyl sulphoxide and its deuterated isotopologue (DMSO-h 6 and DMSO-d 6 ) are performed using synchrotron radiation in the 35,000–80,000 cm −1 region. In the photoabsorption spectrum, Rydberg series converging to the first three ionization potentials of DMSO at 9.1, 10.1 and 12.3 eV corresponding to removal of an electron from the highest three occupied molecular orbitals (14a′, 7a″ and 13a′) are observed. Based on a quantum defect analysis, Rydberg series assignments are extended to higher members as compared to earlier works and a few ambiguities in earlier assignments are clarified. Analysis is aided by quantum chemical calculations using the DFT and TDDFT methodologies. Vibronic structures observed in the spectrum of DMSO-h 6 in the regions 7.7–8.1 eV and 8.1–8.8 eV are attributed to the transitions 7a″→4p at 7.862 eV and 14a′→6s/4d at 8.182 eV, respectively. Photoabsorption spectra of DMSO-h 6 and -d 6 recorded using a broad band incident radiation show prominent peaks, which are identified and assigned to electronic and vibronic transitions of the SO radical. This provides a direct confirmation of the fact that DMSO preferentially dissociates into CH 3 and SO upon UV–VUV excitation, as proposed in earlier photodissociation studies. An extended vibronic band system associated with the e 1 Π–X 3 Σ − transition of the SO radical is identified and assigned. The complete VUV photoabsorption spectrum of DMSO-d 6 is also reported here for the first time.

A consolidated study of the VUV absorption spectra of CH2Cl2 and CD2Cl2 in the 50,000–95,000 cm−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 50,000–60,000 cm−1 is attributed to the valence states 11B2, 11B1 and 11A1. Most of the bands in the 60,000–95,000 cm−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 12.271 eV 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.

We report a comprehensive photoabsorption study of nitrous oxide (N2O) in the vacuum ultraviolet (45,000–95,000 cm−1) region using synchrotron radiation. The observed spectrum comprises of a few valence transitions and low lying Rydberg series converging to the two spin–orbit components (2П1/2,3/2) of the ground state of N2O+. Spectral analysis is aided by extensive quantum chemical calculations of vertical excited states, oscillator strengths and potential energy curves using the time dependent density functional theory. Vibronic bands observed in the first absorption system (45,000–60,000 cm−1) are assigned to hot band progressions in υ 2 ′ originating from v″=1 or 2. New insights into the assignment of the well-formed progression of bands in the X1Σ+→C1П system (60,000–72,000 cm−1) are afforded by consideration of the Renner–Teller interaction. A set of molecular vibrational parameters (ω2=467 cm−1, x22=−2.9, e=−0.24) for the C1П state are derived from a fitting of the experimental data. The 3pπ1Σ+ state at ~77,600 cm−1 shows a large quantum defect (0.96) which is explained as arising due to mixed valence–Rydberg character. In the 85,000–95,000 cm−1 region, a number of absorption features are observed with greater clarity than in earlier photoabsorption studies and assigned to Rydberg series of type nlλ (n=3,4; l=s,p,d; λ=σ,π,δ) and accompanying vibronic bands. This work has resulted in clarification of several discrepancies in earlier Rydberg series assignments. Additionally, the 3pπ 3Σ− Rydberg state at 85,788 cm−1, the valence transition 7σ→3π (1П) at 87,433 cm−1 and the 3dλ Rydberg series in the 91,700–92,600 cm−1 region are assigned for the first time.

The UV–VUV photoabsorption spectrum of dibromomethane (CH2Br2) in the energy region 4.3–11.8 eV (35,000–95,000 cm−1) is investigated using synchrotron radiation. Rydberg series converging to the first four ionization limits at 10.52, 10.74, 11.21 and 11.30 eV corresponding to excitations from the 3b1, 2b2, 1a2, and 4a1 orbitals of CH2Br2 are identified and analyzed. Quantum defect values are observed to be consistent with excitation from the bromine lone pair orbitals. Assignments of the ns Rydberg series are revised and the np and nd Rydberg series are assigned for the first time. Observed vibrational features accompanying the 5p and 4d Rydberg states are assigned exclusively to the totally symmetric (a1) –CBr symmetric stretching mode (ν3) in contrast to the earlier assignment to ν3 and –CH2 bending (ν2) modes. The Rydberg and valence transitions observed in the present experiment are found to be in good agreement with the vertical excited states calculated using the TDDFT method. The calculations are further used to infer the valence transitions responsible for the broad intensity pedestals underlying the Rydberg transitions. The assignments are confirmed using isotopic substitution studies on CD2Br2 whose UV–VUV photoabsorption spectrum is reported here for the first time. This work presents a consolidated analysis of the UV–VUV photoabsorption spectrum of dibromomethane.