Spesyvtsev, R; (2013) Experimental investigation of ultrafast internal conversion in aniline and 1,4-diazabicyclo[2.2.2]octane (DABCO). Doctoral thesis , UCL (University College London).

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Abstract

Pump-probe time-resolved photoelectron spectroscopy is employed to investigate ultrafast non-adiabatic dynamics in aniline, deuterated aniline (aniline-D7) and 1,4- diazabicyclo[2.2.2]octane (DABCO). Aniline molecules are photoexcited over a wide range of wavelengths between 269 and 236 nm. Our observations reveal direct population of the S2(π3s/πσ∗) state. The photoelectron energy and angular distributions obtained in our experiments show an interesting bifurcation of the Rydberg population to two non-radiative decay channels. One of these involves ultrafast relaxation from the Rydberg component of the S2(π3s/πσ∗) state to the S1(ππ∗) state, from which the population relaxes back to the electronic ground state on a much longer timescale. The other channel appears to involve motion along the πσ∗dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S3(ππ∗) state, which undergoes extremely efficient electronic relaxation back to the ground state. Aniline-D7 is photoexcited with 260 nm wavelength light. The photodynamics of aniline-D7 is similar to those observed in aniline. Comparison of the dynamics of aniline and aniline-D7confirm that the relaxation of the π3s component of the S2(π3s/πσ∗) state to the S1(ππ∗) state is a main relaxation pathway. Photodynamics of DABCO molecules were also studied over a wide range of wavelengths between 251 and 234 nm. The ultrafast internal conversion in DABCO between the S2 3px,y(+) and S1 3s(+) Rydberg states follows a biexponential decay. It was found that initial randomly oriented molecules are partially aligned after absorption of linearly polarised light and, therefore, DABCO exhibits a preferential direction of excitation.

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