Reactions of electronically excited alkaline earth atoms

4 Reactions of electronically excited alkaline earth atoms 

In recent years, a number of experiments have been performed using, as reagents, alkaline earth atoms excited by laser or electrical discharge into their metastable electronically excited states [Ba (3D), Sr (lD, 3P), Ca (1D, 3P) and Mg (3P)]. These reactions are summarised in Table 4. 

Reaction Reagent preparation System Product detection Information (J y) fe nt nv Nr Ne Ref. 

The polarisation of the CaCl (B) chemiluminescence from the reaction Ca (1D) + HC1 has been measured [379] to determine the rotational alignment of the CaCl product and indicates a highly polarised distribution of product angular momenta. This is similar to the reactions of alkali atoms with hydrogen halides. 

The reactions of Sr ( P) with HF and HCl have insufficient energy to produce electronically excited halide products and the vibrational distribution of the ground state SrX product has been measured by laser-induced fluorescence [375]. For Sr -I-HF, and for 

Sr -t-HCl, Fv) 0.70 with an extremely narrow vibrational distribution. It appears that a much higher fraction of the reaction energy appears as product vibration than is the case for the reactions of ground state alkaline earth atoms with HX. In the case of the excited atom, reaction takes place on the triplet rather than the singlet surface correlating with ground state products. The transition from covalent reagents 

---

Reactions of electronically excited alkaline earth atoms (see Appendix 1)... 

Although adiabatic correlation arguments [10] could, in principle, be used to understand the observed spin-orbit effects, such considerations have been shown [119-123] to be poor predictors for the product electronic state branching in chemiluminescent alkaline earth reactions. Because of the relatively low ionization potentials of the excited alkaline earth atoms, their reactions proceed by charge transfer to an ionic surface [116]. As the reactants approach but before charge... 

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. 

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. 

---