GAS PHASE X-RAY ABSORPTION SPECTROSCOPY WITH AN ELECTRON YIELD DETECTOR

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GAS PHASE X-RAY ABSORPTION

SPECTROSCOPY WITH AN ELECTRON YIELD DETECTOR

F. Lytle, R. Greegor, G. Via, J. Brown, G. Meitzner

To cite this version:

F. Lytle, R. Greegor, G. Via, J. Brown, G. Meitzner. GAS PHASE X-RAY ABSORPTION SPEC-

TROSCOPY WITH AN ELECTRON YIELD DETECTOR. Journal de Physique Colloques, 1986, 47

(C8), pp.C8-149-C8-151. �10.1051/jphyscol:1986827�. �jpa-00226149�

JOURNAL DE PHYSIQUE

Colloque C8, supplkment au n o 12, Tome 47, dkcembre 1986

GAS PHASE X-RAY ABSORPTION SPECTROSCOPY WITH AN ELECTRON YIELD DETECTOR

F.W. LYTLE, R.B. GREEGOR, G.H. VIA*, J.M. BROWN* and G. MEITZNER*

The Boeing Co. , Seattle, W A 98124, U.S.A.

' ~ x x o n Research and Engineering CO., Allandale,

08801, U.S.A.

ABSTRACT

He used He a t atmospheric pressure as a c a r r i e r gas t o neasure x - r a y a b s o r p t i o n spectra o f gaseous species w i t h remarkably h i g h s i g n a l l e v e l s . The 4 r c o l l e c t i o n e f f i c i e n c y o f e l e c t r o n s c r e a t e d by the a b s o r p t i o n event r e s u l t e d i n an e x c e l l e n t s i g n a l t o n o i s e r a t i o and an estimated s e n s i t i v i t y o f a p p r o x i n a t e l y 1 porn f o r any molecule which can be i n t r o d u c e d i n t o a He stream. Spectra o f Ar and C1-containinq hydrocarbons a r e shown.

INTRODUCTION

The paper o f Kordesch and Hoffman(1) i n t r o d u c e d t h e EXAFS community t o e - y i e l d de- t e c t i o n a t atnospheric pressure. The technique i s r e m i n i s c e n t o f conversion e l e c - t r o n counting i n ~ z s s b a u e r spectroscopy i n which the sample i s laced i n s i d e a p r o p o r t i o n a l counter. I t appears t h a t Shevchik and ~ i s c h e r ( 2 ) a r e a c t u a l l y the f i r s t t o make use o f a s i m i l a r detector. Most r e c e n t l y Guo and denBoer(3) have compared normal a b s o r p t i o n e - y i e l d and i o n y i e l d EXAFS f o r t i i , Fe and Cr, and Sham and Carr(4) have used t o t a l e l e c t r o n y i e l d t o examine t h i n Ni on Fe. Our v e r s i o n o f t h e d e t e c t o r i s shown i n f i g u r e 1. It c o n s i s t s o f a c a v i t y m i l l e d i n a p l a s t i c block. On t h e f r o n t i s an a l u n i n i z e d n y l a r window and on t h e back i s t h e sample p o s i t i o n . The f r o n t window and sample a r e biased a t -45 Y . I n t h e middle o f t h e b l o c k i s a t h i n Ni mesh i n a conducting frame. The s i n n a l i s c o l l e c t e d on t h e mesh and a m p l i f i e d by an electrometer. The p o s i t i v e b a t t e r y t e r m i n a l i s connected t o the a m p l i f i e r ground and the b a t t e r y i s f l o a t e d t o a v o i d around loops. The p o l a r i t y may be reversed t o c o l l e c t p o s i t i v e ions. The i n c i d e n t x-ray beam t r a n s i t s the

f r o n t window, the t i i mesh and impinges upon the sample. The t o t a l y i e l d o f e l e c t r o n s i s c o l l e c t e d on the g r i d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986827

C8-150

JOURNAL DE PHYSIQUE

For spectroscopy of gas samples the s a m ~ l e i s replaced with a t h i n , aluminized mylar window and the sample gas i s mixed with He before flowing through the c e l l . Actually any c a r r i e r gas which does not quench electrons may be used although He and Hz have the advantage of low x-ray absorption cross section. In H 2 the dark current i s lower than in He ( b e t t e r d i e l e c t r i c ? ) . The excellent compilation of data by P e i s e r t and S a u l i ( 6 ) show t h a t low energy electrons may be collected over distances of ap- proximately 1 cm with 45 V and a t atmospheric pressure, which suggested the dimen- sions of the detector. This gas sample-inside-the-detector technique i s reminiscent of the e a r l i e r experiments of Brogren(7) (Geiger counter) and Schnopper(8) (pro- portional counter) which used detectors containing the samole gas.

SAMPLE DATA

The data were measured a t SSRL using S i ( l l 1 ) c r y s t a l s and a

mm entrance s l i t o r ae

5 x lom5 radians. We found i t necessary t o use d i l u t e gas samples t o prevent saturating the detector. For example CC14 was measured a t approximately 1 mn Hg partial pressure, Ar a t 0.1 nmHa.The l i m i t of s e n s i t i v i t y was estimated t o be

1 ppm from the signal of the residual Ar impurity (1 ppm) i n the flush nas. The spectrum of Ar i s shown i n figure 2. In the i n s e t i s an expanded view of the ls3p double ioniza- tion region. Compare t h i s with f i g u r e 5 i n Deslattes, e t a1.(9). The improved signal-to-noise r a t i o and resolution of f i n e d e t a i l s i s remarkable f o r data obtained in a single scan, 1 s / p o i n t . For s o l i d or liquid samples with appreciable vapor pressure we flowed He through a glass tube containing the sample and then mixed i t with the He flush stream. The usual problem was keeping the sample s u f f i c i e n t l y dilute.Spectra of CC14 and C2H4C12 a r e shown in f i q u r e 3. Both the EXAFS and XANES are excellent spectra. The high energy cut-off of the C1 K-edge i s due t o the K-edge of the Ar impurity in the He. The Ar spectrum i s ubiquitous i n t h i s energy region because of i t s presence in a i r ( I % ) , the He flush stream and the He-filled monochromator chamber. However, the sharp I s resonance l i n e a t 3203.3 eV(9) i s a useful c a l i b r a t i o n point.

ACKNOWLEDGMENT

The research of Lytle and Greegor was supported by NSF. !.le thank SSRL f o r beam time. SSRL i s supported by DOE.

REFERENCES

1. M. E. Kordesch and R. W . Hoffman, Phys. Rev. 529, 491 (1984).

2. N. V. Shevchik and D. A. Fischer, Rev. Sci. Inst. 9, 577 (1979) 3. T. Guo and M. L. denBoer, Phys. Rev. m, 6233 (1985).

4. T. K. Sham and R. Carr, submitted t o J . Chem. Phys.

5. F. W. L y t l e , R. 5. Greegor, G. P. Huffman and F. E. Hungins, 1985 SSRL A c t i v i t y Report.

6. A. P e i s e r t and F. S a u l i , " D r i f t and D i f f u s i o n o f E l e c t r o n s i n Gases: A Compilation ," CERll 84-03 (1 984).

7. G. Brogren, Nova Acta Soc. Reg. Sci. Upsal 14, No. 4 (1949), a l s o 'in A. E.

Sandstrom, Hand. der PhysiK XXX p. 208 (1957).

3. H. W. Schnopper, Phys. Rev. 133, A627 (1964).

9. R. D. Deslattes, R. E. L a v i l l a , P. L. Cowan and A. Henins, Phys. Rev. g, 923 (1 983).

Figure 2. Argon K-edge Spectra. The Double F i g u r e 1. Simple e - y i e l d Detector I o n i z a t i o n Region i s Expanded i n

t h e I n s e t

cCl4- C2H4C12----

I I

26.

ENERGY ,eV

F i g u r e 3. Gas Phase C1 K-edge XANES and EKAFS o f CC14 and C2H4C14