Quantum dissipative systems beyond the standard harmonic model: Features of linear absorption and dynamics

Current simulations of ultraviolet-visible absorption lineshapes and dynamics of condensed phase systems largely adopt a harmonic description to model vibrations. Often, this involves a model of displaced harmonic oscillators that have the same curvature. Although convenient, for many realistic molecular systems, this approximation no longer suffices. We elucidate nonstandard harmonic and anharmonic effects on linear absorption and dynamics using a stochastic Schrödinger equation approach to account for the environment. First, a harmonic oscillator model with ground and excited potentials that differ in curvature is utilized. Using this model, it is shown that curvature difference gives rise to an additional substructure in the vibronic progression of absorption spectra. This effect is explained and subsequently quantified via a derived expression for the Franck-Condon coefficients. Subsequently, anharmonic features in dissipative systems are studied, using a Morse potential and parameters that correspond to the diatomic molecule H2 for differing displacements and environment interaction. Finally, using a model potential, the population dynamics and absorption spectra for the stiff-stilbene photoswitch are presented and features are explained by a combination of curvature difference and anharmonicity in the form of potential energy barriers on the excited potential

Modelling quantum dynamics in molecular photoswitches

Quantum dynamics and effects in complex systems is an increasingly important area of research with wide impact to several areas of physics, chemistry, and biology. Progress in this field promises the advancement of our understanding of the fundamental behaviour of nature, and crucial developments for future technologies. Molecular photoswitches present a key example with applications to medicine, molecular motors and machines, and understanding the first stages of vision. Significant challenges arise in the modelling of these complex quantum systems due to the need to account for several degrees of freedom in combination with quantum effects. Furthermore, such systems constitute a difficult regime of quantum dynamics far from equilibrium that necessitates an anharmonic description. This thesis addresses the challenge with the development of efficient quantum stochastic methods. As a result, a method is produced capable of simulating quantum dynamics on several anharmonic surfaces in contact with an environment. Firstly, model systems that go beyond the standard harmonic approximation are used to investigate molecular photoswitch potentials, and tools are identified to predict potential energy surface curvature and assist in the interpretation of linear absorption spectroscopy. Secondly, a stochastic Schrodinger equation approach is developed for the investigation of the absorption spectra of anharmonic systems, in the presence of an environment. Through this it is demonstrated that photoswitching may be stabilised via damping caused by interaction with the environment. Lastly, a numerically exact method, the hierarchy of stochastic pure states, is extended for the study of molecular photoswitch dynamics. It is found that a combination of potential energy surface shape, damped quantum dynamics, thermally activated dynamics, and environment memory play significant roles in the observed dynamics of the stiff-stilbene photoswitch. Subsequently, it is established that dynamics produced with the method and single coordinate models agree well with transient absorption spectroscopy experiments