Time-Resolved Absorption and Double-Resonance Methods

2 Time-Resolved Absorption and Double-Resonance Methods 

While collision-induced transitions in excited electronic states can be monitored through the satellite lines in the fluorescence spectrum (Sect. 13.2.2), inelastic collisional transfer in electronic ground states of molecules can be studied by changes in the absorption spectrum. This technique is particularly advantageous if the radiative lifetimes of the investigated rotational-vibrational levels are so long that fluorescence detection fails because of intensity problems. 

A successful technique for studying collision-induced transitions in electronic ground states is based on time-resolved double resonance [13.82]. The method is explained by Fig. 13.17. A pulsed laser LI tuned to an infrared or 

The time-resolved measurement of /(Az, t) yields the time dependence of the population density Ni(t). Using a cw probe laser the absorption can be measured by the transmitted intensity /(Az, t), or by the time-dependent fluorescence induced by the probe laser, which is proportional to Nj(t) oc Ni(t) P(L2). If a pulsed probe laser is used, the time delay At between pump and probe is varied and the measured values of a(At) yield the refilling time of the level 11). 

Without optical pumping the population densities are time independent at thermal equilibrium. From the rate equation 

A successful technique for studying collision induced transitions in electronic ground states is based on time-resolved double resonance [13.78]. The method is explained by Fig. 13.17. A pulsed laser LI tuned to an infrared or optical transition i) -+ k) depletes the lower level l) = (vj ,Jj ). The depleted level is refilled by collisional transitions from other levels. A second weak probe laser L2 is tuned to another transition i) — m) starting from the depleted lower level i). If the pump- and probe-laser beams overlap within a path length Az in the absorbing sample, the absorption of the probe-laser radiation can be measured by monitoring the transmitted probe-laser intensity 

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