Monochromator bandwidth

Radiation exits the monochromator and passes to the detector. As shown in Figure 10.12, a polychromatic source of radiation at the entrance slit is converted at the exit slit to a monochromatic source of finite effective bandwidth. The choice of... 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

The resolution of a monochromator is the smallest frequency interval the instrument can separate. The limiting resolution is the bandwidth measured at half height when scanning across an infinitely narrow intense source 22). As already mentioned, the broader excitation line width of Ar+ lasers (0.15 to 0.25 cm-1) compared to that of the He-Ne lasers (0.05 cm-1) means a lower resolution limit when the Ar+ laser is used as a Raman source. 

A monochromatic beam of X-rays with about 1 eV bandwidth is produced by the standard beamline equipment, the undulator and the high-heat-load premonochromator being the most important parts among them. Further monochromatiza-tion down to approximately the millielectronvolt bandwidth is achieved with the high-resolution monochromator. The width of a band of a millielectronvolt, however, is much more than the inherent linewidth of the Fe y-radiation, F 10 eV, or the full range of hyperfine-split Mossbauer lines, A m 10 eV. Yet, NFS is detectable because the coherent excitation of the nuclei is caused in the... 

The synchronous spectra (SF) were collected in the 260-460 nm excitation wavelength range using bandwidth of AA=20 nm between the excitation and emission monochromators. All SF and emission spectra were recorded with a 10 nm slit width on both monochromators. The scan speeds of spectra were 500 nm/min. 

Bandwidth is the width of the wavelength band that is allowed to exit a monochromator. The narrowness of this band is called the resolution. High resolution corresponds to a very narrow bandwidth and vice versa. 

The proportionality factor k depends on several parameters, in particular on the optical configuration for observation (i.e. the solid angle through which the instrument collects fluorescence, which is in fact emitted in all directions) and on the bandwidth of the monochromators (i.e. the entrance and exit widths see Chapter 5). 

Bandwidth refers to the rang> of wavelengths of radiation transmitted by a monochromator and is measured at half maximum transmittance. 

Furthermore, if one uses the wavelength modulation technique, which is strictly not surface sensitive but only enhances sharp structures and as the bandwidth of a grating monochromator decreases with decreasing energy, the low frequency peak will appear broad for reasonable slit widths. The... 

The luminescence emission spectrum of a specimen is a plot of luminescence intensity, measured in relative numbers of quanta per unit frequency interval, against frequency. When the luminescence monochromator is scanned at constant slit width and constant amplifier sensitivity, the curve obtained is the apparent emission spectrum. To determine the true spectrum the apparent curve has to be corrected for changes of the sensitivity of the photomultiplier, the bandwidth of the monochromator, and the transmission of the monochromator with fre-... 

Fig. 17. Delayed fluorescence spectrum of 5 X 10-63/ anthracene in ethanol.84 Half-bandwidth of analyzing monochromator was 0.05 ju-1 at 2.5 n K Intensity of exciting light was approximately 1.4 X 10 einstein cm. a sec.-1 at 2.73m-1 (366 mju). (1) Normal fluorescence spectrum (distorted by self-absorption). (2) Delayed emission spectrum at sensitivity 260 times greater than for curve 1. (3) Spectral sensitivity of instrument (units of quanta and frequency).

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

M3, which compensates for aberrations and yields an excellent quality of image b) Czerny-Turner design, similar in conception, incorporating two spherical mirrors. M and jf/4 c) design with a concave grating (7( allowing simultaneous dispersion and focusing of the radiation. The spectral bandwidth of these monochromators depends on the width of the entrance and exit slits, F, and F2-... 

Under the effect of temperature, each atomic transition leads to emission or absorption spread over a narrow interval of wavelengths. This uncertainty around the theoretical value constitutes the natural bandwidth of the line and leads to enlargement of the image of the line seen by a monochromator. The natural bandwidth ranges from 1CT5 nm under ideal conditions to about 0.002 nm at 3 000 K. 

Decreasing the monochromator exit slit width decreases the bandwidth of radiation and decreases the eneigy reaching the detector. Thus, resolution of closely spaced absorption bands is achieved at the expense of decreased signal-to-noise ratio. For quantitative analysis, a monochromator bandwidth that is 1/5 of the width of the absorption band (measured at half the peak height) is reasonable. 

The wider the exit slit in Figure 20-5, the wider the band of wavelengths selected by the monochromator. We usually measure slit width in terms of the bandwidth of radiation selected by the slit. Instead of saying that a slit is 0.3 mm wide, we might say that the bandwidth getting through the slit is 1.0 nm. 

Figure 20-8 Increasing monochromator bandwidth broadens the bands and decreases the apparent absorbance of Pr34 in a crystal of yttrium aluminum garnet (a laser material). [Courtesy M. D. Seltzer, Mlchelson Laboratory, China Lake, CAJ...

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