Study on the above-threshold ionization of atoms driven by laser pulses with a small chirp

Through numerical solution of the time-dependent Schrödinger equation in momentum space, we investigate the above-threshold ionization spectra and photoelectron angular distributions of hydrogen atoms driven by linearly polarized laser pulses with small chirp parameters. The results show that the laser pulse with a small chirp parameter can achieve precise control of the above-threshold ionization process, while the electric-field amplitude, central frequency, and ionization probability near the pulse center undergo only minor changes. We find that a small negative chirp induces a significant low-energy shift of the photoelectron spectrum, whereas a small positive chirp has a negligible effect on the peak position. Through the analysis of the photoelectron emission spectrum contributed by the rising and falling parts of the laser pulse, we find that the falling part with a gradually decreasing frequency in the negative chirped field dominates the photoelectron emission spectrum. The low-frequency characteristics reduce the photon energy input and cause a shift of the energy spectrum peak toward lower energies. In a positively chirped laser pulse, the contributions from the rising and falling parts are comparable in intensity, and the resulting energy shifts occur in opposite directions, canceling each other out and thereby stabilizing the peak position of the total spectrum. In addition, the photoelectron angular distributions exhibit significant differences among non-chirped, positive chirped, and negative chirped laser pulses. These differentiations in both the energy and angular observables provide an effective approach for the precise characterization and detection of ultrafast optical fields.