Ultrafast Quantum Interference in the Charge Migration of Tryptophan.

Extreme-ultraviolet-induced charge migration in bio-relevant molecules is a fundamental step in the complex path leading to photo-damage. In this work we propose a simple interpretation of the charge migration recently observed in an attosecond pump--probe experiment on the amino acid Tryptophan. We find that the decay of the prominent low-frequency spectral structure with increasing pump-probe delay is due to a quantum beating between two geometrically distinct, almost degenerate charge oscillations. Quantum beating is ubiquitous in these systems and, at least on the few-to-tens femtosecond timescales, it can dominate over decoherence the line intensities of time-resolved spectra. We also address the experimentally observed phase shift in the charge oscillations of two different amino acids, Tryptophan and Phenylalanine. Our results indicate that a beyond mean-field treatment of the electron dynamics is necessary to reproduce the correct behavior.