Isotope, electric field, and vibrational state dependence of single rotational level lifetimes of S1 formaldehyde

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Abstract
Additional single rovibronic level lifetimes of S 1 H2CO and D2CO have been measured under collisionless conditions. The H2CO 41 lifetimes vary at least a factor of 150, from 20 nsec to 3.10 μsec. The observed D2CO 41 lifetimes fluctuate about ±20% around a mean value of 6.2 μsec, which is probably close to the pure radiative lifetime. In contrast, the observed D2CO 43 lifetimes vary from 1.09 to 2.46 μsec and the 2143 lifetimes vary from 212 nsec to 1.61 μsec. The onset of rotational state lifetime fluctuations in D2CO thus coincides with the high pressure D2+CO photochemical threshold. All of these results are explained in terms of a collisionless sequential decay mechanism, S1→S0→H2(D2)+CO. The last step probably involves tunneling through a barrier for the lower energies studied. For several H2CO 41 rotational levels application of a uniform external electric field of 0–4.6 kV/cm can change the fluorescence lifetime by at least a factor of 4. This result is understood in terms of small (≲0.05 cm−1) shifts in S1−S0 energy spacings. Quantitative estimates of S1−S0 intramolecular couplings, S0 widths due to dissociation, and S0 level spacings are derived in favorable cases.