Nanosecond transient diffuse reflectance

Figure 1. Schematic of the nanosecond transient diffuse reflectance and transient fluorescence instrumentation.

The kinetics of electron transfer in compond IVc change dramatically when it is ion-exchanged onto the external surface of zeolites Y and L. Figure 5 shows transient diffuse reflectance spectra taken 12-130 psec after nanosecond visible excitation of IVc on zeolite Y (similar spectra are obtained with zeolite L). Again, the observed transient corresponds to a charge-separated state which is formed in 10-15% quantum yield within a few nanoseconds of the laser flash. What is especially striking in this case is the extremely long lifetime of this transient. The decay is biphasic, with one... 

The last few years have seen the emergence of projects aiming at elucidating the photobehavior of monochlorophenols in heterogeneous systems. Two studies were concerned with the behavior of 4-chlorophenol in a surface-adsorbed state, the substrates being silicalite and solid /Tcyclodextrin [40], and cellulose and silica [41]. In both cases, nanosecond transient photolysis with diffuse reflectance detection and photoproduct analysis were the experimental techniques employed. The results of these investigations are instructive in demonstrating the influence of the solid support on the outcome of the photolytic reactions. 

Microcrystalline benzophenone [38] and benzil [16] were two of the first systems studied by nanosecond diffuse reflectance flash photolysis. Both samples gave transient absorptions which were positively identified as triplet-triplet absorptions. In the case of benzophenone an absorption, centred at 540 nm, was obsejrved which has, within experimental error, identical kinetics to the phosphorescence decay, which is predominantly second order. In the case of benzil a transient absorption of 60% at 510 nm was observed after 354 nm excitation. The assignment as triplet-triplet absorption was made on the basis of the absorption and phosphorescence kinetics being virtually identical, namely a mixture of first and second order kinetics. Ikeda et al [39] have also studied microcrystalline benzophenone on the picosecond time scale. Another microcrystalline sample studied is 1,5-diphenyl-3-styryl-2-pyrazoline, in which the triplet-triplet transient absorption was identified within the microsecond time domain [15] (see figure 7(b)). However, as mentioned above (see section 4 and figure 5), the transient absorption due to the excited singlet state has been observed on a picosecond time domain [17]. 

Apart from being the group responsible for the development of the diffuse-reflectance laser fiash-photolysis technique in the temporal range from nanosecond up to seconds [10], Wilkinson et al. were also the first authors to publish transient absorption spectra of opaque materials in the picosecond time domain [11]. 

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