Rotational relaxation in the H2CO Ã 1A2 state by transient gain spectroscopy

A new, pulsed pump–cw probe technique, transient gain spectroscopy, has been used to measure the zero pressure lifetime (279±10 ns) and self‐collision depopulation rate (110.5±3.1 μs−1 Torr−1) for the H2CO A 1A2 41 JKa,Kc =132,12 level as well as the 122,11←132,12(31±8 μs−1 Torr−1) and 142,13←132,12(40±6 μs−1 Torr−1) rotational relaxation rates. The dominant collisional process is rotational energy transfer, which, for the H2CO(A)+H2CO(X) process examined here, appears to follow dipole–dipole scaling and propensity rules. The 110.5 μs−1 Torr−1 depopulation rate in the A state agrees with the vibrationless X state rate obtained from microwave power broadening scaled by the 1.46 D/2.33 D electric dipole moment ratio. The two strongest a‐dipole rotational state‐to‐state rates account for more than 50% of the total removal rate from H2CO A 41 132,12.