Polarization-detected transient gain studies of relaxation processes in v4=1 Ã 1A2 formaldehyde-h2

Polarization-detected transient gain spectroscopy (PTGS), a time-resolved PUMP/PROBE scheme with single rotation-vibration level selectivity in both preparative and monitoring stages, has been used to study total depopulation and rotational state-to-state rates in the H₂CO Ã ¹A₂ v₄=1 vibrational level. For H₂CO(Ã)/H₂CO(X∼) collisions, single rovibronic level depopulation rates in the range 88±2 to 127±6 μs^-1 Torr^-1 (∼10 times faster than the hard sphere gas kinetic rate) were obtained from Stern-Volmer plots which were found to be linear over at least the 0.02-0.2 Torr pressure region. Rotational relaxation between the 5_0,5 and 6_0,6 eigenstates reflects a-dipole propensity rules with the sum of first order ΔJ=+1 and ΔJ=-1 processes accounting for well over 50% of the total removal cross section measured for a single rovibronic level. The agreement between collisional decay rates for the 13_2,12 eigenstate measured previously by the TGS technique and here by PTGS, 110±3 and 106±4 μs^-1 Torr^-1, respectively, suggests that elastic reorientation processes (ΔJ=ΔK_a =ΔK_c =0, ΔM≠0) occur with much less efficiency than inelastic energy transfer. Collisional depopulation from single rotational levels of H₂CO Ã ¹A₂ v₄=1 is a remarkably efficient process, but completely understandable in the small molecule, isolated binary collision limit. None of the multiexponential decay and nonlinear Stern-Volmer effects manifest in previous undispersed fluorescence studies remain when PTGS is used to populate and monitor a single rovibronic level.