Chopper wheels

Fig. 25-2. Double-beam, double-pass transmissometer for measuring smoke density in stacks. A[, chopper wheel A, beam gating wheel A3, aperture D, detector Fj, spectral filter F2, solenoid-activated neutral density filter L, lamp M, half-mirror/beam splitter Rj, solenoid-activated zero calibration reflector R2, retroreflector (alignment bullseye not shown). Design patented. Source Drawing courtesy of Lear Siegler, Inc.

The velocity distributions of both the metal and molecular beams were measured using the same slotted chopper wheel described above by... 

The dual-channel (reference filter) configuration is shown in Fig. 8. The reference filter and the sample filter, which define the spectral region of interest, are mounted on a spinning chopper wheel. As the chopper spins, it alternately positions the filters in the optical path. The signal is demodulated in a similar manner to the dual-beam approach. In order for ihe reference... 

The nondispersive (filter-based) PAS detector consists of very similar components to the original setup used by Alexander Bell an IR light source, a chopper wheel and a measurement cell. In addition, optical filters have been added to improve selectivity, as has a pump to introduce the sample into the measurement cell. 

The IR light source is placed in a parabolic mirror in order to concentrate and focus the IR light inside the measurement cell. The chopper wheel, rotating with a well-defined frequency, will modulate the light, generating light-pulses which... 

The instrument works in a semi-continuous way. First, the pump purges the sample lines and the measurement cell in order to flush out the old sample and bring in the new one. The valves to and from the measurement cell are then closed and measurement starts the IR source is turned on, the chopper wheel starts rotating and the microphones pick up the photo-acoustic signal. The optical filter wheel positions each of the optical filters in the light path, one after the other, until all filters have been measured. Finally the instrument calculates the concentration of each gas, the results are displayed and the whole procedure starts all over again. 

In this system the optical filters are combined to form the optical window through which the IR light enters the cell. Passing the chopper wheel modulates the light, but this chopper wheel is perforated at different distances from the center. When rotating the chopper wheel at constant velocity, three different modulation frequencies are obtained-corresponding to the three optical filters. A microphone picks up the photo-acoustic signal downstream. 

Fig. 7.2 Two-beam experimental setup for femtosecond transient absorption studies using a white light continuum. A commercially available CPA 2101 laser system delivers the pulses. Ultrashort tunable visible pulses are obtained by the NOPA optical parametric converter. A chopper wheel is used to cut every second pump pulse in order to compare the signal with and without the pump. The white light continuum is generated by a sapphire disc. The time delay between the pump and probe pulses is adjusted by the optical delay rail...

Many molecules show absorption of radiation in the infrared range due to oscillations (see Table 6.33), e.g. hydrocarbons such as C2H2, C2H6, CH4 etc., NH3, N20, C2H5OH, C02, CS2, CO, S02, SF6, NO, H20 (as a vapour). The radiation from an infrared source (Fig. 6.130) passes - via a chopper wheel -alternatively a measuring cell with the gas to be analysed or a cell with a reference gas and enters an absorber cell. A membrane capacitor detects the small pressure variations in the absorber cell due to the alternating infrared light path. 

Simultaneous with the publication of Hocker et al., there appeared the results of Yardley and Moore [142] on laser-excited vibrational fluorescence in CH4. A mechanically chopped He-Ne 3.39-micron laser [143, 144] was used to excite the asymmetric stretching [/ = 2948 cm-1 (36.55 X 10-2 eV)] vibration, i>3 (see Figure 3.17). The optical arrangement is shown in Figure 3.18. The He-Ne laser tube, 220 cm in length, is shown on the left. Mx, M2, and Ms are mirrors Bx and B2 are baffles to eliminate stray light Lx and L2 are lenses which focus the laser output into a collimated beam having a diameter of 2 mm, and thence, into a Pyrex fluorescence cell. At the focal point between Li and L2 is a chopper wheel, to produce a nearly perfect square wave modulated at frequencies between 600 and 10,000 Hz. An audio oscillator and a 60-W amplifier are used to drive the synchronous chopper motor. An InSb infrared detector (response time of about 4 nsec) is used to... 

The pulsed primary beam is passed through a skimmer into the main chamber a chopper wheel located after the skimmer and prior to the collision center selects a slice of species with well-defined velocity that reach the interaction region. This section of the beam then intersects a pulsed reactant beam released by a second pulsed valve under well-defined collision energies. It is important to stress that the incorporation of pulsed beams allows that reactions with often expensive (partially) deuterated chemicals be carried out to extract additional information on the reaction dynamics, such as the position of the hydrogen and/or deuterium loss if multiple reaction pathways are involved. In addition, pulsed sources allow that the pumping speed and hence costs can be reduced drastically. 

Fig. 7. Schematic diagram of the interaction region, showing the orientation of the effusive plasma and hyperthermal beam sources, chopper wheel, target surface, and detector.

FIGURE 14.1 Schematic top view of the crossed molecular beam apparatus. The two pulsed beam source chambers and the detector (electron impact + quadrupole mass filter) rotating chamber are visible. In the case of the CN radical beam source, the carbon rod holder and the incident laser beam are also sketched. The chopper wheel and the cold shield are also shown. 

Fig. 8. Crossed beam apparatus of Herman et al. for the study of ion—molecule reactions. A, Ion source B, focusing and decelerating lenses C, molecular beam source D, collision region E, chopper wheel F, energy analyser G, mass spectrometer H, electron multiplier I, scattering chamber J, magnet. (From ref. 103.)...

The neutral beam is pulsed by a rotating chopper wheel (E) at a frequency of about 200 Hz to discriminate the product ions formed in the reaction zone from those produced outside the reaction zone. The source is operated at 55°C and is assumed to provide a corresponding Boltzmann distribution. The energy analyser (F) consists of a fine grid (90 lines per in.) at the potential of the last ion lens, followed by a finer screen (375 lines per in.) to which variable retarding potential is applied. 

Velocity Distribution of the Parent Molecular Beam. To obtain the translational velocity distribution from TOF spectra of photofragments, it is necessary to measure the velocity distribution of the parent molecular beam. In these measurements, the parent molecular beam is set at 0,3 = 0° and the detector is reduced to 0.127 mm. Two methods are used here. The first uses a TOF chopper wheel to chop the parent beam to generate a narrow parent gas beam pulse. The TOF spectrum resulting from this gas... 

To obtain the Vq value for the parent molecular beam, the speed profile for the parent molecular beam obtained by the chopper wheel or the laser hole-burning method is fitted to an assumed number density distribution of the form f v) r exp[ —(r — Lo)V(Ai ) ], where Av is a measure of the width of the speed profile. 

The radiation from the hoUow cathode lamp and the deuterium lamp are directed alternately through a chopper to the (flame or graphite furnace) atomiser. The rotating chopper wheel allows the radiation of either source to pass alternately while the absorption is detected for either of the two beams (Fig. 12.15). The absorbance of the deuterium lamp is then subtracted from the absorbance of the element-specific line source. 

The drift tube dimensions or characteristics were as follows length of 45.0 cm, a voltage divider with 3.34-Mft resistors, electric field of about 400 V/cm, and 78 drift rings (0.12 cm thick, 4.90 cm outside diameter, 2.55 cm inside diameter). The front flange of the drift tube was placed at ground potential, and the detector, and housing of the drift tube, was floated to -20.0 kV. Thus, the capillary of the ESI source was operated only at +5.0 kV with +500 V applied to the chopper wheel. The distance between the ESI source and inlet window of the chopper was 2 mm, and that between the inlet window and inlet flange of the drift tube was 5 mm. 

Fig. 4.4 Schematic diagram of an aperture-type scanning near-field optical microscope. TSL Ti Sapphire laser, GP Grating pair, ODL Optical delay line, CH Mechanical chopper wheel, CWL CW laser, XL Xe lamp, WP Wave plates, FC Fiber coupler, OF Optical fiber, S Sample substrate, NFP Near-field probe, PST Piezo driven stage, OB Objective lens, POL Polarizer, FIL Optical filter, MC Monochromator, CCD Charge coupled device, PD Photodiode...

Dispersive Mid-IR Test Apparatus—The type of apparatus suitable for use in this test method minimally employs an IR source, an infrared transmission cell or a liquid attenuated total internal reflection cell, a wavelength dispersive element such as a grating or prism, a chopper wheel, a detector, an A-D converter, a microprocessor and a sample introduction system. 

Instead of a reflective chopper and deep space, blackened blades can be used. In this case the chopper serves as the reference. It is difficult, however, to obtain paints with high emissivities at all wavelengths. A small residual reflectivity exists, which may be objectionable in some cases. Furthermore, it is difficult to control the temperature of a black chopper. On a rotating chopper wheel it is even difficult to measure precisely the temperature of the blades. Power dissipation in the chopper motor or in the tuning fork may raise the blade temperature above that of the rest of the instrument. For these reasons it is preferable to use a reflective chopper and space or a cavity-type blackbody as a reference. The temperature rise of the chopper blades is then much less critical. 

Technically, early time-resolved luminescence microscopes employed two chopper wheels [29] to isolate the excitation and detection phases in a TRLM cycle as demonstrated in Fig. 4.3. The excitation chopper can however be advantageously replaced with a pulsed light source. 

Another point stemming out of Table 4.1 is that, ideally, the delay and acquisition times should be adapted to the decay time of the luminescent probe. This is not quite easy with chopper-fitted microscopes since the delay and acquisition times not only depend on the rotation speed of the chopper but, also, on the number of blades. Optimum working conditions would therefore require changing the chopper wheel, depending on the luminescent stain, which is not very practical. Microscopes are therefore usually fitted with chopper wheels adequate for one class of LLBs (e.g. lanthanide polyaminocarboxylates, with lifetimes around 600-700 ps). There are other ways of continuously modifying the time delay, but they also have their drawbacks (see below). 

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