Ultrafast spectroscopy of wavelength-dependent coherent photoionization cross sections of Li-2 wave packets in the E-1 Sigma(+)(g) state: The role ofRydberg states

The significance of Rydberg states in the probing (via ionization) of Li-2 wave packets has been studied through quantitative measurements of the rela tive coherent ionization cross sections in a two-color pump-probe femtoseco nd experiment. Following the preparation of a single intermediate rovibroni c state with a cw laser, a femtosecond pump pulse (around 800 nm) creates a single two-state rotational wave packet by coherent excitation of the E(1) Sigma (+)(g) (v = 9; J = 27 and 29) states. The wave packet is then probed through ionization using time-delayed, wavelength tunable pulses (in the re gion 508-690 nm) while the total energy of the system is kept below the dis sociation limit of Li-2(+). The background-free coherent ionization yield ( for each probe wavelength) is measured as the relative oscillation amplitud e of the single quantum beat time-dependent signal. The experimental result s closely follow a relatively simple theoretical model, which is based on t he assumption that the coherent ionization predominantly takes place via th e excitation of high-n bound singly excited Rydberg states in the ionizatio n continuum converging to the X(2)Sigma (+)(g) ground electronic state of L i-2(+). The best interpretation is that the high-n Rydberg states (above n similar to 25) undergo collisional ionization or autoionization and contrib ute to the measured coherent ionization signal, while the low-n Rydberg sta tes undergo predissociation and do not contribute to the measured signal. A n implication of the results is that the final;states of the Li-2 system, a ccessed by the above probe pulses, can be better approximated by a correspo nding set of isolated discrete levels rather than by a continuum. This conc lusion is important to experimental, as well as theoretical, coherent contr ol and wave packet- dynamics studies, in particular, when phase- and amplit ude-shaped pump and probe pulses are employed. This study is also the first to investigate ionization of lithium dimer slightly below the E(1)Sigma ()(g) shelf region with visible light. (C) 2001 American Institute of Physic s.