The Laser Principle

Where g0 and g, are the degrees of degeneracy for each state. It is clear that in general, Ni No, since T 0 and , - e0 0. However by pumping the system with a strong light source or a discharge, it is possible to create a situation where Nt N0. This situation is called a 

let us consider the laser oscillation under pumping conditions. The interaction Hamiltonian, Hu and the wave function ip(r, t), for excitable molecules placed in a radiation field of angular frequency, co, and maximum amplitude, 0, are given by Eq. (1.165)  

The second terms in Eqs. (1.167) and (1.168) are rapidly vibrating non-resonant terms having minor contribution on average, so they are neglected in further calculations. This is called the rotating wave approximation. By solving these simultaneous equations with the initial condition of T0 = T, for / = 0, we obtain  

This equation represents a sinusoidal curve and at the resonant condition, (o=a)o, molecules will repeatedly undergo absorption and emission with a cycle oilniZ. 

In this section, the outlines of the interactions between the radiation field and various molecular systems have been discussed, based on the fundamentals of optics and of the characteristics of the molecular and radiation fields. The interactions of the radiation field with molecular systems are a fundamental aspect of the processes of light absorption and emission. This concept is indispensable when dealing with scattering phenomena. On the other hand, most of the discussion in photophysics and photochemistry is focused primarily on the photophysical and photochemical processes that occur after a molecule has absorbed light. So, in these cases, discussions are often limited to the electronic states of the molecular system. In cases, such as excitation energy transfer, a keen understanding of the interactions between light and materials is needed. 

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