Ultrafast Spectroscopy of Wavelength-Dependent Coherent Photoionization Cross Sections of Li2 Wave Packets in the E1Σ+g State: The Role of Rydberg States

Published

June 1, 2001

Author(s)

R Uberna, Z Amitay, C X. Qian, S R. Leone

Abstract

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.

Citation

Journal of Chemical Physics

Volume

114

Issue

No. 23

Pub Type

Journals

Keywords

coherent photoionization, femtosecond, Li₂

Citation

Uberna, R. , Amitay, Z. , Qian, C. and Leone, S. (2001), Ultrafast Spectroscopy of Wavelength-Dependent Coherent Photoionization Cross Sections of Li₂ Wave Packets in the E¹Σ⁺_g State: The Role of Rydberg States, Journal of Chemical Physics (Accessed April 18, 2024)