The role of ultrafast nuclear dynamics in the study of molecular photoionization

The time-resolved investigation of the photoionization process unveiled, in the last years, unprecedented insights into attosecond (10-18 seconds) electron dynamics in atoms and molecules. One well-established technique in attosecond science that allows such studies is the so-called attosecond photoelectron interferometry. It relies on the absorption of a high-energetic quantum of light (usually indicated as a photon) from an attosecond pulse train in the extreme ultraviolet (XUV) accompanied by the exchange of an additional low-energetic photon (typically in the infrared (IR) spectral range). By recording the photoelectron spectra generated by the two-color field as a function of the delay between the XUV and IR pulses, one gains information – from the intensity oscillations of the photoe- lectron peaks (due to quantum interference) – about the characteristics of the ultrafast motion of the electrons upon ionization.

In contrast to atoms, for molecules, the absorption of an XUV photon, leading to the photoemission of an electron, can additionally trigger an ultrafast nuclear motion in the remaining cation. Due to the usually much slower motion of the nuclei compared to the electrons, one can often consider the electron dynamics decoupled from the nuclear ones. In other words, from the point of view of an electron, the nuclei appear to remain “frozen” in their original position. However, in general, this approximation is not always valid. In some molecular systems, it is possible that electronic and nuclear motion occur at comparable speeds, leading to a strong coupling between these dynamics.

So far, the role of nuclear motion in molecular photoionization and its coupling with electronic ones (accessible with attosecond photoelectron interferometry) is still under investigation. Researchers at the University of Freiburg led by Prof. Giuseppe Sansone in collaboration with colleagues at Max Planck Institute for Nuclear Physics in Heidelberg, Max Born Institute in Berlin, and Charles University Prague have studied the influence of nuclear motion upon molecular photoionization and demonstrated that attosecond photoelectron interferometry is sensitive to such dynamics. Their results have been recently published in the scientific journal Science Advances.

In their work, the researchers investigated the photoelectron spectrograms generated during the photoionization of an equal mixture of methane (CH4) and deuteromethane (CD4) utilizing a photoelectron-photoion coincidence spectrometer. "These molecules prove to be ideal candidates to study coupled electronic-nuclear dynamics on an ultrashort time scale due to ultrafast non-adiabatic dynamics triggered by the photoionization process," Sansone explains. "Importantly, the isotopic substitution in the two molecular targets allows us to selectively modify the timescale of nuclear dynamics, without significantly affecting the electronic properties."

The experimental data, in agreement with theoretical simulations, reveal a variation of the amplitudes and contrast of the oscillations of the photoelectron peaks depending on the specific nuclear evolution in the two isotopologues. Due to the isotopic substitution, the dynamics are faster in CH4 with respect to CD4, leading to a modification in the generation of the two-color photoelectron signal. "The tiny variation was only observable thanks to the excellent stability of the developed attosecond source, which allows for extended acquisition times," says Sansone.

Furthermore, to the surprise of the researchers, the observations indicate that even the ultrafast nuclear dynamics, occurring within just a few femtoseconds (10-15 seconds) in both molecular systems upon photoionization, do not affect the phase (i.e., the time delay) that the electron wave packet accumulates as it escapes the molecular potential. However, they do introduce an effective finite coherence time for the interaction of the two-color field with the molecule.

These remarkable results suggest that nuclear dynamics can affect the coherence properties of the electronic wave packet emitted during the photoionization and open new perspectives to study coupled electron-nuclear dynamics on ultrafast time scales.

More information:

• D. Ertel et al. , Influence of nuclear dynamics on molecular attosecond photoelectron interferometry
  Sci. Adv. 9, eadh7747 (2023). DOI: 10.1126/sciadv.adh7747
• AG Sansone

Fig.: Channel-resolved photoelectron spectrograms measured in methane and deuteromethane.
Credit: Science Advances (2023). DOI:10.1126/sciadv.adh7747