Binding energy of van der Waals- and hydrogen-bonded clusters by thresholdionization 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 threshol d. Separating the Rydberg molecules from promptly produced ions in a delaye d pulsed field ionization scheme leads to state and energy selected ions th at are mass analyzed in a time-of-flight mass spectrometer. We present vibr ational ion spectra of various clusters of indole, phenol and 1-naphthol. B ecause of the mass selectivity dissociation thresholds of the clusters can be measured when the ion internal energy is increased and a breakdown of th e parent ion signal caused by dissociation is observed. We compare results for van der Waals with hydrogen bond energies and find a strong increase fo r the latter in the ionized cluster. Experimental results are explained by changes of the electron density distribution in the molecules after cluster ing and ionization. The influence of the size of the aromatic part of a mol ecule 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 i onic clusters, we observe a decrease of hydrogen bond strength with an incr easing number of aromatic rings in the chromophor, whereas the neutral bind ing energy increases in this case. (C) 2000 Elsevier Science B.V.