Measurement of Inelastic Collisions by LIF

Assume that a vibrational-rotational level (V, J ) in an excited electronic state of a molecule has been selectively populated by optical pumping. If the excited molecules (density N ) suffer inelastic collisions with other atoms or molecules (density Ng) before they emit a fluorescence photon, they are 

The rate equation for the depopulation of level (V jJ ) after the end of the optical pump pulse is given by 

The collision rate is related to the integral cross section by 

Here is an average cross section and v is the mean relative velocity 

At sufficiently low pressures we can use the ideal gas law to relate the density Ng with the pressure p 

---