Double-rotation technique broadening

Dalby and Bennett " which has given accurate probabilities for a series of transitions. The technique is described briefly onpp. 291-2. Accurate determination of concentrations may still be hindered by self-absorption of the radiation, particularly in the case of the hydroxyl radical. Penner and co-workers have overcome the difficulty by the use of a double path technique, and are able to determine the rotational temperature and concentration of hydroxyl radicals in both flame and shock-tube studies. The single and double path emissivities are compared simultaneously, the double path beam being chopped to give modulation at about 5 sec intervals. The method of correction for line widths and Doppler broadening is discussed . 

The complete suppression of the influence of second-order quadrupole broadening requires the application of double rotation (DOR) [34-36] or dynamic angle spinning (DAS) [37-39] techniques. 

Figure 3.19. Illustration of the double-resonance technique. The pump laser operates at line center on the Fi(4) transition to produce HF molecules in the i = 1, / = 3 state. These molecules have a narrow velocity range compared to that associated with the Doppler-broadened line profile. A probe laser on the 2(3) transition, tuned to coincide with the narrow frequency range associated with the velocity class of laser-excited t = 1, / = 3 molecules, may be used to monitor the laser-excited population. The probe laser can also be tuned to monitor the populations of nearby rotational states in the v = level, as shown.

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