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(LS) versus (jj) coupling in P 2p inner-shell excited states of PF3
7 February 1997
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 265 (1997) 490-496
(LS) versus (jj) coupling in P 2p inner-shell excited states of PF 3 N. Kosugi a, R.G. Cavell b, A.P. Hitchcock c a lnstituteJbr Molecular Science, Okazaki 444, Japan b Chemistry, University of'Alberta, Edmonton, Alb.. Canada T6G 2G2 c Chemistry. McMaster University, Hamilton. Ont.. Canada L8S 4MI
Received 14 October 1996; in final form 5 December 1996
Abstract
The P 2p total and parent ion yield spectra of PF3 have been measured and interpreted with ab initio SCF-CI calculations. The PFf yield has only one line at 136.5 eV, without a spin-orbit partner. The ab initio calculations attribute it to the (e- 1, e) IA i state. They predict the (e- i e) 3A i-(e- 1,e) IA ~ state separation is 2.4 eV, much greater than for other P 2p states (0.05-0.5 eV), and much larger than the P 2p-i spin-orbit splitting (0.9 eV). We conclude the 136.5 eV state is the (LS) coupled ( e - 1 e)IA~ state, and the normally dominant P 2p i spin-orbit (jj) splitting can be ignored. The importance of treating core-valence exchange as well as spin-orbit interactions is discussed.
1. Introduction
The 2p (L23) excitation spectra of third row elements and their compounds are generally characterized by the presence of two closely related manifolds of excited states, those associated with the 2p3/2 (L 3) and those associated with the 2pt/2 (L 2) spinorbit coupled ion core [1]. In most studies carried out to date this spin-orbit splitting has been found to vary minimally among various compounds of a given element. For this reason spectral assignments are often guided by identification of corresponding lines in the partner manifolds. In 1985 Sodhi and Brion [2] reported a detailed study of the inner-shell spectroscopy of a number of phosphorus compounds, including PF 3 and PCl 3. In that work a number of similarities between the low-energy region of their P 2p spectra were noted. However, a rather dramatic difference was found, namely that there was a sharp, intense transition at 136.5 eV in PF 3 which did not have a counterpart in the dipole-regime energy loss
spectrum of PCI 3. In contrast variable scattering angle studies of PC13 [2] revealed an a p p a r e n t l y similar state at 135.1 eV which had an angular dependence characteristic of an electric quadrupole transition. The corresponding variable scattering angle study of PF 3 [2] did not show any non-dipole states. Given the expected similarity of the electronic structure of these two species and the otherwise very similar nature of the low-lying regions of these P 2p spectra, these results are rather surprising. In addition, both the 136.5 eV dipole state in PF 3 and the 135.1 eV quadrupole state in PC! 3 appeared to be isolated lines, without obvious contributions from a spin-orbit partner state, although very weak features detected at appropriate separations were proposed as possible spin-orbit partner states [2]. Our goal in this study was to better understand the discrete region of the P 2p spectrum of PF 3 through a high-resolution synchrotron radiation study of the total and partial ion yields, combined with high-level ab initio calculations. Recently several groups have
000%2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0009-2614(96)01472-8
N. Kosugi et al. / Chemical Physics Letters 265 (1997) 490-496
studied the P 2p spectrum of PF 3 at high resolution using either photon [3,4] or electron impact [5] excitation. Several theoretical studies have also been reported [4,6]. A complete description of P 2p excitation would involve an intermediate coupling treatment which incorporated both spin-orbit interaction and a full treatment of electron-electron interactions including core-valence electron exchange which is the origin of the singlet-triplet separation. One can imagine interpreting the spectra through either one of two limiting descriptions: (a) LS coupling in which one ignores the spin-orbit interaction in the open-shell P 2p level and core-valence exchange interaction gives rise to a singlet-triplet (S-T) splitting; and (b) (,jj) coupling in which the P 2p spin-orbit (S-O) splitting is assumed to be constant in all P 2p excited states (characteristic of the ion core) and core-valence exchange is ignored. These two descriptions must connect smoothly through the regime of intermediate coupling. Simplistically, if the S - T separation for a given state is small compared to the S - O splitting (which is 0.9 eV in the P 2p ion core [7]), transitions to this state will appear in both the '2p3/2' and '2Pt/2' manifolds. In contrast, if the S - T splitting greatly exceeds the S - O splitting only one transition might be observed and the LS description should be more appropriate.
2. Experimental 2.1. Synchrotron measurements The total ion yield (TIY) and partial ion yield spectra were recorded using a time-of-flight (TOF) mass spectrometer operated under Wiley-McLaren focussing conditions with a multi-stop time-to-digital converter (TDC) for data acquisition [8]. The measurements were carried out using tunable monochromated synchrotron radiation at three different beamlines (BL) - BL 9.0.1 and BL 6.3.2 at the Advance Light Source (ALS) [9] and BL 9.3 (Canadian grasshopper line) at the Synchrotron Radiation Centre (SRC). The total ion yield spectra were very similar at all three beamlines. The partial ion yields are quite reproducible from the different beamlines,
491
as long as similar ion extraction fields and TOF start conditions are used. Measurements were made using both the photoelectron (energy unselected) signal or an ion extraction pulse as the start signal of the flight time scale. The electron start signal provided much stronger ion pair signals, but it considerably distorted the partial ion yields. The pulse start signal provided ion yields which were more representative of the actual ionic fragmentation process (as determined by comparison to (e,e + ion) yields [10]), although there was some distortion in favour of low kinetic energy ions, caused by preferential accumulation of slow ions in the ionisation region just prior to ion extraction. Extraction fields of 250 V cm-I were used for the partial yield studies. The qualitative character of the PF~yield reported here, namely the existence of only a single line, was found in all operational modes of the TOF [8]. The PF 3 sample was obtained commercially (Matheson) and used without further purification. The sample introduction line was flushed several times to allow the reactive PF 3 species to displace surface-adsorbed species and passivate the internal surfaces. Small amounts of water impurities were detected in the valence ionization region, but these did not affect either the total or partial ion yield results. 2.2. Computations The calculations were carried out using the GSCF3 computer code [11,12]. The core-ionized and coreexcited states were obtained by ab initio self-consistent field calculations with explicit consideration of the core hole; i.e. based on the symmetry-adapted ASCF method in which the ground and core-excited state energies are subtracted to derive excitation energies. Small configuration interaction (CI) calculations were performed to treat degenerate symmetries of various open-shell P 2p excited states correctly, but the spin-orbit interaction was not taken into account. Primitive basis functions were taken from (533/53) and (63/5) contracted Gaussian-type functions of Huzinaga et al. [13]. They were augmented with tight (compact) functions for phosphorus (1 +, ~'s = 182.0 and 16.0; ~p = 33.0; ~'d = 6.4 and
N. Kosugi et a l . / Chemical Physics Letters 265 (1997) 490-496
492
2.1) and with polarization functions for phosphorus (1", G =0.659 and 0.183) and for fluorine (1", ~'d = 1.3). Without these tight basis functions denoted by 1 + the phosphorus orbital contraction upon core hole creation was not sufficiently taken into account and the excitation and ionization energies were overestimated. The contraction s c h e m e s were (521211 + 1 +/411111+/1 + 1 + 1' 1~) for phosphorus and ( 6 2 1 / 3 1 1 / 1 *) for fluorine. Diffuse functions were not included because Rydberg states are not the focus of the present work.
3. Results and discussion Fig. 1 plots the total ion yield spectrum of PF 3 in the region of the discrete P 2p excited states, recorded on BL 6.3.2 with better than 0.1 eV resolution. The energies, term values and proposed assignments of the P 2p spectral features are listed in Table 1. These are in good agreement with the results of other dipole-regime electron energy loss spectroscopy (EELS) [2,5] and photoabsorption studies [3,4]. Table 2 summarises the results of the ab initio
calculations for P 2p excitation of PF 3. The calculation is non-relativistic and thus the 2p3/2-2p~/2 ion core splitting is not incorporated. For most of the P 2p states the S - T splitting is quite small, less than 0.5 eV. Since this is less than the S - O splitting, for these states we assume there will be a pair of transitions separated by the 0.9 eV S - O splitting. However, the (e- ~, e) ~A ~ and (e- ~, e) 3A ~ states are calculated to be separated by 2.38 eV. Since this is almost three times larger than the S - O splitting, the appropriate coupling description for this particular state must be one much closer to LS than (jj). Thus, to a good first approximation, one should expect to detect only one, not two transitions since excitation from the ~At ground state to t h e 3A t P 2p excited state is spin forbidden in photoabsorption and dipole EELS. Fig. 1 also contains a prediction of the P 2p spectrum of PF 3 which is based on the ab initio results and some simple approximations used to generate the spectra of the 2p3/2 and 2p]/2 manifolds from the LS-coupled calculation. The transitions are represented by Gaussian lines of 0.5 or 0.3 eV FWHM (to match experimental counterparts), lo-
Table 1 Energies (eV), term values (eV) and assignments of P 2p excited states of PF 3 Assignment
No.
E (eV) [5]
E (eV) this work
Term value 2P3/2
2p[/2
2p3/2
I 2 3 4 sh 5 6 7
134.93 135.70 136.48 138.1 138.60 138.99 139.40 4pz(a i ) -
134.91 135.63 136.47 '~ 137.8 138.62 138.9 139.40
6.86 6.16
-
(e (c
8 9 sh 10 sh 11 12 sh 2p3/2 13 2p ]/2
4p:(a I) + u 139.78 140.3 140.63 3dz--(a~) 141.35 141.77 141.8 142.67
6.20 3.77 3.06
(a / ~,e)tA t (e l,at) IE, 4sa I (a / J,a I) ~Ai, 4pz(a l) 4sa t
3.30
139.54 139.74 140.4 140.64
2Pl/2
I.e) IAi I,e) IE
3.16 1.94
4pe 3dz--(a~) 4pe
2.03
141.2 141.77 141.9 142.67
a Peak 3 was set to the value of 136.47 eV, based on the EELS calibration [5]. u From XPS [21].
IP b IP ~
N. Kosugi et al. / Chemical Physics Letters' 265 (1997) 490-496 i
'
'
'
i
'
'
i
'
i
,
,
,
i
~
p_
'
'
2plrz
(b)
ab initio calc.
sum ' E (aft,e)
2Ps/2
'E (e-',e)
3A'(e'l'e)//l~IAI(e"' ~ e)/1,.=. 2pl/2
~ 134
136
,Aa (e',e) 138
140
142
Energy (eV) Fig. 1. (a) Total ion yield spectrum of PF 3 in the region of the P 2p edge, compared to (b) the calculated spectrum for P 2p--, valence excitations of PF 3. The predicted P 2p~/2 and P 2p~/2 components are indicated. The approach used to generate the spin-orbit component spectra derived from the non-relativistic calculation is described in the text. The hatched peak indicates the (a~t,e)~A~ feature corresponding to the 136.5 eV spectral lealure. The dashed lines indicate dipole forbidden states which could be detected by non-dipole EELS. The IPs are determined by XPS
[211. cated at positions given by the calculated excitation energies, with areas given by the calculated oscillator strengths (see Table 2). The 2p3/2 and 2p~/2 compo-
493
nents were generated artificially by assuming a standard 0.9 eV spin-orbit splitting for all states for which the singlet-triplet splitting is less than 1.1 eV. Equal line intensities were assumed. Previous work [14-16] has emphasized that the observed variation in the ratio of the intensities of spin-orbit partners is a result of the core-valence exchange interaction. In general intermediate coupling, not (j,j) coupling is required to correctly explain third row 2p core excitation. Because of its large S - T splitting the ~A[ state cannot be correlated with either the 2p]/~ or 2p3/:: manifolds. We have chosen to plot the'LAl state within the 2pl/2 manifold in Fig. 1 since this is at higher energy. The overall P 2p spectrum was generated by summing the 2P3/2 and 2p~/2 contributions. The energy scale for the sum was established by adding the calculated P 2pt/2 IP (135.7 eV) to the term values, then shifting by +4.6 eV to align the first peak with experiment (135.0 eV). Based on the position of the ~A1 state relative to other features in the spectrum, the calculations suggest that the sharp 136.5 eV state corresponds to the ]A~ state, and thus should not exhibit a spin-orbit partner state. Previous interpretations [1-5] have considered the shoulder at 135.5 eV as a weak partner in the 2p3/2 manifold to the 136.5 eV 2p~/: state. Is there any additional experimental evidence to support this spectral assignment? The ion yield signal from the parent ion provides strong support. Fig. 2 compares the TIY spectrum of PF 3 over a wider energy range, in comparison to the partial ion yield spectrum of the parent ion, PFf, recorded using pulse extraction mode detection [10]. There is a single strong resonance at 136.5 eV, at exactly the
Table 2 Predictions of term values (eV), singlet-triplet energy separations (A E(S-T)) and oscillator strengths (O.S.) for P 2p -~ valence excitations of PF 3 derived from SCF-CI calculations Excitation
(ai- I,e) ]E ( e - I , e ) ]E (e - ] e) IA 2 (e - l,e) IA] (e I,a]) IE (a / 1, a l ) JAR
Term value triplet
singlet
- 0.198 -0.106 0.237 - 0.580 4.540 4.209
0.0 0.257 0.288 1.801 4.891 5.296
A E(S-T)
O.S.
Assignment (peak number)
0.198 0.363 0.051 2.381 0.351 1.087
0.0451 0.0377 0.0 0.0339 0.0054 0.0020
1, 2 l, 2 (dipole forbidden) 3
494
N. Kosugi et a l . / Chemical Physics Letters 265 (1997) 490-496 . . . .
•
I
. . . .
I
. . . .
I
. . . .
;
%8 ~r ~
6
0
=u
5
A8
PF3*
q)
% ,-
partial ion yield
6
r31
i-
~4 2 o 0
i
i
•
,
i
. . . .
135
i
140
. . . .
i
. . . .
145
Photon Energy (eV) Fig. 2. Comparison of the TIY spectrum of PF s with that for the production of PF~- as measured using pulse extraction time-offlight mass spectrometry. The yield of PF~- in the 136.5 eV peak is > 20% of the P 2p derived total ion yield.
energy of the feature attributed to the ~A~ state. While there are very weak PF~ signals at the P 2p discrete states at 135.0, 135.6 and 139.5 eV, none of these are of sufficient intensity to be the conventional S - O partner to the 136.5 eV peak. Alternatively one might say that the exchange interaction in the (e-=e t) tA. state is so large that the S - O intensity ratio is totally skewed in favour of the 2p~/2 component. The difference in the descriptions would seem to be a question of semantics since it is the core-valence electron exchange interaction that both perturbs relative intensities of S - O partners and generates the S - T splitting [17]. In the limit of exclusively (jj) coupling, the 2p3/2 component should be twice as intense as the 2pt/2 component. As the core-valence electron exchange interaction increases, the relative intensities change with the 2p~/2 component increasing at the expense of the 2p3/2 component. The non-negligible core-valence elec-
tron exchange is why the first two peaks in the P 2p spectrum (those at 134.9 and 135.6 eV) have almost equal intensities. In the limit of exclusively (LS) coupling, only a single upper state is observed in dipole excitation. The lower state has zero intensity, and the normal picture of distinct 2p~/2 and 2p3/2 manifolds is broken. Why is the ( e - ' e ~) ~A~ state so special? As noted in previous studies of S - T splitting using nearthreshold EELS and computation [ 18,19], the magnitude of S - T splitting can be related to the spatial extent of the excited state, in particular the size and degree of spatial overlap of the open-shell orbitals. In general compact states wilt have a larger S - T splitting. The open-shell orbitals of 2p(e) core and e* valence character which are coupled in the (e-~,e) ~A] state have a similar spatial extent and orbital pattem, and thus the exchange interaction is extremely large, even compared to the other two states derived from the (e-~, e) configuration. There are three unoccupied valence-type orbitals, e* and a~. The term value of the e* orbital is 6 - 7 eV. There is no Rydberg-valence mixing in the e symmetry manifold and the e Rydberg series (Px and p:) show normal spectral behaviour; i.e. normal quantum defect and intensity. On the other hand, the a~ orbital has a term value of 2-3 eV and can be strongly mixed with a~ Rydberg orbitals (s and p:). This prediction is consistent with the results of a preliminary calculation including diffuse functions, in which the Rydberg states are more properly represented. Experimentally, the states above 138 eV appear to have a large Rydberg character since they are quite sharp and can be assigned to pattems with appropriate term values and quantum defects. The calculation with diffuse orbitals shows that the interaction of the a~ orbital with the Pz and d,2 Rydberg orbitals is much stronger than that with the s Rydberg orbital. Therefore, the weak shoulder signals # 4 and # 6 may be assigned to 4s(a I) Rydberg states. The strong peaks # 5 and # 7 (and #8) are assigned to 4p:(a 1) Rydberg and peaks # 9 and #11 to 3d :2(a~ ) Rydberg states, both of which are strongly mixed with the a~ valence orbitals/states. Peaks 8 and 10 (term value ( T V ) = 2.2 eV, quantum defect = 1.5) may be assigned to P 2p excitation to the 4pe Rydberg state. The 3de ( xy, x 2 - y2; xz, yz) Rydberg state is very weak and should not be observed.
N. Kosugi et al./ Chemical Physics Letters 265 (1997) 490-496
These Rydberg assignments are identical to those proposed in the literature [1-5].
4. Summary The presence of a 'single component' feature in the P 2p spectrum of PF 3 has been demonstrated from the PF3+ selected ion yield and explained using ab initio calculations in terms of an exceptionally large core-valence electron exchange interaction for the (e-~,e)~'3A state. LS-coupled states, i.e. single states without an identifiable spin-orbit partner, are predicted to be a general phenomena in the excitation spectra of a certain range of np core edges. Necessary conditions include a small S - O splitting (perhaps less than 2 eV) and a very compact valence excited state such that there is a particularly large singlet-triplet splitting. In fact two other examples of this phenomena were found in a recent combined experimental-theoretical study of the S 2p spectrum of SF6 [20]. However, the experimental evidence in the S 2p spectrum of SF6 is much weaker than in the present case. In addition examples are found in the P 2p spectra of PCI 3 and CF3PCI 2 [10]. Further experimental confirmation of this interpretation of the P 2p spectrum of PF 3 could be obtained by detection of the 3A 1 triplet partner to the 136.5 eV state through near threshold EELS [18,19]. Based on the 2.4 eV S - T splitting of the (e- ~, e ~) A~ state, and the experimental energy for the (e-~,e ~)IA~ state, the (e -~ , e ~) 3A~ state is predicted to occur at 134.0 eV, well isolated from the singlet P 2p excited state signals, which only start at 134.5 eV. Non-dipole EELS studies carried out with large angle, moderate impact energy could also be used to detect the quadrupole ~A2 state in PF 3 which is predicted to occur in the region of the low-lying (e-~,e~)A~ and ( e - ~ , e t ) E states. The quadrupole A 2 state is predicted to have a normal spin-orbit splitting since it has a small exchange splitting. A complete discussion of the time-of-flight mass spectra and partial ion yield signals of PF 3 in the P 2p region is presented elsewhere [10], in comparison to results for P 2p excitation of PCI 3 and CF3PCI 2. The use of partial ion yield spectra as a generally useful tool to aid the interpretation of complex inner shell spectra is noted.
495
Acknowledgements Financial support for this work was provided by NSERC (Canada). We thank A.L.D. Kilcoyne and T. Tyliszczak for their expert work in fabricating the time-of-flight/TDC measurement system. We thank J.D. Bozek (ALS), J.H. Underwood (ALS) and Kim Tan (SRC) for their assistance with beam line operation and the measurements. APH thanks LBNL for support and hospitality during a recent research leave. The ALS is supported by DOE. This work is based in part upon research conducted at the Synchrotron Radiation Center, University of Wisconsin-Madison, which is supported by the NSF under Award No. DMR-95-31009.
References [l] A.P. Hitchcock and D.C. Mancini, J. Electron Spectrosc. 67 (1994) 1. [2] R.N.S. Sodhi and C.E. Brion, J. Electron Spectrosc. 37 (1985) 97. [3] E. Ishiguro, S. lwata, A. Mikuni, Y. Suzuki, H. Kanamori and T. Sasaki, J. Phys. B 20 (1987) 4725. [4] 1. Vayrynen, T.A. Kautila, R.G. Cavell and K.H. Tan, J. Electron Spectrosc. 61 (1992) 55. [5] J.W, Au, G. Cooper and C.E. Brion, Chem. Phys., submitted for publication. [6] H. Nakamatsu, Chem. Phys. 200 (1995) 49. [7] W.H.E. Schwarz, Chem. Phys. 9 (1975) 157. [8] A.P. Hitchcock, A.L.D. Kilcoyne, T. Tyliszczak, E. Riihl, J.D. Bozek, J.T. Spencer and P.A. Dowben, J. Electron Spectrosc., in preparation. [9] J.H. Underwood, E.M. Gullikson, M. Koike, P.J. Batson, P.E. Denham, K.D. Franck, R.E. Tackaberry and W.F. Steele, Rev. Sci. Instrum. (1996) in press. [10] A.P. Hitchcock, N. Kosugi and R.G. Cavell, Chem. Phys., to be submitted. [11] N. Kosugi and H. Kuroda, Chem. Phys. Lett. 74 (1980) 490. [12] N. Kosugi, Theor. Chim. Acta 72 (1987) 149. [13] S. Huzinaga, J. Andzelm, M. Klobukowski, E. RadzioAndzelm, Y. Sasaki and H. Tatewaki, Gaussian basis sets for molecular calculations (Elsevier, Amsterdam, 1984). [14] M.B. Robin, Chem. Phys. Lett. 31 (1975) 140. [15] W.H.E. Schwarz, Chem. Phys. 9 (1975) 157; I1 (1975) 217; 13 (1976) 153. [16] F.M.F. de Grout, Z.W. Hu. M.F. Lopez, G. Kaindl, F. Guillot and M. Tronc, J. Chem. Phys. 101 (1994) 6570. [17] B.T. Thole and G. van der Laan, Phys. Rev. B 38 (1988) 3158.
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[18] J.T. Francis, C. Enkvist, S. Lunell and A.P. Hitchcock, Can. J. Phys. 72 (1994) 879. [19] J.T. Francis, N. Kosugi and A.P. Hitchcock, J. Chem. Phys. 101 (1994) 10429.
[20] J.T. Francis, C.C. Turci, T. Tyliszczak, G.G.B. de Souza, N. Kosugi and A.P. Hitchcock, Phys. Rev. A 52 (1995) 4665. [21] R.N.S. Sodhi and R.G. Cavell, J. Electron Spectrosc. 32 (1983) 283.
ELSEVIER
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 265 (1997) 490-496
(LS) versus (jj) coupling in P 2p inner-shell excited states of PF 3 N. Kosugi a, R.G. Cavell b, A.P. Hitchcock c a lnstituteJbr Molecular Science, Okazaki 444, Japan b Chemistry, University of'Alberta, Edmonton, Alb.. Canada T6G 2G2 c Chemistry. McMaster University, Hamilton. Ont.. Canada L8S 4MI
Received 14 October 1996; in final form 5 December 1996
Abstract
The P 2p total and parent ion yield spectra of PF3 have been measured and interpreted with ab initio SCF-CI calculations. The PFf yield has only one line at 136.5 eV, without a spin-orbit partner. The ab initio calculations attribute it to the (e- 1, e) IA i state. They predict the (e- i e) 3A i-(e- 1,e) IA ~ state separation is 2.4 eV, much greater than for other P 2p states (0.05-0.5 eV), and much larger than the P 2p-i spin-orbit splitting (0.9 eV). We conclude the 136.5 eV state is the (LS) coupled ( e - 1 e)IA~ state, and the normally dominant P 2p i spin-orbit (jj) splitting can be ignored. The importance of treating core-valence exchange as well as spin-orbit interactions is discussed.
1. Introduction
The 2p (L23) excitation spectra of third row elements and their compounds are generally characterized by the presence of two closely related manifolds of excited states, those associated with the 2p3/2 (L 3) and those associated with the 2pt/2 (L 2) spinorbit coupled ion core [1]. In most studies carried out to date this spin-orbit splitting has been found to vary minimally among various compounds of a given element. For this reason spectral assignments are often guided by identification of corresponding lines in the partner manifolds. In 1985 Sodhi and Brion [2] reported a detailed study of the inner-shell spectroscopy of a number of phosphorus compounds, including PF 3 and PCl 3. In that work a number of similarities between the low-energy region of their P 2p spectra were noted. However, a rather dramatic difference was found, namely that there was a sharp, intense transition at 136.5 eV in PF 3 which did not have a counterpart in the dipole-regime energy loss
spectrum of PCI 3. In contrast variable scattering angle studies of PC13 [2] revealed an a p p a r e n t l y similar state at 135.1 eV which had an angular dependence characteristic of an electric quadrupole transition. The corresponding variable scattering angle study of PF 3 [2] did not show any non-dipole states. Given the expected similarity of the electronic structure of these two species and the otherwise very similar nature of the low-lying regions of these P 2p spectra, these results are rather surprising. In addition, both the 136.5 eV dipole state in PF 3 and the 135.1 eV quadrupole state in PC! 3 appeared to be isolated lines, without obvious contributions from a spin-orbit partner state, although very weak features detected at appropriate separations were proposed as possible spin-orbit partner states [2]. Our goal in this study was to better understand the discrete region of the P 2p spectrum of PF 3 through a high-resolution synchrotron radiation study of the total and partial ion yields, combined with high-level ab initio calculations. Recently several groups have
000%2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0009-2614(96)01472-8
N. Kosugi et al. / Chemical Physics Letters 265 (1997) 490-496
studied the P 2p spectrum of PF 3 at high resolution using either photon [3,4] or electron impact [5] excitation. Several theoretical studies have also been reported [4,6]. A complete description of P 2p excitation would involve an intermediate coupling treatment which incorporated both spin-orbit interaction and a full treatment of electron-electron interactions including core-valence electron exchange which is the origin of the singlet-triplet separation. One can imagine interpreting the spectra through either one of two limiting descriptions: (a) LS coupling in which one ignores the spin-orbit interaction in the open-shell P 2p level and core-valence exchange interaction gives rise to a singlet-triplet (S-T) splitting; and (b) (,jj) coupling in which the P 2p spin-orbit (S-O) splitting is assumed to be constant in all P 2p excited states (characteristic of the ion core) and core-valence exchange is ignored. These two descriptions must connect smoothly through the regime of intermediate coupling. Simplistically, if the S - T separation for a given state is small compared to the S - O splitting (which is 0.9 eV in the P 2p ion core [7]), transitions to this state will appear in both the '2p3/2' and '2Pt/2' manifolds. In contrast, if the S - T splitting greatly exceeds the S - O splitting only one transition might be observed and the LS description should be more appropriate.
2. Experimental 2.1. Synchrotron measurements The total ion yield (TIY) and partial ion yield spectra were recorded using a time-of-flight (TOF) mass spectrometer operated under Wiley-McLaren focussing conditions with a multi-stop time-to-digital converter (TDC) for data acquisition [8]. The measurements were carried out using tunable monochromated synchrotron radiation at three different beamlines (BL) - BL 9.0.1 and BL 6.3.2 at the Advance Light Source (ALS) [9] and BL 9.3 (Canadian grasshopper line) at the Synchrotron Radiation Centre (SRC). The total ion yield spectra were very similar at all three beamlines. The partial ion yields are quite reproducible from the different beamlines,
491
as long as similar ion extraction fields and TOF start conditions are used. Measurements were made using both the photoelectron (energy unselected) signal or an ion extraction pulse as the start signal of the flight time scale. The electron start signal provided much stronger ion pair signals, but it considerably distorted the partial ion yields. The pulse start signal provided ion yields which were more representative of the actual ionic fragmentation process (as determined by comparison to (e,e + ion) yields [10]), although there was some distortion in favour of low kinetic energy ions, caused by preferential accumulation of slow ions in the ionisation region just prior to ion extraction. Extraction fields of 250 V cm-I were used for the partial yield studies. The qualitative character of the PF~yield reported here, namely the existence of only a single line, was found in all operational modes of the TOF [8]. The PF 3 sample was obtained commercially (Matheson) and used without further purification. The sample introduction line was flushed several times to allow the reactive PF 3 species to displace surface-adsorbed species and passivate the internal surfaces. Small amounts of water impurities were detected in the valence ionization region, but these did not affect either the total or partial ion yield results. 2.2. Computations The calculations were carried out using the GSCF3 computer code [11,12]. The core-ionized and coreexcited states were obtained by ab initio self-consistent field calculations with explicit consideration of the core hole; i.e. based on the symmetry-adapted ASCF method in which the ground and core-excited state energies are subtracted to derive excitation energies. Small configuration interaction (CI) calculations were performed to treat degenerate symmetries of various open-shell P 2p excited states correctly, but the spin-orbit interaction was not taken into account. Primitive basis functions were taken from (533/53) and (63/5) contracted Gaussian-type functions of Huzinaga et al. [13]. They were augmented with tight (compact) functions for phosphorus (1 +, ~'s = 182.0 and 16.0; ~p = 33.0; ~'d = 6.4 and
N. Kosugi et a l . / Chemical Physics Letters 265 (1997) 490-496
492
2.1) and with polarization functions for phosphorus (1", G =0.659 and 0.183) and for fluorine (1", ~'d = 1.3). Without these tight basis functions denoted by 1 + the phosphorus orbital contraction upon core hole creation was not sufficiently taken into account and the excitation and ionization energies were overestimated. The contraction s c h e m e s were (521211 + 1 +/411111+/1 + 1 + 1' 1~) for phosphorus and ( 6 2 1 / 3 1 1 / 1 *) for fluorine. Diffuse functions were not included because Rydberg states are not the focus of the present work.
3. Results and discussion Fig. 1 plots the total ion yield spectrum of PF 3 in the region of the discrete P 2p excited states, recorded on BL 6.3.2 with better than 0.1 eV resolution. The energies, term values and proposed assignments of the P 2p spectral features are listed in Table 1. These are in good agreement with the results of other dipole-regime electron energy loss spectroscopy (EELS) [2,5] and photoabsorption studies [3,4]. Table 2 summarises the results of the ab initio
calculations for P 2p excitation of PF 3. The calculation is non-relativistic and thus the 2p3/2-2p~/2 ion core splitting is not incorporated. For most of the P 2p states the S - T splitting is quite small, less than 0.5 eV. Since this is less than the S - O splitting, for these states we assume there will be a pair of transitions separated by the 0.9 eV S - O splitting. However, the (e- ~, e) ~A ~ and (e- ~, e) 3A ~ states are calculated to be separated by 2.38 eV. Since this is almost three times larger than the S - O splitting, the appropriate coupling description for this particular state must be one much closer to LS than (jj). Thus, to a good first approximation, one should expect to detect only one, not two transitions since excitation from the ~At ground state to t h e 3A t P 2p excited state is spin forbidden in photoabsorption and dipole EELS. Fig. 1 also contains a prediction of the P 2p spectrum of PF 3 which is based on the ab initio results and some simple approximations used to generate the spectra of the 2p3/2 and 2p]/2 manifolds from the LS-coupled calculation. The transitions are represented by Gaussian lines of 0.5 or 0.3 eV FWHM (to match experimental counterparts), lo-
Table 1 Energies (eV), term values (eV) and assignments of P 2p excited states of PF 3 Assignment
No.
E (eV) [5]
E (eV) this work
Term value 2P3/2
2p[/2
2p3/2
I 2 3 4 sh 5 6 7
134.93 135.70 136.48 138.1 138.60 138.99 139.40 4pz(a i ) -
134.91 135.63 136.47 '~ 137.8 138.62 138.9 139.40
6.86 6.16
-
(e (c
8 9 sh 10 sh 11 12 sh 2p3/2 13 2p ]/2
4p:(a I) + u 139.78 140.3 140.63 3dz--(a~) 141.35 141.77 141.8 142.67
6.20 3.77 3.06
(a / ~,e)tA t (e l,at) IE, 4sa I (a / J,a I) ~Ai, 4pz(a l) 4sa t
3.30
139.54 139.74 140.4 140.64
2Pl/2
I.e) IAi I,e) IE
3.16 1.94
4pe 3dz--(a~) 4pe
2.03
141.2 141.77 141.9 142.67
a Peak 3 was set to the value of 136.47 eV, based on the EELS calibration [5]. u From XPS [21].
IP b IP ~
N. Kosugi et al. / Chemical Physics Letters' 265 (1997) 490-496 i
'
'
'
i
'
'
i
'
i
,
,
,
i
~
p_
'
'
2plrz
(b)
ab initio calc.
sum ' E (aft,e)
2Ps/2
'E (e-',e)
3A'(e'l'e)//l~IAI(e"' ~ e)/1,.=. 2pl/2
~ 134
136
,Aa (e',e) 138
140
142
Energy (eV) Fig. 1. (a) Total ion yield spectrum of PF 3 in the region of the P 2p edge, compared to (b) the calculated spectrum for P 2p--, valence excitations of PF 3. The predicted P 2p~/2 and P 2p~/2 components are indicated. The approach used to generate the spin-orbit component spectra derived from the non-relativistic calculation is described in the text. The hatched peak indicates the (a~t,e)~A~ feature corresponding to the 136.5 eV spectral lealure. The dashed lines indicate dipole forbidden states which could be detected by non-dipole EELS. The IPs are determined by XPS
[211. cated at positions given by the calculated excitation energies, with areas given by the calculated oscillator strengths (see Table 2). The 2p3/2 and 2p~/2 compo-
493
nents were generated artificially by assuming a standard 0.9 eV spin-orbit splitting for all states for which the singlet-triplet splitting is less than 1.1 eV. Equal line intensities were assumed. Previous work [14-16] has emphasized that the observed variation in the ratio of the intensities of spin-orbit partners is a result of the core-valence exchange interaction. In general intermediate coupling, not (j,j) coupling is required to correctly explain third row 2p core excitation. Because of its large S - T splitting the ~A[ state cannot be correlated with either the 2p]/~ or 2p3/:: manifolds. We have chosen to plot the'LAl state within the 2pl/2 manifold in Fig. 1 since this is at higher energy. The overall P 2p spectrum was generated by summing the 2P3/2 and 2p~/2 contributions. The energy scale for the sum was established by adding the calculated P 2pt/2 IP (135.7 eV) to the term values, then shifting by +4.6 eV to align the first peak with experiment (135.0 eV). Based on the position of the ~A1 state relative to other features in the spectrum, the calculations suggest that the sharp 136.5 eV state corresponds to the ]A~ state, and thus should not exhibit a spin-orbit partner state. Previous interpretations [1-5] have considered the shoulder at 135.5 eV as a weak partner in the 2p3/2 manifold to the 136.5 eV 2p~/: state. Is there any additional experimental evidence to support this spectral assignment? The ion yield signal from the parent ion provides strong support. Fig. 2 compares the TIY spectrum of PF 3 over a wider energy range, in comparison to the partial ion yield spectrum of the parent ion, PFf, recorded using pulse extraction mode detection [10]. There is a single strong resonance at 136.5 eV, at exactly the
Table 2 Predictions of term values (eV), singlet-triplet energy separations (A E(S-T)) and oscillator strengths (O.S.) for P 2p -~ valence excitations of PF 3 derived from SCF-CI calculations Excitation
(ai- I,e) ]E ( e - I , e ) ]E (e - ] e) IA 2 (e - l,e) IA] (e I,a]) IE (a / 1, a l ) JAR
Term value triplet
singlet
- 0.198 -0.106 0.237 - 0.580 4.540 4.209
0.0 0.257 0.288 1.801 4.891 5.296
A E(S-T)
O.S.
Assignment (peak number)
0.198 0.363 0.051 2.381 0.351 1.087
0.0451 0.0377 0.0 0.0339 0.0054 0.0020
1, 2 l, 2 (dipole forbidden) 3
494
N. Kosugi et a l . / Chemical Physics Letters 265 (1997) 490-496 . . . .
•
I
. . . .
I
. . . .
I
. . . .
;
%8 ~r ~
6
0
=u
5
A8
PF3*
q)
% ,-
partial ion yield
6
r31
i-
~4 2 o 0
i
i
•
,
i
. . . .
135
i
140
. . . .
i
. . . .
145
Photon Energy (eV) Fig. 2. Comparison of the TIY spectrum of PF s with that for the production of PF~- as measured using pulse extraction time-offlight mass spectrometry. The yield of PF~- in the 136.5 eV peak is > 20% of the P 2p derived total ion yield.
energy of the feature attributed to the ~A~ state. While there are very weak PF~ signals at the P 2p discrete states at 135.0, 135.6 and 139.5 eV, none of these are of sufficient intensity to be the conventional S - O partner to the 136.5 eV peak. Alternatively one might say that the exchange interaction in the (e-=e t) tA. state is so large that the S - O intensity ratio is totally skewed in favour of the 2p~/2 component. The difference in the descriptions would seem to be a question of semantics since it is the core-valence electron exchange interaction that both perturbs relative intensities of S - O partners and generates the S - T splitting [17]. In the limit of exclusively (jj) coupling, the 2p3/2 component should be twice as intense as the 2pt/2 component. As the core-valence electron exchange interaction increases, the relative intensities change with the 2p~/2 component increasing at the expense of the 2p3/2 component. The non-negligible core-valence elec-
tron exchange is why the first two peaks in the P 2p spectrum (those at 134.9 and 135.6 eV) have almost equal intensities. In the limit of exclusively (LS) coupling, only a single upper state is observed in dipole excitation. The lower state has zero intensity, and the normal picture of distinct 2p~/2 and 2p3/2 manifolds is broken. Why is the ( e - ' e ~) ~A~ state so special? As noted in previous studies of S - T splitting using nearthreshold EELS and computation [ 18,19], the magnitude of S - T splitting can be related to the spatial extent of the excited state, in particular the size and degree of spatial overlap of the open-shell orbitals. In general compact states wilt have a larger S - T splitting. The open-shell orbitals of 2p(e) core and e* valence character which are coupled in the (e-~,e) ~A] state have a similar spatial extent and orbital pattem, and thus the exchange interaction is extremely large, even compared to the other two states derived from the (e-~, e) configuration. There are three unoccupied valence-type orbitals, e* and a~. The term value of the e* orbital is 6 - 7 eV. There is no Rydberg-valence mixing in the e symmetry manifold and the e Rydberg series (Px and p:) show normal spectral behaviour; i.e. normal quantum defect and intensity. On the other hand, the a~ orbital has a term value of 2-3 eV and can be strongly mixed with a~ Rydberg orbitals (s and p:). This prediction is consistent with the results of a preliminary calculation including diffuse functions, in which the Rydberg states are more properly represented. Experimentally, the states above 138 eV appear to have a large Rydberg character since they are quite sharp and can be assigned to pattems with appropriate term values and quantum defects. The calculation with diffuse orbitals shows that the interaction of the a~ orbital with the Pz and d,2 Rydberg orbitals is much stronger than that with the s Rydberg orbital. Therefore, the weak shoulder signals # 4 and # 6 may be assigned to 4s(a I) Rydberg states. The strong peaks # 5 and # 7 (and #8) are assigned to 4p:(a 1) Rydberg and peaks # 9 and #11 to 3d :2(a~ ) Rydberg states, both of which are strongly mixed with the a~ valence orbitals/states. Peaks 8 and 10 (term value ( T V ) = 2.2 eV, quantum defect = 1.5) may be assigned to P 2p excitation to the 4pe Rydberg state. The 3de ( xy, x 2 - y2; xz, yz) Rydberg state is very weak and should not be observed.
N. Kosugi et al./ Chemical Physics Letters 265 (1997) 490-496
These Rydberg assignments are identical to those proposed in the literature [1-5].
4. Summary The presence of a 'single component' feature in the P 2p spectrum of PF 3 has been demonstrated from the PF3+ selected ion yield and explained using ab initio calculations in terms of an exceptionally large core-valence electron exchange interaction for the (e-~,e)~'3A state. LS-coupled states, i.e. single states without an identifiable spin-orbit partner, are predicted to be a general phenomena in the excitation spectra of a certain range of np core edges. Necessary conditions include a small S - O splitting (perhaps less than 2 eV) and a very compact valence excited state such that there is a particularly large singlet-triplet splitting. In fact two other examples of this phenomena were found in a recent combined experimental-theoretical study of the S 2p spectrum of SF6 [20]. However, the experimental evidence in the S 2p spectrum of SF6 is much weaker than in the present case. In addition examples are found in the P 2p spectra of PCI 3 and CF3PCI 2 [10]. Further experimental confirmation of this interpretation of the P 2p spectrum of PF 3 could be obtained by detection of the 3A 1 triplet partner to the 136.5 eV state through near threshold EELS [18,19]. Based on the 2.4 eV S - T splitting of the (e- ~, e ~) A~ state, and the experimental energy for the (e-~,e ~)IA~ state, the (e -~ , e ~) 3A~ state is predicted to occur at 134.0 eV, well isolated from the singlet P 2p excited state signals, which only start at 134.5 eV. Non-dipole EELS studies carried out with large angle, moderate impact energy could also be used to detect the quadrupole ~A2 state in PF 3 which is predicted to occur in the region of the low-lying (e-~,e~)A~ and ( e - ~ , e t ) E states. The quadrupole A 2 state is predicted to have a normal spin-orbit splitting since it has a small exchange splitting. A complete discussion of the time-of-flight mass spectra and partial ion yield signals of PF 3 in the P 2p region is presented elsewhere [10], in comparison to results for P 2p excitation of PCI 3 and CF3PCI 2. The use of partial ion yield spectra as a generally useful tool to aid the interpretation of complex inner shell spectra is noted.
495
Acknowledgements Financial support for this work was provided by NSERC (Canada). We thank A.L.D. Kilcoyne and T. Tyliszczak for their expert work in fabricating the time-of-flight/TDC measurement system. We thank J.D. Bozek (ALS), J.H. Underwood (ALS) and Kim Tan (SRC) for their assistance with beam line operation and the measurements. APH thanks LBNL for support and hospitality during a recent research leave. The ALS is supported by DOE. This work is based in part upon research conducted at the Synchrotron Radiation Center, University of Wisconsin-Madison, which is supported by the NSF under Award No. DMR-95-31009.
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