Relaxation processes resonance broadening,

A result of tlie relaxation processes is a shortened lifetime of the spin states giving rise to a broadening of the EPR line, which for most magnetic resonance lines dominated by homogeneous linewidth can be written as... 

An understanding of relaxation processes in the resonance process allows one to explain the line shapes observed experimentally. As will be seen in chapter 7, chemical reactions, for example, proton transfer reactions, can lead to band broadening in the NMR spectrum. Thus, this feature of the spectra has very important applications in the study of fast chemical reactions in solution. 

Fig. 12. Effects of relaxation processes, a) Frequency dispersion in CH. Due to the small vibrational heat capacity in CHt, a relatively small frequency shift is obtained, b) Broadening of the resonance curve in the pressure region where relaxation occurs...

Fig. 5. The cross-relaxation process. n3 represents the four levels not participating in cross-relaxation with neighboring fluorines. Because of inhomogeneous broadening two other cases (not shown here) occur simultaneously 1) laser resonant with n (producing reverse F polarization) and 2) laser resonant with any of the 03 levels (not affecting the fluorines).

Another relaxation process involves the scattering of phonons from the excited state. These interactions do not remove energy from the excited state but serve to limit the coherence of the wavefunction resulting in a broadening of the state. The relaxation processes involve one or more phonons and are familiar to us from electron spin resonance (ESR) studies of the ground state of lanthanide compounds. These interactions have also been shown to be responsible for the thermal broadening of the spectra of trivalent lanthanides in crystals, see Di Bartolo (1968) and Hiifner (1978). These phonon induced effects are ultimately responsible for the thermalization of population of these systems. For a state that is metastable to a radiative process, the phonon relaxation processes vanish as... 

To conclude, even if there exist several processes that affect the vibrational line shape it seems probable that when most of them have been sorted out and with the good agreement between theory and experiment, the lifetime broadening for a chemisorbed CO molecule is of the order of a few cm, corresponding to a lifetime of a few ps. The main vibrational energy relaxation mechanism is creation of electron-hole pairs caused by the local charge oscillations between the metal and the 2n molecular resonance crossing the Fermi level. 

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