Experimental study of the 3¹_g and 4¹_g states of the Rb₂ molecule

We report an experimental study of the highly excited 3(1)Delta(g) and 4(1)Delta(g) electronic states of the Rb2 molecule. Rovibrational levels of the two electronic states were probed using the high-resolution optical-optical double resonance technique by exciting Rb-85(2) molecules from thermally populated levels of the X-1 Sigma(+)(g) ground state through intermediate levels of the B-1 Pi(u) electronic state. The (1)Delta(g) resonances induced by the probe laser were observed by detecting laser induced fluorescence from collisionally populated triplet states lying near the upper (1)Delta(g) states to the a(3)Sigma(+)(u) triplet ground state. The (1)Delta(g) character of the two electronic states was confirmed by showing that the probe transitions to these states abide by (1)Delta <-(1)Pi dipole selection rules and by observing that their lowest rotational level is J=2. A set of molecular constants and a Rydberg-Klein-Rees potential-energy curve were calculated from the observed term values for each electronic state and compared with ab initio predictions.