Molecular Beam Mass Spectroscopy

The MBMS system (195K gif)

We are investigating the gas phase chemistry that occurs during diamond CVD using a novel Molecular Beam Mass Spectrometer.

A Quadrupole Mass spectrometer (Hiden Analytical), samples gas from a hot filament reactor through a 100-µm-diameter orifice. The gas passes from the reaction chamber at 20 Torr into a differentially-pumped region at 1mTorr, and forms a molecular beam. This beam then enters another orifice which is the entrance to the mass spectrometer at 10-8 Torr. Thus, molecules which pass through both holes are then in a UHV region with a very large mean free path, and can travel to the detector without undergoing any collisions. This effectively freezes out any chemical reactions, allowing highly reactive species, such as hydrocarbon radicals, ions and atoms to be detected. Mechanically chopping the beam, and using phase sensitive detection allows increased sensitivity.

Recently we have done work studying standard CH4/H2 gas mixtures, as well as looking at the effect upon the chemistry, and the diamond film, of additions of chlorine- nitrogen and phosphorus-containing gases.

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