FORMATION AND DESTRUCTION OF NEON MOLECULAR IONS IN TOWNSEND DISCHARGES

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FORMATION AND DESTRUCTION OF NEON MOLECULAR IONS IN TOWNSEND DISCHARGES

P.A.M. van der Kraan, J. Dielis, F.J. de Hoog

To cite this version:

P.A.M. van der Kraan, J. Dielis, F.J. de Hoog. FORMATION AND DESTRUCTION OF NEON

MOLECULAR IONS IN TOWNSEND DISCHARGES. Journal de Physique Colloques, 1979, 40 (C7),

pp.C7-15-C7-16. �10.1051/jphyscol:1979707�. �jpa-00219066�

JOURNAL DE PHYSIQUE CoZloque C7, suppZ&ment au n07, Tome 40, JuiZZet 1979, page C7- 15

-FORMATION AND DESTRUCTION OF NEON MOLECULAR IONS IN TOWNSEND DISCHARGES

P.A.M. Van Der Kraan, J.W.H. Dielis, F.J. de Hoog.

Eindhoven University of Technology, Eindhoven, the Netherlands.

+ +

As a part of a more comprehensive mass spectrometric here v , v2 and v- are drift velocities of atomic

-

study of Townsend dischargeswe have investigated the resp. molecular ions and electrons; The primary termolecular association reaction of atomic neon ionization coefficient a is split in a contribution ions in their parent gas as well as the loss of the from direct ionization a , and from associative ioni- resulting molecular neon ions in dissociative colli- zation a2; kc and k are rate coefficients for the

d -

sions under var,ious discharge conditions. Both reac- conversion and the dissociation reaction; n stands tions can be represented by the equation for the electron density.

-f +'

~ e + + 2Ne t ~ e + ~ Ne + Using the boundary conditions n+(d) = n2 (d) = 0 at the anode, we can calculate the dependency of the Molecular ions are also formed by associative ioni-

ion currents at the cathode on the electrode dis- zation from highly excited states of the neon atom

tance d.

1 1 1 - A general trend for the reduced ion current density

In this contribution we will describe the discharge

i.e. the cathode ion current density divided by the model used in the evaluation of the measurements.

discharge current densiry, is chat for smaii elec- With this model we were able to determine values

trode distances, where formation of ions is dominant .for the rate constants of processes (I) at various

with respect to collisional destruction, it increa- values of the reduced field strength. Finally from

ses with measured values of the dissociation rate at

1 - exp(-ad).

different swarm energies a value for the dissocia- tion energy of ~ e was found. ~ +

Discharge model. Ion sampling from Townsend dis- charges between flat parallel electrodes at current densities lower than A/cm has the advantage 2

that the discharge can be described by.a simple mo- del. Cumulative processes can be ruled out and only processes in which ground state atoms are involved are relevant. Space charge effects are not present.

Ion sampling is therefore not hampered by space charge shielding around the sampling hole. As long as the gas density and the reduced field strength are constant, the transmission of the sampling hole for a specific ion is the same. In our model we assume that the diffusion of ions in the field di- rection (represented by the coordinate x), can be neglected with respect to the drift. The atomic resp. molecular ion density n+ resp. n2+ obey

For larger distances the probability for collisional destruction of an ion increases and the reduced ion current dgcreases.

Experiment. The experiment was carried out in a stainle,&s steel vessel where between a flat, gold plated cathode containing the 100 urn diameter sam- pling'hole and a quartzanode covered with tin oxide a non-selfsustaining discharge was maintained.

The usual ultra-high vacuum procedures required in ion collision studies were followed. Ion selection and detection took place with a quadrupole mass spectrometer.

In Fig. 1 the reduced ion current of neon ions vs.

electrode distance at an E/N of 30.5 Td. and a pressure of 2.33 kPa is shown. From these data it was possible to evaluate the termolecular associa-

tion reaction coefficient k by fitting the experi- d

mental points with the solution of (2). The coeffi-

+ dn+ - - ^{ax 2} +

-v - ⁼ a v n (o)e + kdnoq2+ - ^{kcno n} (2a) cient kd has been determined at values of E/N dx I

ranging from 9 Td.up to 30 Td.and at pressures from 2 +

+

* ^{= a ,;n-(o) eax - kdn0n2+ + kc\ n (2b)} 2.0 kPa up to 4.0 kPa. At these values of E/N the -Y2 dx 2

3

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979707

dissociative reaction could be neglected. Values for Based upon the assumption that our consideration on a2 were taken from our own experiments on associati- the population of the vibrational levels will be ve ionization. Drift velocities and values for a less valid at higher swarm energies, also a,fit was were taken from 121,131 and 14 1. The values of kd made for values of kd at E/N up to 120 Td. In this obtained did not show a variation with E/N. In this case a value of 1.1 5 .I eV for the depth of the interval the value was (.46+.04) . I O - ~ ~ ~ ~ S - ! This is potential well of the Ne +-molecule was found.

2 in excellent agreement with values obtained from

other experiments 1 ² 1 ,I 7 1 . 8 The reduced molecular ion current was measured as a

function of d at pressures ranging from 270 Pa up _jf 6 to 930 Pa and E/N-values from 48 Td.up to 245 Td. a.u.

A typical plot of this parameter vs. d is shown in 4

Fig. 2. The value of k used in the analysis was 2 from our own experiment. The values of the dissocia-

tion rate kd, again obtained by least squares fit- 0

1 2

ting of the experimental points to the solution of ³

d(cm) 121 are plotted in Fig. 3 as a function of the ion

Fig.1 Reduced atomic i o n current d e n s i t y a t swarm energy W. This energy was taken from the t h e cathode vs.etectrode distance.

drif tvelocity 121 of molecular ions using Wannier's I I

states should be taken into account. This because Fig.2.Reduced moZecuZar i o n current d e n s i t y the swarm energy may be well above the energy a t t h e cathode vs.eZectrode distance.

difference between the vibrational states. From the 7

expression 151.

jl 5 The dissociation energy. Since very few experimental

4 data on the dissociation energy of neon molecular

ions are known, it is worthwile to determine a value a 3 . for this energy from the behaviour of kd as shown

in Fig. 3. As a first approximation we therefore 2 equate the swarm energy W with a kinetic temperature

T as if the distribution of translational energy of ^I the molecular ions is Maxwellian. In our case also 0

F*..

1 ^{)_\ 0.0}

-

E/N=152 Td

- i p = 4 0 0 ~ a

I I I

the summation is truncated at a vibrational level i o n s vs.the i o n swam energy the distribution of the molecules in the vibrational ^{. 5} d(cm) ¹

measured values of kd we can determine that for E/N

-

up to 215 Td. the collision frequency for dissocia- I

m200 tion is at least one order of magnitudesmaller than _m the collision frequency for elastic collisions. So - I

0 -

we may assume that a distribution of vibrational -10 states reflecting the kinetic temperature T is pre- kd sent. With help from data of Cohen and Schneider 161

we are now able to find that ₁

0.01 eV under the dissociation limit. References:

The dissociation energy found by fitting (3) to the experimental points is 1.4 2 0.2 eV. One should note that the experimental points at higher s w a m energies show deviations from the theoretical model.

+ +

-

,{' t *

_ $4

I

Z ( 1 + (D-EV) /kT) ^.I5 .50

1.'5 kd % (kT) 'I2. exp (-DI~T) . _{p x p (-Ev/kT) '} ^{(3) W(ev)}

Fig.3 The d i s s o c i a t i o n r a t e o f molecuZar neon