A photoionization study of vibrational cooling in molecular beams

The efficiency of cooling vibrational modes for a variety of molecules in a seeded supersonic expansion has been investigated at several backing pressures from 100 to 800 Torr. The internal energy removed by the expansion was monitored by comparing the dissociative ionization onset in a photoelectron photoion coincidence experiment. While the results do not permit simple generalizations, they do show increasing cooling efficiency with increasing size of the molecule and with increasing backing pressure. At 400 Torr backing pressure, the percent of the internal energy removed for methyl formate, 2-propanol, and n-pentane was 50%, 80%, and 92%. Furthermore, a heated nozzle experiment showed that the percent energy removed by the expansion remains constant.