Relaxation processes resonance fluorescence

Such a mechanism has been proposed by Slanger and observed for the first time by Bondybey et al. in the case of diatomic molecules inbedded in rare-gas matrices. In their subsequent work, similar effects have been found for collisional processes in the gas phase. The vibrational relaxation of CO excited to the v = 3 and c = 2 levels of the A w state induced by collisions with He is more efficient by 4-5 orders of magnitude than in the ground-state CO + He system.Moreover, the form of the fluorescence decay from the o = 1 level observed under v = 2 excitation cannot be fitted if a direct v = 2 v=l relaxation path is assumed the induction time of the relaxed emission being much longer than the decay of the resonance fluorescence. 

The Raman scattering (which is called resonance fluorescence when the final molecular state is identical to the initial one g)) is not, however, the only process resulting in spontaneous photon emission. If one repeats the above treatment in a density matrix formalism and allows for intermediate state dephasing, one obtains, for resonant excitation, a fluorescence contribution. In practice, in this case the doorway state is really (not virtually) excited and becomes populated for a significant time interval, as pointed out by Lee and Heller. The system becomes then sensitive to any phase-disturbing perturbation. As a consequence, due to dephasing, the scattering is no more a purely coherent two-photon process, and the Raman emission competes with a relaxed component which is usually called fluorescence. The fluorescence is then simply the spontaneous emission from populated excited states, which have completely lost the memory of the... 

From the theoretical point of view, this relaxation process has been the subject of a large number of quantum dynamics investigations, based on reduced and full dimensional models. Farly works [13-17] reported three- and four-mode models and showed that a simple two-state four-dimensional model provides a qualitatively correct simulation of the UV absorption spectrum [17], These models were used to simulate various spectroscopic signals, including time-resolved transient absorption [18-20], and ionization [21] spectra, fluorescence [22] and resonance Raman spectra [23]. Worth et al. [24-27] performed accurate quantum dynamics simulations based on a model including the twenty-four vibrational modes of the molecule using the MCTDH method. These benchmark results have then been used to test various approximate methods for the simulation of non-adiabatic dynamics of molecular systems [28 0]. 

The de-excitation path available to conjugated organic molecules is controlled by quantum-mechanical rules which are complex. Some molecules will relax spontaneously, other will not (within a reasonable time) without assistance from another material/mechanism. The presence of Oxygen is a special case. Resonant conjugated molecules with two Oxygen atoms will not fluoresce and there only means of de-excitation is by means of a direct transition that is not allowed because of the presence of the triplet state. The nonresonant conjugates normally de-excite thermally via a two-step process. 

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