Chopper, mechanical

For each EA spectrum, the transmission T was measured with the mechanical chopper in place and the electric field off. The differential transmission AT was subsequently measured without the chopper, with the electric field on, and with the lock-in amplifier set to detect signals at twice the electric-field modulation frequency. The 2/ dependency of the EA signal is due to the quadratic nature of EA in materials with definite parity. AT was then normalized to AT/T, which was free of the spectral response function. To a good approximation [18], the EA signal is related to the imaginary part of the optical third-order susceptibility ... 

As an alternative approach, Koirtyohann et al (35) employed a mechanical chopper system for deuterium-background correction similar to the one that is marketed by Perkin-Elmer Co (49) One-half of the rotating chopper Is transparent and the second-... 

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-... 

Frequently a modulation of light is introduced to the system in order to increase the signal to noise ratio. Flash lamps by their construction give pulses of light with repetition, which can be controlled by the user. Other lamps cannot be modulated through their driving current because the emitted radiation would be unstable over time. In this case, the application of an external modulator, e.g. a mechanical chopper, is the only solution. In both cases, the frequency of modulation is rather low - up to kilohertz. 

The procedure is strictly analogous to that used for absorbance measurements in UV and visible molecular spectrometry (p. 355). To avoid interference from emission by excited atoms in the flame and from random background emission by the flame, the output of the lamp is modulated, usually at 50 Hz, and the detection system tuned to the same frequency. Alternatively, a mechanical chopper which physically interrupts the radiation beam, can be used to simulate modulation of the lamp output. 

Most detector systems require that the IR beam be modulated, where the source energy is adequately differentiated in the measured signal from the ambient background. One of the traditional approaches is to use some form of mechanical chopper , usually in the form of a rotating sector wheel, which modulates the beam by blocking the radiation in one or more sectors during a rotation. Note that this is not a requirement for FTIR systems where the interferometer naturally modulates the beam. 

The audio signal resulting from rf amplification and detection is amplified and detected in a phase sensitive manner by using the original modulation phase as a reference. Both mixer vacuum tubes 6S) and mechanical choppers are used. The resulting DC voltage is fed into a recorder. For sufficiently small audio modulation amplitudes the first derivative of the resonance absorption or dispersion results from the narrow-banding technique. 

In order to determine the dynamics of atoms we have to carry out an inelastic neutron scattering measurement. With a reactor source this can be done with a triple-axis spectrometer, which has an analyzer crystal. Tripleaxis refers to the three axes for the monochromator, sample, and analyzer, all moving independently and controlled by a computer. With a pulsed source we use a mechanical chopper, which is a rotating cylinder with a hole perpendicular to the rotating axis that allows neutrons with a chosen range of velocity to go through. The neutrons scattered by the sample are detected... 

The measurement of the growth and decay of fluorescence requires essentially two items (a) modulated excitation source and (b) a detector. The modulation of an excitation source may be accomplished in various ways. These range from simple mechanical choppers to highly sophisticated electronic pulsers. Detectors may be phototubes or semiconducting devices, or even the human eye. The detector itself, in some instances, may be modulated. Of course, the detector chosen must depend upon the spectral range to be studied and the response time desired. 

Some of the alternative TOF instrument designs involve replacing the beryllium filter with either a crystal or a mechanical chopper to monochromate the incident beam. With this change, the spectrometer can be used with a higher incident neutron energy (typically E 50 meV) so that a smaller momentum transfer Q is possible for 5 the same energy transfer (21,22). With a monochromatic incident beam, a beryllium filter is sometimes substituted for the chopper after the sample in order to increase the scattered intensity but with a sacrifice in the,minimum Q attainable. Energy transfers up to 100 meV (800 cm" ) can be achieved with TOF spectrometers at steady state reactors before the incident neutron flux is limited by the thermal spectrum of the reactor. (With hot moderators such as at the Institut Laue-... 

The laser beam is introduced parallel to the axis of the trap and retro-rellected by a mirror at the bottom of the trapping cell (Fig. 2.) A mechanical chopper pulses the laser at typically 1 kHz. The 2S atoms are in the same hyperfine state d and therefore remain trapped. After excitation for a brief period, the population of the 2S level is measured by detecting the Lyman-a fluorescence in an applied electric field. A microchannel plate detector is used to permit single photon detection. Due to the small optical collection efficiency for our geometry, the detection efficiency is limited to 10-5. Nevertheless, signal rates as high a few hundred thousand counts per second laser time have been observed. 

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

H19, Hoffmann, G. W., and Jovin, T. M., Nanosecond-rise-time mechanical chopper for laser light. Appl. Opt. 10, 218-219 (1971). 

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