Electronic spectra and single rotational level fluorescence lifetimes of jet-cooled H₂CO: The ùA₂ ← X̃¹A₁ 2₀¹6₀¹, 2₀¹4₀³, and 2₀¹4₀¹6₀¹

The weakly absorbing transitions of 2₀¹6₀¹ and 2₀¹4₀¹6₀¹ near the strongly absorbing 2₀¹4₀³ transition were favorably detected by laser fluorescence excitation spectroscopy of jet-cooled H₂CO (S₀), because of the higher fluorescence yields of the 2¹6¹ and 2¹4¹O¹ levels. The vibronic origins (ν₀) and rotational constants of the 2¹6¹ and 2¹4¹6¹ levels of H₂CO (S₁) have been determined as follows: 2¹6¹: ν̃₀ = 30252.21 cm^-1; A′ = 9.204 cm^-1; B′ = 1.108 cm^-1; C′ = 0.996 cm^-1; 2¹4¹6¹: ν̃₀ = 30 395.00 cm^-1; A′ = 8.918 cm^-1; B′ = 1.105 cm^-1; C′ = 0.993 cm^-1. The S₁ lifetimes have been measured for a number of SRL's of 2¹6¹ (E_vib ≃ 2064 cm^-1) and they vary in the range of 20-90 ns. This behavior indicates that a moderate degree of the lumpy continuum characteristic is observed even at E_vib ≃ 2000 cm^-1. The S₁ lifetimes of 2¹4³ and 2¹4¹6¹ were ≤15 and 20-30 ns, respectively.