OBSERVATION OF COLLISION-INDUCED CENTER DIPS IN LAMB DIP STARK SPECTROSCOPY

Abstract

While observing laser Stark Lamb dips for vibration-rotation transitions which are nearly coincident with infrared laser lines, we have observed extra dips midway between the dips associated with successive M components. These extra dips have been observed for a transition in the $2\nu_{2}$ overtone band of $NH_{3}$ In the 5$\mu$ region using a CO laser and for five transitions in the $\nu_{3}$ fundamental bands of $CH_{3}F$ and $^{13}CH_{3}F$ in the 10$\mu$ region using a $CO_{2}$ laser. They are similar to those observed by H\”ansch, Shahin and $Schawlow^{1}$ in that they appear midway between normal Lamb dips and that they are due to molecules with a non zero velocity component along the direction of the laser beam. However, in our experiments, the Stark shifted energy levels constitute a four level system and the saturating transition does not affect the other transition directly. We believe that the extra dips observed in such a system are caused by efficient collision induced transitions between different M components, without change of $velocity.^{2}$