Vacuum Ultraviolet Absorption Spectrum of Difluoromethane Reinvestigated

Shastri, Aparna; Sekhar, B. N. Raja; Singh, P.; Deo, M. N.

doi:10.1080/00387010902895012

Cited by 8 publications

(14 citation statements)

References 17 publications

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“…Each of the molecules CH2F2, CH2Cl2 and CD2Cl2 (D = 2 H) are isoelectronic in the valence shell. Continuous absorption for the chloro-compounds at the spectral onset, 10,11 has been attributed 14 to overlapping electronic states, rather than the single state previously assumed. [5][6][7][8][9] The 7 to 12 eV VUV absorption of the series CHkCl4-k (k = 0 to 3), has been generally attributed to excitation of non-bonding electrons on chlorine, and this was consistent with several studies for CH2F2.…”

Section: Introductionmentioning

confidence: 85%

“…These preceding studies [5][6][7][8][9][10][11] indicate the need for a more rigorous theoretical approach to the analysis of the VUV spectrum of CH2F2, as undertaken in the present paper. Our theoretical study enables us to determine the onset of several of the low-lying singlet states, by use of the 2 ) ( δ − = n R time dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) methods.…”

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

“…The decision on whether the CF2 or CH2 group is chosen for the σV axis in C2V symmetry, can affect the apparent excited or ionic state symmetries. 9,10,12,13 We follow our PES study 4 of CH2F2 by using the CF2 (y,z) and CH2 (x,z) planes. The opposite choice interchanges both B2/B1 states and b2/b1 orbital symmetries.…”

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

“…Bands II and III of the VUV show discrete vibrational structure, previously attributed to Rydberg states. 9,10 Seccombe et al 9 assigned a considerable number of Rydberg states to VUV bands I to VIII; we will revise those assignments below. Later, Shastri et al, 11 suggested that vibrational sequences in VUV band II continue into band III, where they apparently become both separated and exhibit lower relative line widths.…”

Section: Introductionmentioning

confidence: 95%

“…[5][6][7][8][9] The 7 to 12 eV VUV absorption of the series CHkCl4-k (k = 0 to 3), has been generally attributed to excitation of non-bonding electrons on chlorine, and this was consistent with several studies for CH2F2. [5][6][7][8][9][10][11] However, in a theoretical study of the series CHkF4-k (k = 0 to 3), Edwards and Raymonda 7 concluded that the first absorption in CHF3, attributed to a Rydberg state, should be associated with the C-H unit alone. The first absorption band relating to the CF3 group was thought to lie significantly higher, while the lowest IE for CHF3 is close to that for atomic hydrogen.…”

Section: Introductionmentioning

confidence: 99%

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The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations

Palmer

Hoffmann

Jones

et al. 2018

The Journal of Chemical Physics

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The vacuum ultraviolet (VUV) spectrum for CHF from a new synchrotron study has been combined with earlier data and subjected to detailed scrutiny. The onset of absorption, band I and also band IV, is resolved into broad vibrational peaks, which contrast with the continuous absorption previously claimed. A new theoretical analysis, using a combination of time dependent density functional theory (TDDFT) calculations and complete active space self-consistent field, leads to a major new interpretation. Adiabatic excitation energies (AEEs) and vertical excitation energies, evaluated by these methods, are used to interpret the spectra in unprecedented detail using theoretical vibronic analysis. This includes both Franck-Condon (FC) and Herzberg-Teller (HT) effects on cold and hot bands. These results lead to the re-assignment of several known excited states and the identification of new ones. The lowest calculated AEE sequence for singlet states is 1B ∼ 1A < 2B < 1A < 2A < 1B < 3A < 3B. These, together with calculated higher energy states, give a satisfactory account of the principal maxima observed in the VUV spectrum. Basis sets up to quadruple zeta valence with extensive polarization are used. The diffuse functions within this type of basis generate both valence and low-lying Rydberg excited states. The optimum position for the site of further diffuse functions in the calculations of Rydberg states is shown to lie on the H-atoms. The routine choice on the F-atoms is shown to be inadequate for both CHF and CHF. The lowest excitation energy region has mixed valence and Rydberg character. TDDFT calculations show that the unusual structure of the onset arises from the near degeneracy of 1B and 1A valence states, which mix in symmetric and antisymmetric combinations. The absence of fluorescence in the 10.8-11 eV region contrasts with strong absorption. This is interpreted by the 2B and 1A states where no fluorescence is calculated for these two states, which are only active in absorption. The nature of the two states, 1B and 2B, is fundamentally different, but both are complex owing to the presence of FC and HT effects occurring in different ways. The two most intense bands, close to 12.5 and 15.5 eV, contain valence states as expected; the onset of the 15.5 eV band shows a set of vibrational peaks, but the vibration frequency does not correspond to any of the photoelectron spectral (PES) structure and is clearly valence in nature. The routine use of PES footprints to detect Rydberg states in VUV spectra is shown to be inadequate. The combined effects of FC and HT in the VUV spectral bands lead to additional vibrations when compared with the PES.

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