Abstract

Where Iп, I0 - intensity of the absorbed and probing radiations; k (v) - spectral factor of absorption; L-thickness of an absorbing layer of gas. Absorption by the gas environment of probing radiation is strictly described by the given expression only in conditions of monochromaticity of radiation, independence of factor of absorption of frequency and concentration of absorbing particles and at absence of photochemical reactions in the gas environment. Calculation of concentration of absorbing particles probably by measurement of size k (v), a line of absorption describing intensity, and parameters of a contour of a line of absorption. For calculation it is necessary to use also as initial preconditions those or others theoretical the approach, describing the form of spectral lines depending on conditions of experiment. In real conditions direct research of a contour of a line of absorption represents rather a challenge. Therefore in practice at definition of concentration of atoms and molecules measure integrated intensity of lines (strips) of absorption. The analytical signal in this case is defined by a difference intensities probing radiation before and after ditches with the absorbing gas environment. Analytical communication between change of intensity of probing radiation and concentration of absorbing particles find experimentally and use in the form of calibration schedules

Sources of probing radiation

For the decision of various problems in absorption gas analyzers use various sources of probing radiation: gas-discharge, thermal, coherent. On character of radiation they can be divided into sources continuous, linear and monochromatic radiation in UF-, visible and spectral ranges. Thermal sources are characterized by a continuous spectrum of radiation in IK a range, high stability of radiated capacity, small consumption of energy and greater terms of operation. Use some versions of such sources: globar, a representing core from carbide silicon; working temperature 1300 To; pin Nernsta representing a core, containing a mix oxides zirconium, thorium, yttrium; usual working temperature 1700 To; lamps of incandescences with tungstic or nikhromovoy a spiral, nikhromovoy up to 1000-1100 To, radiating in visible and near areas of a spectrum. Gas-discharge sources are characterized linear by a spectrum of radiation in UF-, a visible and near range of lengths of waves, and also a continuous spectrum in area of a spectrum. To sources of this type concern: hydrogen or deuterium the lamps representing glass flasks with quartz windows, filled by gas at pressure in some hundreds Па; lamps are sources of a continuous spectrum in visible and UF (up to 200 nanometers)-areas of a spectrum; high-frequency electrodeless the lamps filled by inert gas at pressure in some 100-th shares Pa and substance-source nuclear steams ; lamps are sources linear a spectrum of radiation in visible and areas; Mercury gas-unloading the lamps of the low, high or ultrahigh pressure representing quartz tubes with soldered in electrodes and filled by argon and mercury; Lamps are sources linear a spectrum of radiation which most intensive lines have lengths waves:253,7: 313 : 314; 365,5 : 404.7; 435.8; 546,1; 577 and 579.1 nanometers; Lamps with the full cathode being sources linear a spectrum of radiation which character is defined by the elements which are a part of the cathode or a material raised dust on its surface; the atoms which have formed at evaporation of a material heated of the cathode or owing to dispersion of its superficial layers under influence of ionic bombardment, are raised in the decaying category of a direct current in buffer gas; these lamps use at the analysis of air on the maintenance of metal impurity in the form of metalloorganic connections, aerosols and steams (for example mercury). Monochromatic sources - the optical quantum generators radiating separate lines in visible and areas of a spectrum in a mode of pulse or continuous generation. Sources of such type allow to reconstruct frequency of radiation or is continuous in some range of lengths of waves, or discrete on the several fixed frequencies: Gas-discharge With-laser with generation of radiation in the field of 5-6 microns by power a little mVt; Gas-discharge Не-Nе-lazer with generation of the radiation reconstructed discretely on lengths of waves: 3,39; 4.22; 5.4 microns, capacity 0,5-5 мВт; Lasers on dye (LK), studying on lengths of waves from 0,4 up to 0,6 microns; Light-emitting diodes on the basis of firm solutions of semi-conductor connections of type InGaAs and InAsSbP, 2.6-4,7 microns radiating in a range; capacity of continuous radiation of the order of hundreds мкВт. And pulse-several mVt; Semi-conductor diods lasers of type PbS1-xSеx and Рb1-xSnxSе, 3-30 microns generating in a range; lasers provide continuous reorganization of a narrow line of generation due to change of a current of a feed and temperatures of a semi-conductor element in a range up to 1000 sm-1. The schedule gaussian in dependences from temperatures for diode LED46 is resulted below:

In summary we shall mention a specific source of radiation - the MICROWAVE-generator (klystron) with frequency of 10-25 GHz, used in the some people gas of analytical problems. Thus, applied in absorption gas analyzers sources probing radiation cover wide area of a spectrum. Application diods the semi-conductor lasers is most perspective from our point of view, allowing to scan a narrow line of radiation in rather big range of lengths of waves and generating in the field of a spectrum, overlapping shake-rotary strips of absorption of the majority of gaseous molecules. These characteristics of a source of probing radiation provide a good Basis for achievement of high selectivity and universality gas of analytical techniques.

Receivers of radiation

Used in absorption gas analyzers the receiver and radiant energy can be divided on two groups: thermal and photo-electric. Thermal receivers serve for detecting radiation in area of a spectrum (<30 microns). The thermoelements representing bimetallic devices at which heating arises EDS concern To this group of receivers, proportional to temperature of heating, and also bolometrs, representing resistance with the greater temperature factor of resistance. Thermal receivers are ineffective at measurement of small changes of capacity of probing radiation and possess concerning big by an inertance. As positive properties it is possible to specify weak dependence of sensitivity on length of a wave of registered radiation in a working range and convenience in operation. Photo-electric the receiver also use for detecting radiation in UF-, visible and IK-(up to 14 microns) areas of a spectrum. This group of receivers can be divided into photo cells with an external and internal photoeffect. The first usually serve as receivers of radiation in UF-, and visible areas of a spectrum. The principle of action of such detectors named by photoelectronic multipliers (FEU), is based on issue from the photocathode electrons, getting from photons the energy exceeding work of an output from a surface of the photocathode. Formed electrons are accelerated in an electric floor and are multiplied on system of electrodes - accelerating diods. Signal FEU, thus, is proportional to intensity of radiation of the receiver getting on the photocathode. Most a wide circulation have received FEU with Sb-Сs-photo by a cathode with the maximal sensitivity in the field of from 160 up to 650 nanometers; with the multialkaline photocathode - from 400 up to 870 nanometers; with Ag-Сs-photo by a cathode- 400 up to 1300 nanometers. The constant of time FEU makes 10-8 with, sensitivity 10-14 Vt. Photo-electric receivers with an internal photoeffect usually use for registration of radiation in area of a spectrum. The principle of action of considered detectors is based on ability of semi-conductor elements to change the conductivity at absorption of photons. As such receivers use, for example, PbS-photoresistance with sensitivity in the field of <4 microns or InSb with sensitivity in the field of <7,5 microns working both at room (293), and at lowered (273) temperatures. In wider spectral range (<14 microns) can work detectors of type PbSnTe or HgCdTe, but only at cryogenic (77 temperatures. Thus, applied in absorption gas analyzers receivers of probing radiation cover wide area of a spectrum - from 0.2 up to 30 microns. It is necessary to note, that all the considered receivers are not selective and for exception thermal possess appreciable dependence sensitivity length of a wave of registered radiation.

Literature

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