TIME-RESOLVED PUMP PROBE PHOTOIONIZATION STUDY OF EXCITED-STATE DYNAMICS OF PHENOL (H2O)2 AND PHENOL (H2O)3

Time-resolved pump-probe photoionization mass spectrometry has been used to study the excited-state dynamics of phenol-(H2O)2 and phenol-(H2O)3 formed in a supersonic expansion. Singlet-state lifetimes were measured by using both soft (355 nm) and hard (193 nm) ionization. Phenol-(H2O)2 was found to have an anomalously short singlet-state lifetime (6 ± 1 ns) compared to those of other phenol-water complexes. We interpret this as evidence that phenol-(H2O)2 lacks the type of hydrogen bonding found in phenol-(H2O) and phenol-(H2O)3. In addition, we have used 193-nm ionization with a variable delay to study the fragmentation of these complexes in the triplet manifold. It was found that, after undergoing intersystem crossing from the S1 origin level, both complexes rapidly lose two water molecules. This number is apparently limited by the amount of vibrational excitation acquired in the triplet manifold which, in turn, is fixed by the singlet-triplet energy gap. We also investigated the fragmentation that occurs following 193-nm ionization of the S1 origin level of the complexes. Here, phenol-(H2O)2 rapidly loses only one water molecule while phenol-(H2O)3 loses two. Interestingly, 193-nm ionization could potentially deposit more than twice as much vibrational energy in the ion compared to the vibrational excitation in the triplet manifold. These results suggest that Franck-Condon factors are important in limiting the extent to which these complexes dissociate following ionization. © 1990 American Chemical Society.