Theoretical calculations of RABBIT spectra and associated photoionization time delays in polyatomic molecules*

In the RABBIT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions),  the photoelectrons produced by ionization of a target gas by a train of XUV attosecond pulses are measured in the presence of the IR laser field used to generate that train as a function of the XUV-IR delay. The photoelectron spectrum produced by the XUV pulse train is a replica of the harmonic spectrum with discrete peaks separated by twice the frequency of the driving field. When the IR field is added, additional peaks are produced in the photoelectron spectrum, which appear as sidebands of the former peaks and are due to the absorption or emission of one IR photon (for weak IR intensities). These two paths are indistinguishable, so they interfere, and as a result of this, the amplitude of the sidebands exhibits a periodic modulation with respect to the time delay between the XUV and the IR pulses. This allows one to extract the intrinsic phase difference of the matrix elements corresponding to photoionization from consecutive harmonics.

In this work, we compute photoelectron spectra that can be directly compared with measured RABBITT ones for several polyatomic molecules, as NH3, N2O, and C2H2, and the so called Wigner ionization delays for PAHs and adamantane. We do that by solving the Time Dependent Schrodinger Equation (TDSE) in the framework of the spectral method. This approach allow us to calculate physical magnitudes, such as the RABBITT phase, that are directly comparable with experimental measurements, with which we compare when available, thus providing a deeper insight to the electron dynamics behind such measurements.

We also studied the differences in RABBITT spectra and photoionization time delays between molecules of different sizes, exploring new techniques to obtain structural information.