Laser photoexcitation and photodetection of diatomic molecules

Laser excitation is not only a means for providing the necessary energy for chemistry to take place. We begin the discussion of the selectivity achievable by the excitation and the specificity of the detection for the special case of diatomic molecules. Then the spectrum is simpler in that there is only one vibrational mode as well as only one rotational constant. But once the molecule is electronically excited we will find it necessary to allow for the breakdown of the Born-Oppenheimer separation. 

1 The discrete vibrational energy levels of diatomic molecules 

This is the operative technical term. It is equivalent to our earlier condition that there is a definite initial state and not an ensemble. The optically prepared state is then a pure state in the quantum mechanical sense, namely, it can be expressed as a superposition of states, each with its own phase, hence coherent.  

Photodynamic therapy is based on the in situ excitation of 02(a Ag) by electronic energy transfer from photoexcited organic molecules. 

Even a broad laser can selectively excite one species and not the other. In the presence of a scavenger for the electronically excited ICl, such as acetylene, one can separate the two chlorine isotopes. 

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