Laser flash photolysis with back-reflected excitation light—Analysis and experimental verification of the improvements in excitation intensity and homogeneity by a retroreflector

We have evaluated a method recommended (Bonneau et al., Pure and Applied Chemistry 69 (1997) 979 [12]) as a way to increase the amount of absorbed light and to reduce the inhomogeneity caused by the absorption of the sample (Lambert–Beer inhomogeneity) in laser flash photolysis, namely by folding the excitation beam back onto itself after a first pass through the observed volume. Instead of a mirror, we have used a solid corner-cube retroreflector because the latter is insensitive to misalignment, also evens out beam inhomogeneities to some extent, and can accommodate experiments with more than one laser flash (e.g., two-colour experiments) as well. We analyse the dependence of the achievable improvements on the sample absorbance and loss factors, give criteria for optimization, present experimental verification by spatially resolved excitation measurements at varying laser power, and demonstrate the applicability to fluorescence and absorption measurements, as well as to excitation in the visible and the UV range. In our experiments, the retroreflector amplified the amount of absorbed light by a factor between 1.45 and 1.65 and diminished the Lambert–Beer inhomogeneity by a factor between 2.6 and 4.8; the inhomogeneity caused by a Gaussian beam profile decreased by some 10%. There is a surprisingly simple relationship between excitation enhancement and homogeneity improvement: when the total excitation intensity is increased by the (easily measurable) factor θ, the Lambert–Beer inhomogeneity is reduced to a fraction that is given by 2/θ − 1.