Excitation temperatures, electronic

For a material to be a good conductor it must be possible to excite an electron from the valence band (the states below the Fermi level) to the conduction band (an empty state above the Fermi level) in which it can move freely through the solid. The Pauli principle forbids this in a state below the Fermi level, where all states are occupied. In the free-electron metal of Fig. 6.14 there will be plenty of electrons in the conduction band at any nonzero temperature - just as there will be holes in the valence band - that can undertake the transport necessary for conduction. This is the case for metals such as sodium, potassium, calcium, magnesium and aluminium. 

Figure 5.7 Room-temperature electronic absorption, excitation and emission spectra for 2 in aqueous solution. The excitation spectrum of 2 was recorded by monitoring emission at 400 nm. Reproduced with permission from [31]. Copyright (2004) Royal Society of Chemistry.

Figure 10.8 Room temperature electronic absorption spectrum recorded on a THF solution of 6. Bands are marked by the nominal Russell-Saunders multiplet to which the excitation occurs. (Adapted from Ref. [34], Copyright (2011) Nature Publishing Group.)...

It is uncertain to what extent thermal equilibria are achieved in different parts of the flames. — A number of procedures are (in principle) available to determine flame temperatures The immediate measurement, for example by thermocouples, the thermochemical calculation, line reversal methods for electronic excitation temperatures, determination of vibrational or rotational temperatures. In addition more recent methods like advanced Raman techniques may be applied. 

When the temperature of a molecule is increased, rotational and vibrational modes are excited and the internal energy is increased. The excitation of each degree of freedom as a function of temperature can be calculated by way of statis-hcal mechanics. Though the translational and rotational modes of a molecule are fully excited at low temperatures, the vibrational modes only become excited above room temperature. The excitation of electrons and interaction modes usually only occurs at well above combushon temperatures. Nevertheless, dissocia-hon and ionization of molecules can occur when the combustion temperature is very high. 

More precisely, the highest occupied state at Y = 0 K, since at nonzero temperatures thermal excitations of electrons lead to some population of states above the Fermi energy. 

On pp 289-310 (Ref 21), A.G. Gaydon, Shock-Tube Studies of Processes of Electronic Excitation in Gases reported that the spectrum-line reversal temperature in shock-heated gases can be used to obtain information about efficiencies and processes of electronic excitation of metal atoms at high temperatures. For excitation by molecules, the electronic excitation temperature tends to follow the effective vibrational temperature of the molecules, and reversal temperatures may be low near the shock front if. the vibrational relaxation time is appreciable. Although excitation of metal atoms by cold inert gases has a very small effective cross-section, it is shown that at 2500°K the cross-sections of excitation of Cr or Na by Ar or Ne are around 1/20 of the gas-kinetic cross-sections... 

M. Grotti, C. Lagomarsino and J. M. Mermet, Effect of operating conditions on excitation temperature and electron number density in axially-viewed ICP-OES with introduction of vapors or aerosols, J. Anal. At. Spectrom., 21(9), 2006, 963-969. 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

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