Abstract
We have performed a series of measurements on H 2CO A ̃ 1A 2 v 4 = 1 single rotational levels using Transient Gain Spectroscopy (TGS), which was designed to provide detailed information on state-to-state population transfer and on the relaxation and transfer of rotational alignment. Measurements of the time dependence of the population of the directly populated J K a , K c = 1 0,1 level, and of the population that is collisionally transferred to the neighboring 0 0,0, 2 0,2, and 3 0,3 rotational levels, are performed with both parallel and perpendicular relative PUMP/PROBE polarizations. This procedure allows the rotational-state-resolved populations to be analyzed with a microscopic M-resolved kinetic model at a level of detail unprecedented for a polyatomic molecule. Our analysis is able to distinguish direct, single-collision dealignment from sequential processes that result in dealignment. Nonlinear fitting of these data with a number of kinetic models indicates that there is no detectable contribution from single-collision direct-elastic events to the observed dealignment signals, and that dealignment may be accurately modeled by sequential processes following electric-dipole a-dipole selection rules. Furthermore, state-to-state inelastic rates are found to partition into M J -resolved collisional transfer rates according to tensor opacity rank Λ = 1 (electric-dipole amplitudes), with no detectable contribution from Λ = 2 or Λ = 3. Tensor opacity rank Λ = 1 produces the greatest persistence of the M J quantum number consistent with collisions in an isotropic environment, and this high level of M J conservation is necessary to explain our data. An Energy-Corrected Sudden (ECS) scaling model, with collision duration τ c = 7.0 psec, and basis rate coefficient k 0←1 = 26.6(5) μsec −1 Torr −1 provides an excellent representation of the observed energy transfer, with no detectable contribution from higher order terms, or from terms deviating from ECS scaling.
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