Studies on GeF4 Valence and Rydberg States by Electron Impact

Article pubs.acs.org/JPCA

Studies on GeF4 Valence and Rydberg States by Electron Impact Spectroscopy and Ab Initio Calculations S. Ohtomi,† M. Hoshino,*,† A. Suga,† H. Kato,† D. Duflot,‡,§ P. Limaõ -Vieira,*,†,∥ and H. Tanaka† †

Department of Physics, Sophia University, Tokyo 102-8554, Japan Université de Lille, UMR 8523 - Physique des Lasers Atomes et Molécules, F-59000 Lille, France § CNRS, UMR 8523, F-59000 Lille, France ∥ Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal ‡

ABSTRACT: Electron energy loss (EEL) spectra of GeF4 have been measured with incident electrons at 100 eV for 5° scattering angle and at 30 eV for 30° scattering angle, while sweeping the energy loss over the range 7.0−18.0 eV. Lowlying excited triplet, singlet, valence, and Rydberg states are investigated and the assignments supported by quantum chemical ab initio calculations. This provides the first comprehensive investigation of all singlet and triplet excited electronic states of germanium tetrafluoride up to the first ionization energy. The Rydberg series converging to the (lowest) ionization energy limits of GeF4 are also identified according to the magnitude of the quantum defects (δ).

1. INTRODUCTION Germanium tetrafluoride (GeF4) has been attracting the interest of the international scientific community because of its relevance within group IV tetrafluorides (e.g., CF4, SiF4) widely used as etching agents in chemical plasmas. In these lowtemperature plasmas used for the manufacture of semiconductors, electron induced collisions are responsible for driving the local chemistry. As so, detailed information on the underlying molecular mechanisms with the particular relevance to electron-induced processes is needed to improve our knowledge and understanding of the electronic state spectroscopy of such molecular compounds. From the technical and industrial perspective, this information is useful to take advantage of particular plasma conditions better attuned to achieve a specific performance of the plasma reactors. Electron impact excitation, dissociation, and ionization are prevalent processes in chemical vapor deposition (CVD)1 and plasma etching processes.2 Tanaka and Inokuti3 have reported that molecular dissociation mechanisms triggered by electronic excitation forms the core of CVD, and ionization and charge transfer processes are also responsible to maintain the discharge, whereas inelastic interactions are responsible for thermalization of the secondary electrons produced. Regarding the topic of this paper, a literature survey reveals few available studies. The absorption spectrum of GeF4 shows a broad structureless band in the 110−140 nm (11.3−8.86 eV) wavelength region assigned to valence excitation of the outermost orbital σ* ← 1t1.4 Electron energy loss spectra of SiF4 and GeF4 have been recorded by Kuroki et al.5 in the 10− 30 eV energy region covering the five outermost orbitals, © XXXX American Chemical Society

whereas photoelectron spectra of GeF4 have been recorded by Bassett and Lloyd,6 Jonas et al.,7 Cradock,8 and Lloyd and Roberts.9 Previous experimental work on this molecule includes the grand total cross section (TCS) measurements by Szmytkowski and co-workers10 covering an energy range of 0.5−250 eV. Other relevant studies include the absolute differential cross sections (DCSs) for elastic electron scattering from GeF4 of Kato et al.11 (and references therein) over the energy range of 3−200 eV, the elastic scattering calculations by Mozejko et al.,12 and the calculations of Goswami et al.13 who applied an ab initio R-matrix method at low impact energies and the spherical complex optical potential formalism at intermediate to high energies. More recently, theoretical calculations of elastic differential, integral, and momentum transfer cross sections for GeF4 based on the Schwinger multichannel method with pseudopotentials in the staticexchange and static-exchange plus polarization approximations have been reported for electron energies up to 12 eV.14 Finally, we note experiments on negative ion formation through dissociative electron attachment to the group IV tetrafluorides: CF4, SiF4, and GeF4 by Bjarnason et al.15 In this paper, we report experimental studies on the electronic state spectroscopy of GeF4 by high resolution electron energy loss spectroscopy comprehensive ab initio theoretical calculations on the lowest-lying valence (singlet and triplet), Rydberg, and ionization states. In the next section we Received: September 9, 2016 Revised: October 23, 2016 Published: October 26, 2016 A

DOI: 10.1021/acs.jpca.6b09138 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A provide a brief summary of the spectroscopy of germanium tetrafluoride. In addition to identifying the optical electronic transitions of GeF4, the present work provides reliable electron energy loss data in the range 7.0−18.0 eV. In section 3, we present the experimental details, and section 4 is devoted to the theoretical methods. Section 5 is devoted to the results and discussion with a comparison with other data whenever available. Finally, some conclusions that can be drawn from this study are given in section 6.

2. SPECTROSCOPY OF GeF4 Germanium tetrafluoride calculated geometry optimized at CCSD(T) level with the aug-cc-pVQZ basis set establishes a R(Ge−F) bond length of 1.7300 Å. This value is consistent with the typical value in the gas-phase molecules containing Ge and F atoms from the Handbook of Chemistry and Physics (1.73 Å).16 GeF4 is a tetrahedral molecule with a Td symmetry in its electronic ground state. The symmetry species available to a Td molecule are A1, A2, E, T1, and T2, and the calculated electron configuration of the X̃ 1A1 ground state is [Ne] (1sF core): 1t26 1a12; (valence): 2a12 2t26 3t26 1e4 3a12 4t26 4a12 5t26 2e4 6t26 1t16. Note that the order and orbital labeling may differ from the electron impact spectroscopy work of Kuroki et al.,5 whereas in the case of Bassett and Lloyd,6 and Lloyd and Roberts,9 just orbital numbering is different from the present work. This is due to the fact that the 10 innermost electrons, corresponding to the Ne electronic configurations, were not explicitly included in the present study (see section 4). Examination of the ground-state MOs (Figure 1) shows that the highest occupied molecular orbital (HOMO), 1t1, the second highest (HOMO−1), 6t2, and the third highest occupied molecular orbital (HOMO−2), 2e, have F lone pair character (see Figure 1). We can anticipate that promotion of nF electrons will result in the absence of vibrational excitation features since these do not participate in Ge−F bonding. The (HOMO−3), 5t2, and (HOMO−4), 4a1, have σ(Ge−F) character. The lowest unoccupied molecular orbitals (LUMO), 5a1, and (LUMO+1), 7t2, are mainly of σ*(Ge−F) antibonding character (see Figure 1). The theoretical studies have shown considerable overlap of the lowest Rydberg states with valence states. The calculated transition energies, oscillator strengths, and the main character of the wave function are shown in Tables 1 and 2 for singlet and triplet states (EOMCCSD results). The lowest vertical ionization energies have been experimentally obtained at 16.08 (1t1)−1, 16.50 (6t2)−1, 17.04 (2e)−1, 18.60 (5t2)−1, and 21.30 eV (4a1)−1.8 The calculated vertical IEs at the CCSD(T) geometry are presented in Table 3 and agree reasonably well with the experimental data. The experimental values have been used to calculate the quantum defects associated with transitions to Rydberg orbitals (section 5.3).

Figure 1. Shape of the molecular orbitals (aug-cc-pVQZ-(PP) basis set) of GeF4.

3. EXPERIMENTAL METHOD The electron spectrometer used in this work has been described in detail elsewhere.17,18 Briefly, incident electrons wellcollimated by an electrostatic hemispherical monochromator collide with an effusive molecular beam of GeF4 at right angles. Scattered electrons are energy-analyzed by a second hemispherical electrostatic system. The electron optics was designed by beam trajectory simulations and run through computercontrolled voltages which follow the energy sweeps, in order to maintain the beam focus conditions and to keep the

transmission efficiency constant. This spectrometer has been operated at impact energies from 1.5 to 300 eV and angular range from −6° to +130° with respect to the beam direction. An electron beam current was used at ∼5−10 nA depending on the incident electron energies with an overall energy resolution of 45−50 meV (fwhm), and an angular resolution of ±1.5°. In this study, the true zero degree scattering angle was determined carefully by measuring the symmetry in intensities profile of He 1 S → 21P inelastic excitation. With these procedures, we believe B


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The Journal of Physical Chemistry A

Table 1. Calculated Vertical Excitation Energies (EOM-CCSD Level with aug-cc-pVQZ-(PP)+R basis set) (eV) and Oscillator Strengths (Singlet States) of Germanium Tetrafluoride (GeF4) Compared with Experimental Data and Other Work (Energies in eV) Td

E (eV)

fL

⟨r2⟩a

X̃ 1A1 11T1

9.526

0.0000

75 76

11E

10.228

0.0000

76

11T2

10.333

0.6598

76

21T2

11.931

0.2136

78

21T1 11A2 21E 21T2 31E

13.166 13.171 13.247 13.453 13.567

0.0000 0.0000 0.0000 0.2715 0.0000

101 94 99 118 110

31T1 11A1 31T2 41T2 41T1 51T2 41E 21A2 51T1 51E 1 T2 61T1 61E 1 T2 71T1 21A1 31A2 61T2 81T1 31A1 91T1 71E 71T2 101T1 81E 81T2 91T2 111T1 41A2 121T1 101T2 91E 1 T2 101E 131T1 111T2 141T1 151T1 51A2 121T2 131T2 111E

13.686 13.719 13.721 14.127 14.239 14.421 14.502 14.673 14.689 14.777 14.784 14.793 14.797 14.860 14.882 14.912 14.939 14.952 14.960 14.993 15.087 15.180 15.182 15.211 15.244 15.248 15.270 15.283 15.361 15.391 15.410 15.418 15.441 15.453 15.461 15.477 15.496 15.517 15.526 15.533 15.533 15.543

0.0000 0.0000 0.0004 0.0254 0.0000 0.0111 0.0000 0.0000 0.0000 0.0000 0.1874 0.0000 0.0000 0.0076 0.0000 0.0000 0.0000 0.1976 0.0000 0.0000 0.0000 0.0000 0.0264 0.0000 0.0000 0.0177 0.0631 0.0000 0.0000 0.0000 0.0321 0.0000 0.0382 0.0000 0.0000 0.0039 0.0000 0.0000 0.0000 0.0241 0.0234 0.0000

97 93 100 117 119 144 115 197 197 203 181 178 189 203 218 161 218 221 269 149 195 266 225 229 233 218 227 244 490 473 440 299 370 490 484 472 525 660 578 556 559 357

HOMO (1t1)

HOMO−1 (6t2)

HOMO−2 (2e)

HOMO−3 (5t2)

mixed character

E (eV)

ref 5

10.40

10.42

12.05

12.03

14.36

13.36

5a1/σ* (GeF) 5a1/σ* (GeF) 5a1/σ* (GeF) 5a1/σ* (GeF) 5p 5p 5p HOMO→5p + HOMO−1→5a1/σ*(GeF) HOMO−1 →5p + HOMO →5p + HOMO−2 → 5a1/σ*(GeF) HOMO−1→5p+ HOMO−2→5p 5p 5p

14.4(5) 14.96

5p HOMO → 5s + HOMO−2 →5p HOMO−1→5s + HOMO→5p HOMO−2→5s + HOMO−1→5p 6p 6p 6p 4d 4d 4d 4d 4d

14.4(5)

14.53

14.96

14.94

6p 4d 6p 6s 4d 4d 6p 4d 6p HOMO→5d + HOMO−1→4d 6p 4d 4d

14.96

7p 7p HOMO→5d + HOMO→7p HOMO→5d + HOMO−1→4d HOMO−1→6s + HOMO→5d HOMO→7p + HOMO−1→6p

15.44

5d HOMO−1→6s+ HOMO→5d 5d HOMO→7s 5d HOMO−2→5p + HOMO→5d 5d HOMO−2→5p + HOMO→5d

a

Mean value of r2 (electronic radial spatial extents). b(s) indicates a shoulder structure (the last decimal of the energy value is given in brackets for these less resolved features). C


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4. THEORETICAL METHODS

Table 2. Lowest Valence and Rydberg Triplet Transition Energies (eV) of Germanium Tetrafluoride (GeF4) Calculated at the EOM-CCSD/aug-cc-pVQZ-(PP)+R Level

3

T1

3

T2

3

E

3

T2

Calculations were performed to determine the geometry and excitation energies of the neutral molecules (see Tables 1 and 2) and the vertical ionization energies (Table 3) using the MOLPRO code.26 Because MOLPRO cannot handle nonabelian symmetry groups, the calculations are in fact performed using the smallest abelian subgroup of Td, i.e., C2v. The ground state geometry was optimized at the frozen core Coupled Cluster Single−Double with perturbative Triples (CCSD(T))27 level. For F, Dunning’s aug-cc-pVQZ basis set28 was used, while for Ge, the Dolg Effective Core Potential was used for the description of the 10 innermost electrons29 and the 22 valence electrons were described using the corresponding aug-ccpVQZ-PP basis set.30 The electronic spectra were computed at the Equation of Motion EOM-CCSD level31 at the obtained CCSD(T) geometries. For the electronic excited states (Rydberg), the basis set was augmented by a set of diffuse functions (5s, 5p, 3d) MOs taken from Kaufmann et al.,32 located on the Ge atom and added to the original basis set augcc-pVQZ (aug-cc-pVQZ-(PP)+R). The oscillator strengths ( f L) of the electric dipole transitions were calculated using the length gauge after obtaining the dipole transition moment between the ground (Ψgs) and excited (Ψexc) states using Hansen’s33 standard formula (in a.u.):

effective quantum number (n*)

assignment

E (T)

E (S)

exp.

HOMO (1t1) → 5a1/σ*(GeF) HOMO−1 (6t2) → 5a1/σ*(GeF) HOMO−2 (2e) → 5a1/σ*(GeF) HOMO−3 (5t2) → 5a1/σ*(GeF)

8.924

9.526

−

9.136

10.333

9.38

1.39

9.497

10.228

9.38

1.39

10.672

11.931

10.40

that the accuracy of the angular calibration is better than 0.5°. The GeF4 molecular beam was produced effusively from a single tube with the length of 5 mm and diameter of 0.3 mm, which was kept at slightly elevated temperatures (∼70 °C) throughout the measurements to avoid surface contamination on the nozzle by GeF4 molecules. The energy scale of the incident electron was calibrated by measuring the He 2S Feshbach resonance at 19.37 eV.19 Relative intensities of elastically scattered electrons were normalized to the standard elastic DCS of He20 using a well-established relative flow technique.21−23 On the other hand, the benchmarked inelastic DCSs of He 1S → 21P transition24,25 have been used for normalization for intensities of inelastically scattered electrons at higher impact energies above 100 eV and forward scattering angle smaller than 10°. This normalization requires constant Knudsen number between GeF4 and He to generate two equal gas densities in the collision volume, for which the head pressures behind the nozzle were fixed at about 0.6 Torr for GeF4 and 3.0 Torr for He based on the hard sphere model. The target gas was obtained from Takachiho Chemical Industrial Co., Ltd. with a stated purity of better than 99.5% for GeF4. It was used without additional purification. Background pressures were on the order of 2.0 × 10−5 Pa in the absence of target gas beam, and increased to about 5.0 × 10−4 Pa in the presence of the molecular target.

fL =

2 ΔE |⟨Ψgs|r|Ψexc⟩|2 3

The lowest vertical ionization energies of GeF4 were also obtained at the restricted RCCSD(T) level (Table 3), using partial third order (P3) propagation and outer valence green function (OVGF)34 calculations,35 as implemented in the Gaussian 09 package.36 Gaussian 09 was also used to obtain the lowest triplet transitions at the EOM-CCSD/aug-cc-pVQZ(PP)+R level, since this type of calculation is not available through MOLPRO. Finally, the assignment of the calculated transitions was performed through the calculated spatial extensions of the wave function ⟨r2⟩, as well as examining the shape of the EOM-CCSD natural orbitals (aug-cc-pVQZ(PP)+R calculations).

Table 3. A. Calculated Vertical Ionization Energies for Germanium Tetrafluoride, GeF4 (in eV); (b) Vertical Ionization Energies and Intensities (Pole Strengths, PS) of Germanium Tetrafluoride, GeF4 (Energies in eV)a A T1 (1t1−1)

T2 (6t2−1)

2

Koopmans’ theorem ROHF RMP2 RCCSD CCSD(T)

2

18.472 17.635 15.613 16.791 16.247

18.438 17.598 16.250 17.035 16.605

B 1t1−1 OVGF P3 Expb Expc Expd a

2e−1

6t2−1

5t2−1

4a1−1

Energy

PS

Energy

PS

Energy

PS

Energy

PS

Energy

PS

16.689 16.671 16.03 16.06 16.08

0.942 0.922

16.946 16.950 16.56 16.55 16.50

0.935 0.919

17.348 17.330 17.08 17.06 17.04

0.944 0.925

18.740 18.752 18.54 18.55 18.60

0.940 0.923

21.796 21.808

0.939 0.924

21.30

Obtained at the RCCSD(T) geometry using the cc-pVQZ-(PP) basis set. bRef 6. cRef 7. dRef 8. D


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5. RESULTS AND DISCUSSION Figure 2a,b shows the full range of the electron energy loss (EEL) spectra for germanium tetrafluoride (GeF4) in the 7.0−

above 10 eV which may be mostly related to dissociative and/ or predissociative states contributing to shift upward the EEL spectra. A detailed analysis of Figure 2b DCSs from 100 eV, 5°, and 30 eV, 30°, shows identical behavior of the differential cross sections, which is reminiscent of the nature of the singlet transitions involved in the electronic excitation of germanium tetrafluoride. However, between 8.5 and 10 eV energy loss, there is clear experimental evidence of the triplet character in the low-lying transitions (see Table 2), which is discussed here for the first time and fully supported by theoretical calculations (see section 5.2). It is relevant to mention that our electron energy loss spectroscopy data at low electron impact energy and relatively high scattering angle enhances singlet to triplet transitions against the singlet to singlet transitions. Such spectroscopic evidence has been reported by us on previous occasions with other molecular targets with potential use in the plasma processing industry and fully supported by theoretical calculations.37−39 Rydberg state spectroscopy has been performed with the help of available ionization energy values, although we have obtained vertical ionization energies (IEs) at different levels of theory, the results being presented in Table 3. A close comparison between the different methods in Table 3a shows that these agree reasonably well with each other with the exception of Koopmans’ theorem, ROHF and RMP2. Regarding the ROHF method, the value obtained is prominently high, which reflects the difficulties of the Hartree−Fock method to obtain accurate molecular properties, especially for open shell systems, whereas in the case of the Koopmans’ theorem, the lack of agreement with the other methods is not surprising in view of the deficiency in electron correlation and relaxation. The RMP2 method gives a rather low energy value which is related to the inability of a poor description of the strong interaction of the unpaired electron.40 The lowest-lying experimental vertical IE of germanium tetrafluoride (16.08 eV)8 agrees reasonably well with the RCCSD(T) theoretical predictions, although comparison with RCCSD gives a slightly higher difference (∼4%) from the other three methods (Table 3a). Table 3b presents a close comparison between experimental data and the P3 and OVGF theoretical results. The latter are very close to each other and reproduce reasonable well, to within 0.6 eV, the experimental data as far as the ionic electronic ground state of GeF4 is concerned. We now discuss the valence and Rydberg excitation of germanium tetrafluoride highlighting the most relevant features assigned with the help of theoretical calculations as well as from a direct comparison with the previous data available in the literature.4,5 5.1. Valence Singlet Excitation of Germanium Tetrafluoride (GeF4). Figure 2a shows the electron energy loss spectrum in the 7.0−18.0 eV range at 100 eV and 5° scattering angle, where several bands have been assigned to transitions from occupied molecular orbitals to valence4,5 and/or Rydberg orbitals5 (see Tables 1, 2, and 4). The band peaking at 10.40 eV is in good agreement with the VUV photoabsorption value of 10.419 eV4 and the electron impact spectroscopy value of 10.42 eV.5 We note that previous assignments by Kuroki et al.5 (2t2 → 4σ*(t2)) and Ibuki et al.4 (σ* ← 1t1) agree with the present findings although with a different label of the departing orbital. Here, this transition is assigned to the X̃ 1A1 →11T2, 6t2 → 5a1/ σ*(GeF), i.e., LUMO, valence excitation at 10.333 eV, according to the theoretical calculation in Table 1. This

Figure 2. Electron energy loss spectra (EELS) for germanium tetrafluoride (GeF4) in the (a) range 7.0−18.0 eV obtained at 100 and 30 eV electron impact energy and 5° and 30° scattering angle, respectively; (b) range 7.5−12.5 eV obtained at 100 and 30 eV electron impact energy and 5° and 30° scattering angle, respectively. The vertical bars are the calculated oscillator strengths. The dots and lines show the experimental data points and smoothed data; assignments are from the present calculations (see Tables 1 and 2).

18.0 eV range at 100 and 30 eV electron impact energy and 5° and 30° scattering angle, respectively, together with a magnified view of the 7.5−12.5 eV range. In Figure 2a, the EEL spectra are shown together with the calculated oscillator strengths (see Table 1). Gleaning at Table 1, we observe excellent agreement between experiment and theory, within the exception of the feature at 14.4(5) eV which appears in the EEL spectra as a shoulder structure. Some members of the Rydberg series (see section 5.3) are tentatively assigned for the first time. The major EEL bands are classified mainly as valence transitions of 1 (nF → σ*) character and members of Rydberg series converging to the lowest ionization energies. The broad nature of the EEL features may be indicative of dissociative-like character of the valence transitions, which are not surprising since electronic excitation is mainly restricted to the promotion of fluorine lone pair electrons. Moreover, we note in the EEL profile in Figure 2 the contribution of a background signal E


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the feature at 14.02 eV appears as a shoulder structure in the EEL spectrum at 100 eV and 5° scattering angle (Figure 2a), consistent with the experimental observation of 14.07 eV of Kuroki and co-workers5 at 200 eV incident energy and 3° scattering angle. However, at 30 eV, 30°, the feature becomes clearly visible in Kuroki’s data at 9° scattering angle. We note that our and Kuroki’s data have been obtained with the same electron energy resolution, albeit the poor signal-to-noise ratio in the present experiment making it difficult to obtain smoother EEL profiles. Another interesting aspect when comparing the present data with Kuroki’s is related to the feature at an energy loss of 15.44 eV which appears barely observable at 100 eV, 5°, but then enhanced at 30 eV, 30°. To conclude the comparative work, we have no experimental evidence of a feature identical to that of Kuroki et al.5 at 15.64 eV. Other bands assigned to Rydberg excitations are discussed in section 5.3. Finally, in the electron impact data of Kuroki et al.5 the features at 10.42, 13.36, and 16.28 eV have been assigned to 1t1 → 4σ*(t2), 3t2 → 3σ*(a1), and 2a1 → 3σ*(a1) transitions. Our theoretical calculations do not support these assignments on valence but rather on Rydberg excitations (see Table 1 for the assignments). 5.2. Valence Triplet Excitation of Germanium Tetrafluoride (GeF4). As far as the authors are aware, these are the first measurements of the lowest lying triplet states of germanium tetrafluoride as studied by electron energy loss spectroscopy and theoretical calculations (Table 2). Figure 2a shows the experimental EEL data recorded at 100 eV, 5° scattering angle, and at 30 eV, 30° scattering angle, where in the energy region 8.5−12.0 eV (Figure 2b) we observe significant changes in the shape of the EEL profile. This has been fitted with Gaussian profile curves (not shown in Figure 2b) in order to discriminate the underlying transitions. The experimental maximum value is obtained at 10.40 eV and listed in Table 2. The theoretical calculations report on GeF4 lowest valence/ Rydberg triplet transition energies calculated at the EOMCCSD/aug-cc-pVQZ-(PP)+R level. We tentatively assign the feature at 9.38 eV to the mixed valence-Rydberg X̃ 1A1 → 3T2, 6t2 → 5a1/σ*(GeF) transition (Table 2). A careful analysis of Figure 2b shows that this band becomes discernible at low impact energy and relatively high scattering angle denoting an underlying broad dissociation structure which may confirm a valence-like character. As shown in Table 2, the calculations report the dipole-forbidden valence singlet excitation 6t2 → 5a1/σ*(GeF) feature at 9.136 eV. This means that the singlet excitation is not predominant at this energy, and so, the 9.38 eV feature may be unambiguously assigned to the triplet character. Moreover, we have also added in Figure 2b the calculated 1T1 and 1E transitions, at 9.526 and 10.228 eV, with an oscillator strength f L = 0 (see Table 1) which is in assertion to the triplet character. Regarding the feature at 10.672 eV (experimental value at 10.40 eV), according to the calculations in Table 2, this may correspond to the same mixed valence (1A1 → 3T2, (1t1, nF) → 5a1/σ*(GeF)) transition. Such σ* antibonding valence character assumption seems reasonable since the intensity of this feature does not change appreciably as in the case of the 9.38 eV feature when comparing the EELS data at 100 eV, ∼5° scattering angle and that at 30 eV, 30° scattering angle. Finally, the present electron energy resolution does not allow us to unambiguously resolve any other underlying contributions to the triplet states. From the data in Table 2, we also note that the feature at 9.497 eV may also be assigned to the experimental value at 9.38 eV; i.e., this is due to the valence

Table 4. Energies (eV), Effective Quantum Numbers (n*), and Assignments of the np and nd Rydberg Series Converging to the Ionic Electronic Ground (1t1)−1, First (6t2)−1, Second (2e)−1, Third (5t2)−1, and Fourth (4a1′)−1 Excited States of Germanium Tetrafluoride, GeF4 vertical energya

ref 5

n*

assignment

IE1 = 16.08 eVb 13.37 14.96 15.44

13.36 14.94 −

2.24 3.49 4.61

14.4(5)(s)

14.53

2.89

1t1 → np (t2) 5p 6p 7p 1t1 → nd (t2+e) 4d 5d

IE2 = 16.50 eVb 14.02/14.4(5)(s) 15.44 16.19

14.07/− 15.34 −

2.41/2.58 3.58 6.63

14.96

−

14.02/14.96

14.53

15.44 16.19

15.34 2.92 − 4.00 IE4 = 18.60 eVb

16.19

16.28

17.0(6)(w)

−

18.9(8)(s)

19.01

2.97 IE3 = 17.04 eVb 2.17/2.56

2.38

2.97 IE5 = 21.30 eVb 2.42

6t2 → np (t2) 5p 6p 9p 6t2 → nd (t2+e) 4d 2e → np (t2) 5p 2e → nd (t2+e) 4d 5d 5t2 → np (t2) 5p 5t2 → nd (t2+e) 4d 4a1 → np (t2) 5p

a

(w) weak structure; (s) a shoulder (the last decimal of the energy value is given in brackets for these less resolved features). bVertical value from ref 8.

transition has a considerably large calculated oscillator strength (∼0.6) and a too high term value of 6.16 eV for a Rydberg excitation (6.026 eV in Ibuki et al.4). Moreover, the value of the electronic spatial extent ⟨r2⟩ (76 a.u.2) is very close to that of the ground state. Thus, this transition is assigned mainly to valence character. A close inspection of Figure 2b reveals that at low-energy electron impact and relative high scattering angle, i.e., 30 eV and 30°, the EEL cross section shows interesting behavior reminiscent of singlet to triplet transition. We will thoroughly discuss this in section 5.2. The other major EEL band centered at 12.05 eV can be classified mainly as a mixture of Rydberg-valence transition due to the promotion of an electron from the (HOMO−3)(5t2) to the same unoccupied MO (see assignments in Tables 1 and 4). The calculated oscillator strength is ∼0.21 which is an indication of mainly valence in character, i.e., 5t2 → 5a1/ σ*(GeF). Kuroki et al.5 report this band at 12.03 and assigned to (HOMO−2) to σ* (1e → 3σ*(a1)) The calculations also predict a relative intense oscillator strengths of ∼0.27 and ∼0.18 at higher energies, 13.453 and 14.784 eV (experimental values at 13.36 and 14.4(5) eV, respectively), assigned to mainly Rydberg transitions 6t2 → 5p + 1t1 → 6s and 1t1 → 4d, respectively (see Tables 1 and 4). It is worth mentioning that F


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The Journal of Physical Chemistry A (1A1 → 3E, (2e) → 5a1/σ*(GeF)) transition. Such an assignment seems reasonable within the context of the optically forbidden transitions. 5.3. Rydberg Excitation of Germanium Tetrafluoride (GeF4). The peak positions, En, in the EEL data consisting of structures superimposed on diffuse features extending to the lowest ionization energies (IEs), have been compared using the Rydberg formula

eV to 5s and 5p in good agreement with the energy loss peaks of Kuroki and co-workers5 of 17.25 and 19.01 eV.

6. CONCLUSIONS The present work provides the first comprehensive electron energy loss spectroscopy study of GeF4 from 7.0 to 18.0 eV together with calculations providing more reliable assignment in the spectra. Due to the EEL behavior from 100 eV, 5° and 30 eV, 30° scattering angle, transitions are assigned to singlet− singlet and singlet−triplet states to a combination of valence and Rydberg transitions supported by ab initio calculations on vertical excitation energies and oscillator strengths. The present electron spectroscopy data provides the first experimental evidence of the triplet state nature of the lowest lying excitation of germanium tetrafluoride. The broad nature of the underlying GeF4 EEL structure may be closely related to a dissociative character of the electronically excited states of this molecule.

En = E i − R /(n − δ)2

where Ei is the ionization energy (vertical values), n is the principal quantum number of the Rydberg orbital of energy En, R is the Rydberg constant (13.61 eV), and δ is the quantum defect resulting from the penetration of the Rydberg orbital into the core. The proposed first members of the Rydberg structures are presented in Table 4. Assignments in the spectra for higher members of the Rydberg series, where n ≥ 6 members are expected to lie, is rather complex due to the presence of other valence transitions, and the limited resolution of the present experiments. Given that the assignments are proposed on the quantum defect calculations as our only guide, though we have not tried any attempts to find other highenergy Rydberg members, so the values in Table 4 are just a tentative assignment. The identification of Rydberg states was based more firmly on the symmetry and shape of the single occupied orbitals and the values of the oscillator strengths (Table 1). Briefly, the feature at 12.05 eV is mainly valence in character, 5t2 → 5a1/σ*(GeF), but can have a Rydberg character as predicted by the calculations and assigned to the 5t2 → 5s transition (converging to the ionic electronic ground state), in agreement with the value 12.03 eV from the data of Kuroki et al.5 Note that a t2 → ns(a1) in Td symmetry is an optically forbidden transition and so unlikely to be observed, so in Table 4 we have not included the ns Rydberg series. The lowest-lying members of the np and nd series are assigned at 13.37 and 14.4(5) eV (Table 4); the former and the latter are in good agreement with the values of 13.36 and 14.53 eV from Kuroki et al.5 Furthermore, the features assigned to 5s, 5p, and 4d members of Rydberg series converging to the ionic electronic first excited state are reported at 13.37, 14.02, and 14.96 eV, the first two in reasonable agreement with 13.36 and 14.07 eV from the experimental data of Kuroki el al.5 The features assigned to 5s, 5p, and 4d members of Rydberg series converging to the ionic electronic second excited state are reported at 13.37, 14.02, and 15.44 eV in reasonable agreement with 13.36, 14.53, and 15.34 eV data of Kuroki el al.5 Other members of the Rydberg series, 6s and 7s, can be assigned at 15.44 and 16.19 eV, where the former is in agreement with the value of 15.34 of Kuroki et al.5 Here again, these have not been tabulated because of their optically forbidden nature. Regarding the members of the Rydberg series converging to the ionic electronic third excited state, we have assigned 5s, 5p, and 4d features at 14.96, 16.19, and 17.0(6) eV, with other structures contributing to these features (14.4(5) and 15.34 eV), where the first two agree very well with 14.94 and 16.28 eV from the electron impact data.5 For n = 6, a weak feature can be identified at 17.0(6) eV. Notwithstanding, new Rydberg features are proposed here for the first time which have not been reported before.5 Finally, as far as members of Rydberg series converging to the ionic electronic fourth excited state are concerned, we have assigned the feature at 17.0(6) and 18.9(8)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (+81) 3 3238 4227. *E-mail: [email protected]. Tel: (+351) 21 294 78 59. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted under the support of the Ministry of Education, Culture, Sport, Science and Technology (MEXT) Japan. PLV and MH acknowledge the Japan Society for the Promotion of Science for the FY2016 JSPS Invitation Fellowship for Research in Japan. D.D. acknowledges support from the CaPPA project (Chemical and Physical Properties of the Atmosphere), funded by the French National Research Agency (ANR) through the PIA (Programme d’Investissement d’Avenir) under Contract No. ANR-10-LABX-005 and by the Regional Council “Nord-Pas de Calais” and the “European Funds for Regional Economic Development” (FEDER). This work was performed using HPC resources from GENCICINES (Grant 2016-088620). The Centre de Ressources Informatiques (CRI) of the Université of Lille also provided computing time. PLV also acknowledges the Portuguese National Funding Agency FCT-MCTES through grant UID/ FIS/00068/2013 and his Visiting Professor position at Sophia University, Tokyo, Japan.

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REFERENCES

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Studies on GeF4 Valence and Rydberg States by Electron Impact

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