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, theH2background 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 detectingH2product detection sensitivity is substantially enhanced, making experimental studies ofH2elimination channels in photodissociation processes much easier. Backgrounds atm/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 ofCH3OHat 157 nm excitation to demonstrate the capabilities of low background detectionH2using this new instrument. ©1998 American Institute of Physics.