Covariance-Map Imaging: A Powerful Tool for Chemical Dynamics Studies

Over the past decade or so, the state-of-the-art in the field of chemical reaction dynamics has progressed from studies of few-atom systems to wide-ranging investigations into a variety of photoinduced and collision-induced processes in much larger molecules. Many of these studies are of direct relevance to a wide audience of chemists, spanning fields such as atmospheric chemistry, astrochemistry, synthetic chemistry, and chemical biology. Key to this work has been the technique of velocity-map imaging, which allows complete product scattering distributions to be recorded for the process of interest. Recent advances in camera technology have enabled the development of multimass velocity-map imaging, in which the scattering distributions of all reaction products can be recorded in a single measurement. In addition to the scattering distributions of individual reaction products, the data set now contains information on correlations between the scattering distributions of two or more fragments. These correlations can be revealed using the technique of statistical covariance, yielding an approach known as covariance-map imaging. This review will introduce the reader to covariance mapping and will describe various applications of the technique within the field of chemical dynamics. The underlying concepts will be illustrated through a series of simple simulations, before moving on to describe a number of recent experimental studies in which covariance mapping has been used to obtain mechanistic insight and information on molecular structure on the femtosecond time scale.