New low background crossed molecular beam apparatus: Low background detection of H2

Jim J. Lin, Dennis W. Hwang, Steven Harich, Yuan T. Lee, Xueming Yang
1998 Review of Scientific Instruments  
A low background and almost hydrocarbon free (ϳ1ϫ10 Ϫ14 Torr) molecular beam apparatus with an improved universal detector, based on electron bombardment ionization, has been constructed for crossed molecular beam research. Extremely high vacuum (ϳ1ϫ10 Ϫ12 Torr) for the detector's ionization region is achieved using multiple ultrahigh vacuum pumps. In addition to a home-made liquid nitrogen cryopump and a turbomolecular pump, a two stage cryogenic He cold head ͑ϳ10 K͒ is used to pump the
more » ... r's ionization region. Using this arrangement, the H 2 background in the detector can be reduced by about two orders of magnitude in comparison with previously built similar instruments. Therefore, the signal-to-noise for detecting H 2 product detection sensitivity is substantially enhanced, making experimental studies of H 2 elimination channels in photodissociation processes much easier. Backgrounds at m/eϭ28 (CO ϩ ), 16 (CH 4 ϩ ,O ϩ ), 15 (CH 3 ϩ ), 14 (CH 2 ϩ ), and 13 (CH ϩ ) in the ionization detection region are also significantly reduced. In this article, we will describe the new apparatus in detail and present some preliminary results of the unimolecular decomposition studies of CH 3 OH at 157 nm excitation to demonstrate the capabilities of low background detection H 2 using this new instrument.
doi:10.1063/1.1148821 fatcat:xobpkg374vcipanr42w4q5mugi