Dimer and tetramer formation in an AsH3 cracker studied by calibrated quadrupole mass spectrometry

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Abstract
The reaction of AsH3 in a gas source with a tungsten filament (W-GS) was studied. The reaction products were detected by a quadrupole mass spectrometer and measured as a function of filament temperature and feed pressure. The mass dependence of the transmission of the quadrupole system was optimized and calibrated using a mixture of five noble gases. The results were corrected for ionization efficiency and cracking pattern. Background and noise effects were reduced by beam modulation. Conditions for buildup of an arsenic deposit within the gas source are discussed. Maximum As2 and As4 production was observed at 800 and 630 °C, respectively. The gas source which decomposes the hydride at a tungsten wire is compared with a second one which decomposes the hydride at a tantalum wire (Ta-GS). Operating conditions that yield predominantly arsenic dimers are achieved only with the tungsten wire source. Chemical reasons are discussed to explain the higher dimer yield of the tungsten source.

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