Channel-resolved photoionization time delay of hydrogen atoms in two-color ultraviolet laser fields

We investigate theoretically the phase-dependent photoelectron spectra in resonance-enhanced multiphoton ionization (REMPI) of hydrogen atoms with 200-400-nm ultraviolet laser fields by solving the time-dependent Schrödinger equation. Unlike photoionization time delay in the REMPI of Ne and Ar driven by 400-800-nm laser fields, several Rydberg states are involved [Phys. Rev. A98, 013409 (2018)10.1103/PhysRevA.98.013409; Phys. Rev. A109, 013103 (2024)10.1103/PhysRevA.109.013103]. Here we show that the electron can be released to the continuum state by absorbing three 200-nm photons (i.e., non-resonant channel) or firstly excited to one Rydberg state (i.e., the 3d state) through a two-photon transition and then removed by absorbing another 200-nm photon (i.e., resonant channel). The channel-resolved time delays of the above-threshold ionization (ATI) and sideband (SB) peaks are extracted from the phase-dependent photoelectron spectra. We find that a relative time delay about +165 attoseconds exists between the first resonant SB and non-resonant SB. The phase shifts of the high-energy electrons in these two channels are affected by the continuum-continuum (CC) transitions and the centrifugal barrier effect. Additionally, the intensities of the pump and probe fields affect the phase shifts of high-energy electrons in the non-resonant and resonant channels, respectively. This study demonstrates the possibility of higher-order transitions in ATI of hydrogen atoms at higher laser intensities and offers a theoretical insight for understanding the channel-resolved time delays in atomic REMPI processes.