Research Topics

Experimental Methods

Polarization Selective IR Pump-Probe Experiments

Vibrational lifetimes (population relaxation) and orientational relaxation (anisotropy decay) are measured using wavelength and polarization resolved IR pump probe experiments. The laser equipment is the same as shown in 21. However, only two pulses are used. The method is illustrated in figure 22. The pump pulse excites molecular vibrations. The probe pulse measures the time dependence of the change in absorption caused by the pump pulse. The signal decays for two reasons: molecules decay to the ground vibrational state, and molecules undergo orientational relaxation.

Prior to the monochromator, there is the analyzer polarizer, which is on a computer controlled rotation stage. The polarizer alternates between being parallel to the pump pulse (I_||) and being perpendicular to the pump pulse (I_┴). Another polarizer is located just before the entrance slit of the monochromator. It is set at horizontal polarization so that regardless of the polarization passed by the analyzer polarizer, the monochromator always sees horizontal polarization. This is necessary because the grating diffraction efficiency is polarization dependent. It is necessary to measure the absolute amplitudes of I_|| and I_┴. As shown in figure 22, by taking the appropriate combinations of I_|| and I_┴, the population relaxation and the orientational anisotropy decay can be determined.