Flash photolysis was a technique developed by Eigen, Porter and Norrish (for which they shared the Nobel Prize in Chemistry, 1967). Flash photolysis uses a light pulse (generated from a pulsed laser or flash lamp) for both initiating a chemical reaction as well as for probing the absorbing species that are generated. I believe the term transient absorption spectroscopy includes the technique of flash photolysis, but could also refer to the technique where a pulsed light source excites the sample and the generated species are probed by a continuous light source (which would only be useful if a reaction is slow enough).