NEW LOW-BACKGROUND CROSSED MOLECULAR-BEAM APPARATUS - LOW-BACKGROUND DETECTION OF H-2

A low background and almost hydrocarbon free (similar to 1 x 10(-14) T orr) molecular beam apparatus with an improved universal detector, bas ed on electron bombardment ionization, has been constructed for crosse d molecular beam research. Extremely high vacuum (similar to 1 x 10(-1 2) Torr) for the detector's ionization region is achieved using multip le ultrahigh vacuum pumps. In addition to a home-made liquid nitrogen cryopump and a turbomolecular pump, a two stage cryogenic He cold head (similar to 10 K) is used to pump the detector's ionization region. U sing this arrangement, the H-2 background in the detector can be reduc ed by about two orders of magnitude in comparison with previously buil t similar instruments. Therefore, the signal-to-noise for detecting H- 2 product detection sensitivity is substantially enhanced, making expe rimental studies of H-2 elimination channels in photodissociation proc esses much easier. Backgrounds at m/e = 28 (CO+), 16 (CH4+,O+), 15 (CH 3+), 14 (CH2+), and 13 (CH+) the ionization detection region are also significantly reduced. in this article, we will describe the new appar atus in detail and present some preliminary results of the unimolecula r decomposition studies of CH3OH at 157 nm excitation to demonstrate t he capabilities of low background detection H-2 using this new instrum ent. (C) 1998 American Institute of Physics.