Electric quadrupole allowed transitions from ground S to the first excited D state in the vapors of alkalies have been studied experimentally and theoretically in the past [1-3]. Only recently high quality ab initio potential curves became available [4,5] and this enabled us to model the collision induced shapes of these forbidden lines. In addition to this we performed new high resolution absorption measurements using all sapphire cell [6] in order to obtain absorption coefficient profiles, which could be compared with theoretical shapes.

It is well known that asymptotic transition dipole moments for dipole forbidden S-D transition are extremely small, but sufficiently large for easy observations of forbidden lines at higher atom densities. However, away from the asymptote molecular transition probabilities become large enough and they definitely influence the shape of the relevant line profile.

We shall discuss the results of theoretical calculations, with emphasize on difference potential curves possessing extrema and rapidly changing transition dipole moments. Theoretical shapes of forbidden lines will be compared with experimental absorption profiles, and suggestions for laser induced fluorescence measurements will be given.

Aspects of this work with respect to the detection of ultracold molecules will be discussed for the case of heavier alkali dimers [7].

1. K. Niemax, J.Quant.Spectrosc.Radiat.Transfer 17 (1977) 125.
2. A. Gallagher and T. Holstein, Phys.Rev.A 16 (1977) 2413.
3. R.Beuc: Broadening of dipole forbiden S-D lines in homonuclear and heteronuclear alkali collisional systems,
in Spectral Line Shapes, Vol. 3, ed. F.Rostas (Berlin de Gruyter, 1985) pp. 655-6.
4. N. Spies and W. Meyer, private communication and to be published.
5. S. Rousseau, A. R. Allouche and M. Aubert-Frecon, private communication and to be published.
6. T. Ban, H. Skenderovic, R. Beuc and G. Pichler, Europhy.Lett. 48 (1999) 378
7. A. Fioretti, D. Comparat, A. Crubellier, O. Dulieu, F. Masnou-Seeuws, and P. Pillet, Phys.Rev.Lett. 80 (1998)
4402.