Optical velocity

An IBM IR-98 spectrometer was used for all FT-IR/PAS measurements. The photoacoustic cell used was built at Ames Laboratory. Resolution for all PAS measurements was 8 cm, and no smoothing function was used. Helium passing through a liquid nitrogen trap was used to purge the sample chamber and as conducting gas inside the PAS cell. Carbon black was used as a reference material. Optical velocities corresponding to various mirror velocities of the interferometer are presented in Table I. 

Velocity Mirror Velocity (cm s ) Optical Velocity (cm s ) Velocity Mirror Velocity (cm s ) Optical Velocity (cm-s )... 

Velocity Optical Velocity (cm s ) Thermal Diffusion Length (um)... 

The polyacrylate-sized cotton yarn was also examined using different optical velocities, and the peak intensity ratio. 

The time-line vector is then defined by the optical position and the optical velocity V ast = S/V. The optical velocity is derived from the driver velocity Vscan and the Optical System Level OSL), which depends on the band of interest and is described in Sect.4.5.5.2. With these parameters, the optical velocity is V = 2VscanOSL, and the factor 2 is because of the selected Martin-Pupplet configuration. 

There are many other types of two-beam interferometers besides the one originally described by Michelson (see Chapter 5). Many of these interferometers do not vary the path difference between two beams by a single mirror moving at constant velocity. Except for stationary interferometers used for Fourier transform spectroscopy (Section 5.6), an optical element or combination of optical elements is moved so that the optical path difference is changed at a certain rate, known as the optical velocity or OPD velocity, V. For the Michelson interferometer, V = 2V. In general, the Fourier frequency for radiation of wavenumber v is given by... 

For a Michelson interferometer, the rate of change of retardation is twice the mechanical velocity of the mirror. This parameter is often known as the optical velocity, V, and has the units of centimeters (retardation) per second. Some FT-IR vendors specify the velocity of the moving mirror in their interferometer in terms of its mechanical velocity and some in terms of its optical velocity. Note Users should make sure which of these two parameters is in fact specified.) Still others specify the optical velocity in terms of the corresponding modulation frequency of the laser interferogram although a few commercial interferometers no longer use HeNe lasers, we will denote the frequency of the laser interferogram as /neNe-... 

If the interferometer mirror speed is such that the optical velocity is 0.316 cm s (HeNe laser frequency of 5 kHz), 4000-cm radiation is modulated at 1.25 kHz (see Eq. 2.11). Thus, the response time of a detector for FT-IR spectrometry must be less than 1 ms. Although several cryogenically cooled detectors have response times this low, the only mid-infrared detectors that have an appropriate combination of high speed, reasonably good sensitivity, low cost, good linearity, and operation at or near room temperature are the pyroelectric bolometers. 

To a good approximation, Ry is proportional to lf For a rapid-scanning interferometer, therefore, the optical velocity V should be as low as possible. Two factors usually determine the value of V when a DTGS detector is being used. First, V must be high enough that the SNR of the interferogram at the centerburst is... 

The thermal wave decay depth, and hence the extent of saturation, may be reduced by increasing the modulation frequency, which is done by increasing the optical velocity of the interferometer. As the scan speed is increased, the situation changes from the condition where L > pp to one where L < pp and the relative intensities of the absorption bands becomes closer to their tme values. 

This effect can be observed in the three PA spectra shown in Figure 20.5, which shows how the spectmm of polyethylene (PE) changes as the optical velocity, V (given in terms of the HeNe laser frequency in this figure) is changed from 200 Hz to 20 kHz [5]. When the laser frequency is 200 Hz, the modulation frequencies at 2900, 1460, and 720 cm are about 37, 18, and 9 Hz, respectively. The actual thermal diffusivity of polyethylene varies with its source but is assumed... 

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