Binding energy of van der Waals- and hydrogen-bonded clusters by threshold ionization techniques

Mass analyzed threshold ionization is based on the laser excitation of high long-lived Rydberg states 5-15 cm(-1) below a selected ionization threshold. Separating the Rydberg molecules from promptly produced ions in a delayed pulsed field ionization scheme leads to state and energy selected ions that are mass analyzed in a time-of-flight mass spectrometer. We present vibrational ion spectra of various clusters of indole, phenol and 1-naphthol. Because of the mass selectivity dissociation thresholds of the clusters can be measured when the ion internal energy is increased and a breakdown of the parent ion signal caused by dissociation is observed. We compare results for van der Waals with hydrogen bond energies and find a strong increase for the latter in the ionized cluster. Experimental results are explained by changes of the electron density distribution in the molecules after clustering and ionization. The influence of the size of the aromatic part of a molecule on the strength of the hydrogen bonding to an attached water molecule is investigated for the neutral as well as for the ionized molecules. In ionic clusters, we observe a decrease of hydrogen bond strength with an increasing number of aromatic rings in the chromophor, whereas the neutral binding energy increases in this case. (C) 2000 Elsevier Science B.V.