Knowledge Bank

Triplet Rydberg series of Carbon Monoxide in the range of n=4-9 have been rotationally analysed for the first time using a threestep excitation scheme. The $X1\sum +\to a\prime 3\sum +(0-14)$ intercombination transition was pumped with tunable, coherent VUV radiation generated in Mg vapour by four-wave mixing. With two subsequent spin-allowed steps (see figure) at visible wavelengths via the $c3\Pi$ state, the energy region between 105 400 and $112500cm-1$ was probed. While scanning the wavelength of the third step, the fluorescence $c3\Pi \to a3\Pi (0\to v\prime \prime )$ was monitored. This detection scheme based on laser-reduced fluorescenc allows the study of strongly predissociated levels. While the ns series could not be observed due to destructive interference of the transition moments, the np, nd and nf triplet Rydberg series show up prominently in the spectra, allowing the determination of accurate molecular constants. Except for the lowest members of the np series, strong l-uncoupling was observed. The N-dependent predissociation of the npo levels can be explained by the combined effects of l-uncoupling and a homogeneous perturbation of the npr states. Local perturbations have been observed for the 5po, 5do and 4f states.