Ultraviolet monochromator

The spectral output from several used lamps was examined on a McPherson vacuum ultraviolet monochromator. Strong krypton resonance lines were observed at 1236 A. and 1165 A. with relative intensities of about 4 to 1 respectively. A number of much weaker lines at longer wavelengths were observed which were mostly attributable to H20. Some H20 was apparently released from the borosilicate glass walls during the operation of the lamp. As will be seen later a weak lamp intensity at wavelengths longer than 1236 A. does not affect the ion chemistry of the isobutylene system. 

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. 

A monochromator is useful not only for removing unwanted lines from the X-ray source but also for narrowing the otherwise broad lines. For example, each of the MgXa and AlXa doublets is unresolved and about 1 cY wide at half-intensity. A monochromator can reduce this to about 0.2 cY This reduction of the line width is very important because in an XPS specttum, unlike an ultraviolet photoelectron specttum, the resolution is limited by the line width of the ionizing radiation. Unfortunately, even after line narrowing to 0.2 cY... 

Additions to the PLM include monochromatic filters or a monochromator to obtain dispersion data (eg, the variation in refractive index with wavelength). By the middle of the twentieth century, ultraviolet and infrared radiation were used to increase the identification parameters. In 1995 the FTIR microscope gives a view of the sample and an infrared absorption pattern on selected 100-p.m areas (about 2—5-ng samples) (37). 

Radiations outside the ultraviolet, visible and infrared regions cannot be detected by conventional photoelectric devices. X-rays and y-rays are detected by gas ionization, solid-state ionization, or scintillation effects in crystals. Non-dispersive scintillation or solid-state detectors combine the functions of monochromator and detector by generating signals which are proportional in size to the energy of the incident radiation. These signals are converted into electrical pulses of directly proportional sizes and thence processed to produce a spectrum. For radiowaves and microwaves, the radiation is essentially monochromatic, and detection is by a radio receiver tuned to the source frequency or by a crystal detector. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

The word monochromator is derived from the Latin language, mono meaning one and chromo meaning color. It is a device more sophisticated than an absorption filter that isolates the narrow band of wavelengths from visible and ultraviolet sources. 

A reflectance grating is the most suitable monochromation system for ultraviolet radiation... 

A continuously monitoring detector of high sensitivity is required and those that measure absorption in the ultraviolet are probably the most popular. These may operate at fixed wavelengths selected by interference filters but the variable wavelength instruments with monochromators are more useful. Wavelengths in the range of 190-350 nm are frequently used and this obviously means that a mobile phase must not absorb at those wavelengths. 

The real-photon method is essentially more direct and easier compared to the dipole-simulation method in obtaining absolute values of photoabsorption cross sections (o ), photoionization cross sections and photoionization quantum yields (t],). In the real-photon method, however, there is a practical need to use the big and dedicated facilities of synchrotron radiation where, in many cases, one should change the beam lines equipped with different types of monochromators depending on used photon-wavelengths—and to develop some specific new experimental techniques in the range from the vacuum ultraviolet radiation to soft X-ray. 

The features in C1-C4 normal alkanes discussed in Section 3 seem to be generalized to a wide range of molecules, and thus we conclude that the major part of the photoabsorption cross sections of molecules (cr) is associated with the ionization and excitation of the outer-valence electrons. Hence, there is a strong need to measure the absolute values of a in the vacuum ultraviolet range, particularly in the range of the incident photon energy 10-30 eV, which is covered by the normal incidence monochromator used to monochromatize synchrotron radiation. The photoionization (cr ) and photodissociation (cd) cross sections. 

The method just described is not usually applicable in the ultraviolet because ultraviolet lamps of known spectral distribution are not readily available at present. The spectral sensitivity caii be calculated directly if the values of B L and P, are known. The first of these is obtained from the dispersion curve of the monochromator the second is somewhat difficult to measure—for prism instruments over restricted wavelength regions above 250 m t it is often reasonably constant. The photomultiplier sensitivity, P can be determined by comparison with a thermopile or with the ferrioxalate actinometer.11 12 Direct calculation of S, is subject to inaccuracies due to the accumulation of errors in the measurement of the three separate quantities B L and P,. A more convenient... 

The frequencies of interest for studies of collision-induced absorption range from microwave frequencies to the ultraviolet, depending on the systems and specific transitions considered. Light sources, monochromators, detectors and pressure cells are needed for such studies, which are more or less the same as in the conventional spectroscopies. 

An ultraviolet detector using a flow cell such as that. in Figure 25-19 is the most common HPLC detector, because many solutes absorb ultraviolet light. Simple systems employ the intense 254-nm emission of a mercury lamp. More versatile instruments have deuterium, xenon, or tungsten lamps and a monochromator, so you can choose the optimum ultraviolet... 

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

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