PNNL

Abstract

Rotational coherence spectroscopy (RCS), via time-correlated single photon counting, and two-color resonant two-photon ionization (R2PI) time-of-flight mass spectrometry, have been used to characterize fluorene-(water)1,2 (FL-(water)1,2) van der Waals clusters generated in supersonic jets. Rotational coherence traces have been obtained at excitation energies corresponding to several resonant features in the S1S0 R2PI spectra of FL-(H2O)1,2. RCS simulations and diagonalization of the moment of inertia tensor have been used to obtain the S1 excited state rotational constants and structures of FL-(H2O)1,2 that are consistent with the experimental rotational coherence traces. The RCS results indicate that: (i) the water molecule in FL-H2O bridges the central five-membered ring of fluorene and hydrogen bonds to both aromatic sites; (ii) the water molecules in FL-(H2O)2 form a water dimer that is oriented along the long axis of fluorene and is hydrogen-bonded to both aromatic sites. The S1S0 R2PI spectra of FL-(D2O)1,2 and FL-HDO have also been obtained. The transition is a doublet in the R2PI spectra of FL-H2O, FL-D2O, and a singlet in the R2PI spectrum of FL-HDO. The presence of this doublet in the FL-H2O/D2O spectra, and the absence of such a splitting in the FL-HDO spectrum, is an indication of nearly free internal rotation of the water molecule on a potential energy surface that changes upon electronic excitation. Lastly, the use of RCS and psec time-resolved fluorescence as a tool for assigning features in R2PI spectra that are of ambiguous origin due to fragmentation of higher mass clusters into lower mass channels is demonstrated.