Collision Spectroscopy with Continuous-Wave Lasers

3 Collision Spectroscopy with Continuous-Wave Lasers 

State-selective measurements of the different contributions Rim in (8.3, 8.22) can be performed without time resolution if the absolute value of is known 

In this case the deviation — Nf from thermal equilibrium during the on phase does affect via collisional transfer the population densities Nm of neighboring levels m) (8.21a). Under stationary conditions we obtain in analogy to (8.19, 8.21a-8.21b) for the on-phase of the pump laser 

This equation relates the population change AiV with that of all other levels ), where the sum over j also includes the optically pumped level i). For j i the quantities ANj and Rjm are proportional to the density iVe of the collision partners. At sufficiently low pressures we can neglect all terms ANj Rj for j i in (8.26) and obtain for the limiting case 0 the relation 

The relaxation constant Km can be obtained from a time-resolved measurement (Sect. 8.3.2). The signals from the chopped cw laser experiments give the following information the ratio ANm/ANi is directly related to the ratio of the ac absorption signals of the probe laser tuned to the transition ) / ) and m) / , respec- 

This always guarantees quasi-stationary population densities N- during the on phase and n9 during the off phase of the pump laser. 

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