Action spectroscopy detects absorption of photons by tracing down photon-induced changes in the molecular system. For example, in the case of tag spectroscopy the molecular system is tagged by a weakly bounded inert particle like inert gas atom or nitrogen molecule. If a vibrational mode absorbs a single photon with frequency ν, the resulting energy hν is redistributed between modes, such that this leads to the detachment of the tag. Let us assume the existence of two vibrational modes in the molecule: #1, which is photoexcited, and #2 corresponding to the dissociation of the tag. We also assume that vibrations are classical and thus the total energy (kinetic plus potential) of each mode E n ≥ 0 (n = 1, 2). Upon the photon absorption, the total energy of the system E = E 1 + E 2 increases by the photon energy hν and becomes E = hν = E 1 + E 2 . The energy of the second mode (E 2 ) during redistribution process may overcome the dissociation energy (D). In this case a signal is produced and detected. We can imagine the total number of states N with the redistributed energy hν as a straight line hν = E 1 + E 2 (0 ≤ E 1 , E 2 ≤ hν) in coordinates E 1 -E 2 (see Figure ) and N = hν • cos(45