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Electronic State Spectroscopy of c-C5F8 Explored by Photoabsorption, Electron Impact, Photoelectron Spectroscopies and Ab Initio Calculations P. Lima˜ o-Vieira,*,†,‡ D. Duflot,§ A. Giuliani,|,&,⊥ E. Vasekova,‡ J. M. C. Lourenc¸ o,† P. M. Santos,# S. V. Hoffmann,∇ N. J. Mason,‡ J. Delwiche,| and M.-J. Hubin-Franskin| Laborato´ rio de Coliso˜ es Ato´ micas e Moleculares, Departamento de Fı´sica, CEFITEC, UniVersidade NoVa de Lisboa, P-2829-516 Caparica, Portugal, Centre of Molecular and Optical Sciences, Department of Physics and Astronomy, The Open UniVersity, Walton Hall, Milton Keynes, MK7 6AA, U.K., Laboratoire de Physique des Lasers, Atomes et Mole´ cules (PhLAM), UMR CNRS 8523, Centre d’EÄ tudes et de Recherches Lasers et Applications (CERLA, FR CNRS 2416), UniVersite´ des Sciences et Technologies de Lille, F-59655 VilleneuVe d’Ascq Cedex, France, Laboratoire de Spectroscopie d’EÄ lectrons Diffuse´ s, UniVersite´ de Lie` ge, Institut de Chimie-Baˆ t. B6c B-4000 Lie` ge 1, Belgium, DISCO Beamline, Synchrotron SOLEIL, BP 48, L’Orme des Merisiers, 91192 Gif-sur-YVette CEDEX, France and Cepia, Institut National de la Recherche Agronomique, BP 71627, F-44316, France, Instituto de Telecomunicac¸ o˜ es, IST, AVenida RoVisco Pais, P-1049-001 Lisboa, Portugal, and Institute for Storage Ring Facilities, UniVersity of Aarhus, Ny Munkegade, DK-8000, Aarhus C, Denmark ReceiVed: NoVember 12, 2007
The electronic transitions and resonance-enhanced vibrational excitations of octafluorocyclopentene (c-C5F8) have been investigated using high-resolution photoabsorption spectroscopy in the energy range 6-11 eV. In addition, the high-resolution electron energy loss spectrum (HREELS) was recorded under the electric dipolar excitation conditions (100 eV incident energy, ∼0° scattering angle) over the 5-14 eV energy loss range. A He(I) photoelectron spectrum (PES) has also been recorded between 11 and 20 eV, allowing us to derive a more precise value of (11.288 ( 0.002) eV for the ground neutral state adiabatic ionization energy. All spectra presented in this paper represent the first and highest resolution data yet reported for octafluorocyclopentene. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.
1. Introduction As part of an international effort to reduce the emission of atmospheric pollutants that may lead to global warming the Kyoto Protocol and its amendments have been developed to phase out the use of several anthropogenic gases by 2010, including several species arising from the plasma processing industries. Therefore, in order to meet the requirements of the Kyoto Protocol, industry has been seeking alternative gases that will both operate in current plasma reactors and maintain high performance of current feed gases (e.g., SF6 and CF4). Octafluorocyclopentene, c-C5F8, is among the fluorocarbons suggested as a candidate to replace the traditional feed gases used in the semiconductor industry to prepare silicon wafers for SiO2 reactive ion etching.1 Surface reactions induced by c-C5F8 plasmas have been studied on silicon dioxide and silicon nitride substrates using an inductively coupled plasma source, revealing * Corresponding author. Mailing address: Laborato´rio de Coliso˜es Ato´micas e Moleculares, Departamento de Fı´sica, CEFITEC, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal. E-mail address:
[email protected]. Tel: +351-21 294 85 76. Fax: + 351-21 294 85 49. † Universidade Nova de Lisboa. ‡ The Open University. § Universite ´ des Sciences et Technologies de Lille. | Universite ´ de Lie`ge. & DISCO Beamline, Synchrotron SOLEIL. ⊥ Institut National de la Recherche Agronomique. # Instituto de Telecomunicac ¸ o˜es, IST. 3 University of Aarhus.
larger etching selectivity and polymerization than in a more conventional c-C4F8 plasma.2 However, before c-C5F8 can be widely adopted it must be shown to have little (or no) environmental impact. To compute the global warming potential (GWP) and lifetime of any atmospheric molecule, one requires both knowledge of the electronic state spectroscopy and the absolute photoabsorption cross-sectional values in the visible and vacuum ultraviolet (VUV) region of the electromagnetic spectrum.3 As part of a larger project to investigate the electronic state spectroscopy of several plasma processing molecules and to evaluate their role in global warming in the earth’s atmosphere (see, e.g., ref 4), we have, therefore, recorded high-resolution VUV photoabsorption, electron energy loss, and photoelectron spectra (PESs) of c-C5F8. As far as we are aware, no other spectra have been reported in the literature. Indeed, a review of the literature reveals little experimental data for c-C5F8, these being limited to some measurements of the electron attachment line shapes and cross sections at ultralow electron energies.5 Recent studies on the ionization cross sections of Jiao et al.6 on cationmolecule reactions and Parkes et al.7 on threshold photoelectron-photoion coincidence spectroscopy with a He(I) PES obtained with a lower resolution that in the present work have been reported. Quantum chemical studies via configuration interaction scheme (CIS) with the Hartree-Fock method8 and ab initio calculations on the dissociative attachment resonance energies with comparisons to electron attachment mass spectrometric measurements9 have also been discussed in a recent
10.1021/jp7107939 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/11/2008
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Figure 1. High-resolution VUV photoabsorption spectrum of c-C5F8, recorded using the Århus synchrotron facility, showing valence and Rydberg states in the 6-11 eV energy range.
Figure 2. High-resolution VUV photoabsorption spectrum of c-C5F8, recorded using the Århus synchrotron radiation facility, showing valence and Rydberg states and vibrational progressions in the 8.5-11.0 eV energy range.
work, but the former has proven to disagree with our present calculations (see Computational Method section). Electron energy loss spectra in the present experiments and excitation energies derived from theory are used to help assign the major bands in the photoabsorption spectra. Further information is derived from a high-resolution PES, which is used to help in the identification of Rydberg states in the VUV spectrum. The VUV absolute photoabsorption cross section of c-C5F8 reported in this paper is used to derive its photolysis lifetime in the earth’s atmosphere and compared to other candidates for replacement in the plasma industry. 2. Experimental Section The electronic states of octafluorocyclopentene (c-C5F8) spectroscopy has been studied using a range of different
experimental techniques, including (1) VUV photoabsorption using a synchrotron radiation source, which provides data on optical allowed transitions, (2) high-resolution electron energy loss spectroscopy (HREELS) in electric dipolar interaction conditions, which allow only optically allowed transitions to be monitored, and (3) He(I) photoelectron spectroscopy (PES) to determine the ionization energies. HREELS was used in order to increase the energy range studied, the VUV absorption curve being limited in energy by the transmission limit of the entrance window of the photoabsorption cell (see below). 2.1. VUV Photoabsorption. The high-resolution VUV photoabsorption measurements (Figures 1 and 2) were performed using the ASTRID-UV1 beam line at the Institute for Storage Ring Facilities (ISA), University of Aarhus, Denmark. A detailed description of the apparatus can be found elsewhere,10 so only
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Figure 3. Electron energy loss spectrum of c-C5F8 recorded at the University of Lie`ge in the 5-14 eV energy region in electric dipolar interaction conditions (100 eV incident energy, ≈ 0° scattering angle).
a brief description will be given here. A toroidal dispersion grating is used to select the synchrotron radiation with a full width at half-maximum (fwhm) wavelength resolution of approximately 0.075 nm. The synchrotron radiation passes through the static gas sample at room temperature. A photomultiplier is used to detect the transmitted light. For wavelengths below 200 nm, a flow of He gas is flushed through the small gap between the photomultiplier and the exit window of the gas cell to prevent any absorption by air contributing to the spectrum. A LiF entrance window acts as an edge filter for higher-order radiation, restricting the photoabsorption measures to below 10.8 eV (115 nm). The grating itself provides a maximum wavelength (lower energy limit) of 320 nm (3.9 eV). The sample pressure is measured by a Baratron capacitance gauge. To avoid any saturation effects, sample pressures were chosen such that the transmitted flux was >10% of the incident flux. Gas transmission results are compared to a background scan recorded with an evacuated cell. Absolute photoabsorption cross sections may then be calculated using the Beer-Lambert law:
It ) I0 exp(-nσx) where It is the intensity of the light transmitted through the gas sample, I0 is that through the evacuated cell, n is the molecular number density of the sample gas, σ is the absolute photoabsorption cross section, and x is the absorption path length (25 cm). The accuracy of the absolute cross section is estimated to be better than (5%. 2.2. High-Resolution Electron Energy Loss Spectroscopy. The instrument used at the Universite´ de Lie`ge, Belgium (VGSEELS 400) has been described in detail elsewhere.11 An electrostatic electron energy monochromator defines a narrow energy spread about the mean incident electron energy, and a three element lens focuses the electrons into the collision region. The electron beam intersects the effusive gas beam, which flows through a hypodermic needle, at 90°. The working pressure in the chamber with target gas is ∼1.5 × 10-5 mbar. The analyzer system is also an electrostatic energy analyzer12 with the
scattered electron signal detected by an electron multiplier of the continuous dynode type. Both electron energy selectors work in the constant pass-energy mode. Spectra were recorded for energy losses between 5.0 and 14 eV at step intervals of 10 meV. The electron energy loss scale was calibrated to the “elastic scattering peak”. The resolution, measured as the fwhm of the elastically scattered electron peak was about 40 meV. The apparatus was used with relatively high incident energy electrons (100 eV) and a quite small scattering angle (θ ≈ 0°), such that strictly electric dipole interaction conditions apply. The derived VUV cross section was normalized at 8.57 eV from our VUV photoabsorption value of 7.203 Mbar to obtain absolute cross section values, a method shown to be reliable in previous works (see, e.g., ref 12). Comparison of the HREELS cross section values with those recorded using the synchrotron radiation source thus provides a test for any systematic errors in the optical values arising from the line saturation effect and second-order light from the light source and beam line. These were found to be negligible in this work. The present HREELS spectrum is shown in Figure 3. 2.3. Photoelectron Spectroscopy. He(I) (21.22 eV) photoelectron spectra of c-C5F8 (Figure 4) were taken at the Universite´ de Lie`ge, Belgium. The apparatus has been described in detail previously.13 Briefly, the spectrometer consists of a 180° cylindrical electrostatic analyzer with a mean radius of 5 cm. The analyzer is used in constant energy pass mode. The incident photons are produced by a D.C. discharge in a two-stage differentially pumped lamp. The energy scale was calibrated using argon lines using the values 2P3/2 ) 15.760 eV and 2P1/2 ) 15.937 eV),14,15 and the resolution of the present spectrum is measured from the fwhm of the Ar peaks to be 20 meV, in presence of c-C5F8. The position of the first peak in the first electronic band of the c-C5F8 PES was determined by making use of a least-square fitting with a Gaussian curve. Repeated tests in our laboratory have shown that this procedure is not very sensitive to the choice of the limits used for the fitting, and so the uncertainty of the position of a peak maximum is more or less one energy interval between two successive experimental points, giving, in the case of the c-C5F8 PES, an uncertainty of (0.002 eV. The same procedure was used for
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Figure 4. He(I) photoelectron spectra of c-C5F8 recorded at the University of Lie`ge (a) in the 11-20 eV energy region, (b) in the 10.8-13.0 eV energy region, and (c) in the 13.0-15.0 eV energy region (see discussion in text).
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all the energy values quoted in the paper. The intensities in the spectrum were corrected for the transmission of the analyzing system. The spectrum presented in Figure 4 consists of two parts, with the high-energy region (>15 eV) being recorded without Argon in order to avoid the strong signal due to the presence of the rare gas. The Ar lines shown in the lower energy part of the spectrum are due to Ar features produced by the He(I)β and γ lines, respectively. The PES presented in this paper is the sum of 22 individual spectra. This procedure allowed us to obtain a good signal-to-noise ratio while keeping the pressure in the spectrometer at a very low level (
