Electric dipole moments of excited vibrational levels in the X̃ 1A1 state of formaldehyde by stimulated emission spectroscopy

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Abstract
The electric dipole moments of the 44 (E4ν4=4628.4 cm−1), 2242 (E2ν22ν4)=5770 cm−1), and 2144 (E1ν24ν4=6347 cm−1) levels of H2CO are measured as 2.2723(86), 2.3222(47), and 2.2825(33) D, respectively. This first combination of stimulated emission pumping (SEP) with a static electric field demonstrates the possibility of systematically measuring dipole moments for highly excited vibrational levels of polyatomic molecules in their electronic ground states. The present dipole moment measurements and all previous ones involving overtones and combinations of the C–O stretch (ν2) and out‐of‐plane bend (ν4) are reduced to two types of expansions. The dipole moment μ is expressed both as an expansion in the normal coordinate displacements and as an expansion through squared terms in the v2 and v4 vibrational quantum numbers.