Photomultipliers atomic spectroscopy

The simplest application of these multiplexing methods involves the so-called direct-reading spectrometer , which was used with some success for a short period in atomic spectroscopy [42]. This instrument consists of a dispersion system with an array of exit silts arranged at appropriate locations. Behind each silt Is a photodetector —usually a photomultiplier. These multiplexing methods have also been used In UV-vIsIble spectroscopy, although to a lesser extent they have been Implemented on automatic discrete analysers featuring an optical system of this type with 5-10 channels or wavelengths... 

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

Barrier-layer cells and photomultipher tubes have both been used for photodetection in atomic absorption spectroscopy. The use of barrier-layer cells of course is limited by their sensitivity and the diflBculty encountered in amplifying their output. They will suflBce where determination of the alkali elements is desired only. For most other work photomultiplier tubes are necessary. These are available for a broad spectral... 

Hanselman D. S., Withnell R. and Hieetje G. M. (1991) Side-on photomultiplier gating system for Thomson scattering and laser-excited atomic fluorescence spectroscopy,... 

Optical spectroscopy ranging from Raman through Atomic is under considerable pressure to replace the "tried and true" photomultiplier tube (PMT) with multichannel devices. Two classes of solid state devices, the Charge Coupled Device (CCD) and the Charge Injection Device (CID), hold great promise for meeting this need. Operating modes of these devices are reviewed. Characteristics pertinent to analytical spectroscopy are presented. 

There are a number of different types of photon detectors, including the photomultiplier tube, the silicon photodiode, the photovoltaic cell, and a class of multichannel detectors called charge transfer devices. Charge transfer detectors include photodiode arrays, charge-coupled devices (CCDs), and charge-injection devices (ClDs). These detectors are used in the UV/VIS and IR regions for both atomic and molecular spectroscopy. 

The spectral response of a photomultiplier tube varies with the coating materials used on the photocathode. Spectral responses of various photomultiplier tubes are given in Table 6-3. Chapter 6 also includes a general discussion of photomultiplier phototubes. The 1P28 tube (S-5 response) is sensitive from 2000 to 6500 A and is frequently used for atomic absorption spectroscopy. The Hamamatsu R106 also has an S-5 response but uses a silica window to lower the usable short wavelength response to about 1700 A. The S-20 response of the RCA 4459 permits measurements to 8500 A and is very useful for most of the alkali metals. 

The different sensitive techniques of Doppler-limited laser spectroscopy discussed in the previous sections supplement each other in an ideal way. In the visible and ultraviolet range, where electronic states of atoms or molecules are excited by absorption of laser photons, excitation spectroscopy is generally the most suitable technique, particularly at low molecular densities. Because of the short spontaneous lifetimes of most excited electronic states the quantum efficiency rjk reaches 100% in many cases. For the detection of the laser-excited fluorescence, sensitive photomultipliers or intensified CCD cameras are available that allow, together with photon-counting electronics (Sect. 4.5), the detection of single fluorescence photons with an overall efficiency of 10 —10 including the collection efficiency 5 0.01—0.3 (Sect.6.3.1). 

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