where k p and k t are the propagation and termination rates, respectively, [M ] is the concentration of the compound under investigation, M , reacting with a radical, φ is the quantum yield of photoinitiation, I is the light intensity, and r is the ratio of time in the dark to time in the light . This method has been applied to reactions in gas phase and solution, including polymerization and radical recombination . This method, however, can only be used for some specific types of radical chain reactions, with one requirement being that they can be photochemically initiated . A very common method for measuring the reaction rate of radical reactions in both gas and liquid-phase is laser flash photolysis. In this method a sample is excited by a pulse from a laser, and radical species are monitored by measurement of their spectral absorption. The spectral absorption can be measured with electron spin resonance, in which the unpaired electron of the radical interacts with the nuclei in the molecule leading to a mapping of electron density .