AVS 52nd International Symposium
    Vacuum Technology Tuesday Sessions
       Session VT-TuP

Paper VT-TuP8
Pumping Characteristics of Activated Alumina for Ortho- and Para-Hydrogen Molecules

Tuesday, November 1, 2005, 4:00 pm, Room Exhibit Hall C&D

Session: Vacuum Technology Poster Session
Presenter: T. Okano, University of Tokyo, Japan
Authors: T. Okano, University of Tokyo, Japan
K. Niki, University of Tokyo, Japan
K. Yoshida, University of Tokyo, Japan
T. Ito, University of Tokyo, Japan
K. Fukutani, University of Tokyo, Japan
Correspondent: Click to Email
Hydrogen is the most dominant gas species in ultrahigh vacuum. With respect to the nuclear spin state of hydrogen molecules, the triplet nuclear spin states are called ortho hydrogen (o-H@sub 2@) and the singlet ones are called para hydrogen (p-H@sub 2@). Since the spontaneous transition between a triplet and a singlet nuclear spin state is very small in gas phase, they are considered to be two distinct molecular species. In the present study, we studied on the behavior of o-H@sub 2@ and p-H@sub 2@ in a vacuum chamber. The apparatus consisted of a stainless steal chamber equipped with a mechanical compressor type cryohead. The lowest attainable temperature of the cryohead was 6 K. A small vessel (diameter: 10 mm, depth: 2 mm) was attached on the end of the cryohead and was filled with granules of activated alumina. The state-selective measurement of o-H@sub 2@ and p-H@sub 2@ was made by (2+1) resonant-enhanced multiphoton ionization (REMPI) method. Due to the requirement of antisymmetry of the total wave function of hydrogen molecules, even rotational state J is only allowed for o-H@sub 2@ and odd J-state for o-H@sub 2@. The J-state selective measurement by REMPI made it possible to detect o-H@sub 2@ and p-H@sub 2@ with very high sensitivity. The REMPI laser pulses with a wavelength of 201nm and an energy of 0.4 mJ/pulse is focused at the center of a radiation shield surrounding the activated alumina vessel. The ionized H2 is detected by a microchannnel plate attached to the radiation shield. After admitting hydrogen up to 2x10@super â?"4@ Pa, we started to cool down the temperature of the activated alumina. The densities of o-H@sub 2@ and p-H@sub 2@ in the chamber decreased rapidly at a temperature of 29 K and 25 K, respectively. This deference in the pump-down behavior was ascribed to the deference in the activation energy of desorption of ortho and para H@sub 2@.