The structures of fluorene-(H_2O)_(1,2) determined by rotational coherence spectroscopy

摘要

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-(H_2O)_(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 S_1 <- S_0 R2PI spectra of FL-(H_2O)_(1,2), RCS simulations and diagonalization of the moment of inertia tensor have been used to obtain S_1 excited state rotational constants and structures of FL-(H_2O)_(1,2) that are consistent with the experimental rotational coherence traces. The RCS results indicate that: (i) the water molecule in FL-H_2O resides above the central five member ring and interacts with both aromatic sites; (ii) the water molecules in FL-(H_2O)_2 from a water dimer that is most likely oriented along the long axis of fluorene and is hydrogen-bonded to both aromatic sites. The S_1 <- S_0 R2PI spectra of FL-(D_2O)_1,2) and FL-HDO have also been obtained. The 0_0~0 transition is a doublet in the R2PI spectra of FL-H_2O, FL-D_2O, and a single in the R2PI spectrum of FL-HDO. The presence of this doublet in the FL-H_2O/D_2O spectra, and the absence of such a splitting in the FL-HDO spectrum, is an indication of internal rotation of the water molecule on a potential energy surface that changes upon electronic excitation. Lastly, the use of RCS and 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.