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

Abstract

A low background and almost hydrocarbon free (∼1×10−14 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 (∼1×10−12 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 (∼10 K) is used to pump the detector’s ionization region. Using this arrangement, the H2 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 H2 product detection sensitivity is substantially enhanced, making experimental studies of H2 elimination channels in photodissociation processes much easier. Backgrounds at m/e=28 (CO+), 16 (CH4+,O+), 15 (CH3+), 14 (CH2+), and 13 (CH+) 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 CH3OH at 157 nm excitation to demonstrate the capabilities of low background detection H2 using this new instrument.