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Based on rotationally resolved sub-Doppler spectroscopy and optical–optical double resonance (OODR) experiments on cold Na3 molecules in a collimated supersonic argon beam seeded with sodium, 51 rotational transitions selectively excited by OODR in the complex electronic A 2A2←X 2B2 system of Na3 could be assigned unambigiously. Accurate values of the rotational constants and the molecular geometrical parameters have been derived from these measurements.

S0031-9007(98) In Fig. 2, page 4585, in the calculations for Berry's phase p, there was no distinction between the two types of bars. We include below a correct reproduction of this figure as it was submitted.In addition, the two following printing errors occurred: In column 1 of page 4585, in the first sentence of the last paragraph, "A" was missing in the transition label "A͑1, 0, 0͒ √ X͑0, 0, 0͒." In column 1 of page 4587, in the third sentence of the last paragraph, "other" was missing after the word "weak."The Physical Review Letters online version has been corrected. FIG. 2. Top: the A √ X system of Na 3 and its assignment in terms of transitions A͑y s , y b , y a ͒ √ X͑0, 0, 0͒. The brackets indicate the pseudorotational splittings. Bottom: absolute values of transition moments calculated ab initio. 3560 0031-9007͞99͞82(17)͞3560(1)$15.00

The combination of high resolution laser spectroscopy in collimated cold molecular beams with mass selective detection allows investigations of rotationally resolved molecular spectra of selected molecular species in the presence of molecules with overlapping absorption spectra. This is illustrated by some examples, such as the isotope selective spectroscopy of Ag2–dimers or the sub-Doppler spectroscopy of Na3 in supersonic beam with a broad mass distribution of different clusters. Time resolved laser spectroscopy of excited states of alkali molecules, perturbed by bound dark states or by repulsive states, yields information about singlet–triplet mixing and predissociation rates.

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