Time-resolved brewster angle microscopy for photochemical and photothermal studies on thin-films and monolayers

Transient events in thin films and interfaces have been studied using the technique of time resolved pump-probe nanosecond Brewster angle microscopy. For p-polarized light there is a minimum reflectivity at the Brewster angle. When the interface is viewed with light that is both incident and reflected at the Brewster angle the resulting image is dark. Subsequent small changes is refractive index will then cause an increase in the reflectivity in affected regions providing high contrast images of an altered interface with a dark background level. This is the basis of Brewster angle microscopy. In the present work two synchronized nanosecond pulsed lasers were used in the pump-probe configuration in order to induce changes at an air-liquid interface and to monitor the resulting morphology changes over a range of time delays from nanosecond to milliseconds after laser-excitation. This method can be used to observe morphological changes in phase altering thin-films and molecular monolayers. Further it can be used to obtain information about transient photochemistry even in optically thin materials and nano-films. In the current work the method is used to monitor laser induced processes in phase separating binary liquid mixtures as well as in monolayers of photo-responsive amphiphilic molecules derived from spiropyran on water. The two systems are quite different but provide valuable comparisons.