Experimental and theoretical studies on vacuum ultraviolet absorption cross sections and photodissociation of CH3OH, CH3OD, CD3OH, and CD3OD

Absorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are measured in a 107-220 nm spectral region using synchrotron radiation. Spectra of improved quality for four deuterated isotopomers, coupled with extensive calculations on low-lying excited states of methanol using time-dependent density functional theory with a large cc-pV5Z basis set, enable us to improve assignments of observed spectral features and to better understand the nature of these electronic transitions. Energies and oscillator strengths of all transitions predicted with calculations are consistent with experimental results. Observed isotopic shifts clearly indicate that absorption features in the 163-220 nm region (transition 1 (1)A(')-X (1)A(')) are associated mainly with breaking of the O-H bond, consistent with theoretical predictions. In the 151-163 nm region (transition 2 (1)A(')-X (1)A(')), observed small vibrational spacings (806 cm-1 for CH3OH) associated with the C-O stretching mode can be rationalized with a broad double-well-like potential-energy curve resulting from avoided crossing of Rydberg states 1(1)A(')(2a(')-->3s) and 2 (1)A(')(2a(')-->3p); with isotopic data, further vibrational progressions are identified. Absorption lines in the 140-151 nm region with regular vibrational spacing (similar to1046 cm-1 for CH3OH), likely associated with the CH2 twisting mode, are assigned to nearly degenerate transitions 3 (1)A(')-X (1)A(') and 3 (1)A(')-X (1)A('); the 3 (1)A(') and 3 (1)A(') states are associated with excitations 2a(')-->3p(') and 2a(')-->3p('), respectively. Progressions associated with the torsional mode of the excited state are observed for the first time. For wavelengths smaller than 140 nm, Rydberg transitions with ngreater than or equal to3 are tentatively assigned in accord with their quantum defects that are identical for all isotopomers in each Rydberg series. (C) 2002 American Institute of Physics.