High harmonic generation and attosecond dynamics in small organic molecules and biomolecules

  • Peter Philip Mulholland

Student thesis: Doctoral ThesisDoctor of Philosophy


Developments in laser technology over the past 60 years have enabled experimentalists to study a number of interesting processes that can occur during laser-molecule interactions. Study of such processes can help to shed light on ultrafast electronic and molecular processes, and aid with the development of a range of technologies. However, theoretically describing interactions of molecules with intense laser pulses is challenging. Traditionally, describing such interactions requires the solution of the time-dependent Schr\"{o}dinger equation, but this approach is only feasible for the smallest molecular systems. An alternative approach is to use time-dependent density functional theory (TDDFT). In this thesis we report on calculations of ionization and high harmonic generation (HHG) in a number of small molecules, using the EDAMAME code, which combines a TDDFT description of electronic dynamics with a classical treatment of ionic dynamics. The work is divided into two main areas.

In the first area, we study the response of acetylene to a linearly polarized, mid infrared (mid-IR) laser pulse. The alignment of the molecule relative to the laser polarization direction is shown to have a large influence on HHG in the system. With the molecular axis aligned parallel to the laser polarization direction, we observe a double plateau in the harmonic spectrum, with an inner plateau that arises due to ionization from and recombination back to an excited state. This mechanism is investigated through use of a pump-probe scheme, in which the molecule is excited by a vacuum ultraviolet (VUV) pump pulse before HHG is driven by the mid-IR probe pulse. With the wavelength of the pump pulse suitably chosen, we observe a dramatic enhancement of the inner plateau harmonics, while the outer plateau is relatively unaffected.

In the second area of work, we study the interaction of a mid-IR pulse with three biologically relevant molecules: the nucleobases uracil and thymine, and the radiosensitiser molecule 5-fluorouracil. We firstly compare differences in ionization and HHG between the two nucleobases, and between different molecular orientations relative to the laser polarization direction. On average we observe greater ionization from thymine than from uracil, but comparable HHG. Comparing the responses of the nucleobase thymine and the radiosensitiser 5-fluorouracil, we again see greater ionization from thymine, but larger differences in HHG between thymine and 5-fluorouracil than between uracil and thymine. In all three molecules we find that ionization and HHG are significantly reduced when the laser is polarized perpendicular to the plane of the molecule.
Date of Award2019
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorDaniel Dundas (Supervisor)

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