Chopper, optical spectroscopy

The basic experimental arrangements for photocurrent measurements under periodic square and sinusoidal light perturbation are schematically depicted in Fig. 19. In the previous section, we have already discussed experimental results based on chopped light and lock-in detection. This approach is particularly useful for measurement at a single frequency, generally above 5 Hz. At lower frequencies the performance of lock-in amplifier and mechanical choppers diminishes considerably. For rather slow dynamics, DC photocurrent transients employing optical shutters are more advisable. On the other hand, for kinetic studies of the various reaction steps under illumination, intensity modulated photocurrent spectroscopy (IMPS) has proved to be a very powerful approach [132,133,148-156]. For IMPS, the applied potential is kept constant and the light intensity is sinusoid-... 

Figure 10. Schematics of the experimental setup for intracavity laser absorption spectroscopy (ICLAS). CD chopper driver PM power meter Mj, M2, M3, M4 spherical high reflection mirrors Mp = pump mirror MN monochromator PMT photomultiplier SP silicon photocell PC Pockels cell WF wedged filter LIA lock-in amplifier R recorder MS microscope OF optical fiber S sample (solution on BLM) IEM instruments for electrical measurements (see Figure 2).

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

Prom a spectroscopic standpoint, the main difference between a synchrotron and an FEE is that a synchrotron emits broadband radiation, while the FEL emits monochromatic radiation with several orders of magnitude higher brUUance than a synchrotron. Thus, radiation from a synchrotron can be expanded and passed into a FT-IR spectrometer, for which the interferometer and aU the subsequent optics are no different from those in a conventional FT-IR spectrometer (or microspectrometer.) When a FEL is used for spectroscopy, the monochromatic radiation generated can be modulated in any appropriate manner (e.g., by a tuning fork chopper) before being passed onto the sample and detector. In principle, FELs can... 

Fig. 6. Experimental arrangement for photocurrent spectroscopy. C, chopper E, electrode F, optical filters I, interface L, tungsten or xenon lamp M, grating monochromator PC, microcomputer with peripheral hardware PSD, phase sensitive detector P, potentiostat WG, waveform generator.

In Fig. 5 a typical setup for resonant photoacoustic spectroscopy as used in the authors laboratory is shown. The radiation of a cw laser is intensity modulated by a mechanical chopper of high precision. An electro-optic modulation device may also be employed or the laser beam is modulated directly by modulation of its power supply. As already discussed, vibrational excitation with an IR laser, for example, causes a modulated pressure change in the resonator via fast vibrational relaxation. This acoustic signal is detected with a microphone, because these devices provide the highest sensitivity. Detectors employed in calorimetry to measure the heating of a mpte such as thermistors or thermophi are less sensitive and p( sess a slower rise... 

Since the discovery of the photoacoustic (PA) effect by Bell in 1880, who used the Sun as radiation source, a foot-operated chopper for modulation and an earphone as acoustic detector, the PA effect has found numerous applications as a sensitive and rather simple technique for determining optical, thermal and mechanical properties of all kinds of samples. This article focuses on methods and instrumentation employed in spectroscopic applications. Since photothermal (PT) spectroscopy is discussed elsewhere in the encyclopedia, PT schemes are only briefly mentioned here, whereas emphasis is put on instrumentation used in photoacoustic spectroscopy. 

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