Isotopic dependences of the dielectric strength of gases-new observations, classification, and possible origins

Abstract

It has been found previously that for both uniform and non-uniform fields, the direct current breakdown voltages, Vs, of H2 and NH3 exceed those of D2 and ND3, respectively, for positive polarity. In contrast, it was found that while for uniform fields CH4 and CD4 behave in a similar manner, the direct current breakdown voltage, Vs, of CD4 exceeds that of CH4 in non-uniform fields for positive polarity. The authors have now found that the CH4/CD4 non-uniform field behaviour is polarity dependent (i.e. the Vs of CD4 is lower than the Vs of CH4 for negative polarity). The origin of these observed isotope effects is related to changes in the effective ionisation coefficient, alpha /N, and to changes in the value of the secondary coefficient, gamma . Since the alpha /N changes are due to changes in either or both the ionisation, alpha /N, and the attachment, eta /N, coefficients, they have classified the isotopic pairs of molecules in three groups: non-electronegative (where only changes in alpha /N are important), weakly and moderately electronegative (where changes in alpha /N and/or eta /N are important), and strongly electronegative gases (where no isotopic dependence of Vs is expected). alpha /N depends on mass, especially when electron impact dissociation processes are significant and its isotopic dependence arises from the competition between dissociation into neutrals and auto-ionisation. eta /N depends on mass, especially when dissociative attachment proceeds via a negative-ion state which is attractive, and dissociative attachment is then strongly influenced by the time it takes for the dissociation fragments to separate as opposed to the time of auto-detachment. The latter led them to predict that the Vs of H2S should exceed the Vs of D2S since the dissociative attachment cross section is much larger for H2S than for D2S. This prediction has been confirmed by experiment. Basic data and Vs measurements in quasi-uniform and highly non-uniform fields for both positive and negative polarities suggest that the isotopic dependences of the Vs of gases are apparently of more general occurrence than hitherto recognised.