The doubly excited valence (3p+3p) 2 (1)Delta(g) state of Na-2 is experimentally observed by using optical-optical double resonance spectroscopy. A single line Ar+ laser (a total of nine lines) was used to pump the sodium dimers from thermally populated ground state X (1)Sigma(g)(+) to the intermediate B (1)Pi(u) state. Then, a single mode Ti:sapphire laser was used to probe the doubly excited 2 (1)Delta(g) state. Violet fluorescence emitted from the highly excited states (mainly 2 (3)Pi(g) or 3 (3)Pi(g) states which are transferred from 2 (1)Delta(g) state via collision) to the a (3)Sigma(u)(+) state was monitored by a filtered photomultiplier tube (PMT). A total of 582 rovibrational levels of 2 (1)Delta(g) state were observed, identified, and assigned to the vibrational and rotational quantum numbers in the range of 0less than or equal tovless than or equal to28 and 11less than or equal toJless than or equal to99, respectively. The absolute vibrational quantum number assignment was verified by comparing the totally resolved fluorescence with the calculated Franck-Condon factors between 2 (1)Delta(g) state and B (1)Pi(u) state. Dunham coefficients and Rydberg-Klein-Rees potential curve were derived from these observed quantum levels. The primary molecular constants of Na-2 2 (1)Delta(g) state are T-e=32 416.759(15) cm(-1), omega(e)=124.8484(36) cm(-1), B-e=0.119 158(3) cm(-1), and R-e=3.508 20(5) Angstrom. (C) 2004 American Institute of Physics.