Ultrafast spectroscopy of wavelength-dependent coherent photoionization cross sections of Li2 wave packets in the E1Σ+g state:: The role of Rydberg states

The significance of Rydberg states in the probing (via ionization) of Li-2 wave packets has been studied through quantitative measurements of the relative coherent ionization cross sections in a two-color pump-probe femtosecond experiment. Following the preparation of a single intermediate rovibronic 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 region 508-690 nm) while the total energy of the system is kept below the dissociation limit of Li-2(+). The background-free coherent ionization yield (for each probe wavelength) is measured as the relative oscillation amplitude of the single quantum beat time-dependent signal. The experimental results closely follow a relatively simple theoretical model, which is based on the assumption that the coherent ionization predominantly takes place via the excitation of high-n bound singly excited Rydberg states in the ionization continuum converging to the X(2)Sigma (+)(g) ground electronic state of Li-2(+). The best interpretation is that the high-n Rydberg states (above n similar to 25) undergo collisional ionization or autoionization and contribute to the measured coherent ionization signal, while the low-n Rydberg states undergo predissociation and do not contribute to the measured signal. An implication of the results is that the final;states of the Li-2 system, accessed by the above probe pulses, can be better approximated by a corresponding set of isolated discrete levels rather than by a continuum. This conclusion is important to experimental, as well as theoretical, coherent control and wave packet- dynamics studies, in particular, when phase- and amplitude-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 Physics.