UNIVERSAL CROSSED MOLECULAR-BEAMS APPARATUS WITH SYNCHROTRON PHOTOIONIZATION MASS-SPECTROMETRIC PRODUCT DETECTION

Vacuum ultraviolet radiation was generated from an undulator at the Ad vanced Light Source Synchrotron facility and used for photoionization detection of reaction products in a new universal crossed molecular be ams machine. A description of the machine and its performance is prese nted. Initial experiments on the photodissociation of methylamine (CH3 NH2), ozone (O-3), oxalyl chloride [(OCCl)(2)] as well as the reactive scattering of Cl with C3H8 show many of the advantages of photoioniza tion in comparison to electron impact ionization, which has been exclu sively used in such instruments in the past. ''Momentum matching'' of reaction products is much more easily accomplished than in electron im pact studies due to suppression of dissociative ionization. The tunabi lity of the vacuum ultraviolet radiation can be used to suppress backg round from residual gases especially when it is desired to detect free radical reaction products. Even when the tunability cannot be used to suppress background, the fact that little heat is generated by the io nizing beam allows background to be substantially suppressed by cryoge nic pumping. The energy resolution of the apparatus is comparable to i nstruments that have previously been designed with electron impact ion ization which have more than twice as long a flight path. This new ins trument provides outstanding performance for fundamental studies of ch emical dynamics. (C) 1997 American Institute of Physics.