ISOTOPE, ELECTRIC FIELD, AND VIBRATIONAL STATE DEPENDENCE OF SINGLE ROTATIONAL LEVEL LIFETIMES OF S1 FORMALDEHYDE

Additional single rovibronic level lifetimes of S{sub 1} H{sub 2}CO and D{sub 2}CO have been measured under collisionless conditions. The H{sub 2}CO 4{sup 1} lifetimes vary at least a factor of 150, from 20 nsec to 3.10 {micro}sec. The observed D{sub 2}CO 4{sup 1} lifetimes fluctuate about {+-} 20% around a mean value of 6.2 {micro}sec, which is probably close to the pure radiative lifetime. In contrast, the observed D{sub 2}CO 4{sup 3} lifetimes vary from 1.09 to 2.46 {micro}sec and the 2{sup 1} 4{sup 3} lifetimes vary from 212 nsec to 1.61 {micro}sec. The onset of rotational state lifetime fluctuations in D{sub 2}CO thus coincides with the high pressure D{sub 2} + CO photochemical threshold. All of these results are explained in terms of a collisionless sequential decay mechanism, S{sub 1} {yields} S{sub 0} {yields} H{sub 2}(D{sub 2}) + CO. The last step probably involves tunneling through a barrier for the lower energies studied. For several H{sub 2}CO 4{sup 1} 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 ({approx}< 0.05 cm{sup -1}) shifts in S{sub 1}-S{sub 0} energy spacings. Quantitative estimates of S{sub 1}-S{sub 0} intramolecular couplings, S{sub 0} widths due to dissociation, and S{sub 0} level spacings are derived in favorable cases