Doppler-free two-photon absorption spectroscopy and the Zeeman effect of the A 1B2u←X 1A1g14111 band of benzene

A Doppler-free absorption spectrum and the Zeeman effect of the A 1B2u(v14=1,v1=1)←X 1A1g(v=0) transition of benzene have been measured by means of two-photon absorption spectroscopy with counterpropagating light beams of identical wavelength within an external cavity. Rotational lines were fully resolved, and 647 QQ lines of J=0–43, K=0–43 have been assigned. The molecular constants of the A 1B2u(v14=1,v1=1) state have been determined as A=B=0.181 046 1, C=0.090 548 9, DJ=0.544×10−7, DJK=−1.093×10−7, DK=0.587×10−7, and T0=40 578.2672 cm−1. The Zeeman splittings for lines of a given J were observed to increase regularly with K and reach a maximum at K=J. This demonstrates that the magnetic moment lies along the c axis (perpendicular to the molecular plane). The magnetic moment of the A 1B2u(v14=1,v1=1,J=43,K=43) level was determined to be 0.005μB. The Zeeman splittings of the K=J levels were observed to increase with increasing J. Via analysis of the rotationally resolved Zeeman spectra, it is concluded that the A 1B2u state is mixed with the E2u3 state. This new finding suggests that vibronic interactions between E2u3 and B1u3(T1) and between E2u3 and E1u3(T2) through the mixed E2u3 component, contribute to the B2u1(S1)→3B1u(T1) and B2u1(S1)→3E1u(T2) intersystem crossings, respectively.