Probing the molecular frame of uracil and thymine with high-harmonic generation spectroscopy

In this work, we present the computed high-harmonic generation (HHG) spectra of uracil and thymine molecules, by means of the real-time time-dependent formulation of Gaussian-based configuration interaction with single excitations (RT-TD-CIS). According to the experimental work [Hutchison et al., Comparison of high-order harmonic generation in uracil and thymine ablation plumes, Phys. Chem. Chem. Phys., 2013, 15, 12308], a puke wavelength of 780 nm has been used, together with an intensity of 1014 W cm(-2) and a puke duration of 23 optical cycles. In order to examine the effect of puke polarisation, rotationally averaged (to mimic the gas-phase sample) and single-polarisations have been computed for both molecules. Our results show that the HHG signal for both molecules possibly originates from different ionisation channels, involving HOMO, HOMO-1, HOMO-2 and HOMO-3 orbitals, which he within 4 eV. We characterize the HHG spectrum of thymine, supporting the idea that the absence of the thymine signal in the original work does not depend on the single-molecule behaviour. Present results for uracil are consistent with the experimental data. Moreover, we have observed that states below and above the chosen ionisation threshold provide different contributions to the HHG spectrum in averaged and single-polarisation calculations.

Automated summary

In this study, high-harmonic generation (HHG) spectra of uracil and thymine molecules were computed using real-time time-dependent formulation of Gaussian-based configuration interaction with single excitations (RT-TD-CIS). The results show that the HHG signal for both molecules possibly originates from different ionisation channels, involving HOMO, HOMO-1, HOMO-2 and HOMO-3 orbitals, within 4 eV. Different contributions to the HHG spectrum were observed from states below and above the chosen ionisation threshold in averaged and single-polarisation calculations.