Pump and Probe of Ultrafast Charge Reorganization in Small Peptides: A Computational Study through Sudden Ionizations

The ultrafast migratory dynamics of the nonstationary hole resulting from a sudden ionization of the small tetrapeptides, Trp-(Leu)(3) and Tyr-(Ala)(3), is studied using as input a high level quantum chemistry description of the electronic structure for extended conformers computed for frozen nuclei. The sudden ionization process prepares a localized electronic wavepacket that is a superposition of a few stationary states of the cation that are energetically allowed. The superposition evolves field-free until a second ionization to the dication. The wavelength and polarization of the first ultrashort VUV ionizing pulse can be used to tailor the amplitudes on the states of the cation and the initial localization of the hole. For these molecular chains that extend over 15 angstrom, the most efficient mechanism for charge migration is sequential, involving coherent transitions between neighbor and next neighbor amino-acid subunits. The migration of the hole is probed by a second sudden ionization leading to a dication peptide. Its time scale is in the range of a few to a dozen of femtoseconds depending on the initial state of the cation built by the ionization process. The computed angular distributions provide a clear signature of the field-free dynamics between the two sudden ionization processes. Our results are consistent with the experimental observation that the charge transfer is activated, meaning that an excess energy above the ionization potential of the cation is required for facile migration of charge.