Two-photon spectroscopy in the gas phase: first excited state of naphthalene, ¹B_3u

Molecular two-photon spectroscopy is promising to be a new tool for finding otherwise forbidden spectroscopic transitions. Work in the gas phase is particularly important for a study of vibronic two-photon states. The proof of such a new method, at least in part, must be based on its ability to make new assignments. Benzene has up to now been the first, and only example for new assignments. We now present naphthalene as a second system for this new technique. Assignments of new states are presented and based on a spectrum at a pressure of only 70 mtorr. The analysis makes use of rotational structure, polarization behavior and hot band information. We assign not only six new fundamentals in the first excited singlet state ¹B_3u but also fix some ground state assignments, which were ambiguous in the IR spectrum.