Direct observation of collision induced transitions between the a 1Πg (v=0) and a′ 1∑−u (v=0) levels of N2 via double resonance enhanced multiphoton ionization spectroscopy

We report on the first direct observation of collision induced electronic transitions in a diatomic molecule using double resonance enhanced multiphoton ionization (REMPI) spectroscopy. The collision partner to induce these transitions in N2 between the electronic states, a 1Πg (v=0) and a′ 1∑u− (v=0), is helium. A pump laser selectively excites a rotational level of the a 1Πg (v=0) state by tuning the laser to a suitable line of the (2+2) a 1Πg–X 1∑g+ (0,0) REMPI spectrum and a probe laser records the collision induced transitions to the a′ 1∑u− (v=0) manifold by scanning the (1+1) REMPI x 1∑g−–a′ 1∑u− (1,0) band. Both pulsed lasers are coincident in time to optimize single collision effects. The results show that the collision induced electronic transitions over energy gaps of approximately 1100 to 1200 cm−1 are as efficient as rotational energy transfer and this technique can be used to access high lying electronic states of the molecule.