STSM by Zdenek Masin, The Open University (UK) with Olga Smirnova, Max Born Institute, Berlin (DE)
On October 19th, 2014 (15 days)

High Harmonic Generation from pyrazine

Irradiation of an atom or a molecule by an intense laser pulse leads to the emission of photons with frequency that is an integer multiple of the frequency of the laser field. This is the high harmonic radiation. The spectrum of the radiation depends sensitively on the sub-cycle dynamics in the ion and can be used as a sensitive probe of the electronic and nuclear dynamics. The aim of our collaboration is to construct a theoretical model for the High Harmonic Generation (HHG) for the biological molecule pyrazine.

Our theoretical description is based on the well-known three-step model of HHG. This STSM has allowed us to complete the set of data required to construct the model: transition dipole matrix elements, ionization amplitudes for the low-lying states of the pyrazine cation (carried out by Serguei Patchkovskii, MBI) and the sub-cycle laser-induced dynamics in the cation.

The transition dipole matrix elements were obtained using the UKRmol suite, the UK implementation of the molecular R-matrix method. We have benchmarked our calculations against the known photoionization cross sections for pyrazine. Our results show the strong role of correlation in the photoionization dynamics involving the deeper lying valence states: the scattering model using ~5000 configurations significantly underestimates the partial photoionization cross sections for these states. In order to obtain accurate cross sections for these states a much larger set of ~135000 configurations was needed.

Combining all the obtained data our preliminary analysis has identified several interesting channels that may play role in the process of HHG: ionization into the cationic ground state with recombination into the second excited single-hole state (B1u) and vice versa. However, the potentially most interesting dynamics involves a low-lying satellite state of B2u symmetry: ionization into the satellite state with recombination into the cationic ground state.

Finally, we have developed routines for the UKRmol suite that allow us to visualize the Dyson orbitals produced by the CDENPROP module, see Figure 1.

Further collaboration will focus on analysis of the role of the different cross channels and on generation of the harmonic yields.


Figure 1: Dyson orbitals for ground state of pyrazine cation, 1 Ag, and its two lowest-lying excited single-hole states: 1 B1g, 1 B1u. For each cationic state the label in the parentheses corresponds to the singly occupied molecular orbital in the most dominant configuration in the CAS-CI expansion of the wavefunction.

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