Photoelectron spectroscopy of sulfur L levels in the SF5CF3 molecule

Chemical Physics 353 (2008) 202–208

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Photoelectron spectroscopy of sulfur L levels in the SF5CF3 molecule A. Kivimäki a,*, J. Álvarez Ruiz b, M. Coreno a,c, M. Stankiewicz d, G. Fronzoni e,f, P. Decleva e,f a

CNR-INFM, Laboratorio Nazionale TASC, 34012 Trieste, Italy Departamento de Química Láser, Instituto de Química-Física Rocasolano Consejo Superior de Investigaciones Científicas, Serrano 119, 28006 Madrid, Spain CNR-IMIP, Montelibretti, 00016 Rome, Italy d ´ ski, 30-059 Kraków, Poland Instytut Fizyki im. Mariana Smoluchowskiego, Uniwersytet Jagiellon e Dipartimento di Scienze Chimiche, Università di Trieste, 34127 Trieste, Italy f INFM DEMOCRITOS, and INSTM CRIMSON, Trieste, Italy b c

a r t i c l e

i n f o

Article history: Received 3 June 2008 Accepted 27 August 2008 Available online 31 August 2008 Keywords: SF5CF3 SF6 Photoelectron spectroscopy S 2p S 2s Shake-up transitions

a b s t r a c t The photoelectron spectra of the S 2s and S 2p main lines as well as of the S 2p shake-up satellites of the SF5CF3 molecule have been measured using synchrotron radiation excitation. The main spectral lines have been compared to those of the closely-related SF6 molecule. The S 2p binding energies in SF5CF3 were obtained in reference to those in SF6, while the S 2s binding energies of both SF5CF3 and SF6 were determined with respect to the Ar 2p1 states. The intensity behavior of the spin–orbit-split S 2p photoelectron lines of SF5CF3 was studied in the region of the shape resonance whose maximum is located some 15 eV above the S 2p threshold. The S 2p shake-up satellite spectrum of SF5CF3 has been interpreted with the aid of ab initio configuration interaction calculations within the sudden-limit approximation. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The SF5CF3 molecule has recently attracted keen interest, as this potent greenhouse gas was found in the atmosphere in 2000 [1]. Its global warming potential is about 18,000 times higher than that of CO2, and it ranks in the second place among greenhouse gases only after SF6. The only known source of SF5CF3 is connected to the production of fluorochemicals [2,3]. It has also been suggested that SF5CF3 could form through recombination of ground-state SF5 and CF3 radicals on aerosol particles in the atmosphere [4]. Few core level studies have been published on SF5CF3 so far [5–8], and none of them has dealt with the sulfur L levels. Thus even the S 2p and S 2s binding energies of SF5CF3 are unknown. We have therefore measured the S 2p and S 2s photoelectron spectra (PES) of SF5CF3 to determine these fundamental values. These spectra will be presented and compared with the corresponding ones of the SF6 molecule. The SF5CF3 molecule can be constructed from SF6 by replacing one F atom by the CF3 group. The SF6 molecule has been studied extensively at core edges using photon excitation, electron impact and theoretical methods (e.g., [9–11] and references therein) because it displays very intense molecular resonances both below and above core ionization limits. The above-threshold resonances * Corresponding author. Tel.: +39 0403758476; fax: +39 0403758400. E-mail address: [email protected] (A. Kivimäki). 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.08.014

are usually called shape resonances and they can be explained by two different models. Firstly, the escaping photoelectron can be thought as being temporarily trapped by the potential created by the surrounding atoms of the molecule. Secondly, shape resonances can be described as excitations to empty molecular orbitals embedded in the continuum. Based on its structural similarity to SF6, SF5CF3 may also be expected to show strong excitations to empty molecular orbitals. Our complementary study [12] shows that the S 2p excitation spectrum of SF5CF3 reveals two distinct resonances (shape resonances) above the ionization limit, which are indeed very similar to those of SF6. In the present paper, we have studied how the intensities of the S 2p photoelectron lines of SF5CF3 behave when the photon energy is scanned across the shape resonance whose counterpart in SF6 is 4eg. These results will be compared to those obtained by Kitajima et al. [13] on SF6. The similarities and differences found in the S 2p photoelectron emission in these two molecules will be discussed. Core photoionization is often accompanied by simultaneous transitions of valence electrons to empty orbitals (shake-up processes) or to the continuum (shake-off processes). The shake-up processes result in two-hole one particle (2h–1p) final states and generate discrete structures at kinetic energies lower than those of the main photoelectron lines. In the present work, we have measured the S 2p shake-up satellite spectrum of SF5CF3 50 eV above the S 2p ionization potential. At such a low photon energy both normal and conjugate shake-up satellites [14] may appear in the

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1600 1400

a

SF5CF3 S 2s

Ar 2p

1200

Counts

spectrum. The SF5CF3 molecule belongs to the Cs symmetry group, where molecular orbitals (MOs) are of two possible symmetries (a0 or a00 ) only. Consequently, there is a large number of allowed normal shake-up transitions even in the single-orbital approximation. Even though the experimental spectrum appears relatively simple, it may be expected to be composed of numerous transitions. We have therefore performed ab initio configuration interaction (CI) calculations on the S 2p shake-up transitions in order to interpret the experimental spectrum.

1000 800 600

2. Experimental

3. Results and discussion 3.1. S 2s photoelectron line Fig. 1a shows the S 2s PES of SF5CF3 measured at 320 eV photon energy. The experimental spectrum is shown with dots after the subtraction of a constant background. For energy calibration purposes Ar gas was leaked in to the chamber. The Ar 2p3/2 and 2p1/2 ionization energies in argon are 248.628(4) eV and 250.776(5) eV, respectively [17]. The lifetime widths of the Ar 2p1 states are known (118(4) meV [18]) and they provide means of determining the total instrumental resolution. A Voigt fit of the Ar 2p lines with a fixed Lorentzian width of 118 meV yielded a Gaussian contribution of 280 meV in the line profile. The latter width stands for the total effect due to the photon band width, the electron analyzer broadening and the Doppler broadening. Neglecting the difference of the Doppler broadening between Ar and SF5CF3, the Gaussian width was fixed to 280 meV when fitting

200 0 68

70

72

74

76

78

80

82

Kinetic energy (eV) 3500

b

SF6 S 2s

3000 2500

Counts

The measurements were carried out at the Gas Phase Photoemission beam line at the synchrotron radiation facility Elettra, Trieste (Italy). The beam line operates in the photon energy range from 14 eV to 1 keV. Undulator radiation is monochromatized by a spherical grating monochromator equipped with a planar premirror. One of the five available gratings can be selected according to the photon energy range of interest. The beamline is described in more detail in [15]. The photoelectron spectra of the SF5CF3 and SF6 molecules were measured using a commercial 150-mm hemispherical electron energy analyzer (Vacuum Generators) that has six channel electron multipliers placed on the focal plane of the analyzer. Thus six electron spectra are measured simultaneously, and their sum gives the final result after the proper adjustment of the kinetic energy scales. The electron analyzer was mounted in the pseudo-magic angle; so the electrons were collected in the horizontal plane in the direction that forms an angle of 54.7° with respect to the electric vector of the linearly polarized light and an angle of (90–54.7) degrees with respect to the light propagation vector. The kinetic energy resolution of the analyzer is about 2% of the pass energy used. The S 2p main photoelectron lines were measured with 5-eV pass energy, while the S 2s main line and the S 2p shake-up spectra were recorded with a 10-eV pass energy. The transmission of the electron analyzer was checked at 5-eV pass energy in the kinetic energy range 10–35 eV by measuring the N4,5OO Auger spectrum of Xe at 95-eV photon energy and by comparing the observed intensities of the Auger lines to the literature values [16]. The transmission was found to be constant within error bars, hence no transmission correction has been done to the experimental spectra. The pressure in the chamber was regulated with a leak valve and kept at 9  106 mbar during the experiment. The SF5CF3 sample was obtained from Apollo Scientific Ltd with stated purity of 99%. SF6 gas of purity 99.97% was purchased from SIAD S.p.A. Both gases were used as delivered.

400

Ar 2p

2000 1500 1000 500 0 68

70

72

74

76

78

80

82

Kinetic energy (eV) Fig. 1. The S 2s photoelectron spectra of (a) SF5CF3 and (b) SF6 measured at the photon energy of 320 eV. Constant backgrounds have been subtracted from the measured spectra (dots). The fit results are shown with solid curves. The S 2s line of SF6 was tentatively fitted with four vibrational components (see text for details). Argon gas was leaked in to the chamber for energy calibration, hence the presence of the Ar 2p photoelectron lines.

the S 2s photoelectron line. This resulted in the Lorentzian contribution of 1.89(5) eV. This can be regarded as an approximation for the lifetime width of the S 2s1 state, as a possible vibrational structure is accounted for by the same fitted profile. However, the analysis for the S 2s PES of SF6 indicates that the inclusion of the vibrational excitations may not affect appreciably the lifetime width obtained from the fit (see below). At the photon energy of 320 eV, the post-collision interaction (PCI) can be neglected in the analysis of the S 2s photoelectron line, as the predominant Coster-Kronig decay channel of the S 2s1 state leads to emission at kinetic energies lower than the S 2s line. (The L1–L2,3V Coster-Kronig spectrum of SF5CF3 has not been measured, but its position should not deviate much from that of OCS, which is located at kinetic energies below 50 eV [19].) In contrast, the PCI does affect the positions of the Ar 2p photoelectron lines. Using the analytical formula given in [20] and an average Auger energy of 200 eV, we estimate that the Ar 2p photoelectron lines are shifted by 11 meV towards lower kinetic energies. This shift, although within the error bars of the present experiment, has been taken into account in the determination of the S 2s binding energy in SF5CF3 for which we obtain 242.93(8) eV.

A. Kivimäki et al. / Chemical Physics 353 (2008) 202–208

ative than F and has a smaller capability to draw electrons away from S. Note that the SF bond lengths and FSF bond angles in SF5CF3 are very similar to those in SF6 [27]. The calculated S net populations in the ground states of SF6/SF5CF3 are 0.58/0.60 for S 2s and 0.88/1.01 for S 2p. That should lead to an increased Coster-Kronig decay rate in SF5CF3, explaining the observed trend in the S 2s1 lifetimes. Furthermore, effectively new decay channels, which are beyond the correlation diagram of the electron orbitals between SF5CF3 and SF6, may become available in SF5CF3, as some channels could be symmetry-forbidden in the Oh symmetry, but become allowed in the lower Cs symmetry. 3.2. S 2p photoelectron lines In general, the S 2p PES of a given molecule can display three main lines, as the 2p3/2 line can be split into two components by the molecular field (MF). This has been observed for small S containing molecules: the MF splitting in the S 2p3/2 photoelectron lines were observed to be between 100 and 150 meV for H2S, SO2, CS2 and OCS [28]. However, SF6 has an isotropic geometry with no MF splitting [29], thus its S 2p PES shows only two components due to the spin-orbit interaction. In SF5CF3, the geometry is lowered compared to SF6, so there should be a small MF splitting, which is also predicted by our calculations (see Section 3.4.). The S 2p photoelectron spectra of SF5CF3 and SF6 are shown in Fig. 2. The spectrum of SF6 displays the two spin–orbit split lines

3500

a

S 2p3/2

SF5CF3 hν =256 eV

3000 2500

Counts

The S 2s PES of SF6 was measured with Al Ka excitation by Siegbahn et al. [21]. They reported the S 2s binding energy of 244.7 eV. In a later paper, Gelius [22] published the S 2s and S 2p photoelectron spectra of SF6, SO2 and OCS. The S 2s binding energies were not given but, as estimated from the published spectrum, the S 2s threshold of SF6 appeared slightly lower than in the earlier determination [21]. As there is some uncertainty in the S 2s binding energy of the SF6 molecule, we decided to re-determine it in this work. The S 2s photoelectron spectrum of SF6 measured at the photon energy of 320 eV is shown in Fig. 1b. The first analysis was done in the same way as described above for SF5CF3, and it gave the Lorentzian width of 1.67 eV and the binding energy of 244.21 eV for the S 2s photoelectron line. However, the experimental peak appeared tilted with respect to the fitted curve. An asymmetry could be caused by the excitation of vibrational modes, which commonly occurs upon core photoionisation of molecules. Only the totally symmetric S–F stretch vibration, m1, is dipole-allowed and should be present in the S 2s photoelectron spectrum of SF6. Its energy is 95.9 meV in the molecular ground state [23]. The inclusion of vibrational components with a fixed spacing of 100 meV in the S 2s photoelectron peak improves the fit considerably. Somewhat surprisingly, it barely affects the Lorentzian width of the fitted Voigt profiles. This result is generally not valid, but may apply approximately whenever the extension of the vibrational structure is small compared to the lifetime width of the core hole state. A fit with four vibrational lines (nm1, n = 0, 1, 2, 3) within the S 2s peak is shown in Fig. 1b and it results in a Lorentzian width of 1.69 eV (which was assumed to be the same for each vibrational line). The inclusion of the vibrational structure affects slightly the position of the photoelectron peak: from the fit of Fig. 1b we obtain the ionization energy of 244.17 eV for the vibrationless S 2s1 state. The vibrational lines have reasonable intensity distributions, but the vibrational frequency cannot be obtained from this kind of analysis. In the photoelectron spectrum of Fig. 1b, there are weak structures of unknown origin that could affect the analysis. We are therefore conservative in the error estimation of our final results for SF6, reporting them as the S 2s ionization energy of 244.19(7) eV and the S 2s1 lifetime width of 1.68(5) eV. Note that the new S 2s ionization energy also affects the term values of the S 2s resonances [24], since they are given relative to the S 2s ionization energy. The S 2s binding energy decreases by 1.28(15) eV when substituting one F atom by the CF3 group. This experimental chemical shift is extracted from the results of the analogous fits to the S 2s PES of the two molecules (i.e., one Voigt profile described the S 2s photoelectron line). Theoretically, we calculate the chemical shift of the S 2s levels between SF5CF3 and SF6 to be 1.09 eV when using the orbital energies, and 1.34 eV when using the energies of the relaxed self-consistent-field (SCF) orbitals. The latter value compares well with the experimental result. The increase of the Lorentzian width by 0.2 eV when going from SF6 to SF5CF3 can mainly be ascribed to the decrease in the lifetime of the core-ionized state. Interesting effects of the chemical environment on the decay widths C have been recently discussed for tetrahedral molecules containing hydrogen and fluorine ligands [25,26]. A general argument relates a decrease in C to the decrease of the atomic orbital populations in valence orbitals on the coreionized centre, but it has had mixed success. While the C 1s1 lifetime width is significantly reduced in CF4 with respect to CH4, the opposite is observed for the Si 2p1 lifetime width in SiF4 versus SiH4. In the present context, the shift of the S 2s photoelectron line to a lower binding energy in SF5CF3 as compared to SF6 indicates a larger S atomic population in the former. In fact, the valence electron density on the core-ionized S atom is expected to be larger in SF5CF3, as S is also bonded to a carbon atom that is less electroneg-

2000

S 2p1/2

1500 1000

SF6

500 0 74

75

76

77

78

Kinetic energy (eV) 500

S 2p3/2

SF6 hν =256 eV

400

b

300

Counts

204

S 2p1/2

200

100

0 73

74

75

76

77

Kinetic energy (eV) Fig. 2. The S 2p photoelectron spectra of (a) SF6 and (b) SF5CF3 measured at the photon energy of 256 eV. The total experimental resolution is estimated to be 0.13 eV.

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3.3. Photon-energy dependence of the S 2p photoelectron lines of SF5CF3 Kitajima et al. [13] observed for SF6 that the relative partial photoionization cross section into the 2p1 3=2 state was enhanced at both the spin–orbit components of the 2t2g resonance (S 2p1 3=2 2t2g and 1 2p1 1=2 2t2g), while the photoionization into S 2p1=2 state resonated only at the S 2p1 1=2 2t2g resonance. This indicates that the S 2t state also possesses some character of the S 2p1 2p1 2g 1=2 3=2 ionic core, a manifestation of the exchange interaction between the S 2p1 inner-shell hole and the t2g electron. For the 4eg resonance, 1 Kitajima et al. [13] observed that the S 2p1 3=2 and S 2p1=2 cross sec-

tions have their maxima at the same photon energy, but the width 1 of the S 2p1 1=2 cross section was narrower than that of S 2p3=2 . The 1 and S 2p photoionization cross sections, as ratio of the S 2p1 3=2 1=2 determined across the whole 4eg resonance region, was 1.5 instead of the statistical ratio of 2. Also this effect was attributed to the exchange interaction between the S 2p hole and the 4eg electron. The anomalous intensity ratios between the S 2p spin–orbit components are also reflected in the soft X-ray emission spectra of SF6 measured at the shape resonances [29]. As already mentioned in the introduction, the excitation spectrum of the SF5CF3 molecule above the S 2p ionization limits shows two shape resonances similar to those of SF6. For simplicity, in the following discussion we will refer to them with the same labels as in SF6, albeit in quotation marks: ‘‘2t2g” and ‘‘4eg”. The ‘‘2t2g” resonance is spin–orbit-split, with each component about 3.3 eV above the respective S 2pj ionization limit. Our electron spectrometer can not register reliable electron spectra at such low kinetic energies that would be required for the S 2p PES at the ‘‘2t2g” resonance. Thus we could only track the S 2p photoelectron intensity as a function of photon energy across the ‘‘4eg” resonance, which is centered 15.5 eV above the S 2p3/2 ionization limit. The S 2p photoelectron spectrum of the SF5CF3 molecule was measured with 5 eV pass energy in the photon energy range from 186 eV to 210 eV, with a step of 0.5 eV. The photon energy resolution was slightly relaxed as compared to the spectrum of Fig. 2b, but without a significant effect on the resolution of the S 2p spin–orbit components. The intensities of the two S 2p photoelectron lines, obtained from the fits of the spectra, have been normalized to the photodiode currents and are shown in Fig. 3. Both the S 2p photoelectron lines are enhanced at the ‘‘4eg” shape resonance but the S 2p1/2 line reaches its maximum later. A Gaussian fit (not shown) of the cross section curves gives the energy difference of 1 0.5 eV between the S 2p1 1=2 and S 2p3=2 maxima. We also obtain from the fit that the resonance experienced by the S 2p1 3=2 state is 1.0 eV wider than that of the S 21 1=2 state. In comparison to SF6, there are some differences in the behavior of the S 2p photoelectron lines of SF5CF3 at the ‘‘4eg” shape resonance. In SF6, the S 2p1 1=2 photoionization cross section is larger 1 than statistically expected: the ratio of the S 2p1 3=2 and 2p1=2 photoionization cross sections across the whole shape resonance is 1.5 [13] instead of the statistical value of 2. In SF5CF3, we obtained 1 an average value of 1.9 for the S 2p1 3=2 :2p1=2 intensity ratio across the whole ‘‘4eg” shape resonance region, even though the ratio

8

Intensity (arb. units)

that have similar widths and shapes, i.e., there is no sign of the molecular field splitting, in agreement with the above-given symmetry arguments. However, the peak profiles are slightly asymmetric towards the low kinetic energy side. This asymmetry could possibly be attributed to two factors. Firstly, the S 2p photoelectron lines of SF6 should show a short vibrational structure, which has been observed in the S 2p excitations to the Rydberg orbitals [9]. Secondly, it could be caused by the post-collision interaction (PCI). However, resolving both effects is well beyond the capability of our experiment as the vibrational energy is of the order of 100 meV and the PCI effect at this excess energy (75 eV) is negligible (Auger electrons ejected after the 2p hole decay in SF6 have kinetic energies 100–140 eV [30]). For that reason and observing that the N4,5OO Auger lines of Xe were also found to be asymmetric, we conclude that the asymmetry is at least partly due to the instrumental profile of the electron analyzer. The fit analysis of the S 2p PES gives a spin–orbit splitting of 1.21(1) eV, which agrees with the result of Hudson et al. [9]. Fig. 2a shows the S 2p PES of the SF5CF3 molecule. The 2p1/2 and 2p3/2 photoelectron lines are clearly broader than in the spectrum of SF6 measured under the same conditions. The feature around 74.5 eV is due to the S 2p1/2 photoelectron line of SF6 still remaining in the experimental chamber. Its spin–orbit component is not visible, but it has been taken into account in the fitting of the spectrum. Each S 2p photoelectron line of SF5CF3 was fitted with two Gaussian profiles; the smaller one was needed in order to account for the slight asymmetry in the observed line shape (see Fig. 2a). The binding energies of 178.92(5) eV and 180.13(5) eV were obtained for the maxima of the S 2p3/2 and 2p1/2 photoelectron lines of SF5CF3. The energy calibration was done with respect to the observed S 2p1/21 state of SF6, whose binding energy is 181.48 eV [9]. The S 2p spin–orbit splitting of SF5CF3 is 1.21(2) eV, which is the same as that measured for SF6. The S 2p binding energies decrease by 1.35(5) eV upon substitution of an F atom in SF6 by the CF3 group to form SF5CF3. This chemical shift is about the same as that found for the S 2s levels in these two molecules [1.28(15) eV]. The S 2p chemical shift is calculated to be 1.13 eV employing the orbital energies and, more realistically, 1.31 eV employing the energies of the relaxed SCF orbitals. The full width at half-maximum (FWHM) of about 0.5 eV of the S 2p photoelectron lines in SF5CF3 are mostly caused by unresolved vibrational excitations, as the measured spectrum does not have a line shape with a predominant Lorentzian contribution. While in SF6 only one totally symmetric vibration (the S–F stretch) is expected to appear in the S 2p photoelectron spectrum, there are several totally symmetric modes in SF5CF3 [31]. In the case of the S 2s1 state, we found that the lifetime broadening increased by more than 10% when going from SF6 to SF5CF3. We cannot exclude a similar effect in the S 2p1 states, but a significant lifetime broadening is not expected as the lifetime width of the S 2p1 states in SF6 can be as low as 50 meV [9]. The large width of the S 2p3/2 peak in SF5CF3 also prevents any attempts to determine the MF splitting.

SF5CF3

S 2p1/2 S 2p3/2 S 2pTOT TIY

6

4

2

0 185

190

195

200

205

210

Photon energy (eV) Fig. 3. Dots and triangles display the intensities of the S 2p1/2 and S 2p3/2 photoelectron lines, respectively, of the SF5CF3 molecule in the region of the ‘‘4eg” shape resonance. The total S 2p intensity is presented with open squares. For comparison, the total ion yield (TIY) of SF5CF3 (from [12]) is shown with a solid curve. The base level and the height of the TIY curve have been adjusted for presentation purposes.

A. Kivimäki et al. / Chemical Physics 353 (2008) 202–208

does vary from 1.3 to 2.8 inside the resonance. The same ratio of 1.9 is also obtained from the spectra measured above 205 eV photon energy, so the deviation from 2 is probably within the error limits. In SF6, the two S 2p1 photoionization cross sections peaked at the same photon energy, while in SF5CF3 there is a small energy 1 difference. An energy difference between the S 2p1 1=2 and S 2p3=2 cross section maxima is actually expected within the electron trapping model of the shape resonances, where the kinetic energy of the photoelectrons is assumed to be constant at the shape resonance. This would translate into the energy difference corresponding to the S 2p spin–orbit splitting (1.2 eV), while we observe a clearly smaller value of 0.5 eV. A common feature for SF5CF3 and SF6 is that at the 4eg shape resonance the width of the S 2p1 1=2 cross . In summary, section enhancement is narrower than that of S 2p1 3=2 at the ‘‘4eg” shape resonance, S 2p photoionization behaves in a more expected way in SF5CF3 than in SF6, considering that the S 1 2p1 3=2 :2p1=2 cross section ratio is closer to the statistical value and 1 that the S 2p1 3=2 and 2p1=2 intensity maxima do show a small energy difference. This implies a smaller exchange interaction in SF5CF3. In order to explain the difference in the interaction strengths, let us consider the consequences of the reduced symmetry of the SF5CF3 molecule. The SF6 molecule is a perfectly symmetrical cage in which the presence of the six F ligands helps to create the potential barrier around the S atom. The potential barrier in the SF5CF3 molecule has an imperfection in the direction towards the C atom, where it is probably lower. If it is so, this would lead to a shorter tunneling time of the trapped photoelectron and a shorter lifetime of the shape resonant state. In agreement with this speculation, the ‘‘4eg” shape resonance is clearly wider in SF5CF3, where its fullwidth at half maximum is 5.2 eV [12] (see also Fig. 3), than in SF6 (4.1 eV [9]). This means that an electron in the ‘‘4eg” orbital is more delocalized in SF5CF3, and its interaction with the core hole is lowered. In Fig. 3 we also show the total ion yield (TIY) from [12]. The S 2p photoelectron lines should not explain all the intensity in the TIY, but a part of the total S 2p photoionization cross section is distributed into shake-up and shake-off processes. The first shake-up satellite of SF5CF3 already appears at the binding energy of 186.4 eV (see below). In SF6, the intensity of the lowest-energy shake-up satellite was measured to have 30% of the S 2p single-hole photoionization cross section at the 4eg resonance [32].

3

12x10

10

8

x 0.07 6

4

2

g f

e

c d

h

ba

0 10

20

30

40

50

Kinetic energy (eV) Fig. 4. The S 2p photoelectron lines and shake-up satellites of the SF5CF3 molecule measured at the photon energy of 229.7 eV. The original spectrum is the uppermost one. An estimate of the background (black solid curve) has been subtracted from it. The resulting spectrum (black dots) has been fitted with Gaussian profiles (solid curves), whose sum is the total fit (light solid curve on data points). The intensity of the S 2p main photoelectron lines have been multiplied by a factor of 0.07 for presentation purposes.

Table 1 Experimental and theoretical energies and intensities of the S 2p shake-up satellites in SF5CF3 Experiment

Theory

Label

E (eV)

Int (%)

Group

E (eV)

Int (%)

Main shake-up transitions

a b c

7.5 8.9 11.5

0.8 0.4 3.9

1 10 2

8.3–10.0 11.7–12.3 14.0–14.6

0.40 0.22 0.91

20

15.2–16.2

0.59

HOMO (rS–C) ? LUMO (rS—C ) HOMO ? 35a0 ; 29a0 ? LUMO HOMO ? 35a0 ; HOMO ? 34a0 ; HOMO ? 16a00 29a0 ? LUMO; 24a0 ? LUMO; 14a00 ? 16a00 Occupied outer valence MOs ? virtual valence MOs (16a00 , 34a0 ) Occupied outer valence MOs ? virtual valence MOs (35a0 –37a0 ) and higher virtual MOs (38a0 , 18a00 ) Occupied valence MOs ? higher virtual MOs (40a0 –45a0 , 18a00 –19a00 )

d

16.0

2.8

3

17.0–18.5

1.15

e

20.9

5.8

4

24.0–28.0

2.93

f

24.7

0.7

5

30.0–33.0

1.29

g h

26.2 29.5

1.5 1.1

3.4. S 2p shake-up spectrum of SF5CF3 The S 2p shake-up spectrum of SF5CF3 is shown in Fig. 4. It was measured using 10 eV pass energy for the electron analyzer at the photon energy of ca. 230 eV. The photon energy was selected to be below the S 2s pre-edge resonances, which are located around 240 eV, in order to avoid spectral features due to the L1–L2,3V Coster-Kronig decay. The shake-up spectrum shows some fine structure which becomes more evident after background subtraction. The background-subtracted spectrum (black dots) was fitted with Gaussians (light solid curves) whose positions and widths were allowed to change freely. The number of the peaks (eight) was the minimum to get a good fit. Table 1 gives the shake-up energies of the satellites with respect to the S 2p3/2 photoelectron line (shake-up energy = kinetic energy of the main line – kinetic energy of the satellite) and shake-up intensities as percentages of the total S 2p main line intensity. The intensities of the shake-up satellites obviously depend on the choice of the background and on the transmission of the electron analyzer, which was assumed to be flat, as it was with 5-eV pass energy (see Experimental). Furthermore, above the first S 2p shake-off limit, shake-off transitions become possible and cause a continuous, but unknown intensity towards lower kinetic energies. Thus, the intensities given are subject to large error bars (relative error may be of the order of 30%).

S 2p

SF5CF3 S 2p shake-up photon energy 229.7 eV

Counts

206

Experimental values were obtained from the fit analysis shown in Fig. 4. Theoretical results were computed within the 2h–1p CI approach. The shake-up energies are given in electron volts with respect to the S 2p hole state of the lowest energy, while the intensities are percentages of the total intensity of the S 2p main photoelectron lines. Theoretical intensities (squared overlaps) of the shake-up transitions have been converted to percentages by dividing them by the sum (2.485) of the squared overlap of the three primary holes. Experimental labels and theoretical group designations refer to Figs. 4 and 5, respectively.

The total intensity of all the shake-up satellites obtained from the fit was approximately 17% of the S 2p single-hole intensity. Satellite states have been calculated by ab-initio configuration interaction (CI) calculations within the sudden-limit approximation [33,34]. The final ionic states (core hole and satellites) have been computed employing a 2h-1p CI with one hole localized on one of the three S 2p orbitals, i.e. considering all single excitations in the presence of a S 2p core hole. Relaxed self-consistent-field

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(SCF) orbitals relative to the S 2p1 hole configuration have been employed. For the initial state the ground state SCF configuration has been employed. Intensities have been computed according to the sudden approximation as squared overlaps:

where ak annihilates an S 2p orbital. The Dunning cc-PVTZ basis has been employed for the S atom, fully truncating the outermost primitives to ensure a good description of the relaxation, and a cc-PVDZ basis for the C and F atoms. Higher order CI was impractical for this system, where a very high density of states is obtained already at the 2h–1p level. Up to 1000 CI vectors had to be extracted to cover some 30 eV in shake-up energy. More accurate calculations on small molecules [34–36] show that the inclusion of higher excitations generally lowers shake-up energies and mainly increases the intensities, with some redistribution. However, the picture obtained at the 2h–1p level should be at least semi-quantitative. The shake-off threshold has been estimated by an analogous 2h CI calculation. The Cs symmetry of the molecule splits the S 2p core orbitals into 10a0 , 11a0 and 3a00 . These orbitals were calculated to have ionization energies of 181.213 eV (11a0 ), 181.394 eV (10a0 ), and 181.383 eV (3a00 ). (The approach used is non-relativistic, hence the spin–orbit interaction is not taken into account.) The bonding rS-C orbital (32a0 ) is the highest occupied MO (HOMO) in SF5CF3. There are seven virtual valence MOs (33a0 –37a0 and 16a00 –17a00 ). The MO 33a0 corresponds to the lowest unoccupied MO (LUMO), which is r* antibonding in character and similar to the r* a1g antibonding orbital of SF6. After virtual valence orbitals, there are even higher lying virtual MOs (38a0 ,. . ., 18a00 ,. . .) with more Rydberg character. The coupling of the three inequivalent holes (single excitations + S 2p hole) gives rise to a large number of final states of A0 and A00 symmetry. This is reflected in the calculated spectrum where the shake-up intensity appears spread over many lines in all parts of the spectrum. This intensity fragmentation also derives from the significant mixing of configurations which characterize all the shake-up states. Additional splitting is expected also due to the neglected spin–orbit effect. As the calculated structures do not derive from distinct main transitions, we assign them by grouping the final states in short energy ranges, as indicated in Fig. 5. These states are then characterized by considering the most intense transitions of each group and the main configuration contributing to them, as reported in Table 1. The intensity of each group is obtained summing the overlap squared values of all the transitions present in the relative energy range. Note that the high-energy transitions contributing to peak 5 all fall above the calculated shake-off limit (23.2 eV). Large intensity shake-up structures originating from inner-valence orbital excitations can survive in the spectrum above the shake-off threshold [37], as is clearly seen also in the present experiment. Our calculated total shake-up and shake-off intensity, given by (1 – R0)/R0, is 0.207 (or 21%), where R0 is the average spectral intensity of one S 2p single-hole state. Theoretical and experimental spectra bear many similarities. For an easier comparison, we have plotted the experimental shake-up spectrum in Fig. 5 in the shake-up energy scale. Group 1 evidently corresponds to the lowest-energy experimental peak a, which is thus assigned to have predominantly HOMO ? LUMO character. Group 10 occurs between two more distinct structures (1 and 2) similarly to the peak b in the experimental spectrum. We relate the experimental peak c to structure 2 (composed of groups 2 and 20 ) in the theoretical profile. In all these cases, the maxima in the theoretical profile have ca. 2.5–3.5 eV higher shake-up energies than the experimental ones. The assignment of

c

1500

Counts

ð1Þ

e f d 1000

g

500

b a

h

0 4

0.05

overlap squared

Rlk ¼ jhW1N1 jak jwN0 ij2

2000

3

0.04

2

0.03 5

0.02 1

0.01

4

5

3 2'

2 1'

1

0.00 40

30

20

10

0

Shake-up energy (eV) Fig. 5. The lower panel shows the theoretical S 2p shake-up spectrum of the SF5CF3 molecule calculated within the 2h–1p CI approach. Vertical bars give the squared overlap intensities of the individual transitions. The spectrum is divided in several groups of transitions (1, 10 ,. . .,5). The sum profile (solid curve) was generated assuming a constant width for all the lines (hundreds, many very small). Upper panel shows for comparison the background-subtracted experimental shake-up spectrum from Fig. 4 in the shake-up energy scale.

the theoretical group 3 to the experimental peak d is somewhat questionable, as this would imply that the energy difference between theory and experiment would be slightly smaller than in the previous cases. Both experiment and theory display the highest intensity shake-up feature at rather high shake-up energy (peak e and group 4, respectively). Here the calculated energy is 5 eV too large. Finally, structure 5 in the theoretical profile at ca. 31.5 eV shake-up energy appears similar to the combined peaks f and g in the experimental spectrum, in respect to its intensity and position compared to the highest intensity shake-up structure. The assignments discussed here are also presented in Table 1. 4. Conclusions The S 2p and S 2s photoelectron spectra of SF5CF3 have been measured and compared to those of SF6. The S 2s binding energy of SF5CF3 is determined to be 242.93(8) eV, while that of SF6, redetermined in this work, is 244.19(7) eV. The S 2p3/2 and 2p1/2 binding energies of SF5CF3 are 178.92(5) and 180.13(5) eV, a reduction of 1.35(5) eV from the values of SF6. The spin–orbit splitting of the S 2p ionized state has, within the error bars, the same value of 1.21 eV in these two molecules. Both the S 2p and S 2s photoelectron lines are broader in SF5CF3 than in SF6. We deduce that the S 2s1 lifetime width is 0.2 eV larger in SF5CF3 than in SF6 and suggest that this is due to the increase of the Coster-Kronig decay rates, caused by the increased S 2p and valence electron density on the core-ionized S atom as compared to the SF6 molecule. On the contrary, the excitation of vibrational modes in S 2p photoion-

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ization appears to be the main reason for the broadening of the S 2p photoelectron lines in SF5CF3. The intensities of the spin–orbit-split S 2p photoelectron lines were measured at several photon energies across the ‘‘4eg” shape resonance in SF5CF3. Both the S 2pj components resonate at the shape resonance, but their maxima differ by about 0.5 eV. Furthermore, the width of the S 2p1 1=2 cross section is 1 eV narrower than that of the S 2p1 3=2 cross section. The anomalous behavior is caused by the exchange interaction that, however, appears weaker in SF5CF3 than in SF6, presumably due to the larger delocalization of the ‘‘4eg” orbital in the former. The S 2p shake-up satellite spectrum of SF5CF3 was measured at the photon energy of 230 eV. The spectrum could be reproduced rather well by ab initio CI calculations within the sudden-limit approximation, taking into account all possible single excitations in the presence of an S 2p hole. The number of shake-up transitions in the spectrum was found to be huge already at the 2h–1p level, hence experimental peaks cannot be assigned to distinct main transitions but rather to the groups of transitions that have some common characteristics. Acknowledgments We acknowledge the support provided by the European Community-Research Infrastructure Action under the FP6 ‘‘Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative ‘‘Integrating Activity on Synchrotron and Free Electron Laser Science”. We are also thankful to Christian Leghissa and Salvatore La Rosa (from Sincrotrone Trieste) for assistance in assembly of the experimental apparatus. References [1] W.T. Sturges, T.J. Wallington, M.D. Hurley, K.P. Shine, K. Sihra, A. Engel, D.E. Oram, S.A. Penkett, R. Mulvaney, C.A.M. Brenninkmeijer, Science 289 (2000) 611. [2] M.A. Santoro, Science 290 (2000) 935. [3] L. Huang, L. Zhu, X. Pan, J. Zhang, B. Ouyang, H. Hou, Atmos. Environ. 39 (2005) 1641. [4] W. Carrier, C.S. Jamieson, R.I. Kaiser, Inorg. Chem. 46 (2007) 1332. [5] T. Ibuki, Y. Shimada, S. Nagaoka, A. Fujii, M. Hino, T. Kakiuchu, K. Okada, K. Tabayashi, T. Matsudo, Y. Yamana, I.H. Suzuki, Y. Tamenori, Chem. Phys. Lett. 392 (2004) 303. [6] T. Ibuki, Y. Shimada, R. Hashimoto, S. Nagaoka, M. Hino, K. Okada, I.H. Suzuki, Y. Morishita, Y. Tamenori, Chem. Phys. 314 (2005) 119.

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