WAVE-PACKET DYNAMICS IN THE LI-2 E((1)SIGMA(- SIMULTANEOUS OBSERVATION OF VIBRATIONAL AND ROTATIONAL RECURRENCES WITH SINGLE ROVIBRONIC CONTROL OF AN INTERMEDIATE STATE()(G)) SHELF STATE )

A three-step excitation sequence is used to study the wave-packet dyna mics in the E((1) Sigma(g)(+)) ''shelf'' state of lithium dimer. In th e first excitation step, a continuous wave (cw) dye laser prepares a s ingle rovibrational level (upsilon=14, J=22) in the intermediate Li-7( 2) A((1) Sigma(u)(+)) state. Ultrafast excitation of this single level with a 200 fs laser pulse centered at 803 nm creates a rovibrational wave packet (upsilon=13-16; J=21 and 23) in the shelf region of the E( (1) Sigma(g)(+)) state. The motion of this three-dimensional wave pack et is probed via ionization by a second ultrafast laser pulse of the s ame color. The initial cw excitation step allows precise control of th e states that compose the wave packet. Fourier analysis of the pump-pr obe transients shows 15 frequency components that correspond to energy differences between the levels that constitute the wave packet, Becau se of the large rotational energy splitting, the rotational beats occu r in the same frequency range as the vibrational beats. Experiments pe rformed with parallel and perpendicular pump-probe polarizations provi de a ''magic angle'' transient in which only the pure vibrational beat s are observed, thus aiding in the spectroscopic assignment. The obser ved beat frequencies agree well with conventional high resolution freq uency-domain spectroscopy. Applications of the intermediate-state cont rol of the initial wave packet are discussed. (C) 1995 American Instit ute of Physics.