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# Simulating and controlling non-adiabatic effects in ultrafast photochemistry

Penfold, Thomas James (2010)
Ph.D. thesis, University of Birmingham.

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## Abstract

Photochemistry plays a critical role in many fundamental processes. These reactions necessarily involve excited states and therefore the breakdown of the Born-Oppenheimer approximation means that many photochemical processes are dominated by non-adiabatic effects, such as conical intersections. The study of such reactions is therefore vital to our understanding of many fundamental processes and the interesting topological features which cause them. The photophysics and photochemistry of benzene is a classic example of the richness of competing pathways available to a molecule after photoexcitation. Computer simulations are one way to provide a molecular picture for the dynamics behind the experimental observations. We develop a Vibronic Coupling Hamiltonian previously published [G.A.Worth, J . Photochem. Photobio. 190:190-199,2007]. Using CASPT2 we add dynamic correlation to the description of the excited states, improving their accuracy dramatically. Seven coupled states and all vibrational mode are included in the model and the parameters are obtained by fitting to points provided by the quantum chemistry calculations. The model is shown to be a good fit of the adiabatic surfaces and its accuracy is demonstrated by the calculation of three absorption bands, which compare favourably with the experimentally obtained spectra. Using the calculated Hamiltonian we investigate the ultrafast dynamics of benzene of electronically and vibrationally excited benzene. We observe ultrafast decay which is a result of internal conversion occurring at the S$$_1$$/S$$_0$$ conical intersection at a prefulvene geometry. These results are able to describe most of the dynamical features seen experimentally. Spin orbit coupling is generally a small, but sometimes a vital perturbation to the Hamiltonian. It is often ignored in hydrocarbons due to the size of the static coupling at equilibrium. However these static couplings ignore the vibrational effects which can occur, and which can be important in describing fine details on spectroscopic measurements. A detailed analysis of spin orbit coupling in cyclobutadiene and benzene is presented. Spin orbit coupling values are presented along the important normal modes, which promote the strength of the coupling. The effect of conical intersections on the strength of spin orbit coupling are presented by plotting the vector in normal mode space from equilibrium geometry to the S$$_1$$/S$$_0$$ conical intersection. We further investigate the ultrafast dynamics of benzene by including the triplet manifold and spin orbit coupling to the Hamiltonian. Ultrafast intersystem crossing is observe between S$$_1$$ and T$$_2$$, which are degenerate along the important prefulvene reaction coordinate. These results challenge the accepted view that ultrafast intersystem crossing cannot occur in hydrocarbons due to the size of spin orbit coupling. Coherent control uses shaped laser pulses to control the outcome of chemical reactions. Local control calculates a pulse as a function of the instantaneous dynamics of the system at each time step. By defining some operator, the field is calculated to ensure an increase or decrease in its expectation value. We present the initial implementation of this method within MCTDH quantum dynamics package. Using models of cyclobutadiene, pyrazine and ammonia we demonstrate that this method is effective in controlling chemical reactions and extremely efficient. The simplicity of this approach means that the calculated fields are very easy to relate to the dynamics of each system providing detailed understanding of the processes involved.

Type of Work: Ph.D. thesis. Colleges (2008 onwards) > College of Engineering & Physical Sciences School of Chemistry QD Chemistry University of Birmingham 803
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