Monochromators

The monochromator is a fundamental tool in optical spectroscopy. As we mentioned in Chapter 1, a monochromator is an optical element that is used to isolate the different spectral components of a light beam. Monochromators have two main utilities in optical spectroscopy experiments  

An Introduction to the Optical Spectroscopy of Inorganic Solids J. Garcia Sole, L. E. Bausa, and D. Jaque 2005 John Wiley Sons, Ltd ISBNs 0-470-86885-6 (HB) 0-470-86886-4 (PB) 

When a polychromatic beam reaches the grating, diffraction effects take place, so that the angle at which each spectral component is reflected depends on its particular 

F ure 3.4 The spectral responses of two diffraction gratings with different groove densities. 

Studied by optical spectroscopy means that the radiation involved is spread over a large spectral range, and cannot be monitored by using only one type of detector. As a consequence, much work has been carried out to develop detectors that cover the different spectral ranges required by the optical techniques and the materials studied. 

Various optical elements can be placed in the beam path to tailor the characteristics of the X-ray beam. These can work by diffraction e.g. a monochromator crystal), reflection e.g. a mirror), or absorption e.g. a filter or slits). A monochromator is used to select a particular wavelength, a mirror can focus the beam or suppress higher harmonics, and filters can be used to remove unwanted radiation. 

At synchrotron sources, attenuators such as graphite, aluminium or synthetic-diamond foils can be inserted into the primary beam path to reduce the heat load on an optical element, to prevent saturation of the X-ray detector, or to reduce the rate of radiation damage to the sample. 

The first crystal selects a wavelength from the polychromatic source, which is reflected along the initial direction by the second crystal. The lattice planes of the latter must be perfectly aligned with the first crystal for efficient transmission of the beam. 

Grating optical element with closely spaced lines 

Diffraction bending of light by a grating Refraction bending of light by a lens or prism 

Resolution ability to distinguish two closely spaced peaks 

Dispersion ability to produce angular separation of adjacent wavelengths 

Resolution measures the ability to separate two closely spaced peaks. The greater the resolution, the smaller is the difference (AX.) between two wavelengths that can be distinguished from each other. The precise definition (which is beyond the scope of this discussion) means that the valley between the two peaks is about three-fourths of the height of the peaks when they are just barely resolved. The resolution of a grating is given by 

A double monochromator has two separate gratings which are separated by an adjustable slit. Both of these gratings are mechanically coupled onto the same shaft so that tracking errors are virtually eliminated. The gratings are rotated concurrently and the 

3 Detectors. - Once the Raman signal has been successfully extracted from all other components of the scattered radiation, the Raman signal impinges upon a detector. The type of detector chosen depends on the optical arrangement of the monochromator and the demands of the spectroscopist. If the monochromator is to be used as a spectrograph, then 

The optical multichannel photodiode array most used in Raman spectroscopy is the IPDA consisting of a one-dimensional array of amplified photodiodes. The mechanism behind the IPDA detection is that each photodiode converts photons to separated electron-hole pairs (semiconductor-amplification device). In some solid-state detectors (e.g., 

Standard UV/VIS detectors are photomultipliers and silicon diodes. Silicon diodes are smaller and cheaper, whereas photomultipliers have a higher sensitivity. Most research instruments are based on photomultipliers. A recent development is the use of photomultiplier arrays and CCD cameras as in all other spectroscopic methods. 

An important consideration for all types of instruments is the linear absorption range, i. e. maximum absorption measured at a predetermined accuracy. The photometric accuracy depends on the instrument s electronics, which may be sensitive to ambient temperature and humidity. Spectrometers should be cahbrated from time to time. Neutral density filters of well defined absorbance are commonly used for absorbance cahbration. Solutions of potassium chromate and potassium 

As in the case of dispersive Raman spectrometers (cf Section 4.4.1), it is necessary to calibrate the wavelength scale of dispersive UV/VIS spectrometers. The most accurate standards for checking the UV/VIS wavelengths are lasers of various types. The inexpensive helium-neon laser can be used to check at 632.8 nm. For spectrometers with a deuterium source, spectral lines at 486.6 and 656.1 nm can be used for calibration. A common method for wavelength calibration is the use of optical filters. A filter of didymium glass has many sharp absorption peaks, which can be used as a second wavelength standard (precision within 0.5 nm). 

If measurements have to be done in the UV region below 240 nm, it is necessary to purge the spectrometer with dry nitrogen gas in order to remove oxygen. Oxygen absorbs at wavelengths shorter than 240 nm and is transformed into ozone. Absorption by oxygen molecules inside the instalment can make measurements 

SRM number Type Parameter checked Wavelength range/nm 

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The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

If the sample is placed in the path of the infrared beam, usually between the source and the monochromator, it will absorb a part of the photon energy having the same frequency as the vibrations of the sample molecule s atoms. The comparison of the source s emission spectrum with that obtained by transmission through the sample is the sample s transmittance spectrum. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

An electron prisin , known as an analyser or monochromator, is created by tlie field between the plates of a capacitor. The plates may be planar, simple curved, spherical, or toroidal as shown in Figure Bl.6.2. The trajectory of an electron entering the gap between the plates is curved as the electron is attracted to the positively biased (iimer) plate and... 

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

The construction of a typical monochromator is shown in Figure 10.12. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with... 

Effect of the monochromator s slit width on noise and resolution for the ultraviolet absorption spectrum of benzene. The slit width increases from spectrum (a) to spectrum (d) with effective bandpasses of 0.25 nm, 1.0 nm, 2.0 nm, and 4.0 nm. 

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... 

Typical grating monochromator with inset showing the dispersion of the radiation by the diffraction grating. 

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. 

An instrument for measuring absorbance that uses a monochromator to select the wavelength. 

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

Multielemental Analysis Atomic emission spectroscopy is ideally suited for multi-elemental analysis because all analytes in a sample are excited simultaneously. A scanning monochromator can be programmed to move rapidly to an analyte s desired wavelength, pausing to record its emission intensity before moving to the next analyte s wavelength. Proceeding in this fashion, it is possible to analyze three or four analytes per minute. 

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