Optical Pumping in Molecular Beams

If the laser frequency oj is tuned to a molecular transition (vV,JV - the molecules are excited into the upper level where they can 

In the intersection region of laser beam and molecular beam the pressure 

This illustrates that optical pumping in molecular beams is a very efficient way to completely deplete a specified molecular level (vVjJV). This can be used to gain detailed information on velocity distributions in supersonic beams of molecules in defined quantum states and on relaxation processes, molecular formation, and collision processes in crossed molecular beams. 

The following is an example of one of the many possible applications of optical pumping to a time-of-flight spectrometer with velocity selection for molecules in defined quantum states in supersonic beams [10.241. 

The monochromatic laser beam is split into two beams 1 and 2, both of which cross the molecular beam perpendicularly but at different locations = A and 2 = 8 (see Fig.10.10). When the laser frequency is tuned to a molecular transition (vIj, JIj - V, J ), the molecules passing through the pump beam 1 are optically pumped and there are nearly no molecules left in the depleted level (vIjjJ . ). This means that the fluorescence excited by the probe beam 2 is very low. If the pump beam 1 is interrupted for a short time interval At (e.g., by a fast mechanical chopper) the molecules can pass during this interval At without being pumped. Because of their different velocities they reach the probe beam 2 at different times t = L/v, where L = 22 - The time-resolved fluorescence intensity induced by the cw probe beam therefore reflects the velocity distribution of molecules in the level (vV,JV). 

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Hefter, U., Ziegler, G., Mattheus, A., Fischer, A. and Bergmann, K. (1986). Preparation and detection of alignment with high m selectivity by saturated laser optical pumping in molecular beams, J. Chem. Phys., 85, 286-302. 

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