The 365 nm 41 0 band of formaldehyde-d2: Analysis of rotational structure and Coriolis perturbations

The 365 nm 4₁ ⁰ (υ′= 0 - υ₄″ = 1) band in the A ¹A₂ - X¹A₁ absorption system of formaldehyde-d₂, D₂CO, has been studied under high resolution. Rotational analysis procedures, which include approximations allowing for strong Coriolis perturbations to the υ₄″ = 1 level, are described and compared with more rigorous approaches. Rotational constants for both ground and excited vibronic states are determined by least-squares analysis, and significantly revised ground state vibrational frequencies reported, as follows: ν₄″ = 936.5 ± 0.1 cm^-1, ν₆″ = 989.3 ± 0.1 cm^-1. The coupling parameter ξ₆₄ ^a (2̃Aζ₆₄ ^a), associated with the a-axis Coriolis interaction between υ₄″ = 1 and υ₆″ = 1 levels, is determined to be |ξ₆₄ ^a| = 4.98 ± 0.02 cm^-1. Several resonant Coriolis perturbations, arising from accidental coincidences between certain ν₄ rovibrational levels and levels of ν₆ and ν₃, are discussed and characterized. A simple theory for the intensity distribution in a pair of Coriolis-coupled vibronic bands is used to relate minor intensity anomalies observed in the 4₁ ⁰ band to the apparent absence of the adjacent 6₁ ⁰ band.