Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time

Since the pioneering experiments by Weinkauf and Schlag on electron mobility and dissociation in peptide cations [1], the interplay between local ionisation and molecular reactivity is of considerable interest in many areas of physics, chemistry and biology [2]. The amino acid glycine is an abundant basic building block of proteins and plays a part in the recognition sites on cell membranes and enzymes. When energetic radiation hits a glycine molecule, often one of its electrons gets knocked out. In the resulting glycine ion, the remaining electronic charge begins to redistribute itself, resulting in a time-dependent oscillation of the charge density. To ionise glycine, we used the ultrashort soft X-ray pulses from the free-electron laser FLASH at DESY, each lasting less than five femtoseconds. With these flashes and by applying sophisticated post-analysis data processing algorithms, we could look at the behaviour of one specific of glycine’s 40 electrons from a particular orbital. In the applied time-resolved Auger electron spectroscopic study, events featuring electrons from the 10a’ inner-valence orbital are of interest. The 10a’ orbital spans nearly the full molecular backbone, and in consequence, the transient local electron hole density moves to the same extent [3], thus making this orbital also an excellent candidate for the study of charge-induced chemical dynamics [4]. While the initial knockout of the electron results in a positive charge at a specific atom in the molecule, the following charge oscillation creates a force field that makes the nuclei move as well.The time-frequency spectra of the many-body quantum mechanical wave packets represented by coherent superpositions of electronic states dressed by vibrational excitations have been measured for the first time along different reaction coordinates in the glycine cation. We could show that the observed coherences reveal rich information on the many-body quantum system including ultrafast decay and site-specific couplings that differ in phase.

[1] R. Weinkauf, et al., J. Phys. Chem. 99, 11255 (1995).

[2] H. J. Wörner, et al., Struct. Dyn. 4, 061508 (2017).

[3] D. Schwickert, et al., Science Advances 8, eabn6848 (2022).

[4] D. Schwickert, et al., Struct. Dyn. 9, 064301 (2022).