Tuning the laser-dressed attosecond transient absorption spectra of a singly excited helium state using a shaped attosecond pulse with a spectral minimum

The attosecond extreme ultraviolet (XUV) pulse pump and femtosecond infrared (IR) pulse probe scheme is commonly used to study the dynamics and attosecond transient absorption (ATA) spectra of microscopic systems. In a recent report Proc. Natl. Acad. Sci. USA121 e2307836121 (2024), we showed that shaped XUV pulses with spectral minima can significantly alter the absorption line shape of helium’s 2s2p doubly excited state within a few tens of attoseconds. However, it remains unclear if similar effects could be observed in a singly excited state. In this study, we use shaped XUV pulses to excite helium’s 2p singly excited state and couple the 2p and 3d states with a delayed IR pulse. Comparing these results with those from Gaussian XUV pulses, we find that the ATA spectra for the shaped XUV pulses exhibit more pronounced changes with delay, while the changes for the Gaussian pulses are gradual. We also explain these differences through population changes and analytical models. Our findings show that shaped XUV pulses can regulate the dynamics and absorption spectra of a singly excited state.