Modelling of transient production and decay following laser excitation of opaque materials

Abstract

The generation and decay of transient species in light-scattering materials is examined theoretically. Two limiting cases of transient distribution can be identified for which straightforward mathematical solutions are available: ( a ) where an exponentially falling-off concentration of transients exists beneath the sample surface and ( b ) where a homogeneous transient concentration (a 'plug') exists beneath the sample surface. Between and including these limits, the concentration profile can be calculated by a numerical iterative thin-slice approach. This method also permits calculation of the experimentally available reflectances R of the sample at the analysing wavelength. The size of the transient absorption is governed by the extinction coefficients of the initial absorber and the transient. In the case of a transient species which shows both luminescence and absorption, it is found that the emission intensity and the transient absorption at the same wavelength are proportional to each other. For a transient decaying by a unimolecular pathway, the rate constant can be obtained in case ( a ) from the slope of a plot of In (1 -R ) us. time, while for case ( b ) In [ F ( R ) ] must be plotted against time, where F ( R ) is the Kubelka-Munk function. If the decay follows a bimolecular pathway, the rate constant can be evaluated from the slope of a l / F ( R ) plot us. time only for case ( b ) . In all other cases no straightforward method is available. Experimental data are presented and discussed for benzil microcrystals, an example of case ( a ) where the predicted superimposition of transient decay and phosphorescence is also observed, and for acridine adsorbed on silica gel, an example of case ( b ) .

Intrazeolite photochemistry. VI. Diffuse reflectance laser flash photolysis and product studies of diphenylmethyl radicals on solid supports

“…Laser dose experiments (see later) show that increasing the laser intensity leads to an increase in the intensity of the transient absorption, demonstrating that a saturated plug of totally converted ground state to transient is not produced and, therefore, a Kubelka-Munk treatment is not appropriate (18).…”

Surface Photochemistry: Organic Molecules within Nanocavities of Calixarenes

“…In the first years of eighties Frank Wilkinson and coworkers were able to apply the laser flash photolysis technique to opaque samples [5,[21][22][23], thus allowing the study of a considerable number of photochemical processes in heterogeneous systems. Good examples of those studies are the photochemical reactions in confined spaces, the study of organic molecules adsorbed or included on substrates with catalytic activity, dyes adsorbed or covalently bound to natural or synthetic fibers, among many others.…”

Quantitative Analysis of Pharmaceutical Bilayer Tablets Using Transmission Raman Spectroscopy

“…Classical photon radiation principles such as the Kubelka-Munk (KM) two-flux model and Chandrasekhar Radiation Transport model (CRT) have been widely used in paper coating, printed paper, paint, and other opaque samples [11,12]. However, the need to implement these analyses to develop an analytical procedure to quantify bilayer tablets represents an unusual level of complexity for routine method development and would be burdensome for the industry to easily adopt.…”