Knowledge Bank

A Nd: YAG laser is combined with a harmonic generator, pulsed dye laser and wavelength extender to produce a tunable output beam from 217 to 690 nm. The sample is passed through a pulsed valve into a vacuum chamber to form a supersonic jet which intersects the laser. The emitted fluorescence is collected using an ellipsoidal mirror and focused onto a photomultiplier tube. A computer is used to control the stepping of the laser frequency and the data collection from the boxcar which integates the pulsed fluorescence signal. In a supersonic jet, the excitation transitions originate almost exclusively from the vibrational ground state, thus simplifying the analysts. The high resolution of the laser allows us to tune through the rovibrational states to determine the vibrational potential energy surface in the electronic excited state. In preliminary work, we have recorded the fluorescence excitation spectrum of lodine and found a vibrational temperature of less than 50 K. Current work is underway on molecules with conformationally significant vibrations in the electronic excited state.