Mercury atom, electronic excitation

A mechanism by which electronic energy is sometimes made available for chemical purposes is that which occurs in the so-called Stoss zweiter Art , or collision of the second kind . Cario and Franck found that on exposure of a mixture of mercury vapour and hydrogen to the mercury line 2536-7 A, the absorbed light stimulated the mercury atoms electronically, and that the excited mercury atoms were able on collision with hydrogen molecules to resolve them into atoms. The atomic hydrogen so produced was detected by its great chemical activity. A number of other examples of this phenomenon were found Z. Phyaik, 1922, 21, 161. 

In an atom of the second column of the periodic system, such as mercury, the two valence electrons are in the normal state s-electroiis, and form a completed sub-group. Two such atoms would hence interact in a way similar to two helium atoms the attractive forces would be at most very small. This is the case for Hg2, which in the normal state has an energy of dissociation of only 0.05 v.e. But if one or both of the atoms is excited strong attractive forces can arise and indeed the excited states of Hg2 are found to have energies of dissociation of about 1 v.e. 

The mercury lamp has been the conventional light source used in photochemistry. The ground-state mercury atom, Hg, has two electrons in its highest occupied orbital, the 6s atomic orbital. Excited mercury... 

Recombination of fhe ionized elecfron wifh the argon cation produces an electronically excited argon atom, which can energize and subsequently ionize a mercury atom ... 

The electronically excited mercury atom generated by the recombination of the mercury cation with an electron loses its energy radiatively. The above are only a few of fhe processes fhaf fake place in the lamp, but the combined effect is the emission of light in the UV and visible regions and the generation of heat The heat vaporizes some of fhe mercury mefal. The mercury cations are conducting and the current passing across the electrodes rises until a steady state is reached. 

For example, 6 P, (six triplet P one) state of mercury signifies that the total energy of the state corresponds to n = 6 the orbital angular momentum is L— 1 the multiplicity is three hence it is a triplet energy state and the spins of the two valence electrons must be parallel (S = 1) and the particular value of 3 is 1 (/= 1). Since a normal mercury atom, has a pair of electrons with opposed spin in the S orbital, this must be an excited energy state, where a 6S electron is promoted to a 6P state. 

In this chapter electronically excited atoms are classified into two groups. The lirst group of excited atoms are those that are formed by resonance absorption and decay rapidly by fluorescence if not quenched by collisions with foreign gases. Examples are electronically excited Hg, Cd, H, Ar, Kr, and Xe atoms. Of these Hg(,/>1) atoms and their reactions have been most extensively studied. The mercury sensitized reactions provide a convenient way to generate atoms and radicals in the spectral region where many molecules do not absorb. 

An interesting case is the action of excited mercury on oxygen18 19. The energy required to dissociate the oxygen molecule is 5.13 electron volts or 118 kcal.mole-1. 63Pi mercury atoms are thus unable to cause this dissociation. However, the reaction... 

The exchange of electronic excitation between two atoms frequently results in sensitised fluorescence and one of the earlier examples was the discovery of emission of the fluorescence of atomic sodium, which occurs when a mixture of sodium and mercury vapour is irradiated with mercury resonance radiation at 2537 A... 

To the rare compounds that are not molecular compounds and that must perhaps be regarded as being due to an interaction of the London type belong some very unstable molecules, encountered only in gaseous discharges, for example HgKr, HgA and perhaps also Hg2. These compounds have only a very small heat of dissociation (HgKr 0.8 kcal/mol), and the mercury atom is moreover in an excited state so that a one-electron bond is also not excluded. 

Frish and Kraulinya and, most recently, by Czajkowski, Skardis, and Krause [71] and Czajkowski, Krause and Skardis [96]. Frish and Bochkova [97, 98] studied excitation transfer from the 6 aPr and 6 aP0 mercury atoms excited by collisions with electrons in a discharge, to various states in sodium. Kraulinya [99] optically excited the Hg(6 aPJ state and followed the excitation transfer to sodium by monitoring the intensities of the collisionally sensitized sodium lines. Her results which are quoted within 30% — 50% are summarized in Table 4.5 and are compared with the cross sections determined by Czajkowski, Skardis and Krause [71], The considerable discrepancies between the two sets of results are apparently due to errors arising from the trapping of mercury resonance radiation [100, 28] which must have particularly affected Kraulinya s results, and from the uncertainty in the determination of the mercury and sodium vapor densities in the binary mixture. 

In emission spectroscopy the molecule or atom itself serves as the somce of light with discrete frequencies to be analyzed. In some cases, such as Exp. 39, which deals with the emission spectrum of molecular iodine vapor, excitation by a monochromatic or nearly monochromatic laser or mercury lamp is utilized. For other cases, such as the emission from N2 molecules, electron excitation of nitrogen in a discharge tube provides an intense somce whose spectrum is analyzed to extract information about the electronic and vibrational levels. Such low-pressure (p < 10 Torr) line somces are available with many elements, and lamps containing Hg, Ne, Ar, Kr, and Xe are often used for calibration purposes. The Pen-Ray pencil-type lamp is especially convenient for the visible and... 

Very elegant experiments unequivocally proving the occurrence of electronic energy transfer were performed in 1922 and 1923 by Carlo and Franck [14], When a mixed vapor of mercury and thallium was irradiated with the mercury line at 253.67 nm, the emission lines of thallium could be observed in addition to the anticipated fluorescence spectrum of mercury. Since thallium cannot absorb 253.67-nm light, it must have been sensitized by the excited mercury atoms in order to produce the green fluorescence... 

Two basic schemes have been used for initiating intracluster processes. In the first the reaction is induced by electronic excitation of an atom inside the complex. Jouvet and Soep used this scheme for the study of several reactions of electronically excited mercury atoms Hg( Pi). In the reaction with CI2 they obtained conclusive evidence of the formation of a charge transfer intermediate in the process. In the reaction with Hj a strong dependence of the reactivity on the geometry was observed. It was found that the reaction occurs on the n surface with a C2 symmetry. 

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