Alkaline earth metal atoms electronically excited

Alkaline earth metal atoms have fairly low ionization potentials, as have alkali metal atoms (e.g., 5.21 and 5.14 eV for barium and sodium, respectively [89]). Hence the reactions of alkaline earth metal atoms with oxidizing molecules are also expected to be initiated by an electron transfer and should follow the harpoon mechanism. However, alkali metal atoms are monovalent species, whereas alkaline earth metal atoms have two valence electrons. Hence peculiarities are to be expected in the alkaline earth metal reaction dynamics, especially when doubly charged products such as BaO are to be formed [90]. The second valence electron also opens up the possibility of chemiluminescent reactions, which are largely absent in alkali metal atom reactions [91, 92]. The second electron causes the existence of low-lying excited states in the product. 

Since alkaline earth metal atoms have two valence electrons, it is convenient to distinguish between reaction products, which have a single ionic bond such as BaCl (Ba+Cr) from products having a double ionic bond in the ground state such as BaO (BaO has the structure Ba +O in the ground state and Ba+O in the lowest excited states). 

Dramatic effects of electronic excitation on the reaction mechanisms have been demonstrated in several cases. One of the first reported examples must be recalled here also as it falls outside the scope of this chapter. Electronically excited 0( D) is much more reactive than ground-state 0( P) and inserts into the C-H bonds of methane [162]. Similar state specificity in the reactivity has also been encountered in electron-transfer reactions and seems to be the rule in light systems. Its origin has been explored systematically in alkali and alkaline earth metal atom reactions. Before discussing some of the studies, it is appropriate to survey a much simpler situation where electronic excitation affects the dynamics of the reaction just by changing the location of the electron-transfer region. 

The nature of the electronic configuration in the alkaline earth metals does, however, allow for intriguing research into correlations between the two electrons in the outer shell of these atoms. Simultaneous excitation of the two outer electrons is possible with a single photon, which leads to the provocative idea of a shared principal quantum number. Excitation of one of the electrons to consecutively higher orbits while monitoring de-excitation of the other electron gives information about the border between quantum and classical (planetary) behavior. 

When the atoms of samples are excited to higher electronic energy levels in flames they emit radiation in the visible and UV regions of the electromagnetic spectrum. Emission intensities may be measured to analyze for metals, especially alkali and alkaline earth elements. 

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