Quantitative tests of mixed crystal excition theory. I. Naphthalene monomer1B2u and 3B1u spectra

Abstract

A quantitative, computer processed spectroscopic study, using photon counting, on the first excited triplet and singlet states of dilute isotopic mixed crystals of naphthalene at 2 K is presented for C10H8; 1-DC10H7; 2-DC10H7; 1,4-D2C10H6; 1,4,5-D3C10H5; 1,4,5,8-D4C10H4; 1,2,4,5,8-D5C10H3; a [beta]-D4C10H4 and a [beta]2-D6C10H2 as guests in C10D8 host crystals (and, for comparison, also for the same guests in a durene host crystal). The guest--host relative polarization Rashba formula has been verified quantitatively, and, as an added bonus, the elusive polarization ratio of the pure naphthalene crystal singlet Davydov components has been found to be 80 +/- 20 (b/a), which is in poor agreement with the transition octupole--transition octupole model. The experimental guest energies and their concomitant quasiresonance shifts for bound singlet states (as well as the occurrences of unbound states) are in excellent quantitative agreement (about 1 cm-1) with those calculated using a Green's function formalism based on the ideal mixed crystal approximation and on a restricted Frenkel type dispersion relation derived from resonance pairs. The same Green's function also accounts quantitatively (within 10%) for the guest singlet state exciton localizations (guest excitation amplitudes). The triplet exciton state reveals an orientational site splitting (about 0.7 cm-1) for the 0--0 transition of the I-DC10H7 guest in C10D8 host. The order of the [alpha] and [beta] substituted deuteronaphthalenes in the triplet state is reversed from that of the singlet state. The last two observations are related to the different nature of the lowest [Pi]-[Pi]* singlet and triplet states of naphthalene.