High precision dipole moments in à ¹A₂ formaldehyde determined via Stark quantum beat spectroscopy

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 (μ_a) for individual J = 2 rovibronic levels in the ν₄ out-of-plane bending mode. In the case of 2 _1,1 rotational levels, we find μ_a (4⁰) = 1.4784(7) D and μ_a (4¹) = 1.4678(4) D for H ₂CO. For D₂CO the measured 2_1,1 dipole moments are μ_a (4⁰) = 1.4698(6) D,μ_a(4 ¹) = 1.4693(3) D, and μ_ae(4³) = 1.4786(7) D. The state-specific variations in μ^a 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 (μ_c) in nonrigid à ¹A₂ formaldehyde is also discussed.