Solids vibrational relaxation

The above considerations allow us to construct a simplified energy level diagram [Fig. 3]. The vibrational substates of the various excited electronic states shown in Fig. 3 have been omitted for clarity. Due to the high frequency of collisions among molecules in liquids or solids vibrational relaxation to the zero-point vibrational level of a particular electronic state is extremely rapid. Therefore, it is generally assumed that only molecules in their lowest vibrational levels exist long enough to be important photochemically. 

TR-SFG seems to be an ideal tool to study the surface dynamics of adsorbed CO at solid/liquid interfaces. Although there are several reports of TR-SFG studies on an electrode, they are only of investigations of vibrational relaxation lifetime by IR excitation [34, 65, 67]. 

H. Ueba, Vibrational relaxation and pump-probe spectroscopies of adsorbates on solid surfaces, Progress in Surf. Sci. 55, 115 (1997). 

Figure 15 shows the lifetime as a function of temperature at the critical density of carbon dioxide. With CO2 as the solvent there is no inverted region in which the lifetime becomes longer as the temperature is increased. Instead, the lifetime decreases approximately linearly. Thus the inverted behavior is not universal but is specific to the properties of the particular solvent. The fact that the nature of the temperature dependence changes fundamentally when the solvent is changed from ethane to C02 demonstrates the sensitivity of the vibrational relaxation to the details of the solvent properties. The solid line is the theoretically calculated curve. The calculation of the temperature dependence is done with no adjustable... 

Fig, 2. The sequence of steps leading to fluorescence. After the initial absorption, the upper vibrational states in the excited state (S ) undergo radiationless decay (vibrational relaxation) by giving up energy to the lattice environment and/or surrounding gas-phase molecules, when the ion pair is located at the solid-gas interface. A radiative transition then occurs from the vibrational ground state (v o) of the electronically excited state (5i) to the ground state (Sq) [reproduced with permission from Atkins (46). ... 

E.E.Nikitin and M.Ya.Ovchinnikova, Role of anharmonic dynamics in vibrational relaxation of impurity molecules in solids, ZhimnEksp. Teor.Fiz. 78,1551 (1980)... 

The quantum theory of vibrational relaxation in low-temperature ordered solids is well develojjed, at least for weak interactions. Starting from the harmonic solid, with known normal mode energies , the anharmonic interactions between modes are introduced as an ordered perturbation and the renormalized mode energies are calculated, usually by temperature Green s function methods, for each order of jierturbation. The calculated energy shifts j — are complex. 

---