Grating spectrometers

The reflectivity of a ruled grating depends on the diffraction angle 

In laser-spectroscopic applications the case a = P often occurs, which means that the light is reflected back into the direction of the incident light. For such an arrangement, called a Littrow-grating mount (shown in Fig. 4.20), the grating equation (4.21) for constructive interference reduces to 

Maximum reflectivity of the Littrow grating is achieved for / = r= f) 9=a (Fig. 4.20b). The Littrow grating acts as a wavelength-selective reflector because light is only reflected if the incident wavelength satisfies the condition (4.21a). 

With a blaze angle 6 = a = P = 30° and a step height h = A, the grating can be used in second order, while the third order appears dX P = Pq + 37°. With d = A/ tan 0 = 2A, the central diffraction lobe extends only to Po 30°, the intensity in the third order is very small. 

We now examine the intensity distribution I(P) of the reflected light when a monochromatic plane wave is incident onto an arbitrary grating. 

The collimated, parallel light is reflected by M, onto a reflection grating 

The Littrow grating acts as a wavelength-selective reflector because light is only reflected if the incident wavelength satisfies the condition (4.23). 

The superposition of the amplitudes reflected from all N grooves in the direction 3 gives the total reflected amplitude 

The path difference between partial waves reflected by adjacent grooves is As = d(sina sin/9) and the corresponding phase difference is 

The line profile (p) of the principal maximum of order m around the diffraction angle can be derived from (4.26) by substituting p = P +c. Because for large N, 1( 9) is very sharply centered around we can assume e With the relation 

Because of the diffraction of each partial wave into a large angular range, the reflectivity R P) will not have a sharp maximum at p = a — 20, but will rather show a broad distribution around this optimum angle. The angle of incidence a is determined by the particular construction of the spectrometer. 

The many grooves, which are illuminated coherently, can be regarded as small radiation sources, each of them diffracting the light incident onto this small 

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As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Three different types of grating spectrometer detection sterns are used (Figure 3) sequential (slew-scan) monochromators, simultaneous direct-reading polychroma-... 

Figura 3 Grating spectrometers commonly used for ICP-OES (a) monochromator, in which wavelength is scanned by rotating the grating while using a singie photomultiplier tube (PMT) detector (b) polychromator, in which each photomultiplier observes emission from a different wavelength (40 or more exit slits and PMTs can be arranged along the focal plane) and (c) spectrally segmented diode-array spectrometer.

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

The infrared spectra were recorded with a Grubb-Parson single-beam grating spectrometer provided with a GS 4-type monochromator (resolution... 

The photoluminescence measurements of Thewalt et al. (1985) were performed at 4.2 K with 200 mW of 514.5 nm excitation from an Ar-ion laser in a 4 mm-diameter spot. The spectrum was analyzed with a double-grating spectrometer using a cooled photomultiplier operating in the photon-counting mode. 

The levitated laser dye droplet was optically pumped by a pulsed (pulse length 5 ns, repetition rate 10 Hz), frequency-doubled Nd YAG laser (2 = 532 nm) in free-space optical setup. Droplet light emission was collected by a multimode optical fiber placed at an angle of approximately 50° relative to pump laser beam. Collected light was analyzed in a fixed-grating spectrometer with a resolution of FWHM 0.15 nm. 

Figure 2. Schematic layout of an arrayed waveguide grating spectrometer/demultiplexer.

This hydrogen band at 3 = 0.2 was also studied with a diffraction grating spectrometer, and its optical density was found to be 0.1 and its half-width to be 21 cm.- (Fig. 10). Condon (134) showed that infrared spectra were expected to be induced by high electric ffelds and that the selection rules... 

Apparatus. Preliminary experiments were carried out in a modified Kiselev-type cell [21 ] with a grating spectrometer, PERKIN ELMER model 325. Precise measurements of diffusivities were conducted by means of a fast Fourier Transform IR (FTIR) spectrometer, PERKIN ELMER model 1800 inserted in a complex set-up equipped with UHV, gas dosing and mass flow control systems. Details of the cell and experimental devices will be described elsewhere [22]. 

Let us cite an example to help us judge the equivalence between Fourier and dispersive instruments. A grating spectrometer employing a four-passed 8 x 104-line grating in the first order has a resolving power of 4 x 8 x 104 = 3.2 x 105. At 3200 cm -1 in the near infrared, this instrument has a Rayleigh resolution of 10" 2 cm- L The same resolution can be achieved by a Fourier... 

Fig. 22 Q branch of the RQ4 subband of v4 of CD3F recorded on a grating spectrometer. Trace (a) is the raw data after smoothing and base-line subtraction. The resolution based on the widths of single lines is 0.010 cm-1. Trace (b) is the result of deconvolving this data set with a gaussian of FWHM = 0.010 cm-1. The resolution is 0.0045 cm-1. Trace (c) is a calculated spectrum based on the analysis of an earlier spectrum with a resolution of 0.028 cm-1. The resolution in trace (b) is 0.0045 cm -1 and in trace (e) is 0.0040 cm"1.

Fig. 23 P6 of 28SiH4 recorded on a grating spectrometer. Trace (a) is from a continuous scan of natural-abundance silane at a resolution of 0.020 cm-1. Trace (b) is the same region recorded separately at a resolution of 0.012 cm-1 and is the average of 12 scans.

The data illustrated in Fig. 4(a) are methane absorption lines (0.02 cm-1 wide) observed with a four-pass Littrow-type diffraction grating spectrometer. For these data also, 256 points were taken. The data were obtained at low pressure, so that Doppler broadening is the major contributor to the true width of the lines. The straightforward inverse-filtered estimate with 15 (complex) coefficients retained is shown in Fig. 4(b). Figure 4(c) shows the restored function. The positions and intensities of the restored absorption... 

FIGURE 16 Predispersive grating spectrometer for diffuse reflectance. 

The use of CW tunable semiconductor lasers as a source in IR spectroscopy research makes possible a very great increase in resolving power over traditional IR grating spectrometers. IR studies with laser sources have been done on several gases (e.g., H20,NH3,SF6,N0). The laser line width is typically 1/100th the width of the Doppler-broadened absorption lines of the gases, so the fine details of the IR line shapes are... 

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