Doppler switching

The observation of transient effects is closely connected to the feature of Doppler switching by sudden changes of the ion velocity. Applications include cascade-free lifetime and quantum beat measurements by observation of the free decay after a short excitation region. Following the first experiments by Andra/ the alternative to produce a short excitation pulse by crossing the ion and laser beams has been applied extensively (cf. Part B, Chapter 20 by H. J. Andra). 

The optical pumping of the molecules is now performed with a separate laser beam, which is sent through the sample cell anticollinearly to the probe beam for V-type OODR, but collinearly for A-type OODR (Fig. 5.34). In order to keep the pump transition on the wanted selected transition, at first an ordinary Doppler-free polarization spectrum of the pump laser must be recorded. Therefore, the pump laser beam is split into a pump and a probe beam, and the spectrum is recorded while laser L2 is switched off. Now the pump laser is stabilized onto the wanted transition and the second (weak) probe laser L2 is simultaneously sent through the cell. 

Fig. 7.23 (a) Stark switching technique for the case of a Doppler-broadened molecular transition. 

Figure 7.23 illustrates the Stark switching technique, which can be applied to all those molecules that show a sufficiently large Stark shift [908]. In the case of Doppler-broadened absorption lines the laser of fixed frequency initially excited molecules of velocity v. A Stark pulse, which abruptly shifts the molecular ab-... 

Instead of a gated switch in the fluorescence detector for pulsed excitation, one may also use excitation by a cw laser that is phase-modulated with a modulation amplitude of tt (Fig. 9.81). The fluorescence generated under sub-Doppler excitation in a collimated atomic beam is observed during a short time interval A, which is shifted by the variable delay T against the time to of the phase jump. If the fluorescence intensity T) is monitored as a function of the laser frequency cu,... 

O. Kafri, S. Speiser, S. Kimel, Doppler effect mechanism for laser Q-switching with a rotating mirror. IEEE J. Quantum Electron. 7, 122 (1971)... 

Once the target is ascertained to be present, a wide bandpass filter can be gradually narrowed about 2 /( — /l or 2I/2 — /l1 and thereby used to obtain Doppler information, Alternatively one could, of course, switch to a conventional configuration. 

Fig. 12.20. (a) Stark switching technique for the case of a Doppler-broadened molecular transition, (b) Infrared photon echo for a vibration-rotation transition. The... 

This optical induction free decay can be measured with a beat technique at time r = 0 the frequency cu of a cw laser is switched from co = con to aJ o) 2 out of resonance with the molecules. The superposition of the damped wave at o) 2 emitted by the coherently prepared molecules with the wave at co gives a beat signal at the difference frequency Aco = co 2— o), which is detected [12.70]. If Aco is smaller than the Doppler width, the laser at cu interacts with another velocity subgroup of molecules and produces optical nutation, which superimposes the free-induction decay and which is responsible for the slowly varying envelope in Fig. 12.21b. 

Use dead-time mode. The dead time is a specified period of time after each detected Doppler burst, during which further bmsts will be ignored. Setting the dead time equal to two times the integral time scale will ensure statistically independent samples, while the integral time scale itself can be estimated from a previous series of velocity samples, recorded with the dead-time feature switched off. 

Fig. 12.20. (a) Stark-switching technique for the case of a Doppler-broadened molecular transition, (b) Infrared photon echo for a CH3p vibration-rotation transition. The molecules are switched twice into resonance with a CW CO2 laser by the two Stark pulses shown in the lower trace. The 3rd pulse is the photon echo [12.63]... 

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