An experimental study of collisional transfer of electronic excitation,collisional dissociation of excited molecules and photodissociation in alkali vapour

A single-frequency laser is used to excite Na₂ molecules to the electronic B state. Besides the molecular fluorescence also atomic Na resonance radiation is observed. This is caused by: collisional transfer of electronic excitation from a Na₂(B) molecule to a Na atom, collisional dissociation of Na₂(B) molecules and photodissociation of Na₂ from very high vibration—rotation (v. J) levels of the ground state. We show how the contributions of these processes can be separated experimentally and characterized quantitativily over a wide range of temperatures, using a free-jet expansion. Illustrative results are given for one laser frequency (i.e. one molecular transition). Effective collision cross sections for excitation transfer and for collisional dissociation are given. The probability of photodissociation is compared to the probability of the discrete (B ← X) transition. The relaxation of the number of molecules in high (v. J) levels in a free jet is obtained.