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 3A′ state of 1-bromonaphthalene in durene

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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.