ABSTRACT
Modulation spectroscopy or modulated excitation (ME) spectroscopy can always be applied if a system admits a periodic alteration of its state by the variation of an external parameter, such as temperature (T), pressure (p), concentration (c), electric field (£), electric potential OF), radiant power (O), or mechanical force (F). The response of the system to ME will also be periodic, exhibiting the same frequency as the stimulation. In case of a nonlinear system, the response to a sinusoidal stimulation will also contain multiples of the fundamental frequency. After an initial period of stimulation, the system will reach the stationary state, which is characterized by periodic alterations around a constant mean. In case of incomplete reversibility, e.g., the existence of an irreversible exit in the reaction scheme, the signal amplitudes of the initial components and of the intermediate species will decline as the system is approaching its final state.
