Vibronic spectra of the naphthalene crystal at 1.6 K using two-photon fluorescence excitation

Abstract

This paper presents a study of the two‐photon fluorescence excitation spectrum of the naphthalene crystal at 77 and 1.6 K and of naphthalene in durene at 1.6 K. The anisotropy of many bands is presented leading to assignments for some of the stronger transitions that appear to fit an oriented gas model. All measurements were made for light incident on principal faces of the crystal, and with a single laser beam, having a spectral width of ∼1 cm⁻¹, obtained from a N₂‐pumped dye laser. No new electronic transitions were observed in the two‐photon wavenumber range 31 500–35 500 cm⁻¹. The dominant two‐photon spectrum started from a vibrational band at 1542 cm⁻¹ (measured from the one‐photon 0–0 band) that was shown to have an A_g two‐photon tensor. Isotopic experiments with C₁₀D₈ and C¹³ containing molecules in natural abundance show conclusively that 1542 is a vibrational level of the ¹B_2u state (S₁) of naphthalene. The symmetry of the molecular vibration at 1542 cm⁻¹ is b_2u, and our measurements show that the dominant part of the two‐photon tensor involves transition dipoles along the y (long) axis of naphthalene. Other modes in ¹B_2u for which assignments are suggested are b_1u(3), 1503 (1506 in durene); b_1u(4), 1303 (1276); b_1u(5), 1181 (1194); b_1u(6), 1120 (1102); b_1u(8), 377 (390); b_2u(3), 1409 (1433); b_2u(4), 1542 (1540); b_2u(5), 1226 (1218); b_2u(6), 914 (941); b_2u(7), 1036; b_2u(8), 537; a_u(1), 833; a_u(2), 727; a_u(4), 131; b_3u(1), 857 (863); b_3u(2), 652; b_3u(3), 465. Some of these assignments are uncertain as discussed in the text. They are based on measurements of the anisotropy of the full two‐photon tensor in the principal crystal system. The levels at 377 and 382 cm⁻¹ were identified by their anisotropy to present factor group component states, and the appropriate description of factor group anisotropy in the oriented gas approximation is outlined. The one‐photon 0–0 band levels were observed in two‐photon absorption and a study of their anisotropy is consistent with their interpretation as magneto‐electric dipole two‐photon transitions in which one of the photons couples via electric dipole and the other via magnetic dipole interactions. Comparisons are made between crystal, solution, and gas phase results for the 1542 cm⁻¹ band, and it is shown that the various measured anisotropies are consistent with one another. Some discrepancies arise for the 914 cm⁻¹ band, which is found to have an A_g tensor in the crystal (every two‐photon transition has an A_g tensor in the C_i point group, but these map onto A_g, B_1g, B_2g, and B_3g of the D_2h point group in the oriented gas model). We have suggested the possibility that vibrational levels of the ground state are involved as virtual states in the two‐photon absorption. A relatively strong band reported at 592 cm⁻¹ in the gas does not appear in the solid state spectra. A discussion is presented as to why no perturbations between the S₀ and S₁ vibronic levels are observed such as were seen in one‐photon spectra. The Franck–Condon pattern of the progression starting at 1542 cm⁻¹ of a_g(9), 503; a_g(8), 716; a_g(7), 987; a_g(6), 1149; a_g(5), 1390; and a_g(4), 1430 cm⁻¹ does not mimic the pattern observed for the one‐photon absorption starting at the 0–0 band. We have concluded this is explained by anharmonicity in the combinations b_2u(4) a_g(n) and is not a result of the breakdown of the van Vleck sum rule.