The Doppler-free two-photon absorption spectroscopy of naphthalene with Zeeman effects

The Doppler-free spectrum and the Zeeman effect of the A 1B1u(v4=1:b1u)←X 1Ag(v=0) transition of naphthalene in the 33 576.4–33 578.0 cm−1 range has been measured by means of two-photon absorption spectroscopy with counter propagating light beams of identical wavelength within an external cavity. Rotational lines were fully resolved, and 1098 Q(K)Q(J) lines of J=0–42, K=0–24 were assigned. The molecular constants of the A 1B1u(v4=1:b1u) state were determined. Deviations from the line positions calculated using the molecular constants were observed for several lines. Perturbations were not observed for K=0 but were observed to increase with increasing K. The perturbations were therefore identified as originating from a parallel Coriolis interaction. The Zeeman splittings for lines of a given J were observed to be maximum at K=0 and to decrease with increasing K. Analysis of these results indicate that the magnetic moment lies along the c axis (perpendicular to the molecular plane). Any lines broader than our instrumental resolution (5 MHz) were not observed at H=0 T. The J and K dependence of the Zeeman splittings of the A 1B1u(v4=1:b1u)←X 1Ag(v=0) transitions were observed to be regular. This observation and the small number of perturbed lines, leads to the conclusion that the resonance interaction of the A 1B1u state with the T1 3B3u and T2 3B1u states are small and negligible in the observed region.