Recent progress which allows laser flash photolysis investigation of opaque materials by using diffuse reflectance from analysing sources for the detection of laser induced transient species is described. Experimental details of nanosecond and picosecond diffuse reflectance laser photolysis systems are presented and methods of analysis of data are discussed. The potential of the technique for studying elementary reactions at interfaces is demonstrated with particular reference to bimolecular reactions of (a) the triplet state of acridine adsorbed on various porous silica surfaces and (b) ion-electron recombination following multi-photon ionisation of diphenyl polyenes adsorbed on γ-alumina. The mechanisms of formation and decay of these transient adsorbed species are discussed.

Recently we have extended to heterogeneous, opaque and often highly scattering systems, the application of flash photolysis by using diffuse reflected light instead of transmitted light as the analysing source on timescales extending from several seconds to picoseconds. Laser-induced transient spectra and decay kinetics have been observed from a wide variety of samples including fractions of monolayers of organic molecules adsorbed on catalytic metal oxide surfaces, within inorganic and organic microcrystals, from dyed fabrics and from paper pulp. The potential of the technique to study photochemical reactions at interfaces is demonstrated with particular reference to ion-electron recombination on porous silica surfaces and to photochemical and photophysical processes occurring in dyed fabrics.

Nanosecond laser photolysis measurements of sensitized phosphorescence from oxygen have been used to obtain values for singlet oxygen formation efficiencies during oxygen quenching of excited singlet and triplet states of anthracene and naphthalene derivatives. Oxygen quenching of excited singlet states of anthracene and dicyanoanthracene in cyclohexane has been shown to lead to catalysed production of triplet states with unit efficiency in both cases, but concurrent production of singlet oxygen only occurs in the case of 9,10-dicyanoanthracene with efficiency close to unity whereas the efficiency for singlet oxygen production due to direct oxygen quenching of excited singlet anthracene is close to zero. In contrast to these results, oxygen quenching of the triplet states of anthracene and dicyanoanthracene in cyclohexane yields singlet oxygen with unit efficiency whereas the singlet oxygen formation efficiency during oxygen quenching of triplet 1-ethylnaphthalene is only 0.86 in cyclohexane and drops to 0.51 in acetonitrile. This solvent dependence demonstrates the role which charge transfer interactions play in determining singlet oxygen yields. Further information concerning the decay of excited oxygen-aromatic hydrocarbon charge-transfer complexes have been obtained from picosecond laser pump-probe studies where direct excitation is into the charge-transfer bands of oxygenated 1-ethylnaphthalene. Following the excitation of the charge-transfer complex, the triplet state of 1-ethylnaphthalene is rapidly produced with an efficiency which shows a marked solvent dependency, being 0.4 and 0.8 in acetonitrile and cyclohexane, respectively. The measured yields of singlet oxygen formation following excitation into 1-ethylnaphthalene-oxygen charge-transfer complexes are 0.36 and 0.78 in these two solvents which is greater than that expected on the basis of the measured triplet yields. Mechanisms of quenching of excited states by oxygen which explain these results are discussed.