Photonic quantum

Two-photon absorption occurs when the energy of a molecular transition matches the combined energy of two photons. Quantum mechanically, the absorption probability is proportional to the two-photon transition moment from the ground state, g, to the excited state, n, via intermediate state, m, and can be expressed as follows (Boyd 1992 Abe 2001) ... 

The quantum yield 0 of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express 0 in units of mol einstein, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of 0 are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield 0 over a defined wavelength range. Such chemical systems can be... 

Here, h = (6.626 068 76 0.000 000 52)-10 34 Js is the Planck constant, also known as Planck s action quantum v is the frequency of the photons. Quantum theory is required to calculate the spectral distribution of the energy emitted by a body. Other aspects of heat transfer can, in contrast, be covered by classical theory, according to which the radiation is described as the emission and propagation of electromagnetic waves. 

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