Sub-Doppler Spectroscopy of the trans-HOCO Radical in the OH Stretching Mode

Rovibrational spectroscopy of the fundamental OH stretching mode of the trans-HOCO radical has been studied via sub-Doppler high-resolution infrared laser absorption in a discharge slit-jet expansion. The trans-HOCO radical is formed by discharge dissociation of H₂O to form OH, which then combines with CO and cools in the Ne expansion to a rotational temperature of 13.0(6) K. Rigorous assignment of both a-type and b-type spectral transitions is made possible by two-line combination differences from microwave studies, with full rovibrational analysis of the spectrum based on a Watson asymmetric top Hamiltonian. Additionally, fine structure splittings of each line due to electron spin are completely resolved, thus permitting all three ε_aa, ε_bb, ε_cc spin–rotation constants to be experimentally determined in the vibrationally excited state. Furthermore, as both a- and b-type transitions for trans-HOCO are observed for the first time, the ratio of transition dipole moment projections along the a and b principal axes is determined to be μ_a/μ_b = 1.78(5), which is in close agreement with density functional quantum theoretical predictions (B3LYP/6-311++g­(3df,3pd), μ_a/μ_b = 1.85). Finally, we note the energetic possibility in the excited OH stretch state for predissociation dynamics (i.e., trans-HOCO → H + CO₂), with the present sub-Doppler line widths providing a rigorous upper limit of >2.7 ns for the predissociation lifetime.