[Introduction]Source: Philosophical Transactions of the Royal Society of London. Series A, Mathematical andPhysical Sciences, Vol. 259, No. 1100 (Apr. 21, 1966), pp. 299-301Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/73381 .

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[ 299 ]

THE MOTION OF SLOW POSITIVE IONS IN GASES

I. CRITICAL REVIEW

BY P. G. DAVIES,* J. DUTTON, F. LLEWELLYN JONES AND E. M. WILLIAMS

II. MOBILITIES OF POTASSIUM AND NITROGEN IONS IN NITROGEN

BY P. G. DAVIES,* J. DUTTON AND F. LLEWELLYN JONES

III. MOBILITIES OF IONS IN ARGON

BY P. G. DAVIES*, J. DUTTON, F. LLEWELLYN JONES AND J. A. REESt

IV. DRIFT AND DIFFUSION OF IONS IN HYDROGEN

By J. DUTTON, F. LLEWELLYN JONES, W. D. REES+ AND E. M. WILLIAMS Department of Physics, University College of Swansea

(Communicated by Sir George Thomson, F.R.S.-Received 15 December 1964- Revised 3 August 1965)

CONTENTS

PAGE PAGE

I. CRITICAL REVIEW (ii) Microwave methods 310 1. INTRODUCTION 302 (iii) Pulse methods 311

2. PREVIOUS THEORETICAL STUDIES OF (iv) Other methods 312 MOBILITIES 302 (b) Measurements of diffusion coefficient

(a) Forces of interaction and energy and mean energy 313 distribution 302 4. DATA FOR MOBILITIES 313

(b) Ionic mobility at low values of E/po 304 (a) The variation of mobility with tem-

(i) The Langevin theory 304 perature and E/po for ions in their

(ii) Modification of the Langevin theory parent monatomic gases 313

for atomic ions in their parent (i) Atomic ions 313 monatomic gas 306 (ii) Molecular ions 316

(iii) Quantum mechanical calculations 306 (b) The variation of mobility with tem-

(c) Ionic mobility at high values of E/po 308 perature and E/po for nitrogen,

3. PREVIOUS EXPERIMENTAL INVESTIGATIONS 309 oxygen and hydrogen ions in their

(a) Experimental methods for the parent gases 316

measurement of mobilities 309 (c) Mobility of alkali ions in gases 319

(i) Electrical shutter methods 309 5. CONCLUSIONS 320

* Now at the Science Research Council, Radio and Space :Research Station, Slough.

5- Now at the Department of Electrical Engineering and Electronics, University of Liverpool. +. Now at the B.P. Research Centre, Sunlbury-onl-Thames.

VoL. 259. A. II00. (Price ?1. IS.; $3.15) 37 [Published 21 April I966

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300 P. G. DAVIES AND OTHERS

PAGE PAGE

IIe MOBILITIES OF POTASSIUM 3. INTERPRETATION OF PRESENT MOBILITY

AND NITROGEN IONS IN NITROGEN MEASUREMENTS IN ARGON 336

1. INTRODUCTION 321 4. CONCLUSIONS 338

2. DESCRIPTION OF APPARATUS AND EXPERI-

MENTAL TECHNIQUES 321 IV. DRIFT AND DIFFUSION OF

(a) The electrode system 321 IONS IN HYDROGEN

(b) The current measuring and gas L. INTRODUCTION 339 systems 32 3

2. DESCRIPTION OF APPARATUS 339 (c) Operation of the electrical shutters 323

(i) The out of phase method of pulsing 323 (a) Electrode system 339

(ii) The in phase method of.pulsing 323 (b) Vacuum and current measuring

(d) Operation of the apparatus 324 systems 340

3. RESULTS FOR POTASSIUM IONS IN NITROGEN 325 :3 OPERATION OF THE APPARATUS 340

4. RESULTS FOR NITROGEN IONS IN NITROGEN 327 (a) The drift section 340

(a) Present results 327 (b) The diffusion section 341

(b) Comparison of the present measure- 4. DETERMINATION OF W/D FROM THE

ments with the results of previous MEASURED RATIO OF CURRENTS 342

investigations 328 U. RESULTS AND DISCUSSION 344

(c) Discussion 329 (a) Measured values of current ratios 344

III. MOBILITIES OF IONS IN ARGON (b) Experimental and theoretical ratios

1. POTASSIUM IONS IN ARGON 331 for thermal ions 345

2. IONS FROM A GLOW DISCHARGE SOURCE IN (c) Values of D1Lk and e 347

ARGON 332 (d) Values of Dpo and cross-sections 348

(a) First set 332 (e) Mobilities 349

(b) Second set 332 (f) Ion identity 350

(c) Third set 334 REFERENCES 351

In part I the theoretical and experimental metl-hods for the study of low energy positive ions in gases are reviewed and the available data for ions in atomic and common molecular gases are summarized.

It is shown that a large number of investigations of mobility X have been made, and that at low values of E/po (E the electric field, p0 the gas pressure under standard conditions), there is satisfactory quantitative agreement between theory and experiment for alkali ions in atomic and diatomic gases and for atomic ions in their parent monatomic gases. For all other cases and at high values of E/po, the situation is far less clear and much work remains to be done. In particular, further experimental measurements of mobility are required in which the ion species on which the observations are made are identified directly, and more theoretical quantum mechani- cal computations of mobility, especially when charge transfer occurs would be of interest.

As far as other quantities related to the motion of slow ions such as the diffusion coefficient D, the mean energy 6 and the collision cross-section Q are concerned, the review shows that there is a dearth of knowledge, and experimental determinations of these quantities would be of great value.

In parts II, III and IV an account is given of an experimental investigation of the motion of slow positive ions in nitrogen, argon and hydrogen. The principles of operation of an apparatus for the simultaneous measurement of YX and D for ions are first discussed. The basic feature of this apparatus was the combination of a shutter-type electrode system, similar to that used by Tyndall & Powell (I1930) for the measuremnent of with an electrode system similar to that used by Townsend (Iu925) for the measuremnent of the ratio D/Y? for electrons. In t;he first practical construction of this arrangement it was found that large spurious currents to the ion collector of the diffusion section prevented mneasurements of diffusion, but that accurate measurements of mobility could be

mnade.

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THE MOTION OF SLOW POSITIVE IONS IN GASES 301

This first apparatus was thus used to obtain results for the mobility of ions in nitrogen and argon and these results are discussed in parts II and III, respectively. The apparatus was cali- brated by determining the mobility of potassium ions in nitrogen, since the zero-field mobility of these ions in this gas is well established; SCwas found to remain constant at its zero-field value of 2i55 cm2 V-1 s-I over the range of E/po from 6 to 44 V cm-' mmHg-1. Measurements on the mobility of ions extracted from a glow discharge in nitrogen showed that there was a single ion species present, the value of X' for which remained constant at 2-5 cm2 V-1 s-I over the range of E/po from 4 to 42 V cm-1 mmHg-'. Consideration of mass spectrometric evidence and com- parison of the results with other recent data led to the conclusion that this value of X probably referred to N+ ions.

In argon the mobility of potassium ions was found to remain constant at 2-75 cm2 V-1 s-I for 8 < E/po < 28 V cm-' mmHg-'. When we used as an ion source a glow discharge in argon, which was shown by mass spectrometric analysis to contain small quantities of hydrogen, two ions having zero-field mobilities of 2-9 and 2-05 cm2 V-1 s-I were observed. Analysis of the experi- mental data led to the conclusion that the slower species was formed from the faster species after a sufficient number of collisions with gas atoms. In view of the presence of hydrogen in the argon sample the faster ion with mobility 2-9 cm2 V-1 s-1 was considered to be ArH+, but the identity of the slower ion was uncertain.

In part IV a second redesigned experimental arrangement which successfully eliminated the spurious currents to the ion collector of the diffusion section is described. With this second apparatus measurements of both X and D/JY were obtained for ions extracted from a glow dis- charge in hydrogen. A single ion species with zero-field mobility 10X8 cm2 V-1 S was observed. For E/po < 10 V cm-' mmHg- the mobility remained constant at 10.8 cm2 V-1 s-I and the ratio D/Mremained constant at 0X025 V. For higher values of E/po, S-and the ratio D/l" increased and for E/Po > 25 V cm-1 mmHg-' the ratio D/MP was found to be linearly dependent on E/p0. The results were analysed to give values of D, of the ratio of the mean energy of the ions to that of the gas molecules e, and of the collisional cross-section Q.

The results for e showed that the ions remained in thermal equilibrium with the gas molecules for E/po < 10 V cm-1 mmHg-', but that at higher values of E/po the energy began to increase. As the energy increased, Q decreased rapidly and the mobility increased, both of which are consistent with the assumption of an ion species which undergoes a dissociative reaction with the gas molecules at energies slightly greater than gas kinetic. Although no direct identification of the ions was possible, the above observations were consistent with the identification of the species with zero-field mobility of 10.8 cm2 V-1 s-I as H+, which has been observed in recent mass spec- trometric analyses of ions produced in hydrogen discharges.

37-2

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