Interferometer mirror

The interferometer mirror of a Fourier transform infrared spectrophotometer travels 1 cm. 

Conventional FTIR instruments, in which the interferometer mirror is translated at a constant velocity, are ideally suited to the analysis of steady state infrared emission. However, time resolution of the infrared emission is required in many applications, such as the measurement of absolute rate constants for the formation or subsequent relaxation of a vibrationally excited species. It is then necessary to follow the intensity of the emission (at a particular wavenumber if state-specific rate constants are required) as a function of time. For continuous-wave experiments, crude time resolution... 

The typical polymer or rubber sample would be classified as optically transparent or opaque and thermally thick except possibly for the strongest bands. In this case the signal intensity would be proportional to the product of the optical absorption coefficient (P) and the thermal diffusion length and show a - 3/2 dependence on the modulation frequency (to). The angular modulation frequency is a product of the interferometer mirror velocity and the wavenumber ... 

Fig. 7.1. Layout of the infrared spectrometer showing the Michelson Interferometer Optical System. An FTIR spectrometer s optical system requires two mirrors, an infrared light source, an infrared detector and a beamsplitter. The beamsplitter reflects about 50% of an incident light beam and transmits the remaining 50%. One part of this split light beam travels to a moving interferometer mirror, while the other part travels to the interferometer s stationary mirror. Both beams are reflected back to the beamsplitter where they recombine. Half of the recombined light is transmitted to the detector and half is reflected to the infrared source.

For imaging measurements using a step-scan FT-IR spectrometer, the interferometer mirror is held at a constant position (usually at the zero-crossing of the laser interferogram) while the signal from each pixel is recorded by the ADC. This... 

A double-beam spectrometer is illustrated in Figure lfi-9. The mirrors directing the interferometer beam through the sample and reference cells are oscillated rapidly compared to the inovenieni of the interferometer mirror so that sani ile and reference information can be obtained at each mirror position. The double-beam design compensates for source and detector drills. 

A drawing of this visible/UV interferometer appears in Figure 6. One important design variation which differs from Figure 1 is the fact that the interferometer mirrors used to reflect the radiation back to the beam splitter are now cat s-eye retroreflectors. These... 

It is possible to perform depth-resolved spectroscopy using these FTIR photothermal methods. Two commonly available instrumentation designs are used to perform FTIR photothermal spectroscopy. In constant scan rate FTIR photothermal instruments, the uniform motion of the movable interferometer mirror... 

Laser interferometers have hugely affected the quality control of lenses in the optical-manufacturing industries. Interferometers are the method of choice to measure curvature and smoothness of lenses used in microscopes, telescopes, and eyeglasses. Companies specializing in optical measurements are available for on- or off-site testing of optical components. Astronomy, in particular, has benefited from the precision testing of polished surfaces made possible by laser interferometers mirrors such as the ones used in the Hubble Space Telescope would not be possible without a laser interferometry testing system. 

Because the beam in an FTIR spectrometer is modulated, the signal that the microphone measures is an interferogram, and this can be transformed to give a spectrum. The sample spectrum is usually measured as a ratio against a carbon black spectrum, because this absorbs all of the radiation falling onto it. The result is an absorbance-like spectrum (Fig. 10.76). In addition, because the photoacoustic effect occurs in the surface of the sample, it is possible to obtain depth-related information by altering the interferometer mirror velocity [1487]. 

Representative results are shown in Figure 12.3, with ATR spectra of the same polymers being shown for comparison [8], although it should be noted that the ATR spectra were measured at higher resolution than the photothermal spectra. The distortion of the stronger bands in the ATR spectra of the more polar polymers is caused by the effect of anomalous dispersion when an internal reflection element (IRE) with a relatively low refractive index, presumably ZnSe, was used. Remarkably the highest quality of photothermal spectrum was measured in the case of polypropylene, which has a relatively weak spectrum, and the lowest quality spectrum was measured in the case of Nylon 6, where the effect of photoacoustic saturation [10] is clearly evident. It is interesting to speculate on whether this spectrum and that of polycarbonate would have been improved had the velocity of the interferometer mirror been increased. Spikes in some of the spectra at 1082 and 1804 cm were attributed to supply frequency harmonics. 

Ideally, time-resolved and imaging measurements require that the OPD be held precisely constant at each sampling point. On the other hand, for photoacoustic and most sample modulation measurements, the position of one of the interferometer mirrors is varied sinusoidally with an amplitude of one, two, or more wavelengths of the HeNe laser by a piezoelectric transducer while the average position is held constant at each sampling point. This mode of operation is called phase modulation. 

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