Abstract
Ultrafast electron dynamics are responsible for the physical and chemical properties of molecular systems. They can induce variations in the local reactivity as well as in the nuclear arrangement, both of which ultimately determine the making and breaking of molecular bonds.This thesis presents pump-probe measurements of femtosecond (10−15 s) and attosecond (10−18 s) electron dynamics in biomolecular building blocks of DNA and proteins, namely nucleosides and aromatic amino acids, characterised by a large degree of stability due to their efficiency in converting electronic energy into vibrational energy within picosecond and sub-picosecond timescales. The study of the inherent properties of these molecules requires the removal of any possible influence of the surrounding environment, for instance represented by the introduction of solvents. The production of isolated molecular samples was achieved by laser induced thermal desorption technique, which was found to be capable of evaporating intact and neutral molecules and of generating an usable gas-phase target for spectrometric measurements.
Low intensities of UV laser radiation are sufficient to access the electron wave-packet evolution from the first excited state of the analyte along the corresponding potential energy surfaces. In a bottom-up approach to investigate the photo-physical properties of DNA and peptide systems, the individual UV chromophores of these compounds have been studied up to now. The next step consists of studying de-excitation dynamics of larger building blocks, such as nucleosides and aromatic amino acids. The bonding of the sugar ring to the nuclebases, as well as of the glycine moiety to the aromatic chromophores was observed to facilitate and, in some cases, to speed up the relaxation process. These results are discussed in terms of alternative non-radiative deactivation channels opened up by the added units.
At higher photon energies or intensities, electron motion and charge transfer can be studied by prompt removal of an electron from the highest occupied molecular orbital. These dynamics are fundamental in understanding the variation of local reactivity and, hence, to predict the possible reaction channels promoted by further photo-absorption. In aromatic amino acids, the ring structures are strongly involved in internal conversion processes occurring within tens of femtoseconds. Furthermore, fragmentation patterns of nucleosides produced by UV and IR irradiation gave clear evidence of the presence of an ultrafast base/sugar charge transfer in deoxy-thymidine, whereas it was less probable or unlikely in the other nucleosides due to less favourable energetics.
Much faster dynamics can be initiated by attosecond XUV irradiation, where the superposition of many electronic states produces pure electron motion preceding any nuclear rearrangement. Cutting-edge laser technology has made possible the observation of ultrafast charge migration in tryptophan by looking at the doubly charged immonium ion channel. The signal modulation with period of 4 fs corresponds to the
electron beating between the amino group and the side-chain indole. Oscillation frequency and coherence of this phenomenon are discussed in comparison with the previous experiment in phenylalanine. These results represent the first ever time-resolved measurements of coherent electron motion in complex biomolecules, which pave the way for steering electron dynamics and controlling molecular reactivity.
| Date of Award | Jul 2017 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | The Leverhulme Trust |
| Supervisor | Jason Greenwood (Supervisor) |