[74] Resonance Raman Techniques for Photolabile Samples: Pump-Probe and Flow

Visual pigments and purple membranes—as well as other systems—offer an experimental problem to Raman spectroscopists. These materials are extremely photolabile, therefore measurements cannot be performed fast enough before severe sample degradation occurs. This chapter discusses two techniques that have had success in coping with photosensitive samples. The first can be used where the system is driven by light and perhaps also temperature to two or more species. The resulting Raman spectra can be assigned to a particular species by suitably controlling sample composition using a second irradiating laser beam. This is the pump-probe method. The second technique that is useful for the broad class of photoreactive molecular systems that can be dissolved in solution is the imposition of a sample molecular velocity transverse to the Raman exciting laser beam. In this method, the flow speed is set high enough so that any given molecule has very little chance of absorbing a photon.