Collision Spectroscopy with CW Lasers

State-selective measurements of the different contributions in (13.3,22) can be performed without time resolution if the absolute value of = is known from time resolved investigations, discussed in the previous section. The pump laser may be a CW dye laser which is tuned to the wanted transition i) k) (Fig. 13.20) and is chopped at a frequency f  

In this case the deviation N -Nj from thermal equilibrium during the on-phase does affect via collisional transfer the population densities N , of neighbouring levels m) (13.21a). Under stationary conditions we obtain in analogy to (13.19,21) 

This equation relates the population change AN with that of all other levels j) where the sum over j also includes the optically pumped level i). For j i the quantities ANj and Rj , are proportional to the density Ng of the collision partners. At sufficiently low pressures we can neglect all terms ANj for j i in (13.26) and obtain for the limiting case p - 0 the relation 

The relaxation constant can be obtained from a time-resolved measurement (Sect. 13.3.2). The ratio AN /ANj is directly related to the AC absorption signals of the probe laser tuned to the transition i) p) and (m) 

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