Stark level-crossing spectroscopy of S formaldehyde eigenstates at the dissociation threshold

Abstract

Spectra of S0 D2CO rovibrational eigenstates with 28 300 cm−1 of vibrational excitation are measured by Stark level-crossing spectroscopy. In this new method, the lifetime of a single J, K, M-resolved S1 state is monitored as a function of electric field. Enhanced nonradiative decay causes the S1 lifetime to decrease as S0 states are Stark tuned into resonance. Analysis of the resulting resonance lineshapes yields complete distributions of S0 decay rates (linewidths) and S1-S0 coupling matrix elements. The S0 decay rates represent the first measurements of unimolecular dissociation rates of a polyatomic molecule at the eigenstate-resolved level. S0 decay widths vary from 6.4×10−5 to 3.8×10−3 cm−1 and S1-S0 coupling matrix elements vary from 3.5×10−7 to 4.7×10−5 cm−1, demonstrating that chemical properties of neighboring eigenstates fluctuate by over two orders of magnitude. The observed density of S0 vibrational states is ∼400 per cm−1, six times greater than an estimate including first-order anharmonic corrections. The small increase of level density with J indicates that Ka is nearly a good quantum number for J≤4. The barrier height to unimolecular dissociation on the S0D2CO surface is determined to be 80.6±0.8 kcal/mol, corresponding to 79.2±0.8 kcal/mol for H2CO, in good agreement with ab initio predictions. Quantitative agreement between the magnitude of experimentally determined decay rates and an RRKM rate calculation with all parameters set by ab initio calculation is found.