Elements of quantal scattering theory

Experimental observations of the rainbow effect provided early evidence for the existence (and magnitude) of the shallow, intermolecular potential well, for chemically non-bonding systems. 

To sum up, elastic scattering measurements serve as a probe of the collision dynamics and can reveal detailed information on the intermolecular potential. But to do a complete job we cannot overlook quantal effects. 

In electricity and magnetism, the intensity is given by the square of the corresponding electric field amplitude. In quantum mechanics the probability is expressed as the (absolute) value squared of a corresponding amplitude. We are concerned with the scattering intensity and it is expressed in terms of the scattering amplitudeX ) as 

The amplitude is generally a complex number. Computing it is the business of the Schrodinger equation. The essential point is that when a final outcome can be reached in several different ways, the amplitude to realize the outcome is the sum (= superposition) of the amplitudes for the different routes. For molecular collisions, where classical dynamics is often a realistic description, it is a useful 

But in an experiment that is capable of determining which alternative is actually followed, we are to add intensities. In such an experiment there is no interference. 

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