Systematics in two-photon rovibrational contours of 141 (1B2u←1A1g) in benzene derivatives

Two-photon rotational contours of the principal vibronic band (1410) in the 1B2u←1A1g transition in benzene and its s-C6H2D4, o-C6H4D2, C6H5F, p-C6H4F2, o-C6H4F2 derivatives have been recorded by multiphoton ionization at room temperature with a laser excitation bandwidth of 0.1 cm−1. The main peaks in the rotational contour are shown to result from multiple coincidences of rovibronic transitions due to ‘‘oblate rotorlike’’ and ‘‘prolate rotorlike’’ behavior of the asymmetric rotor molecules. The observed separations between these features agree very well with the predicted ones obtained from simple formulas. These systematics in the two-photon rotational contours are found to hold over a range of the asymmetry parameter from the least κ=1 to κ=−0.9. The theory is used to obtain the rotationless origin for each of the molecules studied.