Double optical modulation apparatus
We exploit a home-built double photomodulation spectrometer to measure temperature-dependent, quasi- steady-state photoinduced absorbance with a continuously tunable photoexcitation source. Scanning the pump modulation frequency (f1) with a mechanical chopper allows measurement of long-lived photoexcitation kinetics via the frequency-dependent photoinduced transmittance of the sample. By modulating the probe source at a fixed frequency (f2), distinct from f1, and detecting the component of the signal transmitted through the sample at the sum frequency (f1 + f2), we detect the pump-induced photoinduced absorbance with high sensitivity. The advantage of double modulation is that signal due to the excitation source only (photoluminescence or pump scatter) does not contaminate the photoinduced absorbance signal improving sensitivity by orders of magnitude. In fact, changes in transmittance as small as 10-7–10-6 can be measured with this technique. The apparatus can be used as a spectrofluoremeter by using only the photomodulated pump source, or as an absorption spectrometer by using only the photomodulated probe source.