Coherent Chemistry: Ultrabroadband Two-dimensional Electronic Spectroscopy

Light driven reactions in molecular systems are central to the existence of life on earth and to its successful continuation. Photosynthesis ultimately supports all life on the planet through the conversion of solar to chemical energy, while artificial solar energy conversion, photovoltaic devices and photocatalysis are fundamental technologies in replacing fossil fuel dependent power generation, and thus ameliorating the effects of global warming. This has led to intense research activity in understanding and ultimately controlling excited state reactions. In this work we address the long standing dream of tuning chemical reactivity using the unique properties of laser light, specifically coherence. The tools that we develop will, independently of this overarching objective, yield the clearest and most detailed insight yet into the nature of excited state chemistry.

The workhorse for all investigations of photochemical mechanism and dynamics is the technique of 'flash photolysis'. Now called transient absorption (TA) the method is, when combined with modern laser technology, capable of sub 10 fs time resolution, and can be used to measure transient spectra from the UV to the mid IR, and beyond. There are now many variants of TA, but the first truly novel extension came about in the early years of this century with the development of two-dimensional electronic spectroscopy (2DES). The essential feature of 2DES is that it allows a correlation of the input (excitation) energy with the output (emission, product absorption, stimulated emission) signal. Thus an excitation at wavenumber x leading to an output at wavenumber y will have a x:y cross peak. Further the temporal evolution yields information on the nature of the coupling between the initial and final state as a function of time. In our case the cross peak may reveal coherent coupling, which in-turn suggests the possibility of control. Such 2D spectra are very familiar from magnetic resonance studies, revealing spin-spin coupling. However, while these studies reveal exquisitely detailed structural information, the excitation energies are too low to affect chemical transformations. The aim of chemistry is not only to interpret molecular behaviour, but also to change it. The energy implicit in electronic excitation is sufficient to initiate chemical reactions, and by applying 2DES to light driven reactions we will provide unique, new and detailed insights into the nature of excited state reactive dynamics.

The advantages of 2DES have already been demonstrated for the important case of electronic energy transfer, where it has provided detailed insight into the pathway and mechanism of the exceptionally fast energy transfer underlying - for example - light harvesting in photosynthesis. The challenge in extending 2DES to the case of chemical change is that the excitation and product signals are energetically far apart, requiring an exceptionally large coherent bandwidth (several hundred THz) to simultaneously excite and probe the reactive system. This necessitates new laser sources, new measurement methodologies and new theory. In this project each aspect is addressed, with the overall objective being to provide the most detailed insight yet into the photochemical dynamics of some of the most important model reactions, such as the electron and proton transfer reactions central to the chemistry of the cell. These measurements will provide unambiguous answers as to whether or not coherence plays an observable role in photochemistry, and can therefore be exploited to modify rates and mechanisms. Even if coherence turns out not to be a key player in these reactions, we will have obtained unprecedented insights into reactivity, with time resolution of only a few fs, the timescale of the fastest nuclear motions.