A study of valence ionization of ammonia with synchrotron radiation

157

Chemical Physics 114 (1987) 157-163 North-Holland, Amsterdam

A STUDY OF VALENCE

IONIZATION OF AMMONIA

WITH SYNCHROTRON

RADIATION

M. Salim BANNA’ Deparhnent

of Chemistry

Vanderbilt Universi&

Hartmut KOSSMANN Department

Nashville, TN 37235, USA

and Volker SCHMIDT

of Physics, Freiburg lJniversi&

7800 Freiburg, FRG

Received 8 December 1986

Angular distributions of photoelectrons from the three valence levels of molecular ammonia, 2a, with a binding energy (BE) of 27.7 eV, le (BE = 16.3 eV) and 3a, (BE = 10.9 eV) have been measured in the 30-130 eV photon energy range. Comparison is made in the case of the 3a, molecular orbital with the calculated atomic N 2p asymmetry parameters. Photoionization branching ratios have also been obtained in this photon energy range and good agreement has been found with previous quasiphotoionization measurements and with the one-center-expansion calculations of Cacelli et al. The satellite structure in the inner valence (2a,) region has been investigated at 60 eV and compared with a number of energy and intensity calculations.

1. Introduction Ionization measurements have been carried out extensively on ammonia and on the other first-row hydrides CH, , H,O and HF. These molecules have traditionally been the subject of numerous theoretical studies and have often been used to test new theoretical approaches to calculating ionization quantities. There is at present considerable interest in the measurement [1,2] and calculation [2] of photoionization cross sections and of angular distribution asymmetry parameters. Part of the theoretical effort now underway should be directed towards providing a bridge between our understanding of the electronic structure of molecules and of the constituent atoms. In this context it is essential that correlations be sought between photoionization in atoms and in simple molecules. Only when such correlations have been established can the full potential of detailed photoionization measurements as a tool for understanding chemical bonding be realized. This goal has stimulated us to undertake experimental studies of a number ’ Alexander von Humboldt Fellow.

of first-row hydrides which are isoelectronic with neon, for which a considerable body of photoionization data exists. Thus our results for water have been reported recently [3] and our data on hydrogen fluoride will be published in the near future [4]. In contrast to water, there appears to be no previous experimental work on the angular distributions of photoelectrons from the valence levels of ammonia *. The present study, which covers the photon energy range between 30 and 130 eV, thus provides needed data and should stimulate calculations of asymmetry parameters. Additionally, we have measured relative photoionization cross sections including those for the inner-valence-ionization states. These measurements serve to extend previous data [6] to higher photon energies and provide a more complete test of theory. 2. Experimental The technique of electron spectrometry with synchrotron radiation (ESSR) was used in the * A recent experimental study has been undertaken by M.N. Piancastelli, C. Cauletti and M.Y. Adam [5].

0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

158

MS. Burma et al. / Valence ionization of ammonia

present study, with photons from a toroidal-grating monochromator coupled to the German storage ring BESSY. It was possible to obtain = 1012 photons/s with a band pass (for 0.3 mm slits) of = 0.3 eV at 60 eV, and 1 eV at 100 eV. Three cylindrical mirror sectors were used as electron analyzers, two of which were rotatable and the third used for normalization with respect to the photon flux and the target density. The spectrometer has been described elsewhere [7]. Asymmetry parameters (/3) and relative photoionization cross sections were obtained using the equation (valid in the dipole approximation) da( B)/dS2 = (a/4~)

[ 1 + a/3(1 + 3P, cos 2(?)] , 0)

relating the differential and integrated cross sections. Here 8 is the angle between the main polarization axis and the photoelectron direction. The Stokes parameter PI, as well as the azimuthal angle X for the rotation of the polarization ellipse relative to the plane of the ring, were determined for each ring injection by measuring the angular distributions in atomic helium. PI was found to be about 70%. By making measurements at three different angles with each of the two rotatable analyzers, a least-squares fit could be performed using eq. (1) to obtain /? and relative u. The latter values have been corrected for possible loss of analyzer transmission and reduction in the electron multiplier detection efficiency at low kinetic energies (= 10 eV) after the correction for the analyzer dispersion was made by dividing peak intensities by the mean kinetic energy. At the quasimagic angle eQM = + arccos( - 1/3P,)

(2)

eq. (1) shows that the intensities are proportional to the integrated cross sections. Measurements at these angles thus allow for a more direct determination of relative cross sections.

[8-111, it is instructive to show a sample spectrum in order to illustrate the routinely achievable resolution and count rates. This is done in fig. 1, for 60 eV photon-energy excitation. The binding energies for the three MOs are 10.9 eV for 3a,, 16.3 eV for le and 27.7 eV for 2a, [lo]. The structure on the high binding energy side of the 3a, peak is due to a small amount of water in the target gas (or possibly the background gas). The spectrum in fig. 1 differs from previously shown PES spectra in that it was obtained at the quasimagic angle rather than at 90 O. 3.1. Angular distributions of valence photoelectrons We have obtained asymmetry parameters (8) for the three MOs over the 30-130 eV photon energy range covered in this study. The results are shown in table 1 and plotted in fig. 2. As in water [3], it is expected that the 2a, p values should eventually approach 2 since this orbital is mainly N 2s. Indeed, as can be seen from table 1, a /3 value of = 1.9 is reached at a photon energy of 130 eV. The values for 3a, are shown in the top part of fig. 2. Since this is a non-bonding orbital consisting mainly of N 2p and partly N 2s, a similarity with the atomic case might be expected. Accordingly, we have plotted in fig. 2 the results of BN2r calculations by Cherepkov et al. [12]. These include many-electron correlations using the random phase approximation with exchange (RPAE) method. The atomic fi agree very well

‘: !?

Although valence photoelectron spectra (PES) of ammonia have been shown many times before

a, :. ,e ;< ,:. : . : .’ ::

:: Y) (D 2 .

:

‘.

!. ::

? 2 :

20,

0

3. Results and discussion

at 60 eV NH3 Ouasimagic Angle

2.0.

IO

25 KINETIC

ENERGY

:

40 W)

:

:. : ._

1 55

Fig. 1. The molecular orbit& of ammonia obtained with 60 eV photons at the quasimagic angle. The weak structure on the low kinetic energy side of the 3a, peak is due to a small amount of water present in the chamber.

MS. Banna et al. / Valence ionization Table 1 Asymmetry parameters for the molecular orbitals of ammonia hv (ev) 30 35 40 45 SO 55 60 65 70 75 80 90 105 120 130

2al

le

3al

l.Sq8) 1.50(4) 1.65(4) 1.62(4) 1.67(4) l.%(4) 1.57(3) 1.61(4) 1.65(S) 1.63(3) 1.69(S) 1.8q2) 1.83(2) 1.86(S)

0.8X4) 0.92(i) 0.98(2) 1.02(2) 1.07(2) lSl(2) 1.12(2) 1X(2) 1.17(2) 1.22(2) 1.19(4) 1.23(S) 1.26(3) 1.31(4) 1.33(8)

1.42(2) 1.47(3) 1.48(2) 1.53(2) 1.58(2) 1.60(2) 1.59(2) 1.59(2) 1.64(3) 1.67(2) 1.62(3) 1.63(S) 1.59(3) 1.62(3) 1.63(10)

with the measured 3a, values, confirming the 2p character of this molecular orbital. The situation is similar in water, where the lone-pair character of the lb, and 3a, MOs is exhibited by a similarity

1 60 KINETIC

120 ENERGY

(eV)

Fig. 2. The asymmetry parameters of the three ammonia MOs (binding energies are 3a, 10.9 eV, le 16.3 eV and 2a, 27.7 eV) as a function of the photoelectron kinetic energy. For clarity, the le asymmetry parameters have been incremented by 2.00 and the 3a, asymmetry parameters by 3.00. The solid line corresponds to the atomic N 2p asymmetry parameter calculated by Cherepkov et al. [12] using the RPAE method.

of ammonia

159

to the atomic 0 2p /I values [3,13]. The theoretical & curve in fig. 2 extends all the way to threshold and shows the rapid rise in p values for 2p electrons. This has been noted before in this region of the periodic table [13,14]. It is interesting to compare the behavior of the j? values for the bonding orbitals in ammonia and water. In the latter, the lb, orbital consists of bonding combinations of 0 2p and H 1s. Roche et al. [13] previously noted, and our recent experimental results confirmed, that the /3 curve for lb, is generally similar to the corresponding 0 2p curve, but is displaced to lower values due to bonding effects. A similar situation occurs in ammonia, as can be seen from the le curve (middle curve) in fig. 2. The 3a, and le curves are alike, but the latter is displaced by = 0.3 unit to lower /? values. This shift is close to the one observed in water [3]. 3.2. Photoionization sections

branching

ratios

and

cross

Electron impact coincidence (quasiphotoionization) methods have been used to study dipole ionization in NH,. Branching ratios for the three valence orbitals have been measured in the 15-50 eV excitation-energy region by Brion et al. [6]. Partial photoionization oscillator strengths could then be obtained by normalizing to the absorption values of Wight et al. [15]. We follow a similar procedure here, using photoabsorption data to calculate absolute partial cross sections from our branching ratios. In this manner we can compare with the recent calculations of Cacelli et al. [16,17]. The branching ratios obtained from our measurements of fi and in some instances from measurements at the quasimagic angle are given in table 2. Our results cover an energy range which partially overlaps that of Brion et al. [6]; thus comparison is made between their data and ours in table 2. The agreement is seen to be satisfactory. Absolute partial photoionization cross sections obtained by normalizing our branching ratios to the photoabsorption data of Samson and Haddad [18], are given in table 3 and compared in some cases to the corresponding quasiphotoionization

160

MS. Banna et al. / Valence ionization

Table 2 Branching ratios (W) for the valence electronic state of NH:

hv WI

W

‘)

le

%

this work 30 =) 35 ‘) (36) 40 45 (46) 50 55 60 65 70 75 80 90 105 120 130

of ammonia

ref. [6] 11.7 11.7 15.5 19.6

13.0 14.3 16.8 19.3 22.0 22.7 23.5 25.3 24.8 25.9 28.5 30.3 32.9

3at

this work

ref. [6]

this work

ref. [6]

70.9 69.0 58.6 56.6 53.7 51.0 48.7 47.5 46.7 44.9 44.5 43.6 41.2 40.3 39.0

72.8 64.2 64.1 59.0 50.8

29.1 31.0 28.3 29.2 29.5 29.7 29.3 29.8 29.8 29.8 30.7 30.5 30.3 29.4 28.1

27.2 24.0 24.1 25.6 29.6

‘) The uncertainty in our ratios is about 5% of the values given in the table for 2a, and about 3% for the other two MOs. b, The energy in parentheses is the one used by Brian et al. [6]. ‘) Since the 2a, MO was not measured at this energy our branching ratios for the other two MOs are probably too high.

results which are obtained by normalizing to the photoabsorption cross sections of Wight et al. [15] from electron energy loss spectroscopy. A plot of these values is shown in fig. 3. Also shown are the theoretical length and velocity curves of Cacelli and co-workers [17]. These authors used one-center

basis sets (centered on the heavy atom, which is the nitrogen atom in this case) to describe both the bound and unbound electrons. For the latter, they used a new kind of oscillating integrable functions. To obtain the oscillator strength density df(c)/dc needed in the calculation of the partial

Table 3 Partial photoionization cross sections for ammonia (Mb) hv (ev)

Aa) 30 35 (36) 40 45 (46) 50 55 60 65 70 75 80 90 105 120

le

2ar

1.13 0.96 0.89 0.82 0.76 0.65 0.56 0.51 0.43 0.34 0.26 0.19

B b, 1.43 1.10 1.10 1.21

Total (T

3at

Aa)

B b,

Aa’

B b,

ref. [15]

ref. [18]

11.34 7.94 5.10 3.80 2.84 2.16 1.68 1.35 1.11 0.90 0.76 0.57 0.38 0.26

12.16 7.68 6.15 4.28 3.07

4.66 3.56 2.46 1.96 1.56 1.26 1.01 0.85 0.71 0.60 0.53 0.40 0.28 0.19

4.54 2.85 2.30 1.86 1.86

16.7 12.0 9.5 7.24 6.15

16.0 11.5 8.7 6.72 5.29 4.24 3.45 2.85 2.38 2.01 1.72 1.31 0.92 0.64

*) This work; obtained from the branching ratios in table 2 using the photoabsorption data from ref. [18] given in column 9. b, From ref. [6]; obtained using the photoabsorption cross sections from ref. [15] given in column 8.

161

MS. Banna et al. / Valence ionization of ammonia

40

‘,

”

2a, 0

IO

‘.’

”

-0..

50 PHOTON

.

90 ENERGY

”

130

(et’)

Fig. 3. Absolute partial photoionization cross sections of ammonia as a function of photon energy. Our results are denoted by solid circles and those of Brion et al. [6] by open circles. The values .used are taken from table 3 and from ref. [a]. The solid lines correspond to the length (L) and velocity-gauge calculations of Cacehi et al. [16,17]. The le cross sections have been incremented by 2.00 Mb and the 3a, cross sections by 30.0 in order to display them on the same scale.

photoionization cross sections as functions of photon energy, they used the Stieltjes method which gives it in terms of the discrete oscillator strengths by the Stieltjes derivative. Rather good overall agreement with experiment can be noted from fig. 3, in which theoretical results obtained by the single-charmel-RPA method [17] rather than by the static exchange approximation [16] are plotted. The former method yields better agreement between the length- and velocity-gauge results and should therefore be more suitable for comparison with experiment [17]. Cacelli et al. [16,17] point out both for water and ammonia that the known [19] inadequacy of the independent-particle description in the inner valence region limits the agreement with experiment. They further pointed out that there was considerable scatter in the experimental data, which were taken from the work of Brion et al. [6] and Wight et al. [15]. Our results are more in accord with those of Brion et al. [6]. We confirm the conclusions of Cacelli et al. [16,17] that the length-gauge results are in better agreement with experiment than the velocity-gauge results, at least for the 3a, and le MOs. The

opposite behavior was noted for water [16,17]. For a good overview of the partial photoionization cross sections up to 1978 the reader is referred to the work by Berkowitz [20] which complements the discussion given here. Relative photoionization cross sections for NH, which fall in the energy range of interest here have previously been measured using photon excitation but with isolated lines rather than the continuously variable synchrotron radiation. Banna and Shirley [lo] reported relative differential photoionization cross sections obtained with the Y MS line with an energy of 132:3 eV. Similarly, Allison and Cave11 [ll] used both helium resonances lines and the Zr MS line (151.4 ev). In both studies the photoelectrons collected were perpendicular to the direction of propagation of the unpolarized photon beam. Since in our work both (I and p values have been measured, the unpolarized-radiation version of eq. (1) can be used to obtain the ul from our results at various energies. These can then be compared with previous values. From table 4 it is clear that agreement is very good in general. In this table we also show some of the calculations carried out on the ul values of NH,. For example, Yarzhemsky et al. [21] used the Table 4 Relative differential photoionization cross sections ( eI ) for the MOs of ammonia MO

hv=41eV this work ‘)

ref. [ll]

ref. [21]

2at

1.00 1.88 0.46

1.0 2.0 not measured

1.00 1.60 0.41

MO

hv=50eV

3ar le

this work ‘)

ref. [ll] b,

2ar

1.00 1.76 0.49

1 2 not measured

MO

hv=132eV

3a, le

3at le 2ar

this work a)

ref. [lo]

ref. [21]

ref. [22]

1.00 1.31 1.21

1.00 1.28 1.23

1.00 1.33 1.21

1.00 1.44 0.61

a) Obtained from our o and 1 results and eq. (1). b, Actually measured at hv = 48.4 eV.

162

M.S. Banna et al. / Valence ionization

Gelius model for molecular orbital cross sections with calculated atomic u and p values (using the random phase approximation with exchange, or RPAE) as well as calculated electron populations. The model expresses the MO cross sections as sums of weighted atomic cross sections. These results are shown for hv = 41 and 132 eV. They agree remarkably well with experiment at 132 eV and satisfactorily at 41 eV. We also give the theoretical values of Fujikawa et al. [22] at 132 eV who made a study of the use of the orthogonalized plane wave approximation for the outgoing photoelectron in the calculation of u. These authors noted the poor agreement between the predicted and experimental deepest MO cross sections at the Y MS energy in a number of small molecules and explained it as due to the occurrence of a minimum in u at around this energy and the difficulty of predicting the position and depth of this minimum by the OPW method. This is confirmed by our experimental data. 3.3. The inner-valence

van

Niessen

Cacelli

Bieri

et -- al

et --

al

region

Finally, we analyze the 2a, region of ammonia, which is expected to be poorly described by oneparticle methods [19]. It is shown in fig. 4, the spectrum being obtained at the quasimagic angle using 60 eV photons *. Both discrete and continuous features are clear on the high binding energy side of the main peak, extending to the limit of our measurement range of = 50 eV. For example, we observe a peak at = 32 eV. Interestingly, a shoulder at this binding energy is also clearly visibly in the Mg KLX (1254 eV) photoelectron spectrum of Barma and Shirley [lo]. Calculations indeed predict the presence of more than one pole with significant strength. Here we show the results of three Green function computations of the positions and strengths of these poles, restricting ourselves to the strongest poles in each case. Bieri et al. [24] obtained the results indicated by solid lines in fig. 4. The extended two-particle-hole Tamm-Dancoff Green function approximation (2ph-TDA) was used. This version is exact to * The experimental inner-valence spectrum shown previously [23] is in error.

of ammonia

BINDING

ENERGY

IeV)

Fig. 4. The 2a, (inner-valence) region of ammonia obtained with 60 eV photons at the quasimagic angle. The vertical bars correspond to the calculated pole positions and intensitiei. Only the relative intensities are meaningful. The top part shows the preliminary results of van Niessen [26], the middle part those of Cacelli et al. [25] and the bottom part those of Bieri et al. [24].

third order in the electron-electron interaction. They agree surprisingly well with the experimental spectrum. Cacelli et al. [25] calculated one-electron Green functions by solving the Dyson equation making suitable approximations for the optical potential. These authors noted the presence of many-body effects in the photoionization from intermediate (inner valence) energy shells but cautioned that the presence of many poles of the optical potential may be an artifact of the large bases employed. Indeed their results, with normalization (intended to improve their optical potential by avoiding the Hartree-Fock approximation in the calculation of the Green functions) appear to predict the opposite intensity behavior from the observed experimental one as can be seen from

M.S. Banna et al. / Valence ionization o/ammonia

fig. 4. On the other hand, without renormalization the correct qualitative trend is obtained. We also show in fig. 4 the main two poles which a very recent fourth-order Green function calculation [26] yields. These results, while preliminary, are in reasonable accord with experiment. Note, however, that none of the theoretical results quoted here include the photoionization cross sections of the states involved. These cross sections are of course needed for a more complete comparison between experiment and theory.

Acknowledgement This work was supported in the FRG by the Bundesminister fiir Forschung und Technologie and in the US by the National Science Foundation (Grant No. CHE 8319476), the Vanderbilt University Research Council and Natural Science Committee. We thank the BESSY personnel for providing us with excellent research facilities. We also wish to thank Professor James Samson, Dr. Ivo Cacelli and Dr. Wolfgang von Niessen for making their results available to us. One of us (MSB) acknowledges generous financial support from the Alexander von Humboldt Foundation.

References 111M.O. Krause, IEEE Trans. Nucl. Sci. 28 (1981) 1215. 121V. McKay, T.A. Carlson and R.R. Lucchese, J. Phys. Chem. 88 (1984) 3188. 131 MS. Banna, B.H. McQuaide, R. Malutzki and V. Schmidt, J. Chem. Phys. 84 (1986) 4739. 141 MS. Banna, R. Malutzki and V. Schmidt, to be published. 151 M.N. Piancastelh, private communication.

163

161C.E. Brian, A. Hamnett, G.R. Wight and M.J. van der Wiel, J. Electron Spectry. 12 (1977) 323. (71 H. Derenbach and V. Schmidt, J. Phys. B. 17 (1984) 83; H. Derenbach, R. Malutzki and V. Schmidt, Nucl. Instr. Methods 208 (1983) 845. 181 K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He1 photoelectron spectra of fundamental organic molecules (Halstead Press, New York, 1981) and references therein. 191 J.W. Rabalais, Principles of ultraviolet photoelectron spectroscopy (Wiley, New York, 1977). WI M.S. Banna and D.A. Shirley, J. Chem. Phys. 63 (1975) 4759. WI D.A. Allison and R.G. Cave& J. Chem. Phys. 68 (1978) 593. WI N.A. Cherepkov, L.V. Chemysheva, V. Radojevic and I. Pavlin, Can. J. Phys. 52 (1974) 349. 1131 M. Roche, D.S. Salahub and R.P. Messmer, J. Electron Spectry. 19 (1980) 273. P41 S.T. Manson, J. Electron Spectry. 1 (1972/73) 413. P51 G.R. Wight, M.J. van der Wiel and C.E. Brian, J. Phys. B 10 (1977) 1863. WI I. Cacelli, R. Moccia and V. Carravetta, Chem. Phys. 90 (1984) 313. 1171 I. Cacelli, private communication. WI J.A.R. Samson and G.N. Haddad, to be published. [I91 L.S. Cederbaum and W. Domcke, Advan. Chem. Phys. 36 (1977) 205. PO1 J. Berkowitz, Photoabsorption, photoionization and photoelectron spectroscopy (Academic Press, New York, 1979). WI V.G. Yamhemsky, V.I. Nefedov, M.Ya. Amusia, N.A. Cherepkov and L.V. Chemysheva, J. Electron Spectry. 19 (1980) 123. P21 T. Fujikawa, T. Ohta and H. Kuroda, J. Electron Spectry. 16 (1979) 285. 1231 MS. Banna, H. Kossmann and V. Schmidt, Proceedings of the 8th International Conference on Vacuum Ultraviolet Radiation Physics, Lund, Sweden, 4-8 August, 1986. 1241 G. Bieri, L. Asbrink and W. van Niessen, J. Electron Spectry. 27 (1982) 129. 1251 I. Cacelli, R. Moccia and V. Carravetta, Chem. Phys. 71 (1982) 199. [26] W. von Niessen, private communication.