Atom-molecule reactions studied in flow systems the hydrogen halide system

2 Atom-molecule reactions studied in flow systems the hydrogen halide system 

The principal reaction discussed above forms oxygen molecules in high vibrational levels of the ground state. This is chemi-excitation but is not chemiluminescence vibration-rotation transitions of homonuclear molecules are forbidden. For such cases electronic absorption spectroscopy is the required technique. For reactions in which a heteronuclear diatomic (or a polyatomic) molecule is excited these transitions are allowed. They are overtones of the molecular transitions that occur in the near infrared. These excited products emit spontaneously. The reactions are chemiluminescent, their emission spectra may be obtained and analyzed in order to deduce the detailed course of the reaction. 

A crucial step in both emission and absorption studies is the conversion of intensities of bands or lines to concentrations of molecules. This is not easy. The transition probabilities for bands connecting excited states are not often known to any reasonable precision. Also, one further difficulty plagues the emission studies. In contrast to absorption studies the population of molecules in the lowest state, v = 0, is not measurable. Relative populations are usually reported. Self absorption measurements can be used to overcome this difficulty43.  

The favorite technique for studying chemiluminescent reactions has been to mix the reagents in a low pressure flow system and then to use the resulting luminous zone as a spectral light source. This technique has been exploited for combination reactions, for group transfer reactions and, quite generally, for the excitation of molecular spectra. 

Production of the reactive atom or molecular fragment has been achieved by several techniques. The cleanest method is decomposition of a molecule (H2,03) on a heated surface. A second method is to use a very fast chemical reaction. A favorite one for oxygen atoms is 

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