Theoretical studies of two phase-independent frequency spectrum models in atomic selective photoionization processes

Abstract The laser frequency spectrum, whose full width at half-maximum in the frequency domain is laser bandwidth, plays a critical role in atomic multi-step photoionization processes, and it has a significant influence on isotopic selective photoionization results. In this study, two phase-independent frequency spectrum models, that is, mode jitter model based on the “mode jitter” phenomenon and the multi-longitudinal mode model based on multi-longitudinal mode output, are proposed for describing the spectral characteristics in the frequency domain. In the mode jitter model, it is assumed that there is a varying longitudinal mode in every laser pulse, with different central frequencies between adjacent pulses in the pulse train; in the multi-longitudinal model, it is assumed that multiple longitudinal modes are output simultaneously with fixed central frequencies. Selective photoionization properties of these two frequency spectrum models and the chaotic field model are simulated and compared with each other under the Lorentzian frequency spectrum condition. Influences of the excitation intensity, laser frequency spectral line-shape, and cut-off frequency on selective photoionization processes are calculated and compared among the above-mentioned three frequency spectrum models. Finally, the measurement of the laser frequency spectrum and the introduction of individual longitudinal mode’s bandwidth are discussed.