A fully quantum theory of ultrafast chemical dynamics.

0a15b67a-5275-42ce-b03b-0177dbaf2f72

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EPSRC
EPSRC
EPSRC
EPSRC
EPSRC

£1,588,485

Sept. 30, 2015

Dec. 31, 2018

In 1929 Dirac stated that: "The fundamental laws necessary for the mathematical treatment of a large part of physics and the whole of chemistry are thus completely known, and the difficulty lies only in the fact that application of these laws leads to equations that are too complex to be solved." We will show that recently developed methods of quantum dynamics can now overcome the difficulty noticed by Dirac. The fundamental outcome of the current project will be to show that the dynamics of a moderately complicated polyatomic molecule can now be described solely on the basis of quantum lows of motion albeit on a very short time scale of few hundred femtoseconds. Recently developed methods of Quantum Direct Dynamics which treat all electrons and nuclei of a molecule on a fully quantum level will be used to simulate the movements of molecules which follow the absorption of a UV photon. On the ultrafast time scale this motion always reveals a wealth of quantum phenomena such as electronically nonadiabatic processes (i.e. the changes in electronic states when electrons forming chemical bonds rearrange) and tunnelling. On the practical side the project will focus on 3 types of experiments.

First, hydrogen photo- detachment from small heteroaromatic molecules studied in the gas phase will be simulated. In these experiments the initial excitation of a molecule by a photon initiates chemistry and causes the dissociation of hydrogen, which is later ionised with time delay of a few tens of femtoseconds and the "ion image" of the reaction is detected. This allows to obtain very detailed information about the dynamics of reaction by analysing the evolution of spatial and energy distribution of its products.

Second, the new pump-probe experiments have been developed to study similar reactions in solution with femtosecond time resolution. These experiments provide time resolved spectral images of chemical reactions which allow to reconstruct the dynamics and to see their mechanisms.

Third, new and unique experiments are becoming possible now with the construction of the new international Free Electron Laser X-ray facilities in Stanford and Hamburg. In the new time resolved X-ray diffraction experiment, an X-ray probe pulse is combined with femtosecond laser pulses in the visible and UV region, which initiate chemistry, and chemical dynamics is followed by measuring the changes in the X-ray diffraction images.

The molecules studied in the above experiments often represent prototypes or building blocks of larger biologically important molecular species and their photodynamics often models the processes in living organisms, which occur under the influence of light. The proposed theory will explain the experiments and will help to unravel new mechanisms of the ultrafast chemistry. On the other hand the fully quantum theory will be benchmarked against experiment and it will be shown that the difficulty pointed out by Dirac can now be overcome.

Potential Impact:
Firstly, the project will help the UK to maintain its leadership in the study of ultrafast photodynamics with imaging techniques many of which were pioneered in the UK. The project will provide a fully quantum theory of the ultrafast gas phase imaging experiments. Both experiment and theory are looking at the molecules which represent building blocks of larger molecular species such as porphyrins, proteins and DNA. New experiments are now under development to study the same building blocks in condensed phase. Linking highly differential gas-phase measurements to observations in the solution-phase is an extremely topical area because of the exquisite information that can be obtained from gas-phase studies, which can then be applied to more realistic scenarios. Interpreting such experimental techniques with state-of-the-art theory and computation will make a transformative and timely contribution to an area, which targets the physical sciences grand challenge, Understanding the Physics of Life.

Secondly, the proposal may have serious impact on a number of experiments based on the use of new light sources, such as the LCLS instrument in Stanford and a new Hamburg Free Electron Laser scheduled to become operational in 2017. The UK has a substantial share in the latter European international project. We will help to provide a theoretical toolbox for simulating and therefore interpreting the emerging ultrafast time-resolved X-ray diffraction experiments. In the future our AIMC-MCE approach may also be useful for a number of other experiments such as time resolved electron diffraction imaging, which is currently under development.

Finally, the current project will benchmark the new Quantum Direct Dynamics approach against experiment, which is an ultimate benchmark, and prove that modern first principles quantum simulations are now capable of accurately describing ultrafast chemical dynamics. This very ambitious statement is based on the previous record of the results obtained with the proposed approach in the model systems and first results obtained with its QDD version. The size of quantum systems treated with AIMC-MCE direct dynamics is much bigger that previously was believed to be possible. Well converged and quantitatively accurate calculations were performed with the level of accuracy, which has not been achieved with other trajectory based method. Thus, the project addresses the central problem of the exponential curse of quantum mechanics, and develops new ways of dealing with it which fits in the theme of Quantum Physics for New Quantum Technologies. New code developments will be incorporated into the AIMS-MOLPRO package, which is already broadly available and is used by a large number of groups worldwide. This will ensure its broad use and international impact.