Precision wavelength

As mentioned in item 5 of Section 9.1, the light sources used in atomic absorption instruments are sources that emit spectral lines. Specifically, the spectral lines used are the lines in the line spectrum of the analyte being measured. These lines are preferred because they represent the precise wavelengths that are needed for the absorption in the flame, since the flame contains this analyte. Spectral line sources emit these wavelengths because they themselves contain the analyte to be measured, and when the lamp is on, these internal atoms are raised to the excited state and emit their line spectrum when they return... 

From precise wavelength measurements of the fluorescence spectrum (which may be performed e. g. by interferometric methods accurate values for the molecular constants can be obtained since the wavelength differences of subsequent lines in the fluorescence progression yield the energy separation of adjacent vibrational and rotational levels as a function of v . From these spectroscopically deduced molecular constants, the internuclear distance can be calculated A special computer programm developed by Zare ) allows the potential curve to be constructed from the measured constants and, if the observed fluorescence progression... 

Lasers, which are discussed in more detail in Chapter 16, have assumed a very important role in electronic spectroscopy in recent years, for two reasons They can deliver large quantities of energy at very precise wavelengths,... 

More common methods for elemental analysis - to determine the elemental contents of a sample - include spectroscopy and spectrometry. Spectroscopy measures changes in atoms that cause a specific light photon to be either absorbed (absorption spectroscopy) or emitted (emission spectroscopy). This light has a precise wavelength or energy, characteristic of a specific element in the periodic table. The simplest (and oldest) form of elemental analysis was not spectroscopic, in fact, but colorimetric. This method was based on the reaction of a strongly colored chemical in a solution. The appearance of a specific color in the solution revealed the identity of the element of interest. If the color intensity is proportional to the amount of that element present, the method can also be used to estimate the amount of the element present. 

Fluorescence is the emission by a fluorophore of a photon from the lowest excited state Si to the ground state So. Emission of the photon occurs with a precise energy E and thus it will be observed at a precise wavelength (A.em). Emission occurs from a population of n excited fluorophores, therefore it will occur with an intensity I... 

They can deliver large quantities of energy at very precise wavelengths, and the power can be delivered in very short bursts—on the order of only 10 second (one femtosecond) in duration. This allows the excitation of molecules with an ultrashort pulse of energy, after which the molecule can be observed as it decays back to the ground state by various pathways. These techniques are now being used to elucidate the mechanisms of chemical reactions, as we will discuss in Chapter 15. 

The absorption spectrum for hydrogen was first measured in 1885 by a Swiss schooimaster, Johann Baimer, who also noticed that the wavelengths of the lines in this spectrum couid be predicted using a mathematical formula. You do not need to know the details of his formula at this stage, instead let s think about the implications of the observation that a hydrogen atom, with just one electron, has a spectrum of discrete iines at precise wavelengths. It means... 

FIGURE 3-18. Echelle-prism spectrum of stainless steel with mercury lamp precision wavelength reference. [From G. J. Matz, Recent Advances in Echelle Spectrometer Analysis, American Laboratory, 4 (3), 75. Used by permission of International Scientific Communications, Inc.] Entrance slit, 25 /im wide by 200 /im high. Polaroid type 55 fine-grain film was used. 

Precise wavelength measurements are not required for qualitative spectrochemical analysis. Usually measurements to 0.05 to 0.10 A will suffice, since the spectroscopist usually relies on the identification of three or four spectral lines to prove the presence of an element in the analytical sample. Use also is made of unknown spectra of elements to compare with the unknown sample. This technique does not require measurements of wavelengths of spectral lines. 

The contents of Chap. 4, which covers spectroscopic instrumentation and its application to wavelength and intensity measurements, are essential for the experimental realization of laser spectroscopy. Although spectrographs and monochromators, which played a major rule in classical spectroscopy, may be abandoned for many experiments in laser spectroscopy, there are still numerous applications where these instruments are indispensible. Of major importance for laser spectroscopists are the different kinds of interferometers. They are used not only in laser resonators to realize single-mode operation, but also for line-profile measurements of spectral lines and for very precise wavelength measurements. Since the determination of wavelength is a central problem in spectroscopy, a whole section discusses some modern techniques for precise wavelength measurements and their accuracy. 

N. Konishi, T. Suzuki, Y. Taira, H. Kato, T. Kasuya High precision wavelength meter with Fabry-Perot optics. Appl. Phys. 25, 311 (1981)... 

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