Approved for public release by the ^

Director of the National Institute of , & o. Standards and Technology (NIST) / on October 9, 201 5.

NATIONAL BUREAU OF STANDARDS REPORT

7381

THE ATTACHMENT OF LOW-ENERGY ELECTRONS

BY

HALOGENATED HYDROCARBONS AND OTHER GASES

by

Paul D. Burrow

Participant in

Junior Scientist and Engineer Programs

1961

and

E. C. Creitz

<nb|>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent

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in the journals of professional and scientific societies. The Bureau itself publishes three periodicals avail- able from the Government Printing Office: The Journal of Research, published in four separate sections,

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July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) (S2. 25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-11-10121 November 15, 1961 7381

THE ATTACHMENT OF LOW-ENERGY ELECTRONS

BY

HALOGENATED HYDROCARBONS AND OTHER GASES

by

Paul D. Burrow

and

E. C. Creitz

IMPORTANT NOTICE- NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documents

intended for use within the Government, Before material in the reports is formally published it is subjected

to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or open-literature

listing of this Report, either in whole or in part, is not authorised unless permission is obtained in writing from

the Office of the Director, National Bureau of Standards. Washington 25, D. C. Such permission is not needed,

however, by the Government agency for which the Report has been specifically prepared if that agency wishes

to reproduce additional copies for its own use. <NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The Attachment of Low-Energy Electrons

by

Halogenated Hydrocarbons and Other Gases

by

Paul D. Burrow and E. C. Creitz

ABSTRACT

A simple method for measuring the attachment of electrons to gas molecules is described.

The method measures the total amount of attachment without differentiating between simple

resonant capture and dissociative resonance

attachment and yields no information on the energies of the electrons which are attached. The

results of measurements on eleven gases are given.

1. Introduction

It has been proposed [l] that inhibition of flame reactions

by halogenated hydrocarbons may be connected, in some obscure

way, with dissociative resonance attachment of the electrons

found in flames. The apparatus described was devised as a means of effecting a rough screening of compounds known to be

flame inhibitors to find out if there are any such compounds

which do not fall into the general class of electron' attaching

gases. Such a screening was thought to be desirable as a preliminary to the very difficult mass-spectrometric determination

of the actural attachment process.

2. Apparatus

The apparatus is shown in Figure 1. It consisted of three

sections of Pyrex tubing cemented coaxially to two pieces of

36 mesh bronze screen, each 7*5 cm square. A source of lCr alpha

particles per second, consisting of polonium chloride on a platinum foil backing, was cemented into a small dimple in the

center piece of glass tubing, midway between the two screens.

The center section of the apparatus was enclosed in an electrostatic shield and the two screens connected to the shielded

input connections of high impedence, feed-back, vacuum-tube,

electrometer

.
2

Operation of the detector unit is as follows: The

5 Mev alpha particles colliding with the gas molecules in

the center section of tubing produce a plasma of positive

ions and electrons. In the absence of gas flow, and

assuming a uniform spacial distribution of alpha particles,

the energies of both the positive ions and electrons decay

rapidly to thermal energies, and the two screen electrodes

behave as Langmuir probes, each assuming the same negative

potential with respect to the plasma, as indicated by Langmuir

probe theory. However, if there is a flow of gas down the tube

in one direction (nitrogen was used in all the measurements

reported here), positive ions will be dragged along by the gas

molecules, while electrons will be relatively unaffected, with

the result that the downstream electrode will be driven positive

with respect to the upstream electrode. The resultant current is measured by the electrometer. If an electron capturing

gas is added to the stream of nitrogen, electrons will be * captured before reaching the upstream electrode, converted into

negative ions and carried along downstream with the positive

ions. Recombination of positive and negative ions in the gas

stream and on the downstream electrode will further decrease

the current through the electrometer. It should be noted that

electrons may be captured at any stage of their energy decay.

This method of measuring attachment makes no distinction between

the various types of attachment.

3* Results

It was found that the apparatus was sensitive to the

flow of nitrogen, as indicated by Figure 2. It was assumed

that the reduction in current at higher rates of flow was the

result of insufficient time of contact with the downstream

electrode to allow collection of all the positive ions. The

correctness of this assumption was substantiated by the insertion of an additional disk of screen into the downstream

delivery tube where it was pushed into contact with the

electrode. The plot of current vs. flow then showed a narrow,

flat top as shown in Figure 3* In “the actual determinations,

the flow was maintained constant at 0.3 J^/min, rather than at

2 <£/min, which corresponded to the center of the flat part of

Figure 3 • This was a result of a compromise between the lower

current obtained at low flow rates, the length of time required

to saturate the system, and the small volume of the cylinder

used to make up the gas mixtures.
- 3

-

The curves of Figures 2 and 3 show a residual current at

zero nitrogen flow rate. This can be accounted for by the

slightly non-symmetrical placement of the radioactive source.

Some trouble was encountered, initially, caused by the

addition of too high a concentration of the attaching gas.

There was a tendency for it to adsorb on surfaces and in the

connecting tubing and, in some cases, it was extremely hard

to eradicate. It was necessary to go to concentrations in

the part-per-million range and, in addition, to substitute

glass for the plastic and rubber tube connections.

The part-per-million gas concentrations were produced by

making dilute mixtures in cylinders, using pressure measurements for the determination of concentrations, and then

diluting further by means of a proportioning flow system using

calibrated flow meters of appropriate ranges.

The results on the eleven gases are shown in Figure 4.

In addition to gases known to be inhibitors, several others

were included. Carbon tetraf luoride is ineffective as an

inhibitor and has a small cross seption for dissociative

resonance capture only at relatively high electron energies [2].

It showed no attachment in the method described here. Methyl

chloride does not attach electrons by the dissociative resonance process[3], but apparently does attach some to form the

molecular ion (see Fig. 4). 0xygen[4] and sulfur hexafluoride

[51 are known to capture electrons at low energies and are not

inhibitors for flame reactions.

4. Conclusions

Because of the non-specific nature of the method, it is

not expected that the gases would be placed in the same order

as for their efficiencies as flame inhibitors, however, none

of the gases which are known to be inhibitors failed to attach

electrons

.
[1] E. C. Creitz, J. Research NBS 6^A, 389-96 (1961).

[2] V. H. Dibeler, R. M. Reese and F. L. Mohler, J. Research

NBS 1Z, H3-18 (1956) .

[ 3 ] V. H. Dibeler, R. M. Reese, J. Research NBS 127-3^

(1955).

[k] D. S. Burch, S. J. Smith, and L. M. Branscomb, Phys. Rev.

112 , 171-5 (1958).

[5] W. M. Hickam and R. E. Fox, J. Chem. Phys. £5, 6^2-7 (1956).

USCOMM-NBS-DC
FIGURE S
liters PER MINUTE « GAS VELOCITY IN UNITS OF 21 CM PER SECOND

FIGURE 2

LITERS PER MINUTE = GAS VELOCITY IN UNITS OF 21 CM PER SECOND

FIGURE 3
-13

o

o

o

x S3d:idlW Nl iN3ddnO

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Aatin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

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NBS