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UBC Theses and Dissertations

Extracting molecular information from spectroscopic data Menzel-Jones, Cian John


This thesis explores new ways with which to utilize molecular spectroscopic data in both the time and frequency domain. Operating within the Born-Oppenheimer approximation (BOA), we show how to obtain the signs of transition-dipole amplitudes from fluorescence line intensities. Using the amplitudes thus obtained we give a method to extract highly accurate excited state potential(s) and the transition-dipole(s) as a function of the nuclear displacements. The procedure, illustrated here for the diatomic and triatomic molecules, is in principle applicable to any polyatomic system. We, also, extend this approach beyond the BOA and demonstrate applications involving bound-continuum transition, and double-minimum potentials. Furthermore, by using as input these measured energy level positions and the transition dipole moments (TDMs), we derive a scheme that completely determines the non-adiabatic coupling matrix between potential energy surfaces and the coordinate dependence of the coupling functions. We demonstrate results in a diatomic system with two spin-orbit coupled potentials, whereby experimentally measured information along with TDMs computed for two corresponding diabatic potentials to the fully spin-orbit coupled set of eigenstates, are used to extract the diagonal and off-diagonal spin-orbit coupling functions. Using time-resolved spectra, we show that bi-chromatic coherent control (BCC) enables the determination of the amplitudes (=magnitudes+phases) of individual transition-dipole matrix elements (TDMs) in these non-adiabatic coupling situation. The present use of BCC induces quantum interferences using two external laser fields to coherently deplete the population of different pairs of excited energy eigenstates. The BCC induced depletion is supplemented by the computation of the Fourier integral of the time-resolved fluorescence at the beat frequencies of the two states involved. The combination of BCC and Fourier transform enables the determination of the complex expansion coefficients of the wave packet in a basis of vibrational energy eigenstates, from simple spontaneous fluorescence data.

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