Investigation of some Rydberg states of ketene by two-photon resonance-enhanced multiphoton ionization spectroscopy

Rydberg states of CH2CO and CD2CO in the 54 000-72 000 cm(-1) spectral range have been reinvestigated using two-photon resonance-enhanced multiphoton-ionization spectroscopy. Improved resolution and sensitivity has allowed for identifications of more Rydberg states than in previous work. Based on an analysis of rotational structures and quantum defects and a comparison with the results of theoretical calculations, transitions to the 3p(z), 4p(y), 4p(z), and 5p(y), three components of 4d, and two components of 4f Rydberg states with a ground-state ionic core ((X) over tilde B-2(1)) are identified. Several transitions have been reassigned. Vibrational wave numbers indicate that the geometry of the [(X) over tilde B-2(1)]3p(y)((1)A(2)) state is almost identical to that of the corresponding cation in its ground electronic state, with C-2v symmetry, whereas that of the [X B-2(1)]3p(x)((1)A(1)) state differs significantly from those of the neutral molecule and the cation in their ground states, consistent with previous quantum chemical calculations that indicated that the [(X) over tilde B-2(1)]3p(x)((1)A(1)) state has C-s symmetry. The energy ordering of the three components of the 3p Rydberg states is found to be 3p(x)<3p(y)<3p(z), in agreement with a previous theoretical prediction using the equation-of-motion coupled-cluster singles and doubles polarized basis set methods and a first-principles vibronic model simulation. Excitations of the vibrational modes of b(1) (e.g., C=C=O out-of-plane bending, CH2 or CD2 wagging) and b(2) symmetries (e.g., C=C=O in-plane bending) are observed in several Rydberg states of CH2CO and CD2CO. (C) 2003 American Institute of Physics.