Quadrupole fine structure in MODR spectroscopy and the determination of the absolute signs of the ZF parameters of excited triplet states. MODR spectrum of the 3<i>A</i>′ state of 1-bromonaphthalene in durene

Abstract

We report measurements of the complete orientational dependence of the 3 cm microwave-optical double resonance (MODR) spectrum of the lowest 3(π,π*) state of 1-bromonaphthalene (1BN) in a durene host crystal at liquid helium temperatures. An analysis of the high-field spin Hamiltonian of triplet states containing quadrupolar nuclei shows that both the absolute signs and magnitudes of each of the zero-field (ZF) parameters as well as the principal values of the g, quadrupole, and hyperfine tensors can be determined using MODR techniques. For 3A′ 1BN, we find X/hc = +0.0434 cm−1, Y/hc = +0.0229 cm−1, Z/hc = −0.0663 cm−1, gxx = 2.003, gyy = 2.002, and gzz = 1.994. A comparison of these values with those obtained for 3B2u naphthalene suggests that the ZF scheme of the lowest triplet state of 1BN is modified by spin-orbit coupling with higher excited states, principally in the triplet manifold.