Volume 34, number 1 OPTICS COMMUNICATIONS July 1980 A TRIPLET SATELLITE BAND IN THE VERY FAR BLUE WING OF THE SELF-BROADENED SODIUM D LINES D. VEZA, J. RUKAVINA *, M. MOVRE, V. VUJNOVIC and G. PICHLER Institute of Physics of the University, Zagreb, Yugoslavia Received 25 March 1980 We report the measurements and the explanation of the origin of the satellite band at 5515 A, in the very far blue wing of the self-broadened sodium D lines. From spectroscopic investigations and by means of the quite recent calculations by Konowalow et al. it is clear that the satellite band originate from the maximum in the difference potential curve c 3Flg- x 3 + r u. Thus it represents the first direct experimental confirmation of the existence of the c 3Hg state. The interest in the complex manifold of the inter- action potential curves of the lowest excited states of the homonuclear alkali dimer systems has been re- cently intensified in connection with their potential application in lasers [2]. This complex manifold in the case of the lighter alkalis reduces at large inter- nuclear separations to four resonance interaction curves with R-3 dependence [3]. The long-range be- haviour of the heavier homonuclear alkali dimers, having a large spin-orbit interaction, is very compli- cated due to the recoupling of the angular moments [4,5]. Recent theoretical calculations [6] of the quasi- static wings of the selfbroadened first resonance lines of the alkali atoms show satisfactory agreement with recent experimentally observed far wing profiles in the case of rubidium and cesium [7]. Such agreement confirms the existence of all interaction potential curves in the long-range region, which are involved in the formation of the quasistatic wings of the self- broadened lines. However, in the short-range region, where the singiet-triplet splitting occurs due to the exchange interaction, only the spectroscopically ac- cessible singlet system with several lowest excited states was known. There is no apparent reason why the triplet-triplet transitions were not observed be- fore, since the existence of such transitions is directly * Permanent address: Tvornica elektri~nih £arulja, Zagreb, Yugo slavia. assured from their involvement in the formation of the quasistatic wings of the self-broadened first resonance lines of alkali atoms. The situation has been quite re- cently changed when the 3 Eg-X 3 y+ band system was observed in the near infrared for the potassium dimer [8]. The location of that band system in the spectrum is on the longer wavelength side of the well- known A 1E+-X leg band system (beyond 1.1/am), which partly explains the difficulties in spectroscopic detection. In addition, the observation of the bound- free-bound triplet absorption bands in lithium dimer [9] brought an indirect experimental evidence of the existence of the c 3Ilg molecular state. According to the ab initio calculations [ 10] for the eight lowest- lying potential curves of Li2, the c 3Ilg state is a re- pulsive potential curve with a shoulder between 6 and 8 Bohr of internuclear separations. Since the lowest triplet state x 3~ + has a small potential well, with a depth of about 290 cm -1, the difference potential curve c 31-1g-X 3~ + possesses one minimum at about 5.5 Bohr and one maximum at about 7.5 Bohr. The extrema in the difference potential curve usually pro- duce the satellite bands in the vicinity of the atomic spectral lines in the case of the free-free transitions or cause the appearance of the "head of heads" with- in the molecular band system of the bound-bound transitions. The minimum at 5.5 Bohr falls in the in- temuclear region where both potential curves are very repulsive. Hence, at relatively small kT values, as usual- 77