Spectrometer free spectral range

Spectra were recorded on a multipass Fabry-Perot spectrometer, described in detail elsewhere 11,12,22), using the 514.5 nM line of an Ar" laser. The spectrometer was operated at an optimized free spectral range in each case with a finesse of approximately 40. Spectra were recorded digitally and instrumental distortions were corrected by the method of Lindsay, Burgess, and Shepherd (23). Temperatures above 293 K were obtained with an electrically heated jacket around the sample cell and the temperature was measured with a thermocouple placed in the sample near the scattering volume. Lower temperatures were obtained with an Oxford Instruments CF-104 light-scattering cryostat. Temperature control and measurement was accurate to 0.1°C. 

The free spectral range of a spectrometer is the wavelength interval of the incident radiation for which a one-valued relation exists between A and the position x(X) of the entrance slit image. Two spectral lines with wavelengths Ai and A2 = Ai cannot be distinguished without further information. This means that the wavelength k measured by the instrument must be known be-... 

Fig. 5.58. (a) Schematic diagram of computer-controlled laser spectrometer with frequency marks provided by two FPI with slightly different free spectral ranges and a lambdameter for absolute wavelength measurement, (b) Scheme for wavelength determination according to (5.93c)... 

This shows that two wavelengths X = d sin3/m and X2 = d sin3/(m + 1) appear at the same angle 3 in the spectrometer output. The free spectral range... 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

This equation allows contact-free determination of the sample temperature. In order to evaluate it, the measured intensities have to be transformed into absolute values, and they must be corrected with respect to the spectral sensitivity of the spectrometer (see Secs. 3.5.4 and 6.8.4.2.3). As shown in Fig. 2.4-2 the optimum range of the Raman bands employed for this purpose is depending on the expected temperature range. The intensity ratio has to be of a value which can be measured with a large signal-to-noise ratio. For the measurement of sample temperatures near room temperature Raman lines in the range 0. .. 500 cm are useful, for temperatures of about 100 K Raman lines with a wavenumber of less than 200 cm should be employed. 

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