The Momentum Transfer Cross Section for Electrons in Helium Derived from Drift Velocities at 77°K

Self Cite

The Bradbury-Nielsen time-of-f1ight method has been used to measure the mobilities of six species of negative ions in oxygen at 293 K for gas pressures within the range 1· 3-22 torr and E/N values within the range 3-130 Td. The species were identified as 0-, 0;, 0;;-, 0;, CO;;-and CO; by the addition of known quantities of selected contaminants and by the removal of condensible impurities via a liquid nitrogen trap. The present mobility data for 0-, 0;, 0;, CO; and CO; agree with those of Snuggs et at. (1971) to within the combined limits of experimental error over the common range of E/N values. However, the present data for 0; are in significant disagreement with those of Snuggs et at. for this ion. The zero field reduced mobilities (cm2 S-1 V-I) were estimated to be 2·55±0·03 for 0;,2· 30±0·02 for 0';:-,2·51 ±0·03 for CO;, 2·38±0·02 for CO; and 3 ·01 ±0·04 for a species of unknown identity. Previous measurements of the mobility of negative ions in oxygen, made without direct mass identification, have been examined and the identifications of the ions are discussed.

Section: Experimental Apparatusmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Mobility of Negative Ions in Oxygen (293 K)

Elford¹,

Rees²

1974

Self Cite

“…(2) The discrepancy found by Crompton, Elford, and Robertson (1970) in the region v --+ ° between the integrands appearing in the evaluations of W{ and W; most probably arises more from the shortcomings of the assumption (10) than from the inadequacy of the Legendre truncation technique. (3) In the region v --+ 0, the condition of validity of approximation (10) Sesta's (1968, 1969) integral equation for g (v).…”

Section: An Examplementioning

confidence: 99%

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mentioning

confidence: 99%

On the Drift Velocity of Electrons in a Gas

Paveri-Fontana¹

1972

In a recent paper, Crompton, Elford, and Robertson (1970; hereafter referred to as CER) considered certain questions concerning the steady-state distribution of electrons moving in a neutral gas under the influence of a uniform electric field E. The present communication comments on some aspects of the error discussion in the Appendix of the paper by CER. The analysis will be restricted to the case of isotropic scattering. Two different definitions of the drift velocity are considered by CER, namely( 1) andwhere f (v) is the conventional distribution function, g( v) is the distribution function immediately after collision, andare "path integral" expectation values. In equations (3) and (4) a = -I e/me 1 E, (v)/T -f(V)/T(V).

“…In view of the accuracy of present day experiments (Crompton, Elford, and Robertson 1970), it is of interest to estimate the importance ofj2,j3, .... A systematic method of approximating these terms is presented here and it is shown that they are insignificant. The amount of numerical work required increases with the field strength, but the structure of the formulae and the results for low fields, which are presented below, suggest that for the electrons the two-term approximation remains valid for all values of field strength.…”

Section: Introductionmentioning

confidence: 99%

On the Validity of the Two-term Approximation of the Electron Distribution Function

Robson¹,

Kumar²

1971

The Boltzmann equation for electrons moving in a neutral gas under the influence of an externally applied field is solved by expanding the electron distribution function in terms of Legendre and Sonine polynomials. The solution is given in terms of infinite matrices which have elements ordered by the Sonine polynomial index, and which are dependent upon the field strength. From the structure of the formulae, it is possible to infer that truncation of the Legendre polynomial expansion after two terms is a good approximation at all field strengths. This is supported by calculations of the electron drift velocity at low field strengths, which show that the error introduced by making the two-term approximation is small, even when the deviation from equilibrium is significant. The convergence of the Sonine polynomial expansion is shown to be strongly depende:r;J.t upon field strength, and large matrices are required in the drift velocity formula at even small field strengths.