A method of storing and delivering ammonia and the use of electromagnetic radiation for desorption of ammonia from a chemical complex. Solid metal ammine complexes are applied for safe and high-density storage of ammonia to be released for use as reducing agent in selective catalytic reduction of NOx in exhaust gases. The compositional formula of the metal ammine complexes is M(NH3)nXz, where M2+ represents one or more metal ions capable of binding ammonia, X represents one or more anions, n is the coordination number (from 2 to 12), and z the valency of the metal ion (and thus the total number of compensating anion charges). Ammonia is released non-thermally by photon-activation using electromagnetic irradiation of the complex bond between ammonia coordinated to the metal ion.