Relaxation of Gas Excitation

A gas atom which is in an excited state has a finite lifetime due to the possibility of decay to the lower levels. Also, its induced dipole moment can undergo random perturbations disrupting its phase. If an atom can be modelled by a two-level system, both the lifetime of the excited state, Ti, and the dephasing time, T, contribute to the optical linewidth = 2/Ti according to Eq. (2.134). For a pure radiative decay T2 = 2Ti. Collisions in the gas provide an additional relaxation channel. At moderate gas pressures, the col-lisional broadening near the atomic transition frequency is proportional to the number density of gas atoms, n, and is determined as 

The collisional broadening in a monatomic gas is characterized by values of cr of the order of (1-5) x 10 cm. If the perturbing particles are atoms or molecules of a foreign gas, then typically a 10 -10 cm. A similar mechanism of broadening occurs also for vibrational and rotational transitions in molecules. The corresponding cross-sections are much less, about (1-3) x 10 cm.  

Close to the surface the relaxation mechanisms considered in Section 2.2.4 become prominent. Usually, the relevant relaxation times are much shorter than those far away from the surface. An atom which gets adsorbed on the surface for a time longer than the relaxation times T and T will be desorbed being in its ground state with completely quenched induced dipole moment. 

As a result, the atomic state populations as well as the polarization in the atomic flux departing from the surface are different from those in the flux arriving at it. This distinction carries information on the relaxation processes in the close vicinity of the surface. It is reasonable to introduce memory lengths which determine the characteristic distances at which this information is still contained in the departing flux. Then the population memory length can be defined as l = VjT, whereas the polarization memory length h = VjTi (Bordo and Rubahn 2004). 

Davison, M. St Slicka, Basic Theory of Surface States, Oxford University Press, Oxford, 1992. 

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