iflrs report has 'been prepared

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i fKiny publication, NATIONAL BUREAU OF STANDARDS REPORT

v

8932

SOME NEGATIVE IONS FORMED BY ELECTRON ATTACHMENT

by

Robert M. Mills

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

. 02-11-4212121 July 15, 1965 8932

SOME NEGATIVE IONS FORMED BY ELECTRON ATTACHMENT

by

Robert M. Mills

IMPORTANT NOTICE NATIONAL BUREAU OF

for use within the Governmen

and review. For this reason, whole or in part, is not autl Bureau of Standards, Washin

the Report has been specifica

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

;ress accounting documents intended

is subjected to additional evaluation

ire listing of this Report, either in the Office of the Director, National

by the Government agency for which

I copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

SOME NEGATIVE IONS FORMED BY ELECTRON ATTACHMENT

by

Robert M. Mills

ABSTRACT

The predominant negative ions formed by

electron attachment are given for 21 compounds of

interest in studying flame inhibition mechanisms.

Electron energies used in the ion source are distributed roughly over the same range as those

found in a flame, i.e., 0 to 1 ev. The electron

bombardment source has cylindrical symmetry and was

designed especially for this low energy work.

SOME NEGATIVE IONS FORMED BY ELECTRON ATTACHMENT

Robert M. Mills

A quantitative study has been made of the predominant negative

ions formed in an electron bombardment ion source. The compounds used

are of interest in the study of flame inhibition mechanisms. The energies of the bombarding electrons in the source are distributed roughly

over the same range as those believed to exist in most flames, i.e.

0 to 1 electron volts. A time- of- flight mass spectrometer (1), designed

especially for this low energy work, was used to identify the ions.

Figure 1 is a diagram of the ion source. The sample gas is introduced through a leak not shown in the diagram. The source has approximately cylindrical symmetry with a tungsten filament at the center to

supply electrons. The first grid surrounding the filament draws the

electrons away from the filament. Large, grid-covered windows in the

aluminum cap allow electrons through into the circular ionization

space between the aluminum cap and the electron collector. This ionization space is nearly field free since the aluminum cap and the

electron collector are maintained at the same potential. Thus the

energy of the bombarding electrons is determined by the voltage between

the filament and the aluminum cap. In this work, the electron energy

was adjusted for maximum ion current.

Heating power for the filament is supplied by half-wave rectified,

60-cycle current. During the half cycle when current flows, the I R

drop through the filament biases the filament so that electrons do not

reach the aluminum cap, thereby reducing the electron energy spread

due to the voltage drop across the filament.

1

Negative ions are drawn out of the ionization region by a small

positive voltage on the draw-out grid. They are then focused by three

focusing plates shown in the figure. Some electrons are also drawn

out and focused on the orifice along with the negative ions, but they

are eliminated at the entrance and exit ends of the mass spectrometer

drift tube by small permanent magnets. These magnets are not strong

enough to deflect the ions. The orifice in Figure 1 helps to maintain

-3

the pressure drop between the source (approximately 10 torr) and the

drift tube region (approximately 10 ^ torr)

.

The geometry of this source makes it possible to utilize a large

fraction of the electrons emitted at any radial angle from the filament

and eliminates the need for a collimating magnet. In addition, the

cylindrical configuration reduces the space charge problem associated

with low energy electrons . The ion current versus electron energy

curves go through a sharp maximum which is typical of the electron

attachment process, indicating that the ions are produced by the bombarding electrons rather than the interaction of the hot filament surface with the surrounding gas sample. This type of dependence of ion

current on electron energy should not exist if the hot filament is

responsible for the ionization.

2

Table I presents the chemical compounds investigated together

with the ions identified in this work. Some of the compounds listed

have been studied earlier by other researchers. These compounds were

included to verify the instrument's performance. Compounds containing

more than one halogen usually form negative ions with the heaviest

halogen atom. Fe(CO)y CIO^F, and P0C1

3

are dis tinguished from the

others by their forming negative molecular ions after dissociation.

3

TABLE Is SUMMARY OF RESULTS

SAMPLE NEGATIVE IONS

OBSERVED

SAMPLE FURITY

AND SUPPLIER

REFERENCE

1 . CC1

4

Cl" (Baker 1512) (B) (2) (3) (10)

2. CF

2

Br

2

Br" CF

2

Br

2

- 98.9%

CFCl

2

Br - 1.1% (M)

3. <*2C1

2

Cl" CF

2

C1

2

- 99.8%

Hydrocarbons - 0.1% (M) (3)

4. CF

3

Br Br" CF Br - 99.8%

c

2

hf - 0.1% (M)

(4)

5. CHBr

3

Br" Stabilized with

Dipheny lamine

(Eastman 45) (E)

6. chci

3

Cl" Spectro Grade

(Eastman S 33)

(E)

7. CHFC1

2

Cl" CHFC1

2

- 99.7%, CC^F -0.1% (3)

CHC1F + CH F

2

- 0.2% (M)

8. CH

2

Br

2

Br" (Eastman 1903) (E)

9. CH

2

BrCl Br" (Eastman 5698) (E)

10. CH^Br Br" CH

3

Br - 99.7%

CH

3

C1 - 0.3% (M)

(5)

11. ch

3

i I" (Eastman 164) (E) (5)

12. C H BrCl

2 4

Br" (Eastman 567) (E)

13. C

2

H

5

Br Br" Ether Free

(Eastman 114)

(E)

TABLE I: SUMMARY OF RESULTS (continued)

SAMPLE NEGATIVE IONS SAMPLE PURITY REFERENCE

OBSERVED AND SUPPLIER

14. C

8

F

16° Vie0

' 3M Chemical Co. label FC-75 (6)

Reported to be a cyclic ether

15. C1V

>ii r\ - j

F

C10

3

F - 98%

Inerts (including Moisture)- 2% (2) (3)

16. Fe(CO)

5

Fe(CO)'

4

Obtained from City Chemical Co.

17. pci

3

Cl" SO. -0 . 0005%. Fe-0 . 0003%

4

Heavy Metals - 0.0005%

18. FOCI

3

P0C1" SO^O.01%, Fe- 0.001%

Cl" Heavy Metals - 0.002%

19. SF

6

SF

6

SF - 98% (M)

6

(3) (7) (8)

SF

5

20. TiCl

4

Cl" (Fisher T 308) (F) (9)

(B) Obtained from Baker Chemical Co.

(E) Obtained from Eastman Distillation Products Industries.

(F) Obtained from Fisher Scientific Co.

(M) Obtained from Matheson Co.

References

(1) R. M. Mills, J. Res. NBS 67C, 283, (1963).

(2) R. M. Reese, V. H. Dibeler and F. L. Mohler, J. Res., NBS 57 .

367, (1956).

(3) W. M. Hickam and D. Berg, Advances in Mass Spectrometry. Edited

by J. D. Waldron, Pergamon Press, 1959, p. 458.

(4) V. H. Dibeler, R. M. Reese, and F. L. Mohler. J. Res. NBS 57, 113,

(1956) .

(5) V. H. Dibeler and R. M. Reese, J. Res., NBS 54, 127, (1955).

(6) J. von Hoene, W. M. Hickam, J. Chem. Phys., 32 , 876, (1960).

(7) W. M. Hickam and R. E. Fox, J. Chem. Phys. 25. 642, (1956).

(8) D. Edelson, J. E. Griffiths, and K. B. McAlee, Jr., J. Chem. Phys.

37, 917, (1962).

(9) J. Marriott, R. Thorburn, and J. Craggs, Proc. Phys. Soc., (London) B67, 437, (1954).

(10)

R. E. Fox and R. K. Curran, J. Chem. Phys. 34, 1595, (1961).

5

FIGURE

I

SKETCH

OF

ELECTRON

IMPACT

ION

SOURCE