The S1 + S,-, electronic transitions of toluene involving also some internal rotational levels were observed for the firsr time in the multiphoton ionization spectrum in a supersonic jet. A large population in several low-lying internal rorational levels and a strong coupling between electronic motion and the internal rotation are suggested.
1 _ Introduction
The internal rotation of the methyl group of toluene has been an interesting subject for a long time. The levels associated with the internal rotation have never been observed by any means. In the electronic spectrum, there ti the possibility of appearance of transitions between the internal rotational levels of the ground and excited states. However such transitions are not observed in the room-temperature absorption spectrum. They might show up if all the hot bands are swept out.
Recently, it has been demonstrated by several workers [l-6]
that supersonic free expansion provides a good means of preparing samples of ultracold molecules in the gas phase. In addition, multiphoton ionization (MPI) spectroscopy had proved to be a powerful tool for obtaining the absorption spectrum of a gas al the very low pressure that exists in a supersonic jet 171. By using these two techniques, we were successful in observing electronic transitions involving internal rotational levels of toluene for the first time.
2. Experimental
The experimental apparatus is the same as that