Optimization of wave packet coefficients in Li2 using an evolutionary algorithm: The role of resonant and nonresonant wavelengths

Abstract

Using feedback and an evolutionary algorithm (EA), the weak field pump–probe photoionization signal at a single time delay is optimized in Li2. A single launch state is prepared via excitation with a cw laser, from which a pump pulse excites a superposition of two rotational states on an excited electronic potential energy curve: E 1Σg+(vE=9, JE=27 and 29). The EA modifies the phase pattern versus wavelength of the ultrafast pump pulses using a pulse shaper with a 128 pixel liquid crystal spatial light modulator. Limitations of frequency resolution for the pulse shaper create an effective temporal window in which pulses can be shaped. Optimization of the photoionization signal at pump–probe time delays outside of this temporal pulse shaping window involves phase shifts of only the two frequencies resonant with the transition of the wave packet states, effectively introducing a phase shift in the wave packet recurrences. For pump–probe time delays inside the pulse shaping window, optimization of the photoionization shows the influence of not only resonant but also nonresonant wavelengths. In this regime, the phase shift of wave packet recurrences as well as the time-dependent wave packet amplitude coefficients are optimized. First order time dependent perturbation theory is used to explain the mechanism by which the photoionization is maximized. These experiments are the first to use an optimization routine as a tool to identify a variety of simultaneous mechanisms that underlie the maximization of a process.