RESONANCE-ENHANCED 2-PHOTON EXCITATION OF CAI

Induced fluorescence following visible (620-655 nm) laser excitation o f the Cal radical has been detected not only in the same region (B, A- X transitions), but also in the UV (315-330 nm). The UV two-photon exc itation spectrum consists of narrow bands appearing at laser frequenci es located within certain bands of the DELTAv = 1, 0 sequences of the B2SIGMA+-X2SIGMA+ and A 2PI1/2-X2SIGMA+ systems. The main peaks are te ntatively assigned to resonance-enhanced excitation of a single vibrat ional level of the lowest Rydberg D2SIGMA+ state from successive vibra tional levels of the ground state. The excitation process is a one-col or two-photon optical-optical-double-resonance via B2SIGMA+ and A 2PI1 /2 intermediate levels. This analysis is supported by the absorption s pectrum observed long ago by Walters and Barratt. The absorption and l aser excitation complementary data have been used to derive approximat e molecular constants for the D state.