Partial oscillator strengths for the photoionization of N2O and CO2 (20–60 eV)

Chemical Physics 34 (1978) 141-151 0 North-Holland PubIishin~ Company

PARTIAL OSCILLATOR STRENGTHS FOR THE PHOTO%NIZATION OF N20 AND CO, (20-60 eV) C.E. BRION and K.H. TAN Departrzrent of Chemistry. The Urtiversity of British Columblir. Varzcouver, Gmada V6T I WS Received 3 July 1978

The photoelectron branching ratiosand the partial oscillator strengths (cross sections) for photoionisation of the valence orbitals of N20 and CO2 have been obtained in the energy range 20-60 eV using the magic angle dipole (e, 2e) method. In addition to single electron ionization processes there is a large contribution from multiple electron transitions at higher energies in agreement with recent theoretical predictions. The photoionization efficiency and the dipole oscillator strength for total photoabsorption have also been measured.

1. Introduction

The magic angle dipole (e, 2e) method [1,2] provides an effective quantitative simulation of photoelectron spectroscopy (PES). In the (e,2e) technique the electron energy loss is analogous to t&able LJVlight which

is normally

only available

above

20 eV in the

form of monochromated synchrotron radiation. As demonstrated elsewhere [l-3] the dipole (e,2e) results are equivalent to those that would be obtained by optical methods with sampling of photoelectrons at 54.7” and will be discussed 2s such. Binding energy spectra obtained in the coincidence mode yield photoelectron branching ratios as a function of photon energy. In addition a relative photoabsorption spectrum may be obtained directly from the Bethe-Born corrected forward (non-coincident) energy loss spectrum [1,4]. This relative photoabsorption spectrum is readily placed on an absolute oscillator strength (cross section) scale [l] either by sum-rule normalization or by normalization at 2 single (low) energy to an optically determined photoabsorption cross section. Partial photoionization oscillator strengths (cross sections) are readily generated from the product of the branching ratios and the absorption data. These methods have been used in a series of studies of the photoionization and photoabsorption of atoms and molecules including Ar[S] , CH4 161, NH, [7], and H,O [3]_

They are extended to N,O and CO, in the present paper. There have been a number of reported measurements of the photoelectron branching ratios and partial photoionization cross sections of N20 and CO2 over a iimited energy range. Using a Lozier tube apparatus and a monochromated discharge lamp Bahr et al. [8] have obtained results for both molecules at a number of discrete wavelengths corresponding to energies up to 21 eV. Brion and Yee [9] have obtained branching ratios for both molecules at 21.2 eV using a transmission calibrated photoelectron spectrometer sampling photoejected electrons at 90’. These results [9] have been corrected for angular distribution effects using the data of Carlson and McCuire [lo]. Using a magic angle cylindrical mirror analyser together with a monochromated many line light source, Samson [l l] has obtained photoelectron branching ratios for CO, at a series of energies up to 27 eV and also at the He11line at 40.8 eV. The new data [ 111 supersedes the earlier, less quantitative, PES measurements [12,13]. While the present work was in progress Gustafsson et al. [14] reported PES measurements for CO2 between 20 and 40 eV using synchrotron radiation. In this latter work [14] the complex geometry results in partial cross sections which are dependent on the various anguIar asymmetry parameters p(E). These energy dependent parameters are unknown except at 21 eV [lo] and their

142

C.E. Brian, K.H. Tan/Oscillatorstrengthsfor pkotoionizationof rV20 and CO2

resultant neglect introduces some uncertainties the results [14].

into

PES studies of binding energy spectra using He II radiation [15] and X-rays at high [16,17] and intermediate [18,19] energies have indicated the presence of a number of extra peaks due to multiple electron processes (i.e. simultaneous ionization and excitation) arising from electron correlation effects. The terms configuration interaction (CI), satellite or shake-up have also been used to describe these phenomena. We shall, in general, refer to all peaks associated with such processes as multiple electron transitions (MET). Although Gustafsson et al. [14] noted the existence of

several such peaks in the binding energy spectra, no spectra are shown nor was allowance made For such processes in deriving the branching ratios and partial cross sections. He I1 binding energy spectra [IS] indicate that multiple electron photoionization processes are significant even at 41 eV although no quantitative assessment is possible from the work since the photoelectron spectrometer used was not calibrated fortransmission efficiency as a function of electron energy. In our previous studies [3,5] using the dipole (e,2e) method it has been found that the equivalent photon energy can be extended to at least 60 eV while still retaining reasonable statistical accuracy. This energy range is useful for the study of the multiple electron transitions and inner valence orbitals most of which are expected to have threshold energies in the 25-40 eV range. These processes are of particular interest in view of recent many body Green’s function calculations by Domcke et al. [20,21] which predict significant breakdown of the simple molecular orbital picture for photoionization. The effect is particularly prominent for ionization of inner valence orbitals. Furrher calculations [22] have indicated that a rich structure is to be expected in the binding energy spectra of CO2 and N20 between 20 and 40 eV. These calculations [20-221 have led to the suggestion [21] that the distinction between main tines and satellite lines may in fact be inappropriate in cases where the spectral intensity is spread out over many lines_ Rather it should be recognized that in an adequate many-body description ionization of a so-called “one electron orbital” may lead to a number of fmal ionic states where no one state can be singled out as the “parent”_ This view: point is perhaps more realistic than using such terms as “shake-up” and the more recently coined “shake-

down” for lower energy peaks which arose initially from a consideration of inner shell spectra where because of large energy separations mixing of fmal states is not extensive. A relatively new type of electron spectroscopic technique, namely the BINARY (e,2e) method* [23261, which measures momentum distributions and binding energies, is finding increasing use for assigning orbital symmetry [27] and investigating the “parentage” of satellite structure [23,25] _Giardini-Guidoni et al. [29] have recently applied the binary (e,2e) method to the assignment of inner valence orbitals and_satellite structure in the binding energy spectrum of CO,. The implications of these results for the present work are discussed in a later section of this paper.

2. Experimental

The (e,2e) coincidence spectrometer and the techniques involved for data aquisition have been described in earlier publications [1,7]. The intensities of all peaks in the binding ener,q spectra were corrected for the ejected electron analyser transmission efficiency using the correction curve shown earlier [7]. The overall energy resolution (fwhm) is 1.3 eV for the coincidence studies and 0.9 eV for the absorption since this latter measurement only involves one energy analyser. Results were obtained in the range 17-60 eV_ The results are presented in figs. 6-9 as dipole oscillator strengths as well as optical cross sections. The tabular data may be transformed using the conversion factor o(IO-18 cm2) = 109.75 df/dE (eV_1). Gas samples were obtained from commercially available cylinders. The binding energy spectra showed no significant peaks due to impurities.

3. Results and discussion 3.1. Binding energy spectra The energies and Franck-Condon widths for the first four IP’s of CO, and N,O are well known from * The binary (e. 2e) technique is a large momentum transfer electron impact method and should not be confused with the dipole (e, 2e) method (small momentum transfer) used in the present work.

C.E. Brian. K.H. Tan/Oscillator strengths

s

0

P

2T

- x iFi 5 , 7UITr6U

6

E=56eV

!

I

I

N20 El

(b)

A

Fig. 1. Binding energy spectra for the vaIenceorbit&

of N20

and CO*at 56 and 60 eV, respectively(corrected for transmission). h&h resolution PES at 584 a [30,9]. The [P’s for the two inner orbitals are less well defined as is discussed below. The valence shell configuration of CO, can be written [30,31] as (3~~)~ (20,)~ (40,)~ (30,)~ (1~)~ (1Q4; lx;_ The features corresponding to the first four one electron IP’s are shown in the 60 eV binding energy spectrum shown in fig. la (peaks X, A-C) together with a considerable amount of structure (peaks I-VI)

due to ionization of the two inner orbitals and

multiple electron transitions. The energies of the various peaks above 20 eV are in reasonable agreement with the values reported in other publications [14-16, 18,291 using various ionizing sources. From an examination of spectra at various energies we estimate the following energies for the MET peaks, all ?O.SeV; I(23.5), 11(26.5), 111(30.0), LV(32.0), V(35.5) and VI(38.0). The binary (e, 2e) studies of CO, ionization by Giardini-Guidoni et al. [29] provide significant insight into possible spectral assignments. Even at the modest resolution used (fwhm = 3 eV) considerable structure was observed [29] in the 20 -40 eV range of the binding energy spectra. In particular a large broad peak (intrinsic fwhm = 4 ev) was observed at x 33 eV corre-

forphotoionizationof N20

and CO2

143

spending to peak IV of fig. ia. Prominent structure above 35 eV was clearly separated into two components at about 37 f 0.5 eV (fwhm = 2 eV) and at 39 5 0.5 eV (fwhm = 3 eV) by running the binding energy spectra at different angles (corresponding to different momenta of the “knocked out”electrons). Potts and Williams [IS] have assigned a peak at 38.5 eV as being due to both 20, and 3u_ but this would seem to be incorrect in view of the b&ary (e, 2e) measurements. Calculated [P’s for the 2u,, and 30~ orbitals are 39.2 and 41 .l eV [32] and this splitting is compatible with that observed by Giardini-Guidoni et al. [29] and also with peaks V and VI in fig. la of the present work. The expected momentum distributions of the 20, and 30~ orbitals are very different and using this consideration Giardini-Guidoni et al. [29] have shown that the peak at 233 eV (peak III of fig. la) contains a significant contribution from ionization of the 30~ orbital in addition to other components. The many body calculations (see introduction for further discussion) by Domcke [22] show a large number of peaks in the binding energy spectrum of COZ, between 22 and 40 eV in addition to the four intense bands at lower energy. The calculated binding energy spectrum (shifted by about 2 eV to lower energy) shows a remarkable resemblance to the region of the spectrum above 25 eV in fig. la. On the basis of this theoretical work peaks VI and V would arise from ionization of the 30~ and 20, orbitals respectively whiie peak IV would have contributions from both orbitals. The Predictions are consistent with the binary (e, Se) results [29] discussed above. Peaks I and II are calculated [22] to be associated with ionization of the In, orbital. Various other smaller contributions from all orbitals other than the 14 are calculated [22] to be present above 30 eV. Following the descdption given by Brundle and Turner [30] and using the ideas of Mulliken [3 l] the valence shell configuration of N,O can be written as (4~)~ (5~)~ (6~))’ (11~)~(7& (2~)~; lx+. Removal of electrons from each of the six valence orbitals in turn would lead to six ionization potentials (IF?) based on a single particle picture. The first four of these can be clearly seen (X, A-C) in fig. lb which shows the binding energy spectrum of N20 at an equivalent photon energy of 55 eV. However, it can also be seen that there is considerable extra structure (at least five broad bands I-V can be identified) in the region be-

144

C.E. Brian, K.H. Tan/Oscillator strengths for photoionization

7i

-

of N20 and CO2

E=30eV

IO

15

25

20 BINDING ENERGY

t&b

Fig. 2. Binding energy spectrum of N20 at 30 eV (corrected for transmission). Dots are experimental points, dashes are computed fitted contributions for individual states and the solid line is the total computer fitted cwve.

tween 23 and 40 eV. These can be ascribed to ionization of the two inner orbitals and to multiple electron transitions. From an examination of the (e, 2e) binding energy spectra at various “photon”energies we estimate the following energies for the MET peaks, all 50SeV; 1(24-O), II(28.5), III(33.0), IV(35.Q V(38.0). Corresponding peaks are observed by Potts and Williams [I51 using He II PES with peaks at 33.7 and 373 eV being assigned to the 50 and 40 orb&, respectively. The structure above 20 eV also appears in the various photoelectron spectra [15,17,i9,33] with minor variations in peak positions and intensities. The dipole (e, 2e) binding energy spectrum of N,O shown in fig. 2 suggests that peak I has at least two’ components in agreement with the He II spectrum [IJ]. A detailed study of this spectrum by binary(e, ?e) spectroscopy [23-261 would provide further useful information concerning the assignments. The calculations by Domcke [22] for N20 indicate that in addition to the four main bands a rich structure of lines is to be expected above 20 eV. Allowing for an approximate shift of 2 eV to lower energy, as in the case of CO,, the calculations [22] predict the following major contributions to the peaks in fig. lb I(ln); II( III(5o); IV (50); V(4u). Various other smaller peaks arising from all six orbitals are predicted to occur between 20 and 42 eV_ The binding energy spectra of both CO, and N20 are in reasonable agreement with the theoretical padictions [22] thus supporting the suggestion of breakdown of the simple molecular orbital picture especial-

Fig. 3. Binding energy spectrum of CO2 at 30 eV (corrected for transmission). Dots are experimental points, dashes are computer fitted contributions and the solid line is the total computer fitted curve.

ly for ionization of the inner valence orbitals. The binary (e, 2e) results [29] for CO, provide further evidence in support of this idea. With these considerations in mind, it is evident that a more detailed study of these spectra both by high resolution, tuneable energy, PES and also binary (e,2e) spectroscopy [23-261 would be of great interest_ 3.2. Photoelectrou bramhing ratios and dipole

oscillator strengths The A, B and C states of N20t are partially resolved as can be seen in the binding energy spectrum at 30 eV shown in fig. 2. Similarly from fig. 3, which shows part of the spectrum of CO, obtained at 30 eV, it can be seen that the A, B and C states are contained under a single envelope. A computer curve fitting deconvolution program was used to obtain the separate peak areas (A, B and C for N20; [AtB] and C for COz)after the curves had been corrected for analyser transmission efficiency. The intrinsic peak widths due to unresolved vibrational structure (Franck-Condon) widths were obtained from high resolution photoelectron spectra PO] which also gave the expected values of the vertical IP’s. Utilizing this information together with the known instrumental resolution the spectra were Ieast-squares fitted to combinations of gaussian functions. In figs. 2 and 3 the points are the experimental values. The dashed lines are the computer deconvoluted peaks with the solid lie being the total computed spectrum. In the case of CO, the A and B

C.E. Brim. K.H. TanfOscilIator strengths for photoiotdzation

of N20 and CO:!

145

ENERGY (eV)

Pig. 4. Photoionization branching ratios for NaO. Dots - this work; CROSSES - PES. ref. [9 ] ; squares - PES, ref. [8] _The dashed line is the fluorescence yield for the A state from ref. [34] and the open triangle is from fluorescence measurements at 584 A, refs. 136,371.

Fig. 5. Photoionization branching ratios for COl. Dots - this work; open circles - PES, ref. [14]; open triangles - PES, ref. [ill; crosses - PES, ref. [9]; squares - PES, ref. [g].The dashed line is the total fluorescence yield for (A+B) from refs+ [39,40].

states are too close in energy (17.6 and 18.1 eV, respectively) for satisfactory deconvolution and are presented throughout this paper as a summed contribution. The higher regions of the spectra above 22 eV were integrated after transmission correction and reported as a combined contribution for each molecule since the structure is dense and only partially resolved. The area computations have been used to obtain the photoelectron branching ratio curves shown in figs. 4 and 5. Numerical values are given in tables 1 and 2. Discussing first the case of N20 it can be seen that the 24 eV satellite peak (figs. lb and 2) is clearly separated and the branching ratio for this band can therefore be determined separately. However, it should be noted that for reasons of consistency, when comparing with CO,, the 24 eV peak is also included in the total multipie eiectron transition (MET) intensity in the lower section of fig.4. The branching ratios report-

ed by Bahr et al. [S], beIow 21 eV, appear to follow the trend of the present data. The measurement by Brian and Yee [9] at 21.2 eV is also in good agreement with the present work. The branching ratios smoothly decrease with energy although a few broad structures are observed which are possibly due to autoionizing bands. Sharp autoionizing structures wilI be smeared out by the ener,T resolution but it should be noted that their contribution will still be contained under the branching ratio (and partial ionization and absorption cross section) curves. Rapid variations in cross section due to autoionization have been reported to occur below 20 eV [14]. Lee [34] has measured the cross section for the production of N20’(A+ X) fluorescence arising from photoionization. The fluorescence yield, calculated using the total photoabsorption cross section [3.S], is shown as the dashed line in the A *Zf section of fig. 4.

146

C-E. h-ion, K.H.

TanlOsciNotor strengthsforphotoionization of N20

and CO2

Table 1 Photoelectron branching ratios (70) for electronic states of NzOf

Table 2 Photoelectronbranchingratios (%) forelectronicstatesof

Energy (ev)

X’II

A2Z+

17.1 19.0 19.6 21.2 22 23 24 25 26 21 28 29 30 32 34 36 38 40 41 42 44 46 48 50 52 54 56 58 60

56 42 41 35 33 33 34 36 35 36 36 34 34 34 34 34 35 34 35 35 31 32 33 31 31 31 28 28 27

44 41 35 25 19 19 19 18 18 18 17 19 18 18 15 14 15 15 13 I5 13 14 13 11 13 11 12 10 9

B211

17 25 26 26 21 27 27 26 26 26 26 26 25 25 25 22 22 21 20 20 19 17 18 17 16 17 17 16

COf

C2Z+

24eV

Total MET

Ener,.g (eV)

-

-

_ -

21.2

28

66

6

-

22 23 24 25

26 30 35 36

65 59 56 54

8 11 9 10

-

26 27 28 29 30 31 32 34 36 38 40 41 42 44 46 48

36 35 31 31 29 31 30 32 30 28 29 31 30 30 29

28

52 52 50 50 49 46 47 44 43 45 44 43 43 42 43 39

9 10 14 13 13 15 15 16 13 14 13 10 10 10 8 7

20 25

28

38

9

25

27 24 27 28 26

34 34 33 32 30

8 9 10 10 10

31 33 29 31 34

15 22 21 21 16 16 16 15 14 14 14 15 14 14 14 14 15 15 1.5 15 14 15 15 13 14 13

_

_

3 6 5 7 7 7 6 7 5 6 4 8 6 5 6 4 4 4 7 4 4 6

3 6 5 7 7 9 9 11 14 15 16 18 16 21 20 22 26 24 27 30 32 35

There is excellent agreement with our measured A state branching ratio above 25 eV. Although the shapes of the two curves are the same at lower energies there is some difference in the absolute magnitudes. Monahan

and Wauchop [36] and also Eland [37] have measured the A state fluorescence at 2 1.2 eV obtaining a yield of 20% which is in close agreement with the present (e, 2e) result for the A state branching ratio. In a further study of fluorescence from photodissociation fragments of N20 Lee et al. [38] report that below 60 eV less than 2.5% of the fluorescence is from dissociative prcZucts_ It is noteworthy that the multiple electron :ransition intensity accounts for more than 30% of the total photoionization in the region of 60eV and decreases appro.ximately linearly with energy with no obvious thresholds corresponding to the onsets of new processes. This doubtless reflects the high density of states due to multiple electron processes.

50 52 54 56 58 60

Total MET

.

4 4 5 6 9 7 8 8 13 14 14

16

16

18

Similar considerations apply to the CO2 branching ratios shown in fig. 5 (and table 2) where it can be seen that in general there is excellent agreement with the many-line magic angle PES results of Samson [ 111 and also the synchrotron radiation studies of Gustafsson et al. [14]. The discrepancies with the latter work in the region above 30 eV are perhaps due in part to the neglect of the MET structure by these authors [14] while other minor variations may be due to the unknown /3dependence implicit in the synchrotron work. Agreement is generally slightly better with Samson’s data (also fl independent like the present work) except at the single point at 40 eV where the MET contribution is probably overestimated by Samson [l l] since it lies on the limit of the low electron energy “ramp” at He II energies. Reasonable agreement is also obtained with the single PES data point of Brian and Yee [9] et al. Our branching ratios for CO; (A+B) are smallei than those reported by Gustafsson et al. [14] -above

C.E. Brian,K.H. TanfOsciIlator strengthsforphotoionization of h’,O and CO2

0

!

I

I

IO Fig. 6. Photoabsorption

I I I 30 40 ENERGY (eV1

I

20

and photoionization

efficiency

I

I

50

I

I

147

‘0

60

(insert) of NaO. Dots - this work; solid line is photoabsorption

-

synchrotron absorption studies from ref. [35].

I6

20

Fig. 7. Photoabsorption and photoionization photoabsorption - synchrotron absorption

SO ENERGY 4tL

50

60

efficiency (insert) of COP. Dots - this work; triangles - from ref. [ll]; studies from ref. [35].

solid tine is

148

C.E. &ion, K.H. Tan/OscUator strengthsfor photoionization of N20 and CO2

Table 3 Oscillator strengths for partial photoionization and photoabsorption of N20 a) Energy W) 17 19

19.6 21.2 22

Partial osciktor strength (10” ev-‘) X?l

A2Z+

B21-I

23.22 15.58

18.24 15.21

6.30

14.80 11.66 10.55

12.63 8.34 6.08

9.02 8.67 8.32

Total absorption c ?Z+

24 eV

Total MET

(lo-2 ev-‘) 41.46

37.08 36.08 33.35 31.98

5.00 7.03

23

9.86

5.68

8.07

6.28

29.89

24 25 26

9.73 10.20 9.57

7.73 7.65

6.00 4.54

0.85

0.85

28.61 28.34

7.11

4.37

1.64

1.64

27.33

27

9-78

28 29 30

9.55 8.89 8.80

5.44 5.10 4.92 4.88 4.51 4.97 4.66

7.06 6.90 6.80 6.72

4.35 3.97 3.66 3.63

1.36 1.86 1.82 1.81

1.36 1.86 1.82 2.32

27.15 26.51 26.15 25.88

32

34

8.61 8.43

36 38

4.56

6.33

3.54

1.52

2.28

25.33

7.34 6.89

3.72 3.02 295

6.20 5.40 4.33

3.72 3.02 2.75

1.73 1.08 1.18

2.72 3.02 2.95

24.78 21.59 19.68

40

6.35

2.8 1

4.11

2.61

6.25 6.16 5.26 5.08

2.32 2.64 2.20 2.22

3.75 3.52 3.39 3.02

2.50 2.63 2.54 2.38

0.75 1.43

2.99

41 42 44 46 48

1.06 0.85 0.95

3.22 2.82 3.56 3.17

18.68 17.86 17.59 16.95

5.14 4.60 4.44 4.26 3.63 3.22 2.95

2.02 1.63 1.86 1.51 1.55 1.15 0.98

2.65 2.67 2.43 2.20 2.20 1.95 1.75

2.33 2.08 2.15 2.07 1.69 1.60 1.42

0.62 0.59 0.57 0.97 0.52 0.46 0.66

3.43 3.86 3.43 3.72 3.88 3.67 3.83

so 52 54 56 58 60

15.85 15.58 14.85 14.31 13.76 12.94 11.48 10.93

a) o(XIb) = 105.75 df/dE (A’-‘).

30 eV (see fig. 5). The magnitude of the difference

is related to the percentage of the total ionization going into the higher (MET) ionic states (see table2 and Iig. 5) which is larger (e.g. 14% at 40eV) than Gustafsson et al. [14] estimate. As a result the (A+B) branch-

ing ratio that we obtain is in good agreement with the sum derived from the photofluorescence measurements of Lee et al. [39] for the A-tX transition and of Carlsson et al. [40] for the B+X transition out to 60 eV. This sum is shown as the dashed line of fig. 5. Although the branching ratio sum (A+B) is the same by both types of measurement a large and so far unexplained difference exists in the (A/B) cross section ratio as determined by PES (0.65, refs. [11,14]) and photofluorescence (==2 20.5, refs. [39,40]). A detail-

ed discussion of this has been given by Gustafsson et al. [14]. AIthough the A and B states cannot be separated in the present work it is of interest to note that the quantitative high resolution PES studies by Brion and Yee [9] give a value of 0.63 for the (A/B) ratio in agreement with the other PES data [12-14]_ Finaily it should be noted that as in the case of N20 there is a large contribution above 20 eV from the MET processes. Figs. 6 and 7 show the oscillator strength (cross section) for total photoabsorption of N20 and CO,, respectively. These were obtained from the forward scattering (non-coincident) energy loss spectra by kinematic correction according to the Bethe-Born theotywith allowance fcr the instrumental scattering geometry. The resulting relative optical absorption oscillator strengths were normalized at a singk point an@ (at

C.E. Won.

K.H. Tan/OscilIator strengths for photoionization

of NzO and CO?

149

Table 4 Oscillator strengths for partial photoionization and photoabsorption of CO;! a) Energy (ev) 21.2 22 23 24 25 26 27 28 29 30 31 32 34 36 38 40 41 42 44 46 48 50 52 54 56 58 60

Partial oscillator strength (10” cV_‘) x2rIg

A211&-B2EU

c2xg

Total MET

9.37 8.41 9.07 10.20 9.94

22.07 21.03 11.85 16.33 14.91

2.00 2.59 3.33 2.62 2.76

-

9.65 9.44 8.20 8.05 7.40 7.60 7.41 7.73 7.24 6.25 5.81 5.90 5.41 4.89 4.34 4.14 4.05 3.89 3.34 3.56 3.37 2.87

14.03 14.02 13.17 12.98 12.50 12.27 11.61 10.62 10.38 10.05 8.82 8.19 7.75 6.85 6.42 5.76 5.50 4.89 4.74 4.36 3.84 3.31

2.34 2.70 3.69 3.38 3.32 3.68 3.71 3.86 3.14 3.12 2.61 1.90 1.80 1.63 1.19 1.03 1.30 1.15 1.26 1.32 L-20 1.10

_ 1.08 1.08 1.31 1.56 2.30 1.71 1.98 1.93 3.14 3.12 2.8 1 3.04 2.89 2.93 2.99 3.69 3.63 4.46 4.60 3.84 3.73 3.74

Total photoabsorption (10’ eV-' ) 33.44 32.35 30.25 29.16 27.61 27.15 26.97 2633 25.97 25.51 24.51 24.69 24.15 24.15 22.32 20.06 19.04 18.04 16.31 14.94 14.76 14.49 14.40 13.94 13.21 12.03 11.02

a) a(Mb) = 109.75 dfldE (eV_l). 35 eV) on the synchrotron

data reported

by Lee et al.

It can be seen that the shape of the electron impact simulation curve follows the results of Lee et al. very closely especially in the case of N20. Cole and Dexter [41] have recently reported photoabsorption data for N20 above 35 eV and the cross sections are about 10% lower than the values reported by Lee et al. [3S] _The recent results of Samson [I 11 on CO, are also shown in !ig_7 and again the agreement between the various sets of data is very good. The present work follows the detailed shape of Samson’s data slightly better. The measured photoabsorption oscillator strengths are listed in tables 3 and4. For both molecules the relative ionization efficiency can be determined by summing the partial ionization peak areas at each energy (having normalized the beam current and collection time) and dividing these [35].

numbers by the corresponding total forward scattering (non-coincident) intensities (the Bethe-Born correction cancels in this operation and thus the result is independent of this)- It can be seen that the relative ionization efficiency (see inserts to figs. 6 and 7) is effectively constant for both molecules as is expected. It has therefore been normalized to unity in accord with the measurements by Cairns and Samson [43] for CO,. Since the ionization efficiency (pi) determination involves a number of measurementsthe statistical accuracy is correspondingly less. Nevertheless this determination together with the photoabsorption measurements provides a consistency check. The branching ratios shown in figs. 4 and 5 (tables 1 and 2) are next multiplied by the total photoabsorption (since vi= 1) to obtain the partial oscillator strengths (cross sections) shown in figs. 8 and 9 (also

forpftotoionizationofNz0

150

C.E. Brian. KU. TanfOsciifatorstrengtfzs

Kl

20

30

40

50

Q. 9. Partial oscillator strengths (cross sections) for the photoionization ofCO2. Dots - this work; triangles - PES, data of ref. [ll]; open circles - PES, data of ref. [14].

60

ENERGY (eV) Fig- 8_ Partial oscillator strengths (cross sections) toionization oTN20. Dots - this work.

and CO2

for the pho-

in tables 3 and 4). From fig. 9 it can be seen that the results for CO, agree quite well with the work of Samson [I 1] and Gustafsson et al. 1141 considering again the points raised in the earlier discussion comparing branching ratios. Other than the few low energy points reported by Bahr et al. [S] no other data has been published for NIO to the best of our knowIedge. The broad bump in the CO; (C ?Zgf) partial oscillator strength between 30 and 40 eV is noteworthy in that the oscillator strength of the C state of NZO( (fig_ 8) shows a quite different behaviour. It is possible that there is a shape resonance in the case of CO; (C *Cd). Such phenomena have been predicted theoreticafiy [43-45] and observed experimentally [46,47] for CO and N2. No calculations for CO2 and N20 have been reported to date. There is a large contribution to the total photoionization from the MET peaks in the region approaching 60 eV. It can also be seen that the a oscillator strengths (cross sections) drop much more quickly with increase in energy as

compared with those for u orbitals. This is in accord with the observations of UPS and XI’S [17].

Acknowledgement Financial support for this work was provided by the National Research Council of Canada and the North Atlantic Treaty Organization. We wish to acknowledge useful discussions with Dr. M.J. van der Wiel. We also express our thanks to Dr. J.A.R. Samson. for making his results available to us prior to publication. Dr. A.P. Hitchcock and Mr. J.P.D. Cook have provided helpftil comments concerning the computer deconvolution procedures and this manuscript. We are indebted to Dr. W. Domcke for providing us with the results of calculations prior to publication.

CE. Brian, KH. Tun/Oscilkfor strengths forphotoionizotion

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