Probing the Molecular Frame of Uracil and Thymine with High Harmonic Generation Spectroscopy

In this work we present computed high-harmonic generation (HHG) spectra of uracil and thyminemolecules, by means of the real-time time-dependent formulation of Gaussian-based configurationinteraction with single excitations (RT-TD-CIS). According to the experimental work [Hutchinsonetal., Phys. Chem. Chem. Phys. Comparison of high-order harmonic generation in uracil and thymineablation plumes, 2013,15, 12308] a pulse wavelength of 780 nm has been used, together with anintensity of 1014W/cm2and a pulse duration of 23 optical cycles. In order to examine the effect ofpulse polarisation, rotationally-averaged (to mimic the gas-phase sample) and single-polarisation havebeen computed for both molecules. Our results show that the HHG signal for both molecules possiblyoriginates from different ionisation channels, involving HOMO, HOMO-1, HOMO-2 and HOMO-3orbitals, which lie within 4 eV. We characterize the HHG spectrum of thymine, supporting the ideathat the absence of thymine signal in the original work does not depend on the single-moleculebehaviour. Present results for uracil are consistent with the experimental data. Moreover, we haveobserved that states below and above the chosen ionisation threshold provide different contributionsto the HHG spectrum in averaged and single-polarisation calculations