Ultrafast Spectroscopy of Wavelength-Dependent Coherent Photoionization Cross Sections of Li2 Wave Packets in the E1Σ+g State: The Role of Rydberg States
R Uberna, Z Amitay, C X. Qian, S R. Leone
The significance of Rydberg states in the probing (via ionization) of Li2 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 E1ς+g (Ņ=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-ground 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 fo high-n bound singly excited Rydberg states in the ionization continuum converging to the X2ς+g ground electronic state of Li2+. The best interpretation is that the high-n Rydberg states (above ndifference}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 Li2 System, accessed by the above probe pulses, can be better approaximated 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 E1ς+g shelf region with visible light.
, Amitay, Z.
, Qian, C.
and Leone, S.
Ultrafast Spectroscopy of Wavelength-Dependent Coherent Photoionization Cross Sections of Li<sub>2</sub> Wave Packets in the E<sup>1</sup>Σ<sup>+</sup><sub>g</sub> State: The Role of Rydberg States, Journal of Chemical Physics
(Accessed February 20, 2024)