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 3pz, 4py, 4pz, and 5py, three components of 4d, and two components of 4f Rydberg states with a ground-state ionic core (X̃ 2B1) are identified. Several transitions have been reassigned. Vibrational wave numbers indicate that the geometry of the [X̃ 2B1]3py(1A2) state is almost identical to that of the corresponding cation in its ground electronic state, with C2v symmetry, whereas that of the [X̃ 2B1]3px(1A1) 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 [X̃ 2B1]3px(1A1) state has Cs symmetry. The energy ordering of the three components of the 3p Rydberg states is found to be 3px<3py<3pz, 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 b1 (e.g., C=C=O out-of-plane bending, CH2 or CD2 wagging) and b2 symmetries (e.g., C=C=O in-plane bending) are observed in several Rydberg states of CH2CO and CD2CO.