The low energy two-photon spectrum of pyrazine using the phosphorescence photoexcitation method

The two-photon phosphorescence excitation spectrum of neat pyrazine crystal at 1.6 K has been examined in the region 6800-4250 A. A discussion of two-photon allowed and vibronically induced transitions is given. The theoretical discussion shows that, due to the added possibility of vibronic mixing in the intermediate state, in general more non-totally symmetric vibrations are expected to show intensity in a symmetry forbidden two-photon spectrum than in a forbidden one-photon spectrum. An estimate of the relative intensities of allowed and vibronically induced (n, π*) two photon transitions in pyrazine have been carried out using the Cl energies and MO transition moments obtained by Wadt and Goddard. Comparison of these results with the normalized spectrum obtained with polarized light indicates the absorptions observed in the region 30 000–35 000 cm⁻¹ are analyzable in terms of a single electronic transition with a forbidden origin which coincides with the ¹B_3u ← ¹A_g one-photon transition origin. Several of the prominent false origins appearing in this region have been tentatively assigned and indicate that, unlike symmetric modes, frequencies of asymmetric vibrations are significantly altered in the excited state. A lower limit of 0.8 eV is set for the ¹B_3u-¹B_2g splitting which results from the interactions of the two pyrazine lone pair orbitals.