Partial photoionization cross-sections for CO2 and N2O from 20 to 6OeV

Journal of Electron Spectroecopy and Related Phenomena, 16 (1979)241-246 0 Elsev~erSclent~f~cPubhshmgCompany,Amsterdatn-Prmted m TheNetherlands

241

PARTIAL PHOTOIONIZATION CROSS-SECTIONS FOR CO2 AND N20 FROM 20 TO 60eV

C E

BRION and K H

TAN

Department of Chemistry, The Unlverslty of British Columbia, Vancouver V6T lW5 Canada

ABSTRACT Measurements of the partial photolonlzatlon cross-sections of the molecules CO

and N 0 are reported for the energy range ZO-60eV Large contrlbutlons to the 2 2 photolonlzatlon cross-section from multiple electron transitions are observed, particularly above 40eV, ln accord with the results of recent many-body Green's function-calculations

The cross-sectlon for total photabsorptlon as well as the

ionization efflclency have also been measured

INTRODUCTION In a series of experiments [1,2,3] we have shown that the magic angle dipole (e,2e) method? provides an effective quantltatlve slmulatlon of photoelectron spectroscopy

The dipole (e,2e) method involves detection of both the scattered

and the elected electrons following molecular lonlzatlon in a low momentum transfer collision wiUh a fast electron

Using colncldence counting, binding energy spectra

are readily generated at a series of values of the electron energy 10s~

The

latter quantity 1s analogous to a continuously variable photon energy as provided for example, by a monochromated synchrotron source are obtalned from the binding energy spectra

Photoelectron branching ratios

The photabsorptlon cross-sectlon IS

obtained by klnematlc correction of the forward scattering (non-colncldent) electron energy loss spectrum as prescribed by the Bethe-Born theory [1,2,4] Partial photolonlzatlon cross-sectlons are generated from the product of the total photabsorptlon and the branching ratlo

FInally a relative photolonlzatlon

efficiency may be obtalned by dividing the normalized areas of the blndlng energy spectra by the total photabsorptlon at each energy

Using this method we have

recently reported measurements for NH3 [31, H20 c51, CH4 C61 and Ar [71

T'I'he dipole (e,2e) technique 1s a small momenuum transfer electron impact method and should not be confused with the binary (e,2e) method (large momentum transfer) used in the measurement of molecular orbltal momentum dlstrlbutlons

242

C E BRION, K H TAN

EXPERIMENTAL A schematic representation of the dipole (e,2e) spectrometer 1s shown In figure 1

Details of the design and operating prelnciples have been fully dls-

cussed elsewhere [1,2,3]

The intensities of the peaks In the bInding energy

spectra were corrected for the elected electron analyser transmission efficiency using the correctlon curve obtained In earlier work [3]

An energy resolution of

1 3eV (FWHM) was used for the colncldence measurements with a corresponding value of 0 9eV (FWHM) being involved in the absorption determination

Gas samples

were obtalned from commercially available cylinders

forward

onolyser

Figure 1

SchematIc diagram of (e,2e) spectrometer

RESULTS The blndlng energy spectrum for CO2 obtained at an equivalent photon energy of 60eV 1s shown in figure 2 (lower part)

In addition to the partially resolved

structure corresponding to the 1d;uestfour ionization potentials (X, A, B and C) it can be seen that there 1s a broad band of satellite structures above 20eV due to multiple electron transltions (MET) the following energies, all 20 5eV,

These higher energy structures occur at

23 5, 26 5, 30 0, 32 0, 35 5 and 38 OeV

Similar bands have been observed In photoelectron spectra obtained using W and X-ray line sources [S-11]

Slmllar results are obtained (see figure 2, lower part)

for N20 with peaks occurring above 20eV at the following energies, all f0 SeV, 24 0, 28 5, 33 0, 35 5 and 38 OeV

These results are also in accord with the photo-

electron spectra [9, 12-141 In both molecules two inner valence orbltals (30 and 2ou for C02, 4~ and 56 g for N20) might be expected to have ionlzatlon energies In the 35 to 40eV region Recent many-body Green's function calculations by Domcke et al [IS] have lndlcated that a rich spectrum of lines 1s to be expected in the 20-40eV region due to the breakdown of the simple molecular orbital picture [16,17]

These calculations

predict that lonlzatlon of each one electron orbltal may lead to several ion states

243

I

E=56eV

I

15

I

1

N,O

I

30 d5 20 25 BINDING ENERGY

1

I

40

Figure 2 Blndlng energy spectra for the valence orbltals of N20 and CO2 at 56 and 60 eV respectively (uncorrected for transmission)

and that, particularly in the case of the two inner orbltals, the dlstlnctlon between parent and satellite peak 1s blurred due to the more even lntenslty dlstrlbutlon

The present results are in accord with the theoretical predlctlons

Following computer deconvolutlon, using half widths and peak posltlons from high resolution photoelectron spectroscopy [IS], the relative peak areas were used to obtain the photoelectron branching ratios shown in figures 3 (C02) and 4 (N20) Results for CO2 are generally in good agreement with the data of Samson et al [19] obtained using a many line light source

The dlscrepancles with the synchrotron

data of Gustafsson et al above 30eV are due to the neglect by these authors [s] of the multiple electron transltlons (MET)

It can be seen from figures 3 and 4

(also tables 1 and 2) that the multiple electron transitions (le

satellite + inner-

orbltals) make an increasingly important contrlbutlon to the total lonlzatlon above 25eV The cross-sections for partial photolonlzatlon and total photoabsorptlon, obtained as outlined in the lntroductlon to this article, are shown In tables 1 and 2

The photolonlzatlon efficiency was found to be constant (unity) within

experimental error above 20eV published later [20]

More detailed and complete results will be

Figure 3 Photolonlzatlon branching ratios for CO2 Dots - this work, open circles PES, reference 8,0pen triangles - PES

Flgure 4 Photolonlzatlon branching ratios for N20 Dots - this work

Table I Cross Sections (Mb) for Partial Photolomzatlon Partial Photolomzatlon Energy(eV) 21 2 22 23 24 25 26 27 28 29 30 31 32 34

X211 10 9 9 11 10 10 10 9 8 8 8 8 8

28 23 96 20 91 59 36 00 84 12 34 13 48

A211u+B2C tl 24 23 19 17 16 15 15 14 14 13 12 12 11

22 08 59 92 36 45 39 45 25 72 37 74 66

Cross SectIon

C2Z 2 2 3 2 3 2 2 4 3 3 4 4 4

and Photoabsorptlon of CO2

20 84 65 88 03 57 96 05 71 64 04 07 24

Total METa

1 1 1 1 2 1 2 2

19 18 44 71 52 88 17 12

Total Photoabsorptlon 36.7 3s 5 33 2 32 0 30.3 29.8 29 6 28.9 28 5 28 0 26 9 27 1 26 5

245

PARTIALPHOTOIONIZATIONCROSSSECTION Table 1

Continued 7 6 6 6 5 5 4 4 4 4 3 3 3 3

36 38 40 41 42 44 46 48 50 52 54 56 58 60

85

11 11 9 a a 7 7 6 6 5 5 4 4 3

86 38 48 94 37 76 54 45 27 67 91 70 15

3 3 2 2 i 1 1 1 1 1 1 1 1 1

39 03 68 99 51 52 05 32 04 37 20 79 22 63

3 3 3 3 3 3 3 4 3 4 5 4 4 4

45 43 86 09 98 79 31 13 43 26 38 45 32 21

26 24 22 20 19 17 16 16 15 15 15 14 13 12

45 43 08 34 17 22 28 05 98 90 05 21 09 11

5 5 0 9 a 9 4 2 9 a 3 5 2 1

a - total lonlzatlon above 22eV Table 2 Cross Sectlons (Mb) for Partial Photolonlzatlon and PhotoabsorptIon of N20 Partial Photolonlzatlon Cross Section Energy(eV) 17 19 19 6 21 2 21 6 22 23 24 25 26 27 28 29 30 72 34 36 38 40 41 42 44 46 48 50 52 54 56 58 60

X21i 25 17 16 12 11 ii lo 10 11 10 10 10 9 9 9 9 a 7 6 6 6 5 5 5 5 4 4 3 3 3

48 10 24 80 96 58 a2 68 20 50 73 48 76 66 45 25 06 56 97 86 76 77 57 64 05 a7 68 98 53 24

A*E+

B211

20 16 13 9 7 6 6 5 5 5 5 4 5 5 5 4 3 3 3 2 2 2 2 2 1 2 1 1 I 1

6 9 9 9 9 a a a 7 7 7 7 7 6 6 5 4 4 4 3 3 3 2 2 2 2 2 2 1

02 69 86 15 a5 67 23 97 60 40 36 95 45 11 00 08 32 24 08 55 90 42 44 22 79 04 66 70 26 08

a - total lonlzatlon above 22eV

92 90 52 99 13 a6 48 40 80 75 57 46 38 95 80 93 75 51 12 X6 72 31 91 93 67 42 42 14 92

c2c+

5 49 5 78 7 72 6 a9 6 59 4 98 4 80 4 77 4 36 4 02 3 98 3 89 4 08 3 32 3 02 2 87 2 74 2 a9 2 79 2.61 2 56 2 28 2 36 2 27 1 85 1 76 1 56

Total

Total METa

0

93

1 1 2 2 2 2 2 3 3 3 3 3 3 3 3 4 3 4 4 4 4

80 49 04 00 55 50 99 32 24 28 53 09 91 48 76 24 77 08 26 03 20

Absorption 45 40 39 36 35 35 32 31 31 30 29 29 28 28 27 27 23 21 20 19 19 18 17 17 16 15 15 14 12 12

5 7 6 6 4 1 8 4 1 0 a 1 7 4 8' 2 7 6 5 6 3 6 4 1 3 7 1 2 6 0

C E BRION, K H TAN

246

ACKNOWLEDGMENTS Financial support for this work was provided by the National Research Council of Canada and the North Atlantic Treaty Organlzatlon Samson and Dr

W

We wish to thank Dr

J A R

Domcke for maklng their results available to us prior to

publication

REFERENCES

Cl1 C E

&Ion and A Hamnett "Continuum Optical Oscillator Strength Measurements by Electron Spectroscopy in the Gas Phase 'I in Advances in Chemical Physics, "The Excited State in Chemical Physics" Vol 2 Wiley (New York) 1978, to be published c21 A Hamnett, W Stoll, G R. Branton, C E Brlon and M J Van der Wiel, J Phys B 9 (19763 945 c31 C E Brlon, A Hamnett, G R Wlght and M J Van der Wiel, J Electron Spectrosc 12 (1977) 323 M Inokutl,Rev Mod Phys 43 (1971) 297 I:] K H Tan, C E Brian, Ph E fin der Leeuw and M J Van der Wiel, Chem Physics 29 (1978) 299 M J VanTer Wlel, W Stoll, A Hamnett and C E Brion, Chem Phys Letters [d 37 (1976) 240 K H. Tan and C.E. Brlon, J. Electron Spectrosc. 13 (1978) 77 El] T Gustafsson, E W Plummer, D E Eastman and W Gudat, Phys Rev A -17 (1978) 175 191 A W Potts and T A Wllllams, J Electron Spectrosc 2 (1976) 3 Cl01 C J Allan, LI Gellus, D A Allison, G Johansson, H Siegbahn and K Slegbahn, J Electron Spectrosc I (1972/73) 131 D A Allison and R G Cavell, J Chem Phys 68 (1978) 593 U Gellus, J Electron Spectrosc 5 (1974) 98y J W D Connolly, J Chem Phys 58 (1973) 4265 R G Cavell, unpublished data, p=vate communlcatlon W Domcke, Private communication L S Cederbaum, J Schlrmer, W Domcke and W von Nlessen, J Phys B 10 (1977) L549 Cl71 w Domcke, L S Cederbaum, J Schlrmer, W von Nlessen and J P Maler, J Electron Spectrosc , in press 1978 f181 C R Brundle and D W Turner, Int J Mass Spectrom Ion Phys 2 (1969) 195 J A R Samson - to be publlshed C E Brian and K H Tan, to be published, Chemical Physics