Chemical ionization of CF4, C2F6 and C3F8

-.

: 31.1

Jau&al if &k Spectrometry~a+~o~~Physics, -14 (1974) 311-312 .- .. @ Elsevier ScientificDublishing Company, Amsterdam - Printed in The Netherlands

International

Short

coIllpuilication

Chemical

._

ionization

:

,_

of CF,, -C,F,

aitd C3F,*

J. L. FRANKLIN Deparitient S. D.

of Chemistry,

Rice

Unicersity,

Hou~fon,

Texas

77001 (UXA.)

DEY

Saha Institute of Nuclear

Physics,

Calcutta

7&?009 (India)

(Received 15 March 1974)

Chemical ion&z&ion of the lower perfluoro-carbons Cc,, CzF6 and C3Fs were studied in a methane plasma. A 60” sector type mass spectrometer having a radius of curvature of 12 inch was used for this purpose. Its electron bombardment type ion source was specially designed to handle 1 torr of gas pressure inside rhe source. The elec-tron entrance slit, ion exit slit and the slit separating the chamber housing the ion source and the analyscr tube had widths 0.-007”, 0.007” and 0.020” respectively_ The two chambers were evacuated separately by one 4” and another 2” oil diffusion pumps. With this differential pumping arrangement and the slit dimensions, it was possible to maintain 1 .torr gas pressure inside the ionization box, 2 x low3 torr just outside the ionization box and 10B5 torr in the analyser chamber. The ion. detector was an electron multiplier in conjunction with a vibrating reed electrometer and a pen recorder_ Gas was introduced into the- ion source through a glass tube which was inserted through the gas..inlet opening and it went up to the ion repeller plates. This way of introducing the gas just behind the repel1e.r plates helped to.minimize the gas loss and thus maintain desired pressure differences in the various parts.of the apparatus. The electron and ion accelerating voltages were 150.V 2nd 1.5 kV respectively. The field -.at the repeller plates_was 6.25 V/cm. Mass scanning was done by varying. the magnetic field. y .. To get a pure methane mass spectrum [l J,~thein~trument.was operated*with a~continuousflow of methane gas for about 24 hours. .Having obtained the -pure _ methane~spectrti; ;h L e gas pressure inside t.heion source was raised to about 1 torr .. : * T&work

_. -_~ iv2 Dr_.&was . .d&&while . ..

at Rice fJniyersity as-a post~do&oral fellow.

.,

312 TABLE

0 1

CHEMICAL

IONIZATION

SPECTRA

Reactant gas, methane, Added gas

PCH.+

OF T

CF,,

1

C2Fs

AND

CxFe

torr.

IOJl

Remarks

51

CFCHZ+ CF2H+

alkylene addition hydrogen addition

45 51 79

CFCHz+ CF2H+ C&CH5+

CH5+ addition

mle 45

45 51 79 97 115 133 151 170

CFCHz + CF2;;+ C;2F=CHs + CF&z%+ C2F4CH3* CrF&H2+ CxFsH+ CaF7H+

ethylene addition alkyl addition alkylene addition hydrogen addition

and the spectrum obtained was compared with the published data [l]. Gas pressure inside the ion source was estimated from the ratio of the peak heights of CH,+ and CM,+ [l]_ The fluorocarbons, CF,, C,F, and C,F, were mixed with methane in separate gas handling systems one after another and the spectra of the combinations CH,-CF4, CH,--C,F6 and CH,--C,F8 were obtained. The pressure of ;he added gas was increased in steps, the methane gas pressure having been maintained at 1 torr, until the electron bombardment spectrum of the added gas started to appear. Plots of the CHSi and C,H5 f intensities as a function of the increased fluorocarbon pressure show that the intensity of CzH5+ does not change appreciably. The CH, + intensity, however, falIs sharply indicating that it takes part in chemical ionization reactions unlike C,H5+. The mass analysis of the peaks arising due to reactions between methane and the fluorocarbons, given in Table 1, shows that the hydrogen transferjalkyl exchange/ethyl addition take place bet-ween the reactant and the added gas characteristic of thz chemical ionization reaction. 1 F. H. Field and S. B. Munson, J. Amer.

Chem. Sot.,

87 (1965)

3289.