MOLECULAR COOLING AND SUPERSONIC JET FORMATION IN LASER-DESORPTION

A method has been developed to study molecular cooling during the laser desorption (LD) process. It involves the use of resonant two-photon ionization (R2PI) spectroscopy to examine the molecular population distribution among internal states of the molecules generated from LD. A continuous-flow probe is developed to introduce sample and matrix through a capillary tube and onto a stainless steel frit, upon which laser desorption is carried out with a CO2 laser. The sample molecules expand into the acceleration region of a reflectron time-of-flight mass spectrometer, where R2PI is performed with a tunable dye laser. It is demonstrated that small sample molecules are internally cooled during the gas expansion process in LD. The molecular cooling is believed to be the result of a supersonic jet expansion. It is further shown that the jet expansion process in LD is similar to that observed in a molecular beam experiment with a pulsed nozzle source. In addition, it is found that velocity distributions depend on the extent of the molecular cooling. Finally, the relation between the internal temperature and the translational temperature is examined.