# Modeling of Photochemical Reactions in a Focused Laser Beam, II

## TL;DR

This paper presents a method to measure the rate of light-induced degradation of fluorophores using laser illumination and fluorescence detection.

## Contribution

A novel technique is introduced to determine photodegradation rates independently of other model parameters using synchronous detection.

## Key findings

- The photodegradation rate can be determined from the ratio of quadrature to in-phase fluorescence components.
- Photodegradation rate depends on solution viscosity, suggesting an intermolecular interaction mechanism.
- Uncertainties in the estimated rate are analyzed based on measurement errors in the detection technique.

## Abstract

A method is described for obtaining the rate constant of the photodegradation process of fluorophores illuminated by a focused laser beam. The explicit kinetic equations, describing the population dynamics of excited singlet and triplet states, are averaged over the illuminated volume to describe the resulting fluorescence signal. The illumination is modulated at frequencies from 1 Hz to 100 Hz. Synchronous detection of the resulting fluorescence yields in-phase and quadrature components. The measurement of the ratio of quadrature to in-phase components at several power levels yields information on the photodegradation rate. Specifically it is shown that the data can be interpreted in a manner which yields the value of the photodegradation rate independently of other parameters entering the model. Experiments are performed with erythrosine B which has a large intersystem crossing rate to the triplet state. Measurements in solutions with different viscosities show that the photodegradation rate depends on the viscosity. This is interpreted as evidence for an intermolecular interaction mechanism. We explore the uncertainty of the estimated photodegradation constant taking into account the uncertainties of the measurements used in the synchronous detection technique.