The effects of isotope substitution and nuclear spin modifications on the spectra of complexes of tetracene with hydrogen molecules in ultracold 0.37 K He droplets

Abstract

The van der Waals complexes consisting of single tetracene chromophore molecules with an attached H(2), HD, or a D(2) molecule have been assembled inside cold (0.37 K), large ( approximately 1.5 x 10(4) atoms) helium droplets. Their laser-induced fluorescence spectra exhibit typically three well isolated fairly sharp [deltanu(full width at half maximum) approximately 0.5 cm(-1)] bands in the spectral region 22220-22300 cm(-1). Their positions differ for each isotopomer and also are different for each of the ortho- and para-spin modifications. The common feature (except for D(2)) with the largest redshift at about 30 cm(-1), found also in other related free complexes, is attributed to a strongly bound site above one of the two central benzene rings. The other major features come in pairs spaced 3 cm(-1) apart and are not found in similar gas phase studies. This doublet is assigned to a less tightly bound peripheral site with either slightly different configurations or states of the aduct or possibly the He atoms which are stabilized by the surrounding helium bath. The common feature and one branch of the doublet exhibit a pronounced narrow fine structure with spacings of only 0.1 cm(-1), which is nearly the same for all complexes as well as for the bare chromophore, and maybe be due to partially resolved rotational structure of the bands.