Electron spin resonance experiments on AgCl emulsions doped with [Ru(CN)₆]⁴⁻ are performed. This transition metal complex acts as a shallow trap for electrons. The effective mass model is used to explain the electron spin resonance spectral changes, i.e., intensity and line width, after UV-illumination. Changes in line width are explained in terms of exchange and motional narrowing. These effects are studied as a function of concentration, temperature, and UV-irradiation. Finally, the concentration dependence of the stability of shallowly trapped electrons is studied using thermal anneal experiments.