Vibronic origin of 1¹4¹ and the coriolis effect on the excited-state lifetimes of the rotational levels of 5¹ in ùA₂ H₂CO

The 5₀¹ and 1₀¹4₀¹ bands of the S₁-S₀ transitions of jet-cooled H₂CO have been rotationally analyzed from a fluorescence excitation spectrum. The previously unobserved ^rR₁, ^rR₀, ^pQ₁, and ^pP₁ subbands of 1₀¹4⁰¹ have been identified, and the vibronic origin was determined to be at ṽ₀ ≃ 31 159.96 cm^-1. The Coriolis-perturbed intensities of some rovibrational transitions of the 5₀¹ and 1₀¹4₀¹ bands obtained from a Fourier transform absorption spectrum of H₂CO taken at room temperature are compared to the unperturbed intensities calculated from an asymmetric rotor program. The single rotational level (SRL) lifetimes of the 5¹ and 1¹4¹ rotational levels have been measured, and it is shown that the vibrationally pure 5¹ decays at least 3 times slower than the vibrationally pure 1¹4¹. The rotationally cold molecules with the greatest vibrational purity give the highest vibronic selectivity for radiative and nonradiative processes.