The thermal energy reaction of atomic neon ions with molecular nitrogen

383 international

Q

Jburnal if Mass Spectromotry and Ion physics, 12

(1973) 383-388

EIsevier Scientific Publishifig Company, Amsterdam - Printed in The Netherlands

THE THERMAL MOLECULAR

N. G. ADAMS, Department

ENERGY

REACTION

OF ATOMIC

NEON

IONS WITH

NITROGEN

D. SMITH

AND

A. G. DEAN

of Space Research, Unittersity of Birmingham, Birmingham (E,zgIarzd)

(First received 23 May 1973; in final form 2 July 1973)

*

ABSTRACT

In a recent paper by Adams et al.‘, anomalous experimental resuIts for the reaction of Ne’ ions with N2 were presented which emphasized the discrepancy which exists between the rate coefficients previously determined for this reaction (refs. 2-4). A model which attempts to explain this anomalous behaviour in terms excited states (possible primary of the reverse reaction from Nz.+ electronically products of the reaction) was also proposed. More recently, Albritton et al.’ and Ferguson and Fehsenfeld 6 ;lave shown experimentally that the rate coefEcient of this reaction is extremely sensitive to the degree of vibrational excitation of the N2 molecule. Consequently we have reappraised our earlier data in the light of this additional information. To obtain the data, a stationary afterglow technique1 .was used, which involved the ionization of a sample of neon gas with a small admixture of nitrogen in a cylindrical Pyrex vessei by means of a 10 ps pulse of 10 MHz radiofrequency radiation capacitively coupled into the gas. The dominant ion produced by this ionization process was Ne’ which subsequently can decay by the following processes : Ambipolar Conversion:

Termolecular

diffusion: Nei

Ne+ +e -+ wall recombination

t2Ne

association:

Charge transfer:

(1)

+ Nezi +Ne Ne’ + N2 +Ne

Ne’ +vz(u

(2) + Ne

(3)

= 0, 1, 2 . . . ) -+ Nzf +- Ne.

(4)

Process (I) is an accurately calculable diffusion coefficient7. At the pressures

+ NeN,’

contribution from the known ambipolar used, (2) is a very small, accurately cal-

384 culable contribution using the known rate coefficients. The experimental results (see below) indicate that the Ne’ decay rate was clearly dependent on the N2 concentration but independent, to within error, of the Ne concentration. We therefore infer that (3) contributes insignificantly and that any change in the Ne’ decay rate on addition of Nz is solely due to process (4). The individual ion density decay rates in the plasma were determined after mass spectrometric analysis of the diffusive ion current passing through an orifice located in the walls of the plasma container. A detailed review of this technique is given by Smith and Plumb’. Since the only processes by which Ne+ ions are lost from the plasma are single mode, ambipolar difTusion to the walls of the containing vessel and twobody reaction with N,, the temporal variation of the Ne’ concentration would be expected to be described by the expression [Ne+]

= [Neo+ ] exp (-11)

where [Ne+] and me,‘] are the concentrations respectively. % is the exponential decay constant

of Ne+ at time f and time zero of Ne* and is given by

where R, describes the diffusive loss rate of Nei ions with electrons, k is the rate coefficient for the two-body reaction and w,] is the concentration of N2_ The diffusive loss of ions can take place in a series of modes but, in the stationary afterglow, the most slowly decaying mode - the fundamental mode - is finally established after which the decay process is perfectly exponential. For a constant gas pressure (in this case effectively neon only) this diffusive contribution is constant, being essentially independent of the small addition of Nz. Thus if R is obtained for a series of N, partial pressures, then a plot of % against [Nz] would be expected to yield a straight line of slope k with intercept %n if there is a constant value of k or a single reaction process (see later) as has been obtained for many similar reactions’= ’ O. The variation of A with [N,] for the reaction under discussion is not however of this simple form but is as illustrated in Fig. 1, I? increasing rapidly with [Nz] at small N, partial pressures and becoming almost independent of Ir\r,] at higher NZ pressures. A constant Ne pressure of 0.45 torr was used throughout. Additional measurements at a constant N, pressure of 3.4 mtorr with Ne pressures in the range 0.08 to 0.6 torr showed 3, to be independent of Ne pressure to within experimental error. If the variation of ). with [N2] is considered to be due to a variation in the rate coefficient k with N, pressure then, allowing for the constant diffusive loss of Ne+, values of the rate coefficient, k, ranging from about 1 x IO-” cm3 s-l at low N-, pressures to about 5 x lo-l3 cm3 s-l at an N, pressure of 45 mtorr are obtained as indicated by the solid lines in Fig. 1. Previous studies of the reaction of Ne’ ions with N, have yielded the widely divergent values for k of 2.9x lo-l2 cm3 s-l from the stationary afterglow measurements of -Msrk and

385 Ne+N, Ne

presswe

0.45 ton-

N2 pressure

(m;orr)

7320304059

Fig. I_ Variation of the decay constant, 2, for the loss of Ne“ as a function of N, reduced partial pressure for reactions occurring in Ne-N2 mixtures at 300 K. The solid lines are those used to determke the rate coefEcient, k, at several N2 pressures. The net rate coefficient deduced from the slope of each line is as indicated.

Oskam4 in Ne/N, mixtures and the upper limit of IO-l2 cm3 s-r previously obtained in the flowing afterglow by Ferguson et al2 and-by Hemsworth et aL3. The situation has been significantly clarified by the measurements of Albritton et al. (refs. 5, 6) which have shown that the rate coefficient of this reaction varies with the vibrational state of the ground state N2 molecule, and specifically that the rate coefficient is less than 10-l” cm3 s- 1 for the lowest level (LJ = 0) increasing to about 2 x 10-l’ cm3 s- ’ for u = 2. In the light of the reappraisal presented below, this effect is now thought to be more likely than that suggested in our previous publication’. In this reappraisal it is suggested that, although nitrogen was in all cases a minor neutral constituent, it controlled to a large extent the electron energy distribution in the radiofrequency discharge since there is a greater efficiency of electron energy transfer to the vibrational and rotational states of N, via inelastic collisions Hence the relative populations of the vibrathan to neon via elastic collisionsll. tional levels of the nitrogen ground electronic state would be expected to be nitrogen partial pressure dependent, higher vibrational levels being populated at the lower NZ partial pressures. However the net concentration of all vibrationally excited N2 molecules would increase as the N2 partial pressure increased, the lowest vibrational levels becoming relatively more highly populated_ Since we measure the net loss rate of Net ions then, AR, the reactive contribution to the decay constant A,, is given by

At =

c kDJJ, ”

where ?c” and

IN,],

refer to the loss rate coefficient

of Ne + ions with the oth

386 vibrational state of Nz and the concentration of molecules in that state respectively. Thus if, with increasing N, partial pressure, the rapidly increasing number of Nz molecules in low vibrational states is insufficient to compensate for the lower value of the rate coefficient for reaction with N, molecules in these states, then a much reduced net rate coefficient at the higher N, partial pressures would result. This can be seen to be in qualitative agreement with the variation of the experimentally determined net rate coefficients, examples of which are given by the slopes of the solid lines in Fig. 1. This model, however, relies on a quasiequilibrium distribution amongst the vibrational levels of the N, molecules during the afterglow time over which Ne $- ion semi-logarithmic decay curves were obtained, since it is only under this condition that the observed exponential decrease in Ne+ ion density over greater than an order of magnitude would be obtained. If this distribution is Boltzmann then, from the data of Albritton et a1.5*6, the variation of the net rate coefficient with vibrational temperature may be constructed and from this the vibrational temperature determined as a function of pressure for the present stationary afterglow measurements. This reduction in the variation is reproduced in Fi g_ 2 and shows the consequential vibrational temperature with increasing N, concentration. Ne+ -

N,

Ne

pressure 0.45 torr

1OCCJ

0

Fig.

I

2. Variation

pressure

I 1.0

I

I

210 N2

I

3.0

pressure

of the NZ vibrational

as deduced

from

,

(mtorr)

I

I

41)

-10

I

I

I

203040

I

3

temperature,

the data of Albritton

T “, as a function of N2 reduced partial results of Fig. 1. et al.5*6 and the experimental

It is interesting to note that, using the Langmuir probe technique developed in these laboratories12 to measure electron temperature in the afterglow, the electron temperature during the period of observation of the Ne’ decay rate was found to be, within error, equal to the gas temperature. This indicates that the anomalous behaviour is not explicable in terms of elevated electron or gas kinetic temperatures (close coupling exists between the electrons and the translational motions of the neutral gas)_ However, it does not exclude the possibility of vibra-

387 tionally excited N, being present in the afterglow since the theoretical estimates of Klimov and Krinber, ml3 obtained in part from experimental data have shown that the ener,g coupling between vibrationally excited N2 and electrons is small. Consequently electron temperature measurements wouId not be expected to reflect the vibrational temperature of the N, molecules. On the assumption that the result of Mark and Oskam is also high due to the presence of vibrationally excited N2 then their result is consistent with a vibrational temperature of about 1600 K. Since their measurements were made at N, partial pressures in the range 0.3 to 9.5 mtorr, this indicates that their ionization technique produces less vibrational excitation than the present radiofrequency discharge under the conditions of these measurements_ In addition, no features or trends are apparent in the data reported by MBrk and Oskam which could obviously be attributed to the involvement of vibrational excitation of Nz in the loss of Nef ions. However, one perhaps significant difference between the conditions of their measurements and the present measurements is the constant ratio of the neon to nitrogen partial pressures employed by Mgrk and Oskam. It is also of interest to compare this variation of vibrational temperature with the earlier stationary afterglow measurements of Copsey et al.‘” for the reaction of Oi with N, in which vibrational excitation of the IV2 also plays a vital role. Data on the vibrational temperature dependence of this reaction” has suggested that these stationary afterglow measurements were conducted at a vibrational temperature of approximately 1600 K. These measurements were conducted over a N2 partial pressure range similar to the present measurements but in the presence of O2 and He. It can be concluded therefore that the neutral species He, Ne, O2 and N2 and electrons in the concentrations used in the above experiments are ineffective in de-exciting vibrationally excited levels of the ground electronic state of nitrogen to temperatures less than 1000 K. This is a subject in which little supporting data is apparently available although some evidence is available which shows that de-excitation of vibrationally excited, electronically excited N,(A3C,‘, u = 0 to 7) by ground state Nz, O2 and HZ0 is inefficient16. It is also clear from these experiments that, in work involving discharges CORtaining nitrogen, considerable care must be taken to account for or attempt to avoid the effects of vibrational excitation and that the presence of this excitation cannot be readily detected by electron temperature measurements. This type of difficulty may, of course, be circumvented by the use of the flowing afterglow in which the neutral reactant gas is titrated into a flowing plasma and is thus not excited in an electrical discharge.

REFERENCES

1 N. G. ADAMS, A. G. DEAN AND D. SMITH, Int.J. Mass Specirom. Ion Phys., 10 (1972) 63. 2 E.E.FERGuso~~~,F.C.FEHSENFELDA~A. L.SCHMELTEKOPF,MCYUZ. C'hem.Ser.,80(1969)83.

388 3 R. S. HEMSWORTH, R. C. BOLDEN, M. J. SHA~V AND N. D. 237. 4 T. D. MKRK AND

K.

J. OSKAM, 2. P&s.,

247 (1971)

TWIDDY,

Chem. Phys. L&r., 5 i1970)

84.

5 D. L. ALBRITTON, Y. A. BUSH, F. C. FEKSENFELD, E. 6. FERGUSON, T. R. GOVE-,

M. MCFAR-

LAND AND A. L. SCH.*LTEKopp, 25th Gaseous Electronics Conference, University Ontario. London, Ontario (October 1972). Aiso J. Chenz. P&s., 58 (1973) 4037.

of Western

6 E. E. FERGUSON A?XI F. C. FEHSENFELD, personal communication, 1972. 7 D. Sarr~~, A. G. DEAN AE~DN. G. ADAM% Z. P&x_, 253 (1972) 191_

8 D. SMITH AND P. R. CROMEY, J. Phys. B, 1 (1968) 638. 9 D. SMITH AND 1. C. PLUMB, J. Phys. D, 6 (1973) 1431.

10 D. SMITH AND R. A. FOURACRE, Planet. Space Sci., 16 (1968) 243. 11 H. S. W. MASSEY, Electronic and Ionic Impact Phenomena, Vol. 2; Clarendon 1969, Chap. II. 12 A. G. DEAN, D. SMITH AND I. C. PLUMB, J. Phys. E, 5 (1972) 776. 13 N. N, KLlhfoV AND 1. A. KRINBERG, Geomagn. Aeron., 10 (1970) 420.

Press, Oxford,

14 M. J. COPSEY, D. S,MI~ AND J. SAYERS, Planet. Space Sci., 14 (1966) 1047. 15 A. L. ScHhw.-r%opp, E. E. FERGUSON AND F. C. FEHSENFELD, J. Chem. Phyz, 4S (1968) 2966. 16 J. W. DREYER AND D. PERNER, J. Cftelrr. Phys., 58 (!973) 1195.