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The vacuum ultraviolet (VUV) photoabsorption spectra of the dihalomethanes \(\rm CH_2X_2 \ (X=Cl, Br, I)\) are studied using synchrotron radiation in the energy region 6–11.8 eV (\(\sim\)49,000–95,200 cm\(^{−1}\)). A detailed comparison is made to identify similarities, differences and trends in the spectra and excited state structure of these three molecules. The electronic spectra of the dihalomethanes in this region are dominated by Rydberg series of \(ns\), \(np\) and \(nd\) type. Quantum defect analysis reveals that the Rydberg series in all the dihalomethanes originate from the four outermost halogen lone pair non-bonding orbitals. On going from Cl to I, the energy difference between the first four ionization potentials decreases which as a consequence leads to spectral congestion and complications in spectral assignments. In all three molecules, several Rydberg transitions are accompanied by vibrational structure. A notable common feature seen in the vibronic structure is the excitation of the \(\nu_3\) (\(\rm C\)\(\rm -\)\(\rm X\) symmetric stretch) mode to form extensive progressions. Additionally, the \(\nu_1\) (\(\rm C\)\(\rm -\)\(\rm H\) symmetric stretch), \(\nu_2\) (\(\rm CH_2\) bend) and \(\nu_8\) (\(\rm CH_2\) wag) modes are observed in \(\rm CH_2Cl_2\) and \(\rm CH_2I_2\), although they do not form long progressions. Quantum chemical calculations of ground and excited states are used to support the analysis and an improved theory-experiment comparison is provided for \(\rm CH_2Br_2\).

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.

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.

The electronic absorption spectrum of diiodomethane in the 30 000–95 000 cm−1 region is investigated using synchrotron radiation; the spectrum in the 50 000–66 500 cm−1 region is reported for the first time. The absorption bands in the 30 000–50 000 cm−1 region are attributed to valence transitions, while the vacuum ultraviolet (VUV) spectrum (50 000–95 000 cm−1) is dominated by several Rydberg series converging to the first four ionization potentials of CH2I2 at 9.46, 9.76, 10.21, and 10.56 eV corresponding to the removal of an electron from the outermost 3b2, 2b1, 1a2, and 4a1 non-bonding orbitals, respectively. Rydberg series of ns, np, and nd type converging to each of the four ionization potentials are assigned based on a quantum defect analysis. Time dependent density functional theory calculations of excited states support the analysis and help in interpretation of the Rydberg and valence nature of observed transitions. Density functional theory calculations of the neutral and ionic ground state geometries and vibrational frequencies are used to assign the observed vibronic structure. Vibronic features accompanying the Rydberg series are mainly due to excitation of the C-I symmetric stretch (ν3) and CH2 wag (ν8) modes, with smaller contributions from the C-H symmetric stretch (ν1). UV absorption bands are assigned to low lying valence states 11B2, 11B1, 21A1, 31A1, 21B1, and 21B2 and the unusually high underlying intensity in parts of the VUV spectrum is attributed to valence states with high oscillator strength. This is the first report of a comprehensive Rydberg series and vibronic analysis of the VUV absorption spectrum of CH2I2 in the 50 000–85 000 cm−1 region. The VUV absorption spectrum of CD2I2 which serves to verify and consolidate spectral assignments is also reported here for the first time.