Cluster ion dip spectroscopy of hydrogen bonded phenol(H2O)n clusters, n=0–4

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Abstract
Employing a laser based time‐of‐flight mass spectrometer system, ion dip spectra for phenol (Ph), Ph(H2O)1, Ph(H2O)3, and Ph(H2O)4 were obtained in the range of 500–1300 cm−1 from a variety of initially pumped states. Dramatic enhancement of the signal‐to‐noise ratio of the cluster ion dip spectra relative to that of the bare phenol is attributable to the increase in the excited state singlet lifetime of the hydrated phenol chromophore. Several dips in the Ph(H2O)1 spectrum exceed the ‘‘saturation’’ limit of 50%, indicating that significant relaxation of the downpumped ground state is occurring via low frequency vibrational modes of the H2O solvent ‘‘bath.’’ Excitation of the hydrogen bond stretch (σ 10=156 cm−1 ) in the S1 state of the Ph(H2O)1 cluster reveals that the ground state (S0) hydrogen bond stretch, σ 01, is 151(±1) cm−1, a mode which appears to be built off of phenol fundamental and combination bands. A second intermolecular band is also evident at 141(±2) cm−1. There is no evidence of an analogous wag mode when pumping the S1 bend (β10), suggesting that the intermolecular modes in S1 are highly coupled. Attempts to obtain ion dip spectra for Ph(H2O)2 went unrewarded, presumably due to the anomalously short S1 lifetime of the Ph(H2O)2 cluster. Spectra for Ph(H2O)3 and Ph(H2O)4 were obtained which show prominent phenol bands, with low frequency (∼10 cm−1 ) progressions built off of these bands. The intermolecular hydrogen bond stretch for Ph(H2O)3 and Ph(H2O)4 in the ground state are 189(±1) cm−1 and 185(±1) cm−1, respectively.