Alignment chemiluminescent reaction

The DIPR model is often used to help in understanding the stereodynamics of direct reactions [82-85]. The important parameter of the model is thus the electron-transfer probability as a function of the molecular orientation. The same parameter, which actually defines the best geometry of the system in the electron transfer step, also plays a role in determining the product alignment in chemiluminescent reactions [86, 87]. A new model has been introduced recently, the anisotropic impulsive model [88]. It is conceptually close to the DIPR model, and also helps to determine the preferred angle of approach between the reactants. 

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 other route applies when the reactivity of laser-excited species is to be studied. It is then possible to polarize the excited orbital and observe how this polarization affects the dynamics of the reaction. This was first demonstrated by observing the alignment-dependent chemiluminescence in reactions of aligned Ca(4s4p P) with halogen-containing molecules [224, 225]. This work will serve to rationalize the branching to chemiluminscence observed in reactions induced in van der Waals complexes (see Section 2.6.1). It has been extended very recently to other molecular reactants [196]. 

Figure 22.8 Chemiluminescence emission intensity in the Ca( Pj) -L HCl reaction, as a function of the laser-induced alignment of the Ca p-orbital. Adapted from Rettner and Zare, J. Chem. Phys., 1982, 77 2416, with permission of the American Institute of Physics...

The effect of alignment of the metal atom electron has been probed by Rettner and Zare [54] in somewhat more direct fashion in the reaction of Ca(4s4p Ip ) with CI2 and other reactants. Aligned Ca( P ) was prepared by polarized laser excitation. For the O2 reaction, chemiluminescence and chemi-ionization cross sections were found to be largest for n approach of the 4p electron, suggesting that these channels are reached through the B2 surface. 

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