Excitation temperature, atomic spectroscopy

Nuclear magnetic resonance spectroscopy is a technique that, based on the magnetic properties of nuclei, reveals information on the position of specific atoms within molecules. Other spectroscopic methods are based on the detection of fluorescence and phosphorescence (forms of light emission due to the selective excitation of atoms by previously absorbed electromagnetic radiation, rather than to the temperature of the emitter) to unveil information about the nature and the relative amount specific atoms in matter. 

How would emission intensity be affected by a 10 K rise in temperature In Figure 21-14, absorption arises from ground-state atoms, but emission arises from excited-state atoms. Emission intensity is proportional to the population of the excited state. Became the excited-state population changes by 4% when the temperature rises 10 K, emission intensity rises by 4%. It is critical in atomic emission spectroscopy that the flame be very stable or emission intensity will vary significantly. In atomic absorption spectroscopy, temperature variation is important but not as critical. 

In atomic spectroscopy, absorption, emission, or fluorescence from gaseous atoms is measured. Liquids may be atomized by a plasma, a furnace, or a flame. Flame temperatures are usually in the range 2 300-3 400 K. The choice of fuel and oxidant determines the temperature of the flame and affects the extent of spectral, chemical, or ionization interference that will be encountered. Temperature instability affects atomization in atomic absorption and has an even larger effect on atomic emission, because the excited-state popula-... 

The viewing region of the plasma can achieve a temperature of 5000-6000°C and is reasonably stable. The sample solution is aspirated into the core area between the two arms of the Y where it is atomised, excited and viewed. This technique keeps with the atomic spectroscopy theory in that the measurements are obtained by emission from the valence electrons of the atoms that are excited, and the emitted radiation consists of short well-defined lines. All these lines fall in the UV or VIS region of the spectrum and identification of these lines permits qualitative/quantitative detection of elements. 

Atomic spectroscopy can be divided into several broad classes based on the nature of the means of exciting the sample. One of these classes is generally known as atomic emission spectroscopy, in which excitation is thermally induced by exposing the sample to very high electric fields. Another class is known as flame emission spectroscopy or flame photometry, in which excitation is thermally induced by exposing the sample to a high-temperature flame. These methods differ from atomic absorption spectroscopy, in which the absorption of light from a radiation source by the atom is observed rather than the emission from the electronically excited atom. 

In another field, the term buffer (but not buffer solution) is used for substances that are added to the samples introduced to the excitation source in atomic spectroscopy to prevent any change in the excitation conditions due to accompanying elements. Mainly this refers to maintaining constant temperature or electron density. 

Atom-ion equilibria in flames create a number of important consequences in flame spectroscopy. For example, intensities of atomic emission or absorption lines for the alkali metals, particularly pota.ssium, rubidium, and cesium, are affected by temperature in a complex way. Increased temperatures cause an increase in the population of excited atoms, according to the Boltzmann relationship (Equation fl-l). Counteracting this effect, however, is a decrease in concentration of atoms resulting from ionization. Thus, under some circumstances a decrease in emission or absorption may be observed in hotter flames. It is for this reason that lower excitation temperatures are usually specified for the determination of alkali metals. 

Classical emission spectroscopy is based on excitation of atoms or molecules into higher electronic states by electron impact (in gas discharges), photon absorption or thermal excitation at high temperatures (in star atmospheres). Excitation by narrow-band lasers may result in the selective... 

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. 

Therefore, the fraction of atoms excited critically depends on the temperature of the flame thereby emphasizing the vital importance of controlling the temperature in Flame Emission Spectroscopy (FES). 

At temperatures above absolute zero, all atoms in molecules are in continuous vibration with respect to each other [26]. Infrared spectroscopy is an absorption spectroscopy. Two primary conditions must be fulfilled for infrared absorption to occur. First, the energy of the radiation must coincide with the energy difference between excited and ground states of the molecule, i.e., it is quantized (Fig. 14.2). Radiant energy will then be absorbed by the molecule, increasing its natural vibration. Second, the vibration must entail a change in the electrical dipole moment (Fig. 14.3). 

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