Electronic Excitation to Singlet States of 1,3-C - American

Article pubs.acs.org/JPCA

Electronic Excitation to Singlet States of 1,3‑C4F6, c‑C4F6 and 2‑C4F6 by Electron Impact - Electron Energy-Loss Spectroscopy and ab Initio Calculations P. Limaõ -Vieira,*,†,‡ K. Anzai,† H. Kato,† M. Hoshino,† F. Ferreira da Silva,‡ D. Duflot,§ D. Mogi,∥ T. Tanioka,⊥ and H. Tanaka† †

Department of Physics, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal § Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, Université Lille 1, F-59655 Villeneuve d’Ascq Cedex, France ∥ Research & Marketing Management Department, New Products Development Division, Kanto Denka, Kogyo Co., Ltd., Chiyoda-ku, Tokyo 100-0005, Japan ⊥ Shibukawa-Area Laboratory, Development Research Laboratory, New Products Development Division., Kanto Denka Kogyo Co., Ltd., Shibukawa City, Gunma 377-8513, Japan ‡

S Supporting Information *

ABSTRACT: We report on the first measurements of the electron impact electronic excitation cross sections for C4F6 isomers, hexafluoro-1,3butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6), and hexafluoro-2butyne (2-C4F6), measured at 100 eV, 3° scattering angle, while sweeping the energy loss over the range 2.0−15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C4F6 isomers and classified according to the magnitude of the quantum defects (δ).

I. INTRODUCTION Halogenated hydrocarbons under photolysis are a source of atmospheric halogen radicals widely recognized to contribute significantly to stratospheric ozone depletion.1 They have considerable long residence lifetimes in the atmosphere and high global warming potentials (see, e.g., ref 2). Thus, under the regulations of the Montreal Protocol and its amendments, these have been phased out and alternatives had to be found.3 Perfluoroalkanes can easily polymerize and so they have been largely used in the industry for production of perfluorinated polymers in particular as water repellents. These are known to be a source of CF2 and CF3 radicals under electron and photon interactions, and so they can act as potential feed gases for the plasma etching of silicon dioxide. Eden and co-workers4 have reported a detailed analysis of hexafluoropropene (C3F6) vacuum ultraviolet (VUV) photoabsorption in the wavelength range 115−320 nm (10.8−3.9 eV), showing the dissociative nature of the lowest lying excited states that may yield CFx radical formation. C4F6 has three isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6), and hexafluoro-2-butyne (2© 2012 American Chemical Society

C4F6), where 1,3-C4F6 has been considered to be used in selective etching processes of silicon oxide (SiO2) on Si and silicon nitride (Si3N4) layers for fabricating contact holes in ultralarge integrated circuits.5,6 As far as authors are aware, there is just one study in the literature related to c-C4F6 and 2C4F6 performance as plasma etching gases.6 In such plasma reactors, electron driven reactions need to be fully addressed and the primary motivation for measurements of electron scattering cross section data for C4F6 isomers, is the need for a reliable database of information that can be used to model such interactions and applications. Hexafluoro-1,3-butadiene (1,3-C4F6), has an annual production estimate of ∼0.01 Tg/yr. Acerboni et al.7 have reported that the total amount of atmospheric emissions are neither yet known nor homogeneously distributed in the troposphere, meaning that these compounds may react relatively fast in the Earth’s atmosphere. Hexafluoro-1,3-butadiene emissions are Received: August 1, 2012 Revised: October 2, 2012 Published: October 17, 2012 10529

dx.doi.org/10.1021/jp307599y | J. Phys. Chem. A 2012, 116, 10529−10538

The Journal of Physical Chemistry A

Article

interaction region through a 5 mm long capillary with a 0.3 mm inner diameter. After the electron interaction with the target gas, the scattered electrons are energy analyzed with a hemispherical electron analyzer, which can rotate about the gas jet, and detected by an electron multiplier. Both the electron monochromator and the energy analyzer are enclosed in separate differentially pumped housings. This greatly reduces the effect of background gases and improves the stability of the spectrometer, particularly when reactive gases are being studied. The typical base pressure in the main chamber was 2.0 × 10−5 Pa and, upon gas admission, this increased to a pressure of 2.0 × 10−4 Pa. In addition, the spectrometer and molecular beam source are heated to a temperature of about 50 °C, reducing any possible contamination during the measurements. The gas samples were supplied from Kanto Denak Kogyo Co., Ltd. for 1,3-C4F6 and Hydrus Chemical Inc. for 2-C4F6 and c-C4F6 and used as delivered. The stated purity was 99.9% for 1,3-C4F6 and 98% for 2-C4F6 and c-C4F6, respectively. In the current experiments the energy resolution of the incident electron beam was set to 30−40 meV (fwhm), with incident electron currents of a few nanoamps (depending on the initial electron energy). The incident electron energy was calibrated with respect to the 19.365 eV, 2S resonance in He30 and with respect to the 2Πg resonance in N2 for the vibrational excitations around 2.4 eV.31 The hemispherical electron analyzer is placed on a turntable stage and can be rotated from −10° to +130°, with respect to the incident electron beam, with an angular resolution of about ±1.5° (fwhm). For the energy loss measurements both the electron energy of the incident beam and the angle of the analyzer were fixed, with the intensity of the scattered electron signal being measured “in sync” with the energy loss. In this study two approaches were used to obtain absolute differential cross sections (DCSs) for electronic excitation of the C4F6 molecules. In the first method, the absolute scales were obtained by the relative flow technique32 using the theoretical elastic cross sections of helium.33 Note that this normalization method was used for scattering angles larger than 10°, where one can easily detect the elastically scattered electrons. For angles smaller than 10°, where the elastic signal-to-noise becomes problematic, a second normalization technique was used to measure the intensity ratio for electronic excitation of the targets relative to the excitation of the 21P state in helium. The 21P state DCS,34 could then be employed to fix the corresponding absolute scales of the electronic excitations in the EELS.

considered to have a negligible global warming potential, 0.027 relative to CO2 (5.8 × 10−5 to CFC-11), due to the short atmospheric average lifetime of ∼2 days and high reactivity with OH radicals, with a rate coefficient of (1.1 ± 0.3) × 10−11 cm3 molecules −1 s −1 .7 Despite the wide ranging industrial applications of these molecules, detailed spectroscopic analysis of 1,3-C4F6 is scarce. As far as we are aware, the electronic ground state configuration of this molecule has not been identified and the only previous published far UV spectra are that of Rutner and Bauer8 and Pottier et al.9 in the limited range, 4.59−6.20 and 4.96−10.78 eV, respectively. However, a number of works focused on the theoretical studies of the molecular structure and vibrational frequencies10−15 have proved instructive for the nonplanarity of the molecule’s carbon skeleton. Brundle and Robin He I photoelectron spectra for 1,3-C4F6, report a vertical ionization energy of 10.4 eV.16 Hexafluorocyclobutene, c-C4F6, has also deserved a few studies on the molecular structure quantum chemical calculations,17,18 and accurate gas-phase equilibrium structures on the ground-state potential energy surface.19 The rate coefficient of (8.6 ± 1.6) × 10−14 cm3 molecules−1 s−1 for reactions with OH and an estimated 100 year time horizon global warming potential of 42 (relative to CFC-11) have been reported recently.20 The infrared and Raman vibrational spectra of c-C4F6 have been recorded by Klaeboe et al.,21 whereas Christophorou and co-workers present temperature dependence of electron attachment and detachment in c-C4F6.22,23 As far as hexafluoro-2-butyne (2-C4F6) is concerned, we report on the molecular structure work of Chang et al.11 and the (12.35 ± 0.01) eV adiabatic ionization energy of Delwiche et al.24 as provided by He I photoelectron spectra, with a detailed vibrational analysis of the first ionic band. Further electron attachment rates to perfluorocarbon compounds have been reported (see, e.g., ref 25) and a brief letter from Illenberger and co-workers on the formation and dissociation of negative ions for the three isomers of C4F6.26 Electron scattering previous experimental work on these molecules (1,3-C4F6 and 2-C4F6) include the grand total cross section (TCS) measurements by Szmytkowski and Kwitnewski27,28 covering an energy range in the 0.5−370 eV. To our knowledge there are no previous experimental measurements of electron energy loss spectroscopy (EELS) for the C4F6 molecules in the energy range covered in the present work. In this paper we focus our attention on the singlet excited states of hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6), and hexafluoro-2-butyne (2-C4F6). To complement and help to interpret our experimental results, we performed ab initio calculations to determine vertical excitation energies of the electronic states. In the next section we provide details on the experimental apparatus and operating procedures. In section III we present a brief discussion on the computational method and in section IV the experimental results are presented together with a discussion and comparison with other results where that is possible. Finally, some conclusions that can be drawn from this study are given in section V.

III. COMPUTATIONAL SECTION All the calculations were done using the MOLPRO program.35 The ground state geometries were optimized at the frozen core MP2, CCSD,36 and CCSD(T)37 levels (Table 1) using Table 1. Calculated Relative Energies (kcal/mol) for the Ground State Geometries Optimized at MP2, CCSD, and CCSD(T) Levels with the aug-cc-pVDZ Basis Set for (a) Hexafluoro-1,3-butadiene (1,3-C4F6), (b) Hexafluorocyclobutene (c-C4F6), and (c) Hexafluoro-2butyne (2-C4F6)

II. APPARATUS AND OPERATING PROCEDURES The electron spectrometer used in the present work has been described in detail elsewhere,29 so only a brief discussion will be given here. A monochromatic electron beam is generated with a hemispherical electron monochromator and crossed at right angles with an effusive molecular beam that enters the 10530

symmetry

molecule

MP2

CCSD

CCSD(T)

C2 C2v D3d

2-C4F6 c-C4F6 1,3-C4F6

24.90 9.18 0.00

20.57 6.30 0.00

19.98 6.02 0.00



Article

Table 2. Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of Hexafluoro-1,3-butadiene (1,3C4F6) Compared with Experimental Data and Other Work (Energies in eV) (Details in Text) state

E (eV)

fL

⟨r2⟩a

assignment

6.535 6.606 7.193 7.538 7.841 7.896 8.394 8.619 8.647 8.977 9.075 9.166 9.580 9.653 9.675 9.778 9.857 9.896 9.939 9.957 10.209 10.219 10.323 10.343 10.404 10.588 10.699 10.810 10.868 11.093 11.133 11.188 11.189 11.275 11.337 11.346 11.569 11.620 11.676 11.678 11.774 11.821 11.961 12.074 12.094 12.170 12.173 12.225 12.232 12.290 12.313 12.318 12.412 12.414 12.482 12.559 12.617 12.638 12.691

0.0125 0.1830 0.0031 0.1541 0.1620 0.1766 0.0221 0.0492 0.0083 0.0047 0.0140 0.0279 0.0276 0.0017 0.0016 0.0062 0.0285 0.0694 0.0631 0.0132 0.0001 0.0030 0.0005 0.0209 0.0100 0.0006 0.0115 0.0005 0.0103 0.0001 0.0039 0.0410 0.0116 0.0052 0.0307 0.0046 0.0137 0.0005 0.0040 0.0046 0.0191 0.0103 0.0204 0.0055 0.0241 0.0008 0.0001 0.0104 0.0090 0.0405 0.0000 0.0085 0.0227 0.0485 0.0515 0.0003 0.0363 0.0115 0.0113

128 124 136 136 126 138 145 148 146 155 145 154 163 140 158 157 150 158 152 127 160 151 140 167 147 154 155 165 155 165 163 151 116 163 138 151 139 120 128 156 154 130 165 171 147 129 133 127 139 144 139 149 126 123 132 137 139 132 137

Ground state πa → π*a πa → π*b πa → 3sa πb → 3sa + πa → π*b πb → π*b πb → 3sa +πa → σ*CF πa → 3pb πa → 3pa πa → 3pb + πb → 3pa πb → 3pb +πa → 3da πb → 3pb +πa → 3da πa → 3pb +πb → 3da πb → 3db +πa → 3da πb → 3pa +πa → σ*CC/CF πa → 3da πb → 3pa +πa → 3db πa → 3da πb → 3pa +πa → 3db πa → 3da +πb → 3pb n(F)/σ(CC) → π*b πb → 3db +πa → 3da πa → 3db na(F)/σ(CC) → 3sa + πb → 3db +πa → 3da πa → 4pb +πb → 3da na(F)/σ(CC) → 3sa + πb → 3db πa → 4pb +πb → 3da πa → 3db +πb → 3da πa → 4pb +πb → 3da πb → σ*(CF) +πa → 3da πa → 3db +πb → 3da πb → 3db +πa → 3da πb → σ*(CC) nb(F) → σ*(CF) πb → 4pb +πa → 4da nb(F) → σ*(CF) πa → 4db +πb → 4da πa → 4db +πb → 4da na(F) → π*a + nb(F) → π*b na(F) → π*b πb → 4pb πb → 4da na(F) → 3sa + nb(F) → π*b πb → 4db πb → 4da πb → 4pb +πa → 4da nb(F) → π*b nb(F) → 3sa + na(F)/σ(CC) → 3pb nb(F) → 3sa + na(F)/σ(CC) → 3pb na(F) → 3sa nb(F) → 3sa + na(F) → π*b na(F) → 3db + πb → 3db+ πa → 3da πa → 4db na(F) → 3pa na(F)/σ(CC) → π*b na(F)/σ(CC) → π*b + πa → 4db na(F)/σ(CC) → 3pa + nb(F) → π*b na(F)/σ(CC) → 3pa + nb(F) → π*b na(F)/σ(CC) → π*b + na(F) → 4pb na(F)/σ(CC) → 4db + πa → 4pb

1

1A 21A 11B 31A 21B 41A 31B 41B 51A 51B 61A 71A 61B 81A 71B 91A 81B 101A 91B 111A 101B 121A 111B 131A 121B 141A 131B 141B 151B 151A 161B 161A 171A 181A 191A 171B 181B 191B 201A 201B 211A 211B 221A 231A 221B 241A 251A 231B 241B 261A 251B 271A 261B 281A 271B 281B 291A 301A 291B 301B

10531

E (eV) exp

quantum defect

6.18

E (eV)9

6.17

7.18 7.69 7.69

0.94

8.47

0.34

8.29

9.26

0.48

9.17

6.215

0.76

9.97

12.13

E (eV)41

∼ 8.11

9.290

9.92

1.19

12.76



Article

Table 2. continued a

Mean value of r2 (electronic radial spatial extents in atomic units, au2).

Table 3. Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of Hexafluorocyclobutene (c-C4F6) Compared with Experimental Data (Energies in eV) (Details in Text) state

E (eV)

f La

⟨r2⟩b

11A1 11B1 11A2 11B2 21A1 21A2 21B1 31A1 31B1 41A1 21B2 41B1 31A2 51A1 41A2 51B1 61B1 31B2 71B1 81B1 61A1 41B2 51B2 51A2 71A1 81A1 91A1 101A1 91B1 61B2 101B1 61A2 71A2 81A2 91A2 101A2 71B2

7.448 7.915 8.298 8.661 9.444 9.815 9.856 9.950 10.195 10.325 10.340 10.398 10.449 10.689 10.722 10.850 10.858 11.010 11.032 11.148 11.201 11.380 11.449 11.516 11.691 11.768 11.803 11.804 11.843 11.875 11.587 11.802 12.063 12.192 12.553 12.265

0.0239

0.0095

134 121 123 133 156 160 149 122 135 149 130 120 155 169 140 154 145 165 145 137 162 164 135 119 159 157 153 136 121 145 164 142 166 153 132 151

81B2 91B2 101B2

12.351 12.483 12.708

0.0187 0.0352 0.0161

170 150 143

0.2955 0.0034 0.0013 0.0030 0.0030 0.1499 0.0002 0.0052 0.0088 0.0076 0.0535 0.0762 0.0036 0.0000 0.0303 0.0098 0.0031 0.0013 0.0003 0.0009 0.0142 0.0615 0.0002 0.0061

E (eV) exp

assignment ground 7b1 → 3sσ (a1) 14a1 → πa(CC) (LUMO, a2) 7b1 → πa(CC) (LUMO, a2) 14a1 → 3sσ (a1) 7b1 → 3pσ (b2) 7b1 → 3pσ (a1) 7b1 → 3pπ (b1) σ(CC) (b2) → πa(CC) (LUMO, a2) 7b1 → σa(C2F4) (b1) 14a1 → 3pσ (b2) + σ(CC) (b2) → 3sσ (a1) 14a1 → σa(C2F4) (b1) σ(CC)/n(F) (a1) → πa(CC) (LUMO, a2) 14a1 → 3pσ (a1) 7b1 → 3dπ (b2) 7b1 → 3dσ (a1) 7b1 → 4sσ (a1) σ(CC) (a1) → 3sσ (a1) + σ(CC) (b2) → 3pσ (b2) 7b1 → 3dσ (a1) 14a1 → 3pπ (b1) σ(CC)/n(F) (a1) → σa(CF) (a1) 7b1 → 3dπ (a2) 14a1 → 3dπ (b2) n(C2F4) (a2) → 3sσ (a1) n(C2F4) (a2) → πa(CC) (LUMO, a2) 7b1 → 3dσ (b1) + 14a1 → 3dσ (a1) 14a1 → 3dσ (a1) 7b1 → σa(C2F4) (b1)+ 14a1 → 4sσ (a1) n(C2F4) (b1) → 3sσ (a1) n(C2F4) (b2) → πa(CC) (LUMO, a2) 7b1 → 4dσ (a1) 7b1 → σa(C2F4) (b2) σ(CC)/n(F) (a1) → 3dπ (a2) + n(C2F4) (a2) → 3sσ (a1) 7b1 → 3dπ/σa(CC) (b2) 14a1 → 3dπ (a2) n(C2F4) (a2) → 3sσ (a1) + σ(CC) → σa(C2F4) (b1) σ(CC)/n(F) (b2) → 3sσ (a1) + σ(CC)/n(F) (b2) → 3dπ (b2) + σ(CC)/n(F) (a1) → 4sσ (a1) 7b1 → 4dπ (a2) n(F) (b1) → 3dπ (a2) + 7b1 → πa(CC) (LUMO, a2) + σ(CC) (b2) → 4pσ (a1) σ(CC)/n(F) (a1) → 3pσ (b2) + σ(CC)/n(F) (b2) → 3sσ(a1)

7.15

quantum defect 1.27

8.02

9.85

0.77

9.85

10.51

1.18

12.13 (s)

a

No fL value means a forbidden electric dipole transition bMean value of r2 (electronic radial spatial extents in atomic units, au2. (s) means a shoulder structure;

Dunning’s aug-cc-pVDZ atomic basis set.38 The electronic spectra were computed at the EOM-CCSD level38 at the obtained CCSD(T) geometries (Tables 2−4). For a better description of Rydberg excited states, a set of diffuse functions (6s, 6p, 4d), taken from Kaufmann et al.,39 localized at the mass center of the molecule, was added to the original basis set (augcc-pVDZ+R basis set). The oscillator strengths of the electric dipole transitions were calculated using the length gauge. Finally, the lowest vertical ionization energies of C4F6 isomers were also obtained at the RCCSD(T) level (Table 5).

IV. RESULTS AND DISCUSSION A. Brief Summary of the Geometries and Electronic Configurations of C4F6 Isomers. The calculated relative energies for the ground state geometries optimized at MP2, CCSD, and CCSD(T) levels with the aug-cc-pVDZ basis set for (a) 1,3-C4F6, (b) c-C4F6, and (c) 2-C4F6 are presented in Table 1. The most stable structure by increasing order is 2-C4F6 > c-C4F6 > 1,3-C4F6. A.1. Hexafluoro-1,3-butadiene, 1,3-C4F6. 1,3-C4F6 has symmetry C2 in the electronic ground state, and its structure 10532



Article

Table 4. Calculated Vertical Excitation Energies and Oscillator Strengths (Singlet States) of Hexafluoro-2-butyne (2-C4F6) Compared with Experimental Data (Energies in eV) (Details in Text) D3d

C2h

1

1 A1g 11A1u 11Eu 21Eu 11A2u 11Eg 21Eg 11A2g 21A1g 31A1g 21A2g 31Eu 31Eg 41Eu 41Eg 51Eu 41A1g 21A1u 31A2g 51Eg 61Eu 21A2u 71Eu 31A1u 41A2g 31A2u 51A1g 51A2g a

E (eV)

f La

⟨r2⟩b

assignment (D3d; C2h)

126 126 138 132 118 151 152 152 174 174 127 116 132 139 164 118 136 125 114 147 120 151 175 134 175 134 150

HOMO (6eu; 13bu + 7au); SHOMO (1a2g; 6bg) HOMO → LUMO HOMO → LUMO HOMO → 3sσ HOMO → LUMO SHOMO → LUMO HOMO → 3pπ HOMO → 3pπ HOMO → 3pπ HOMO → 3pσ HOMO → 3pσ n(F) → LUMO + σ(CC) → LUMO n(F) → LUMO n(F) → LUMO + HOMO → σa(CC) HOMO → σ*(CC) HOMO → 3dσ + σ(CC) →LUMO n(F) → LUMO n(F) → LUMO+ n(F) → 3sσ + n(F) → 3pσ n(F) → LUMO n(F) → LUMO HOMO → LUMO n(F) → LUMO HOMO → 3dπ + n(F) → LUMO HOMO → 3dπ n(F) → 3sσ HOMO → 3dπ σ(CC) → 3sσ n(F) → 3sσ

1

1 Ag 11Au 21Au + 31Au + 31Bu 21Ag + 31Ag + 31Bg 41Ag 51Ag 41Bg 41Au + 61Ag + 51Au + 71Ag + 61Au + 81Ag 71Au 71Bg 91Ag + 81Au + 81Bu 91Au + 101Au 91Bg 101Bu 101Ag 101Bg

11Bu 21Bu 11Bg 21Bg

41Bu 51Bg 51Bu 61Bg 61Bu

81Bg 71Bu 91Bu

6.900 7.240 9.545 10.595 10.767 10.878 10.972 11.066 11.082 11.082 11.554 11.657 11.684 11.741 11.888 12.104 12.138 12.158 12.203 12.240 12.241 12.364 12.365 12.446 12.450 12.500 12.957

0.0002 0.0767 1.6807

0.0000 0.0189 0.0263

0.0158 0.0001 0.0157

0.0861

quantum defect

7.09 9.31 9.94

1.01

11.29

−0.04

No f L value means a forbidden electric dipole transition. bMean value of r2 (electronic radial spatial extents in atomic units, au2).

is shown in Figure 1a with the bond lengths in Å. The calculated electron configuration of the X̃ 1A ground state is as follows: (i) core orbitals (1a)2 (1b)2 (2b)2 (2a)2 (3b)2 (3a)2 (4a)2 (4b)2 (5b)2 (5a)2; (ii) valence orbitals (6a)2 (6b)2 (7a)2 (7b)2 (8a)2 (8b)2 (9a)2 (9b)2 (10a)2 (10b)2 (11a)2 (12a)2 (11b)2 (12b)2 (13a)2 (13b)2 (14a)2 (14b)2 (15a)2 (16a)2 (15b)2 (17a)2 (16b)2 (17b)2 (18a)2 (18b)2 (19a)2 (19b)2 (20a)2. The highest occupied molecular orbital (HOMO) and the second highest occupied molecular orbital (SHOMO) have πa(CC) and πb(CCCC) character, respectively. The lowest unoccupied molecular orbital (LUMO), 20b, is mainly of πb* antibonding character localized on the (CC) group, and the (LUMO+1) has been assigned to 21a with πa* antibonding character. These are shown as supplementary data. The lowest vertical ionization energies have been experimentally obtained at 10.4 (20a)−1, 11.4 (19b)−1, 14.45 (σ)−1, 15.75, and 16.3 eV (nF)−1.15 The calculated vertical IEs at the RCCSD(T) level are presented in Table 5 and agree reasonably well with the experimental data. In the present work, the experimental values have been used to calculate the quantum defects associated with transitions to Rydberg orbitals (section C). The theoretical studies have shown strong overlapping of the lowest Rydberg states with valence states resulting in complex intensity distribution in the electronic spectrum (Table 2). The calculated transition energies, oscillator strengths, and the main character of the wave function are shown in Table 2 (EOM-CCSD results). A.2. Hexafluorocyclobutene, c-C4F6. Hexafluorocyclobutene structure is shown in Figure 1b, where the bond lengths are in

Table 5. Comparison of Calculated Vertical Ionisation Energies with Available Experimental Values of C4F6 Isomers: (a) Hexafluoro-1,3-butadiene (1,3-C4F6), (b) Hexafluorocyclobutene (c-C4F6), (c) Hexafluoro-2-butyne (2-C4F6) (Values in eV) (a) Hexafluoro-1,3-butadiene (1,3-C4F6) IE (eV)16 experimental values ionic state

IE (eV)

adiabatic

vertical

20a−1 10.218 ≈9.5 19b−1 11.311 (b) Hexafluorocyclobutene (c-C4F6)

2

A 2 B ionic state B1 2 A1 2 B2 2 A2

10.4 11.4 IE (eV)

7b1−1 14a1−1 12b2−1 6a2−1 (c) Hexafluoro-2-butyne (2-C4F6)

2

11.679 12.590 14.607 15.315 IE (eV)24

Ionic state C2h Au/2Bu Ag 2 Bg a

E (eV) exp

experimental values

D3d

2

2

2

2

Eu A1g 2 A2g

6eu−1 8a1g−1 1a2g−1

IE (eV)

adiabatic

verticala

12.578 15.356 15.675

12.35 ± 0.01 14.88 ± 0.02 14.88 ± 0.02

12.76

Value extracted from the He I photoelectron spectrum of ref 24.

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character of the wave function are also shown in Table 3 (EOM-CCSD results). As far as authors are aware, there are no experimental values for the lowest ionization energies of hexafluorocyclobutene. However, we have calculated vertical values at the RCCSD(T) level and the results are shown in Table 5. These values have been used to calculate the quantum defects associated with transitions to Rydberg orbitals (section C). A.3. Hexafluoro-2-butyne, 2-C4F6. Due to its staggered geometry,11 2-C4F6 has symmetry D3d in the electronic ground state and its structure is shown in Figure 1c with bond lengths in Å. The calculated electron configuration of the X̃ 1A1g ground state is as follows: (i) core orbitals (1eu)4 (1a2u)2 (1eg)4 (1a1g)2 (2a1g)2 (2a2u)2 (3a2u)2 (3a1g)2; (ii) valence orbitals (4a1g)2 (4a2u)2 (2eu)4 (2eg)2 (5a1g)2 (5a2u)2 (6a1g)2 (6a2u)2 (7a1g)2 (3eu)4 (3eg)4 (4eu)4 (4eg)4 (5eu)4 (7a2u)2 (5eg)4 (8a1g)2 (1a1u)2 (1a2g)2 (6eu)4. Because the MOLPRO code cannot handle nonabelian symmetry groups, all the calculations were in fact performed using the largest abelian subgroup of D3d, i.e., C2h. The results have been converted back to D3d, as described by Herzberg.40 The highest occupied molecular orbital (HOMO) is the doubly degenerate 6eu corresponding to the π double bond of the central carbon atoms. The second highest occupied molecular orbital (SHOMO), 1a2g, has σ(CCCC) character, with a large contribution of fluorine nF lone pairs. The lowest unoccupied molecular orbital (LUMO), 6eg, which is also doubly degenerate, has a π* antibonding character on the CC part, with bonding character on each C−C part. These are shown as Supporting Information. The lowest adiabatic ionization energy has been experimentally obtained from Delwiche et al.'s24 He I photoelectron spectrum at 12.35 eV, and we have extracted a vertical value of 12.76 eV to be used in calculating the quantum defects associated with transitions to Rydberg orbitals (section C). Here, as in the previous C4F6 isomers, the calculations show a mixing of valence and Rydberg states. The calculated transition energies, oscillator strengths, and the main character of the wave function are also shown in Table 4 (EOM-CCSD results). In Table 5 we show the calculated vertical IEs. As far as the second calculated IE is concerned, we note that the value is slightly higher than the obtained in the experimental work of Delwiche et al..24 B. Neutral Excited States. The present full ranges of the EEL spectra of hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6), and hexafluoro-2-butyne (2-C4F6) are shown in Figure 2, together with the calculated oscillator strengths. Generally speaking, we observe a ∼0.5 eV shift between experiment and theory, which is reasonable for the level of accuracy. Some tentative members of the Rydberg series are assigned for the first time. The major EEL bands, generally speaking, can be classified mainly as valence transitions of 1(π → π*), 1(nF/σCC → π*), and 1(π → σ*) character and members of Rydberg series converging to the lowest ionization energies. Of particular interest from a detailed analysis of Figure 2, is the fact that the lowest band is only visible in 2-C4F6 and is shifted progressively to higher energies in the sequence order 1,3-C4F6 > c-C4F6 > 2-C4F6, where it becomes increasingly overlapped with the next band. Moreover, in the same sequence the relative intensity between the two highest bands is 1.3 < 2.0 < 2.6. Although we do not show it in Figure 2, but the rather identical intensities of the DCS from 100 eV, 3° and 30 eV, 10°,

Figure 1. Ground state geometries optimized at MP2, CCSD, and CCSD(T) levels with the aug-cc-pVDZ basis set for (a) 1,3-C4F6, (b) c-C4F6, and (c) 2-C4F6. Bond lengths are in Å. For clarity, only the CCSD(T) values are reported.

Å, and the molecule has C2v symmetry in its electronic ground state. The calculated electron configuration of the X̃ 1A1 state is as follows: (i) core orbitals (1b2)2 (1a1)2 (1a2)2 (2b2)2 (2a1)2 (1b1)2 (3a1)2 (3b2)2 (4a1)2 (4b2)2; (ii) valence orbitals (5a1)2 (5b2)2 (6a1)2 (6b2)2 (2b1)2 (2a2)2 (7a1)2 (8a1)2 (7b2)2 (8b2)2 (9a1)2 (3b1)2 (10a1)2 (11a1)2 (3a2)2 (9b2)2 (4b1)2 (4a2)2 (12a1)2 (10b2)2 (11b2)2 (5b1)2 (5a2)2 (6b1)2 (6a2)2 (13a1)2 (12b2)2 (14a1)2 (7b1)2. The highest occupied molecular orbital (HOMO), 7b1, corresponds to the π(CC) bond, whereas the SHOMO, 14a1, is of σ character in the ring CC bond. The lowest unoccupied molecular orbital (LUMO), 1a2, is essentially the π*(CC) orbital. These are shown as Supporting Information. The ab initio calculations in Table 3 show a considerable overlapping of the lowest valence and Rydberg states. The calculated transition energies, oscillator strengths, and the main 10534



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Figure 3. Electron energy loss spectrum in the range 5.0−10.0 eV for hexafluoro-1,3-butadiene (1,3-C4F6) with Gaussian fitting profiles of the lowest bands. The vertical bars are the calculated oscillator strengths.

(Figure 3) reveals another electronic state with a vertical transition at 8.47 eV. Such assignment seems reasonable because the EEL characteristic shows a different slope from the right-hand-side tale of the previous transition peaking at 7.69 eV. As such, we tentatively assign the 8.47 eV structure to the (11A → 51A, πa(20a) → 3pa) transition (see section C), although it may also accommodate contributions from the (11A → 41B, πa(20a) → 3pb) and the (11A → 51B, πa(20a) → 3pb + πa(19b) → 3pa), the latter with a considerable lower oscillator strength than the others. Below the lowest ionization energy (10.4 eV15) the EEL spectrum reveals two broad features at 9.26 and 9.97 eV, respectively, in good agreement with those reported at 9.17 and 9.92 eV.9 The first has been assigned in Table 2 to a Rydberg transition and will be discussed further below in section C, the second (9.97 eV) is tentatively assigned to the (11A → 101B, nF/σCC → πb*(20b)) transition. The rather broad feature centered at 12.13 eV is tentatively assigned to a (na(F) → πb*(20b)) transition. Because the fluorine lone pair electrons being promoted do not take part in the chemical bond, the transition should not be accompanied by vibrational features. It is also worth noting that in the high energy region, several underlying excited and ionic states may contribute to the broadness of the observed structures. In the present EEL spectrum of hexafluoro-1,3-butadiene, we also report other features at 12.7 and 13.2 eV that remain unassigned, although we tentatively suggest some Rydberg contribution below. B.2. Valence Excitation of Hexafluorocyclobutene (c-C4F6). As far as the authors are aware, any spectroscopic data on hexafluorocyclobutene (c-C4F6) that may help with the present assignments is scarce or even unknown. However, the band analysis is performed with the help of ab initio calculations in Table 3. From Figure 2, the c-C4F6 EEL spectrum mostly resembles 1,3-C4F6 where the bands are now shifted toward higher energies. The lowest lying singlet excited state with a maximum at 7.15 eV is ascribed to the (11A1 → 11B1, π(7b1) → 3sσ(a1)) transition (see section C). The most intense feature in all C4F6 isomers is assigned to a (π → π*) transition which, as far as c-C4F6 is concerned, has been labeled (11A1 → 11B2, π(7b1) → π*CC(1a2)) peaking at 8.02 eV. In the EEL spectrum we also distinguish at least three other broad features at 9.85, 10.51, and 12.13 eV, which have been tentatively assigned to valence, Rydberg, and/or a mixing

Figure 2. Electron energy loss spectra (EELS) in the range 4.0−14.0 eV for hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (cC4F6), and hexafluoro-2-butyne (2-C4F6), obtained at 100 eV electron impact energy and 3° scattering angle. The vertical bars are the calculated oscillator strengths.

show the nature of the singlet transitions involved in the electronic excitation of the C4F6 isomers. B.1. Valence Excitation of Hexafluoro-1,3-butadiene (1,3C4F6). The lowest lying singlet excited state has been reported by Pottier and co-workers9 in the 5.45−6.70 eV energy region with a maximum at 6.17 eV, in very good agreement with the present value of ∼6.18 eV. They have assigned this transition to (1A1 → 1B2, σ → π*) in contrast to our calculations predicting a (11A → 11B, πa(20a) → πb*(20b)) transition (Table 2). The second band reported by Pottier et al.9 in the 6.70−8.18 eV energy region with a maximum at 7.66 eV and assigned to the (π → π*) transition, is in agreement with the present value of ∼7.45 eV. However, a recent high resolution VUV photoabsorption spectrum,41 has revealed that the 6.5−8.0 eV feature is asymmetric around its maximum at 7.606 eV, suggesting the contribution of another underlying state. In Figure 3 we show an expanded view of the low-lying excited bands between 5.0 and 10.0 eV, where the DCS has been fitted with Gaussian profile curves to resolve the underlying transitions. The maximum values of the features are listed in Table 2. As such, and in the light of the current calculations, we tentatively assigned the second band to a combination of a (11A → 21B, πa(20a) → πb*(20b) + πb(19b) → 3sa) and (11A → 41A, πb(19b) → πb*(20b)) transitions at 7.18 and 7.69 eV, respectively. Regarding the second transition we do not discard the possibility that it may also be assigned to (11A → 31B, πb(19b) → 3sa + πa(20a) → σCF*) (Table 2). Although the calculations indicate a mixed valence/Rydberg character, the rather high calculated intensity is primarily due to the important valence π* and/or σ* character of the MOs. The far UV spectrum of Pottier et al.9 reports a shoulder structure at 8.29 eV. A careful analysis of the EEL profile in Figure 2 and the Gaussian fitting performed in this region 10535



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calculations as our only guide, we cannot propose assignments for these bands with confidence, so Tables 2−4 are just tentative assignments. The ⟨r2⟩ values in Tables 2−4 were obtained by evaluating the electric quadrupole moment and as such, the calculated values depend on the choice of the origin of coordinates used in the multipolar expansion. These values for Rydberg and valence states are not very different, i.e., for the former are around 120− 130 au2, whereas for the latter these have slightly larger values. In fact, the identification of Rydberg states was based more firmly on the symmetry and shape of the mono-occupied orbitals and the values of the oscillator strengths. Because it is not possible to show the entire MO’s of the excited states, we have added to the Supporting Information (Figure II) some illustrative examples of Rydberg states for the D3d isomer. C.1. Hexafluoro-1,3-butadiene (1,3-C4F6). Rydberg series converging to the ionic electronic ground state are associated with the vacation of πb(19b) orbital. The first member of the πb(19b) → nsa series (Table 2) appears at 7.18 eV, with a quantum defect δ = 0.94. The feature at 7.69 eV is tentatively assigned to a πb(19b) → 3sa transition. However, it shows a rather low quantum defect for an ns orbital that may indicate the valence nature of the transition. The n = 3 members of np Rydberg series converging to the ionic electronic ground (20a)−1 and first (19b)−1 excited states of 1,3-C4F6 have been assigned in Table 2. As such, we assign the 8.47 eV structure to the (11A → 51A, πa(20a) → 3pa) transition with a quantum defect δ = 0.34, and the 9.26 eV to the (11A → 71A, πb(19b) → 3pb) transition with δ = 0.48. Note that the 9.26 eV feature attributed to a Rydberg transition is in agreement with the early analysis of Pottier et al.9 far UV data. In the high resolution (∼4.5 meV at the midpoint of the energy range studied, 5.0−11.0 eV) VUV photoabsorption spectrum,41 such transitions are accompanied by weak vibrational structure, with a mean energy spacing of 0.185 eV due to CC symmetric stretching mode. However, in the EEL spectrum and even at a reasonable modest 30−40 meV resolution, such fine structure would become noticeable. This is not the case and the only explanation for that is due to the considerable broad nature of the 8.47 and 9.26 eV features that may hinder such contributions. The structure at 12.13 eV is tentatively assigned to a Rydberg transition converging to 16.3 eV16 ionization energy, with a rather high quantum defect for a ns series, δ = 1.19. However, in section B.1 we have proposed a valence character for the transition, which seems more reasonable due to the similar calculated oscillator strength, f L = 0.0405, in comparison with the (101B) with f L = 0.0132. Finally, in the present EEL spectrum of hexafluoro-1,3butadiene, we also report other features at 12.7 and 13.2 eV that may tentatively be assigned to 3s Rydberg members converging to 16.3 and 16.6 eV ionization energies, with δ = 1.05 and δ = 1.00, respectively. C.2. Hexafluorocyclobutene (c-C4F6). Due to the lack of any experimental values for the ionization energies, we have attempted some assignments for Rydberg series based on the ionization values calculated in Table 5. The first member of the π(7b1) → nsσ(a1) series (Table 3) converging to the lowest ionization energy, appears at 7.15 eV, with a rather high quantum defect δ = 1.27. The structure at 9.85 eV is tentatively assigned to the (11A1 → 31A1, π(7b1) → 3pπ(b1)) transition with δ = 0.77. Due to the slightly higher quantum defect for a np series, this feature is

of such transitions. The 9.85 eV feature is here proposed to be due to the (11A1 → 41A2, π(7b1) → σ*(b1)) transition with a considerably high oscillator strength ( f L = 0.1499), whereas the 12.13 eV is observed as a shoulder structure. This is tentatively assigned to a transition that, according to the calculations in Table 3, may comprise the contributions of valence and Rydberg excitation. Two other features are observed at 12.88 and 13.39 eV but remain unassigned. B.3. Valence Excitation of Hexafluoro-2-butyne (2-C4F6). The electron energy loss spectrum of 2-C4F6 is shown in Figure 2, and generally speaking we observe five features. The first three have been assigned in Table 4, and the last two at 13.15 and 14.38 eV remain unassigned. Of considerable relevance is the lowest band that is only discernible in this isomeric form of C4F6. Its maximum is centered at around 7.09 eV and is assigned to the Eu component of the π(6eu) → π*(6eg) transition, showing the lowest oscillator strength ( f L ≈ 0.0002). The A1u component of the same transition is calculated to occur at 6.900 eV but is dipole-forbidden, according to symmetry selection rules. Due to the doubly degenerate nature of Eu, Table 4 shows the results for such degeneracies where the f L values are kept with several decimal places. In a recent work on the A-band methyl halide dissociation via electronic curve crossing studied by means of electron energy loss spectroscopy,42 the very weak spin−orbit coupling (0.01 eV) for CH3F, was far beyond the experimental resolution to be discernible. As a consequence, the second component of the E state could not be resolved. For the fluorinated compounds, the Jahn−Teller splitting is expected to be small owing to the very weak spin−orbit coupling, and so the Eu degenerate state will not be experimentally resolved. Hexafluoro-2-butyne's (2-C4F6) most intense structure at 9.94 eV shows a considerable shift toward higher EEL relative to those for c-C4F6 and 1,3-C4F6, of ∼1.9 and 2.6 eV respectively. This feature is assigned to the A2u component of the π(6eu) → π*(6eg) transition, where we note the highest f L value (Table 4) for all C4F6 isomers. Such an intense oscillator strength has also been reported in the hydrogen analogue, CH3CCCH3, but-2-yne.43 The 11.29 eV weak structure is due to the contribution of a Rydberg (see section C) and a valence transition, the latter which may correspond to (11A1g → 51Eu, σCC → π*(6eg)). C. Rydberg Features. The EELS data consist of structures superimposed on diffuse features extending to the lowest ionization energies (IEs). The peak positions, En, have been compared using the Rydberg formula: En = E i − R /(n − δ)2

(1)

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 the quantum defect resulting from the penetration of the Rydberg orbital into the core. The proposed first members (n = 3) of the Rydberg structures are presented in Tables 2−4. Assignments in the spectra for higher members of the Rydberg series, where n ≥ 4 members are expected to lie, is rather complex due to the presence of other valence transitions, and the limited resolution of the present experiments. Though no attempts have been made to find other high energy members. To the authors’ knowledge, no other calculations are available for higher members of Rydberg series converging to the ionic excited states of the C4F6 isomers. Therefore, with quantum defect 10536



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BPD/68979/2010 and together with PL-V the PEst-OE/FIS/ UI0068/2011 grant. P.L.-V. also acknowledges his Visiting Professor position at Sophia University, Tokyo, Japan. Part of this project was supported by the French ANR agency under contract No. ANR-11-LABX-0005 "Chemical and Physical Properties of the Atmosphere". This work was also performed using HPC resources from GENCI-CINES (Grant 2012088620) made by GENCI (Grand Equipement National de Calcul Intensif). This work forms part of the EU/ESF COST Action CM0805 programme “The Chemical Cosmos”.

mainly due to the valence transition character as discussed above. At 10.51 eV we assign this feature to (σ(CC) (a1) → 3sσ(a1) + σ(CC) (b2) → 3pσ(b2)), with a rather high quantum defect for an ns series, δ = 1.18. According to the calculations, the 12.13 eV feature may be due to a transition to valence and Rydberg states, the latter showing a rather high quantum defect values for nd and np series. However, it seems more reasonable to assign this feature to a valence transition as proposed in section B.2. C.3. Hexafluoro-2-butyne (2-C4F6). Regarding Rydberg assignments in Table 4, the first member of an ns series is tentatively attributed to π(6eu) → 3sσ, (11A1g → 21Eu), with a quantum defect δ = 1.01. The 11.29 eV feature has been assigned to a valence transition (see section B.3). Due to the proximity of this feature with the lowest ionic state at 12.76 eV, it is also tentatively assigned to the first member of the π(6eu) → 3dσ series, with a quantum defect δ = −0.04. However, the lowest ionic band in 2-C4F6 shows considerable extended vibrational excitation,24 which we would expect to see in such Rydberg state. As reported in section C.1 for 1,3-C4F6, the lack of vibrational features can be attributed to the rather broad nature of the EEL features.

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V. CONCLUSIONS The present work provides the first complete EELS of C4F6 isomers from 2.0 to 15.0 eV. Due to the rather identical intensities of the DCS from 100 eV, 3° and 30 eV, 10°, the transitions are mostly from singlet to singlet states. The observed EELS structures have been assigned to a combination of valence and Rydberg transitions further to ab initio calculations on vertical excitation energies and oscillator strengths. These predict significant mixing of Rydberg and π* and σ* states.

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ASSOCIATED CONTENT

S Supporting Information *

The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), second highest occupied molecular orbital (SHOMO), and second lowest unoccupied molecular orbital (LUMO+1) of C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (cC4F6), and hexafluoro-2-butyne (2-C4F6) are shown in Figure I. Additionally, the lowest Rydberg states for the D3d isomer, hexafluoro-2-butyne (2-C4F6) appear in Figure II. This information is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+351) 21 294 78 59. Fax: (+351) 21 294 85 49. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted under the support of the Japanese Ministry of Education, Sport, Culture and Technology. H.K. acknowledges the Japan Society for the Promotion of Science (JSPS) for his fellowships as grants-in-aid for scientific research. F.F.S. acknowledges the Portuguese Foundation for Science and Technology (FCT-MES) for postdoctoral grant SFRH/ 10537



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Electronic Excitation to Singlet States of 1,3-C - American

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