ABSOLUTE ELECTRON IMPACT IONIZATION CROSS SECTIONS OF N, O AND Ne

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ABSOLUTE ELECTRON IMPACT IONIZATION CROSS SECTIONS OF N, O AND Ne

G. Glupe, W. Mehlhorn

To cite this version:

G. Glupe, W. Mehlhorn. ABSOLUTE ELECTRON IMPACT IONIZATION CROSS SEC- TIONS OF N, O AND Ne. Journal de Physique Colloques, 1971, 32 (C4), pp.C4-40-C4-43.

�10.1051/jphyscol:1971408�. �jpa-00214609�

JOURNAL DE PHYSIQUE

Colloque C4, supplkment au

no

10, Tome 32, Octobre 1971, page C4-40

ABSOLUTE ELECTRON IMPACT IONIZATION CROSS _SECTIONS OF N, 0 AND Ne (*)

G. GLUPE and W. MEHLHORN

Institute of Nuclear Physics, University of Miinster, Germany

Rbsumb. - Les sections efficaces d'ionisation par impact klectronique ont ktk mesurkes en valeur relative et absolue au moyen des transitions Auger pour la couche

K

de

M, 0

et Ne et des knergies electroniques comprises entre

E ⁼

1,5

EK

et

E =

10,5 KeV (dans le cas de

M

et Ne) et 13 KeV (dans le cas de 0). Une cible gazeuse est utilide dans tous les cas. Les rksultats expkrimentaux sont cornparks aux valeurs thkoriques calculkes par Burhap, Rudge et Schwarz (thkorie quantique) et par Gryzminski (thkorie classique).

Abstract.

-

By

means of Auger transitions the relative and absolute electron impact ionization cross sections of the

K

shell of

N, 0

and Ne have been measured for electron energies from

E =

1.5

EK

up to

E =

10.5 keV (in the case of N and Ne) and 13 keV (in the case of 0). In all cases gaseous targets have been used. The experimental results are compared with theoretical values calculated by Burhop and Rudge and Schwartz (quantum theory) and by Gryzinski (classical theory).

1.

Introduction.

- During the last years there was a growing interest, experimentally [l] as well as theoreticcally 121, in electron impact ionization cross sections. Most experiments determined ionization cross sections of the outermost shells, whereas there is little experimental knowledge of inner shell cross sections. Figure 1 presents in graphical form elements

FIG. 1.

-Elements and reduced energy regions of

K

shell ionization cross section measurements.

and energy regions (in units of binding energy

E,)

which have been investigated so far [3a-k]. The expe- rimental cross sections for elements of medium and large Z agree satisfactorily with theoretical values calculated by Arthurs and Moiseiwitsch 141 with relativistic theory. Nonrelativistic values calculated

Ni [3d], Cu [3e], Sn [3k]. Very recently Hink and Ziegler [3b] found also for A1 the nonrelativistic values by Burhop to be smaller than the experimental values by about 30-60 %. The same discrepancy has been shown earlier by Glupe and Mehlhorn [3a]

for C, N, 0 and Ne. In order to test the validity of the present nonrelativistic theory of impact ionization by electrons we repeated the measurements of K shell cross sections of N,

0

and Ne with smaller experi- mental errors and for the larger energy region from

E =

1 . 5 E K u p t o 10.5 keV(forNandNe)and 13 keV

(for 0 ) .

2.

Method and Experiment. -

In all cases, except for C, N, 0 and Ne, inner shell ionization cross sections have been determined by the emission of K X-radiation. In the present investigation we measured K shell cross sections by means of the emission of K shell Auger electrons. This method, introduced earlier [3a], has advantages over the former method in all cases, where the ionization energy of the inner electrons is small, less than 1 keV, because in these cases no accurate fluorescence yields are available.

E. g., the flourescence yield of neon is known to be

U, =

0.025

$.

0.015 161, here the error is 60 %.From this the probability of an Auger process is

aK

=

1

- W, =

0.975 + ^0.015,

by Burhop [5] are considerably smaller than experi-

mental values even for elements of medium

2,

e. g. here the error is only 1.5 %. This demonstrates clearly the higher accuracy of the new method for low

2.

(*)

This work has been financially supported by the Deutsche In order to avoid energy losses of the low energy Forschungsgemeinschaft . Auger electrons we used gaseous targets (NZ, 0 2 , CO,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971408

ABSOLUTE ELECTRON IMPACT IONIZATION CROSS SECTIONS C4-41

Ne) with pressures of about 1

X

10-3 torr. The appa-

ratus used in this investigation has been discribed earlier

[7].

The Auger electrons were analyzed by means of an electrostatic cylindrical mirror analyzer with an energy resolution of 1 % and were measured by a Faraday cage followed by a vibrating reed elec- trometer. The output voltage of the electrometer was recorded by an XY-recorder, the X-axis being given by the spectrometer voltage, which has been varied continuously.

The target gas pressure has been measured by a MC Leod gauge. In order to reduce systematical errors in pressure readings by the mercury diffusion stream from the gauge to the spectrometer

[8],

we cooled the walls of the MC Leod by solid COz just above the mercury reservoir. Thus the error of the pressure measurements is believed to be less than 3 %.

3.

Results and Discussion. - a) RELATIVE CROSS SECTION.

- Figure 2 presents as example the recorder

FIG. 2.

-

K-Auger spectra of NZ and neon excited by 3,2 keV electrons.

plots of the K Auger spectra of NZ and Ne. In measu- ring relative cross sections all parameters were held constant except the intensity and energy of the pri- mary electron beam. Evaluating relative cross sections we used the peak heights of the dominant Auger line of the spectrum. The only correction applied to the relative cross sections is due the intensity loss of pri- mary electrons on the way from the target volume to the Faraday cage and was always less than 3 %.

The main error of relative cross sections is given by the uncertainty of the background below the Auger spectrum, the total error of relative cross sections is certainly less than 2 %.

Figure 3 presents the experimental relative cross sections of N, 0 and Ne (also relative cross sections of C are included). For E/EK >

3

all experimental values fall to within 1 % on a general curve given by the center of the shaded band of figure 3. For energies E/EK < 3 the error increases to about 2 % due to the

m

^{E w i M}

---.

Ihdge, Schwartz

---.

^GryzinsVi

FIG. 3.

-

Relative experimentaI and theoretical ionization cross sections as a function of reduced primary electron energy.

uncertainties of the background. The width of the shaded band corresponds to an error of 2 %. For comparison we calculated theoretical ionization cross sections using Burhop's formula

[5]

(quantum theory with Born approximation) and Gryzinski's [g] formula derived by classical theory. Rudge and Schwratz [l01 calculated ionization cross sections of the 1 S-electron of H, He' and a fictitious hydrogenic ion with nuclear charge

2 =

128 using quantum theory and Born exchange approximation. Their tabulated values of reduced cross sections QR(E/EK) can be transformed to absolute values by

Because the change in Q, going from

Z =

1 to Z

=

2 is larger than the change going from Z

=

2 to Z

=

128 we used for all elements investigated (Z

=

7, 8, 10) the reduced cross sections Q, calculated for Z

=

128.

The error given by this procedure is probably in the order of or less than 1 % [3b]. All theoretical curves were fitted to the experimental curve at E/EK

=

19.

From figure 3 it can readily be seen that for large E/& the energy dependence of the values given by Rudge and Schwartz [l01 agrees with that of the experi- mental values, whereas the values given by Burhop [5]

and Gryzinski [9] decrease faster than the experimen- tal values. The maximum of experimental cross sec- tion is a t EIE;,

=

4, this agrees with Gryzinski's values but neither with Burhop's values nor with the values by Rudge and Schwartz (Q,,, at E/EK

=

3).

h) ABSOLUTE CROSS SECTION.

- Absolute cross sec- tions Q have been evaluated by means of the following equation

Here I,,,,, is given by the total area of the Auger

spectrum corrected for the dispersion of the spectro-

C4-42 G. GLUPE AND W. MEHLHORN

meter and the loss of intensity due to the scattering of

Auger electrons on the way through the spectrometer, I,, is the intensity of primary electron beam at the site of the target volume,

n(p, T) is the number of target particles per cm3, n, gives the number of atoms under investigation in one target particle,

A is given by the area of the spectrometer window function, which is related to the transmission of the spectrometer and

w,

is the fluorescence yield of the

K shell.

The quantity A has been determined experimentally by measuring the intensity of elastically scattered electrons, IelaStic, by He atoms at various energies under the same conditions of the spectrometer as in the case of Auger electrons. The peak area IelaStic of the elastic scattered electrons is given by

From eq.

(3) the quantity A has been evaluated

using the theoretical values of differential elastic cross sections dQelaStic calculated by Khare and Moisewitsch [l l ] for He. The theoretical values agree to within 2 % with experimental values, measured very recently by Bromberg [12], in the angular range of our spectrometer. The total error of absolute cross sections are estimated to be 5 %, given mainly by the uncertainties of gas pressure (3 X), the shape of the background (2 %) and the differential elastic cross sections of He (2 X).

Figures 4, 5 and 6 present the absolute ionization cross sections of N, 0 and Ne as functions of reduced energy E/E,. For comparison also theoretical cross

FIG. 4.

-

Absolute experimental and theoretical ionization cross sections of nitrogen.

FIG. 5.

-

Absolute experimental and theoretical ionization cross sections of oxygen.

sections given by Burhop [5], Rudge and Schwartz [IO], and Gryzinski

[9]

are plotted. In all three cases the Burhop values are smaller than the experimental values by more than 30 %. The Gryzinski values agree well with experimental values for E/E, < 10, whereas the values by Rudge and Schwartz are slightly better for E/EK > 10. These results are in concordance with those found recently for the K shell cross sections of A1 by Hink and Ziegler [3b]. Also there the Burhop values were too small whereas the values by Gry- zinski and Rudge and Schwartz agreed much better with the experimental values.

,---..

/

Experiment

I

I

-

^Burhop

I

l

- ----

⁼

I Rudge, Schwartz

I

11

--- -

^Gryzinski

FIG. 6 .

-

Absolute experimental and theoretical ionization cross sections of neon.

ABSOLUTE ELECTRON IMPACT IONIZATION CROSS SECTIONS

References

[l] KIEFFER (L. J.) and DUNN (G. H.), Rev. Mod. Phys., 1966, 38, 1. LOTZ

(W.),

IPP 1/47, Institut fiir Plasmaphysik, Garching b. Miinchen, 1966.

[2] RUDGE (M. R. H.), Rev. Mod. Phys., 1968, 40, 564.

[3a] GLVPE (G.) and MEHLHORN (W.), Phys. Letters, 1967, 25A, 274 (C, N, 0, Ne).

[3b] HINK (W.) and ZIEGLER (A.), Z. Phys., 1969, 226, 222 (AI).

[3c] FISCHER (B.) and HOFFMANN (K.-W.), Z. Phy~., 1967,.

204, 122 (AI, Mn, Cu, Se, Ag, Sn).

[3d] SMICK (A. E.) and KIRKPATRICK (P-), Phys. Rev., 1945, 67, 153 (Ni). POCKMAN (L. T.), WEBSTER (D. L.), KIRKPATRICK (P.) and HARWORTH (K.), Phys. Rev., 1947, 71, 330 (Ni).

[3e] GREEN (G. W.), Proc. 3rd Int. Symp. X-ray Microc., Standford 1962 (Academic Press New York);

Thesis Cambridge 1962 (Ni, Cu, Ag).

[3f ] HANSEN (H.), WEIGMANN (H.) and FLAMMERSFELD

(A.),

Nucl. Phys., 1964, 58, 241 (Zr, Sn, W, Pb).

[3g] WEBSTER (D. L.), HANSEN (W. W.) and DUVENECK (F. B.), Phys. Rev., 1933, 43, 839 (Ag). CLARK (J. C.), Phys. Rev., 1935, 48, 30 (Ag).

[3h] HANSEN (H,) and FLAMMERSFELD (A.), Nucl. Phys., 1966,79,135 (Ag, Sn,

W,

Au, Pb).

[3i] RESTER (D. H.) and DANCE (W. E.), Phys. Rev., 1966, 152, 1 (Ag, Sn, Au).

[3k] MOTZ (J. W.) and PLACIOUS (R. C.), Phys. Rev., 1964, 136A, 662 (Sn, Au).

[4] ARTHURS (A. M.) and MOISEIWITSCH (B. L.), Proc.

Roy. Soc. (London), 1958, A 247,550.

[S] BURHOP (E.

H.

S.), Proc. Cambridge Phil. Soc., 1940, 36,43.

[6] FINK

(R.

W.), JOPSON (R. C.), MARK (Hans) and SWIFT (C. D.), Rev. of Mod. Phys., 1966,38, 513.

[7] MEHLHORN (W.), Z. Phys., 1965,187,21.

[g] SCHRAM (B. L.), DE HEER (F. J.), VAN DER WIEL (M. J.) and KISTEMAKER (J.), Physica, 1965, 31, 94.

[g] GRYZINSKI (M.), Phys. Rev., 1965, 138, A 336.

[l01 RUDGE (M.

R.

H.) and SCHWARTZ (S. B.), Proc.

Phys. Soc., London, 1966, 88,563.

[l l] KHARE (S. P.) and M O I S E ~ S C H (B. L.), Proc. Phys.

Soc., 1965, 85, 821.

[l21 BROMBERG (J. P.), J. Chem. Phys., 1969, 50, 3906.

DISCUSSION

VAN DER WIEL, M. J.

-

Did YOU include in your evaluation of the Auger Spectrum area the contribu- tion of Auger satellite lines ?

The intensity of these lines amounts to about

10-20 %

of the normal lines, in the gases you mentioned.

Therefore a deviation between your experimental value and the calculated ones is to be expected, since the calculations only take into account pure 1 S-ionisation and no two-electron processes, in which the 1 S-hole is accompanied by one in the outer shell.

CARLSON: T.

A. - Experimentally, it has been determined at Oak Ridge that the sum of satellite lines in the Auger spectrum of neon amounts to about

20 %

of the normal Auger lines indicating a similar degree of doubt t o single ionization.

Answer : Yes, we included also Auger satellite lines.

In theoretical calculations of ionization cross sections one considers the ionization process only as a two-particle process, i. e. one neglects in theory all electrons of the atoms except the one, which is ionized. If shake-off theory and sudden approximation is valid to explain the ratio of double to single ioniza- tion, then treating the ionization process as two- particle process includes all shake-off transitions which are induced by the primary ionization in the inner shell. That is, experimentally the total Auger intensity, including satellite lines, correspond to theore- tical values based on the two-particle process.