The high resolution technique of Stark quantum beat spectroscopy is used to examine the electric dipole moment function for the first excited singlet state (à 1A2) of formaldehyde-h2 and formaldehyde-d2. The high precision of these measurements (i.e., better than 5 parts in 104) enables detailed determination of a-axis dipole moment components (μa ) for individual J=2 rovibronic levels in the ν4 out-of-plane bending mode. In the case of 21,1 rotational levels, we find μa (40)=1.4784(7) D and μa (41)=1.4678(4) D for H2CO. For D2CO the measured 21,1 dipole moments are μa (40)=1.4698(6) D, μa (41)=1.4693(3) D, and μa (43) =1.4786(7) D. The state-specific variations in μa revealed by this study reflect the structural influences exerted by the pervasive S1∼S0 nonadiabatic interactions and the pyramidally distorted equilibrium configuration which characterize the à state of formaldehyde. The origin and experimental manifestation of the out-of-plane dipole moment component (μc ) in nonrigid à 1A2 formaldehyde is also discussed.
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Vaccaro et al. (1989) studied this question.
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