chemical physics of discharges - Argonne National Laboratory
chemical physics of discharges - Argonne National Laboratory chemical physics of discharges - Argonne National Laboratory
64 for the probe facing the cathode. We Cannot distinguish between (a) and (b), nor is it possible to calculate the density of such high energy electrons without knowledge of their velocities. The observed exponential decrease in ie min through the negative glow of pure neon (Fig. 4) can be written as: id = io exp(-a n(d-do)) I in which io is the residual current at do cm from the cathode, d is the distance of the probe surface from do and n is the concentration of neon atoms per unit volume. Thus a has the dimensions of cm2 and is an experimental cross section for loss of high energy electrons, as a first approximation. From the slope of the exponential curve in Fig. 4 we calculate that a = 8.7 x cm2. This is a reasonable value since the cross section for ionization of neonll by 100 eV electrons is 7.58 x cm2. The addition of 0.07% xenon brings about a 30% reduction in I the negative residual current and the exponential slope increases so that the experimental a = 1.03 x cm2. This cannot be accounted / for by assuming that the only additional loss is that ar sing from the ionization of xenon. At a concentration of only 7 x atom %, this would imply a cross section for ionization of xenon equal to 228 x 10-'6 , cm2 - about a factor of 40 higher than the known of xenonll for 100 eV electrons. A more reasonable inference is that the loss of neon metastables by the Penning reaction: Ne*(3P2,0) + Xe * Xe+ + Ne + e- (5) is responsible for the lower values of ie min, and that the change in slope more nearly reflects the actual stopping power of neon for high energy electrons. Since excitation as well as ionization can result from impact of a high energy electron, a cross section of the order of 1 x cm2 is of the expected magnitude for an electron of approx- imately 100 eV. It would be expected that the addition of a small amount of xenon would not significantly change either the cathode fall or the number of directed high energy electrons passing into the negative glow. Thus, if one assumes that the reduction in the residual electron current is caused by reaction (5) it is possible to obtain an approx- imate value for the Concentration of metastables in the beginning of the negative glow. Referring to Fig. 4, for d = 2 cm, -Ai, the reduction in the residual current is 0.02 VA. If the neon metastables have thermal velocities, and if one considers a reasonable yield of about 0.03 for the grid emission (geometric factor x efficiency), then a minimum concentration of 1 x 10i3/m3 is obtained for the metastables. It is then possible to calculate the reactive collision frequency between Ne* and Xe by assuming the same cross section for deactivation as Sholette and Muschlitz12 observed for the He* - Xe Penning reaction, namely 12 x cm2. This results in a calculated lifetime for the neon metastables of 0.5 m sec. This Is somewhat shorter than the lifetime of 0.875 m sec measured by Blevis et a1.13 for the decay of Ne(3P2) in pure neon. We conclude that xenon can intercept the metastable neon atoms in the required time and that our assignment of the Penning reaction is reasonably justified as a major contributing (4) I 1 i(
65 ' cause of the reduction in the residual negative current. Similar argUmentS can be used to account for the.decrease in the residual current when the probe faces the anode. The effect is even more ' marked in this case, particularly at the beginning of the positive column. The treatment of the positive ion residual current is not ' as straightforward, since there arises the additional complication 7 ,of electron impact ionization in the space between the grih and the collector. To a rough approximation we can account for about 60% of i' the current with the probe facing the cathode by this process, but { the remainder can be a combination of secondary emission from the ? collector by photons, by metastable atoms and by energetic electrons. ' The very small currents observed for the probe facing the anode suggest that the release of electrons by metastable impact is much less than 9 would be expected for the metastable density obtained above. Apparently i the impact of metastables with the platinum surface produces secondary electrons with a much lower efficiency than does the stainless steel 1 grid. The reason is not clear, but a contributing cause may be due to the higher work function of platinum. \ I 1 4 1. '~ 2. ' 3. ; 4. ~ 5. 6. ; 7. \ 1. 8 . t 9. REFERENCES N.R.C. No. ...., 'N.R.C. Postdoctorate Fellow. R.L.F. Boyd, Proc. Roy. SOC. m, 329 (1950). R.L.F. Boyd and N.D. Twiddy, Nature 173, 633 (1954). D.H. Pringle and W.E.J. Farvis, Proc. Phys. SOC. =, 836 (1955). P.F. Knewstubb and A.W. Tickner, J. Chem. Phys. 36, 674 (1962). P.F. Knewstubb and A.W. Tickner, J. Chem. Phys. 36, 684 (1962). I. Langmuir and H. Mott-Smith, Jr., G.E. Review 2J,449 (1924). H. Mott-Smith and I. Langmuir, Phys. Rev. a, 727 G. Medicus, J. Applied Phys. 27, 1242 (1956). N.D. Twiddy, Proc. Roy. SOC. E, 379 (1961). .;' 10. N.D. Twiddy, Proc. Roy. SOC. a, 338 (1963). (1956). i 12. W.,P. Sholette and E.E. Muschlitz, Jr., J. Chem. Phys. 36, 11. D. Rapp and P. Englander-Golden, J. Chem. Phys. '13, 1464 (1965). 3368 (1962). I i 13. B.C. Blevis, J.M. Anderson, and R.W. McKay, Can. J. Phys. 35, 941 (1957).
- Page 13 and 14: 13 field per electron is and per co
- Page 15 and 16: . 11. 5. CharRe-Transfer and Ion-Mo
- Page 17 and 18: 17 In the followin:: sections much
- Page 19 and 20: above. With that EIN, an estimate o
- Page 21 and 22: 21 region in w!iich large, nighlv l
- Page 23 and 24: i ’ understanding: 23 (a) Electro
- Page 25 and 26: Obviously, the secondary ion must h
- Page 27 and 28: 27 (22) Franklin, J. L., Munson, M.
- Page 29 and 30: Tables 1-6 present examples of rela
- Page 31 and 32: Table 8 Some Ions Formed by Process
- Page 33 and 34: 33 velocity of the reacting partn r
- Page 35 and 36: 35 must be added to the Langevin cr
- Page 37 and 38: 37 In our laboratory a microwave di
- Page 39 and 40: + . 4 . r 39 as well as N in a cor0
- Page 41 and 42: ION-MOIECULE REACTION RATES MEASUFI
- Page 43 and 44: 43 excitntion 2onditions so that th
- Page 45 and 46: 45 In 2 like manner Fig. 3, showing
- Page 47: 47 Absorption Spectra of Transient
- Page 50 and 51: 50 maximum fiela. In this manner, a
- Page 52 and 53: 52 3 % %- %- 5- a- 7 h W P H 0 L v)
- Page 54 and 55: References I I . A.B.Callear, J.A.G
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- Page 67 and 68: INTRODUCTION I' Attachment of polar
- Page 69 and 70: I 69 EXPERIMENTAL The mass spectrom
- Page 71 and 72: + Figure 1 Mixed water and methanol
- Page 73 and 74: 73 ' , radius might be expected bec
- Page 75 and 76: 75 Negative Ion Mass Spectra of Som
- Page 77 and 78: A b 1 1 I H I I I FILAMEN T CONTINU
- Page 79 and 80: 79 for positive and negative ions,
- Page 81 and 82: 51 out to answer some of the questi
- Page 83 and 84: 93 INTERACTIONS OF EXCITED SPECIES
- Page 85 and 86: L 4 I*C z 0'2 = a, a U x I m 4 m 0
- Page 87 and 88: I L I, ; > > E Fig. 3 I50 200 150 1
- Page 89 and 90: I 300 r 1 - 200 i io ;' a cn I I. 0
- Page 91 and 92: where DISCUSSION OF XESULTS PERTAIN
- Page 93 and 94: 93 where the bar indicates values c
- Page 95 and 96: 'I i 'I 0 2. 4 6 8 2 [ N]* x' I 0-2
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- Page 99 and 100: for chemiluminescent excitation in
- Page 101 and 102: 101 Chemiluminescent Reactions of E
- Page 103 and 104: 103 All gases were taken directly f
- Page 106 and 107: 10; He: + N2 -. 2He + h': (8) whi!?
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- Page 110 and 111: B. Ions and Electrons I Consider a
- Page 112 and 113: ' 112 axial diffusion through the d
65<br />
' cause <strong>of</strong> the reduction in the residual negative current. Similar<br />
argUmentS can be used to account for the.decrease in the residual<br />
current when the probe faces the anode. The effect is even more<br />
' marked in this case, particularly at the beginning <strong>of</strong> the positive<br />
column.<br />
The treatment <strong>of</strong> the positive ion residual current is not<br />
' as straightforward, since there arises the additional complication<br />
7 ,<strong>of</strong> electron impact ionization in the space between the grih and the<br />
collector. To a rough approximation we can account for about 60% <strong>of</strong><br />
i' the current with the probe facing the cathode by this process, but<br />
{ the remainder can be a combination <strong>of</strong> secondary emission from the<br />
? collector by photons, by metastable atoms and by energetic electrons.<br />
' The very small currents observed for the probe facing the anode suggest<br />
that the release <strong>of</strong> electrons by metastable impact is much less than<br />
9 would be expected for the metastable density obtained above. Apparently<br />
i the impact <strong>of</strong> metastables with the platinum surface produces secondary<br />
electrons with a much lower efficiency than does the stainless steel<br />
1 grid. The reason is not clear, but a contributing cause may be due to<br />
the higher work function <strong>of</strong> platinum.<br />
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I<br />
1<br />
4<br />
1.<br />
'~ 2.<br />
' 3.<br />
; 4.<br />
~ 5.<br />
6.<br />
; 7.<br />
\<br />
1. 8 .<br />
t 9.<br />
REFERENCES<br />
N.R.C. No. ...., 'N.R.C. Postdoctorate Fellow.<br />
R.L.F. Boyd, Proc. Roy. SOC. m, 329 (1950).<br />
R.L.F. Boyd and N.D. Twiddy, Nature 173, 633 (1954).<br />
D.H. Pringle and W.E.J. Farvis, Proc. Phys. SOC. =, 836 (1955).<br />
P.F. Knewstubb and A.W. Tickner, J. Chem. Phys. 36, 674 (1962).<br />
P.F. Knewstubb and A.W. Tickner, J. Chem. Phys. 36, 684 (1962).<br />
I. Langmuir and H. Mott-Smith, Jr., G.E. Review 2J,449 (1924).<br />
H. Mott-Smith and I. Langmuir, Phys. Rev. a, 727<br />
G. Medicus, J. Applied Phys. 27, 1242 (1956).<br />
N.D. Twiddy, Proc. Roy. SOC. E, 379 (1961).<br />
.;' 10. N.D. Twiddy, Proc. Roy. SOC. a,<br />
338 (1963).<br />
(1956).<br />
i 12. W.,P. Sholette and E.E. Muschlitz, Jr., J. Chem. Phys. 36,<br />
11. D. Rapp and P. Englander-Golden, J. Chem. Phys. '13, 1464 (1965).<br />
3368 (1962).<br />
I<br />
i<br />
13. B.C. Blevis, J.M. Anderson, and R.W. McKay, Can. J. Phys. 35,<br />
941 (1957).
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