High resolution photoabsorption spectrum of hexafluoro-1,3-butadiene (1,3-C4F6) as studied by vacuum ultraviolet (VUV) synchrotron radiation

Chemical Physics Letters 550 (2012) 62–66

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High resolution photoabsorption spectrum of hexafluoro-1,3-butadiene (1,3-C4F6) as studied by vacuum ultraviolet (VUV) synchrotron radiation F. Ferreira da Silva a, D. Almeida a, E. Vasekova b, E. Drage b, N.J. Mason b, P. Limão-Vieira a,b,⇑ a b

Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal Centre of Earth, Planetary, Space and Astronomy Research, Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

a r t i c l e

i n f o

Article history: Received 8 August 2012 In final form 31 August 2012 Available online 8 September 2012

a b s t r a c t In this Letter we present a high resolution VUV photoabsorption spectrum of hexafluoro-1,3-butadiene (1,3-C4F6), over the wavelength range 113–247 nm (11.0–5.0 eV). The spectrum reveals several new features not previously reported in the literature. The assignment of the observed valence and Rydberg transitions and the associated vibronic series is presented based on our recent ab initio calculations on the vertical excitation energies of C4F6 isomers. The dominant excitation has been assigned to the t01 (a) 11A, 3pa pa(20a)) and (71A 11A, 3pb pb(19b)) transitions, with C@C stretching mode in the (51A mean energies of 0.201 and 0.188 eV, respectively. The measured absolute photoabsorption cross section has been used to calculate the photolysis lifetime of 1,3-C4F6 in the upper stratosphere (20–50 km). Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Under the Montreal Protocol and its amendments, halogenated hydrocarbons have to be phased out of commercial use and therefore several governments have been putting considerable pressure on the plasma industry to seek for more environmentally friendly plasma processing molecules. In order to comply with these directives the industry relies increasingly upon the development of plasma models to design more efficient and affordable systems while also meeting increasingly severe environmental legislation. Such models require large databases of fundamental electron, photon and ion interaction with all plasma species, therefore we have observed a significant increase in spectroscopic studies of several potential replacement molecules with particular attention to those yielding CFx radicals in plasma reactors [1]. Such spectroscopic data not only gives information on the electronic state spectroscopy of a molecule but also enables us through its absolute cross sectional values to determine the atmospheric lifetime under photolysis. Over the last decade, we have undertaken a series of spectroscopic investigations on such plasma relevant molecules either by photon or electron impact (see e.g. [2–12] and references therein) with the aim of populating such a database. In this Letter we report a detailed spectroscopic study of the optical spectroscopy of one such candidate molecule, hexafluoro-1,3-butadiene (1,3-C4F6). Nakamura and co-workers [13] have reported on the efficiency of fabricating contact holes in ultra large integrated circuits ⇑ Corresponding author at: Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. Fax: +351 21 294 85 49. E-mail address: [email protected] (P. Limão-Vieira). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.08.075

through selective etching processes, especially in silicon oxide (SiO2) on Si and silicon nitride (Si3N4) layers using 1,3-C4F6 as a feed gas. Currently hexafluoro-1,3-butadiene has a modest annual production, estimated at 0.01 Tg/y and the atmospheric degradation mechanism and global warming potential studies of Acerboni et al. [14], shows that the main sink mechanism for this compound is due to the high reactivity with OH radicals (kCF2CFCFCF2+OH = (1.1 ± 0.3)  10–11 cm3 molecules–1 s–1) such that its emissions are considered to contribute only 0.027 relative to CO2 to global warming. However 1,3-C4F6 is not homogeneously distributed in the troposphere and the total amount of atmospheric emissions are far from being completely known [14]. Recently we have reported on the first measurements of the electron energy loss spectroscopy (EELS) for C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6) and hexafluoro-2-butyne (2-C4F6), obtained at 100 eV, 3° scattering angle, while sweeping the energy loss over the range 2.0–15.0 eV, from which pseudo-photoabsorption spectra may be derived [15]. However, as far as we are aware, there are only two previously published far UV photoabsorption spectra. Rutner and Bauer [16] and Pottier et al. [17] reported VUV spectra over the narrow energy ranges of 4.59–6.20 and 4.96–10.78 eV, respectively. Regarding the molecular structure of hexafluoro-1,3-butadiene, there have been several contributions agreeing with the skew s-cis preferred ground state geometry [18–22], which proved instructive for the nonplanarity of the molecule’s carbon skeleton. Wurrey et al. [23] reported on the infrared spectra of gaseous and solid 1,3C4F6 recorded from 30 to 4000 cm1, together with Raman spectra for the gaseous, liquid and solid phases. The vertical ionic electronic ground and first excited states of 1,3-C4F6, have been determined using He(I) photoelectron

F. Ferreira da Silva et al. / Chemical Physics Letters 550 (2012) 62–66

spectroscopy at 10.4 and 11.4 eV, respectively, by Brundle and Robin [24]. Other studies include electron attachment rates to perfluorocarbon compounds from Christodoulides and co-workers (see e.g. Ref. [25]), (dissociative) electron attachment on the three isomers of C4F6 by Suzer et al. [26] and the grand total cross section (TCS) measurements by Szmytkowski and Kwitnewski [27,28] in the energy range 0.5–370 eV. In this Letter we focus our attention on the absolute VUV photoabsorption cross sections, where the fine structure observed in the 8.0–10.0 eV energy region is assigned here for the first time. In the next section we provide a brief summary of the structure and properties of 1,3-C4F6. In Section 3 we present a brief discussion of the experimental details and in Section 4 the experimental results are presented together with a discussion and comparison with other results. Section 5 compares the present data with other absolute photoabsorption cross sections and the photolysis rates of these molecules are calculated from 20–50 km altitude in the Earth’s atmosphere. Finally some conclusions that can be drawn from this Letter are given in Section 6. 2. Brief summary of the structure and properties of 1,3-C4F6 According to recent ab initio calculations at the EOM–CCSD level [15], hexafluoro-1,3-butadiene (1,3-C4F6) has symmetry C2 in the electronic ground state. The symmetry species available to a C2 molecule are A and B and all transitions are optically allowed by selection rules. The calculated electron configuration of the outermost valence orbitals of the 1A ground state is . . . (18b)2 (19a)2 (19b)2 (20a)2. The highest occupied molecular orbital (HOMO) and the second highest occupied molecular orbital (SHOMO) have pa(C@C) and pb(C@CAC@C) character, respectively. The lowest unoccupied molecular orbital (LUMO), 20b, is mainly of pb antibonding character localized on the (C@C) group, whereas the (LUMO + 1) has been assigned to 21a with pa antibonding character. The theoretical studies in Ref. [15] have shown that the lowest Rydberg states may overlap with valence states resulting in complex intensity distribution in the electronic spectrum. It is therefore necessary to separate features in the photoabsorption spectra due to Rydberg states from those arising from valence states, such states may be identified through knowledge of the ionization states and application of quantum defect theory. The two lowest vertical ionization energies, which are needed to calculate the quantum defects associated with transitions to Rydberg orbitals, have been experimentally determined to be 10.4 (20a)1 and 11.4 eV (19b)1, respectively [24]. 3. Experimental details A high-resolution VUV photoabsorption spectrum of hexafluoro-1,3-butadiene, 1,3-C4F6 (Figure 1), was recorded using station 3.1 at the UK Daresbury Synchrotron Radiation Source (SRS). The experimental apparatus has been described in detail elsewhere [29] so only a brief review will be given here. Briefly, VUV radiation passed through a static gas sample and was converted by a sodium sallicylate window into visible radiation, which was then detected by a photomultiplier to measure the transmitted light intensity. The incident wavelength was selected using a toroidal dispersion grating with 2000 lines/mm providing a resolution of 0.1–0.2 nm, corresponding to 7.5 meV at the midpoint of the energy range studied. A LiF entrance window acted as an edge filter for higher order radiation restricting the photoabsorption measurements to 11.0 eV (113 nm). The grating itself provided a maximum wavelength (lower energy limit) of 320 nm (3.9 eV). The sample pressure was measured using a capacitance manometer (Baratron).

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To ensure that the data was free of any saturation effects, the absorption cross-sections were measured over different pressure ranges with typical attenuations of less than 10%. The synchrotron beam ring current was monitored throughout the collection of each spectrum and background scans were recorded with the cell evacuated. Absolute photoabsorption cross sections were obtained using the Beer–Lambert attenuation law: It = I0 exp (nrx), where It is the radiation intensity transmitted through the gas sample, I0 is that through the evacuated cell, n the molecular number density of the sample gas, r the absolute photoabsorption cross section, and x the absorption path length (25 cm). The accuracy of the cross section is estimated to be ±5%. Only when absorption by the sample is very weak (I0  It), does the error increase as a percentage of the measured cross section. The gas sample of 1,3-C4F6 was purchased from Apollo Limited with a stated purity of P99%. The gas was used as delivered.

4. Electronic state spectroscopy of hexafluoro-1,3-butadiene (1,3-C4F6) The VUV spectrum of hexafluoro-1,3-butadiene is shown in Figure 1. The major absorption bands are a mixture of Rydberg states and valence transitions of (p⁄ p) character. A detailed discussion of these transitions is given below.

4.1. Valence excitation of hexafluoro-1,3-butadiene (1,3-C4F6) In the 5.45–6.70 eV energy region, Pottier and co-workers [17] reported the lowest lying excited state at 6.17 eV, in reasonable agreement with the present value of 6.215 eV. This has been previously assigned to a (p⁄ r) transition [17] in contrast to the (11B 11A, pb⁄(20b) pa(20a)) transition from Ref. [15]. The second absorption band reported by Pottier et al. [17] in the 6.70–8.18 eV energy region with a maximum at 7.66 eV and assigned to a (p⁄ p) transition, is in good agreement with the present value of 7.57(7) eV and is the most intense band in the VUV spectrum of hexafluoro-1,3-butadiene (1,3-C4F6). This has been assigned to the (41A 11A, pb⁄(20b) pb(19b)) transition. A closer inspection of Figure 1 reveals that this structure is asymmetric, suggesting either a weak contribution from vibrational excitation, which seems reasonable due to a mixing excitation to a 3sa Rydberg orbital and/or contributions from another underlying states. Recently we have explored this latter possibility using EELS data and ab initio calculations and we have proposed the presence of a (31B 11A, 3sa pb(19b) + rCF⁄ pa(20a)) transition. A detailed description can be found in Ref. [15]. The far UV spectrum of Pottier et al. [17] reports a shoulder structure at 8.29 eV, whereas the present VUV photoabsorption spectrum clearly shows the presence of another electronic state superimposed by a rather weak vibrational structure (Figure 1 and Table 2). This has been assigned to a Rydberg transition that is discussed and assigned below. The mean separation energy of this vibrational structure is 0.201 eV and the ground state frequency is reported to be 0.223 eV (1796 cm1) [23] corresponding to t01 (a) C@C stretching. The absorption structure at 9.5 eV, is assigned to (71A 11A, 3pb pb(19b)) from the ab initio calculations in Ref. [15] with a maximum of 12.45 Mb at 9.290 eV (Table 1). Alternatively, due to the rather significant background underlying this transition, this may also accomodate the (71B 11A, r⁄CC/CF pa(20a)) transition. This band shows vibrational structure (Table 2) in which only one active mode has been assigned to the totally symmetric t01 (a) C@C stretching with a mean energy of 0.188 eV.

F. Ferreira da Silva et al. / Chemical Physics Letters 550 (2012) 62–66

8 Photon energy [eV]

3 nsb

nυ1

3

0

5

10

15

20

25

30

35

40

5

6

7

nsa

3

nυ1

np a

3

nd a

9

np b

4

4

3

4

5

10

IE1 (20a -1 )

IE2 (19b -1 )

11

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Cross Section [Mb] Figure 1. VUV photoabsorption cross section with vibrational progressions and Rydberg series assignment in the 5.0–11.0 eV absorption band of hexafluoro-1,3-butadiene (1,3-C4F6).

4.2. Rydberg transitions in hexafluoro-1,3-butadiene (1,3-C4F6) The VUV spectrum above 6.0 eV consists of a few structures superimposed on a diffuse absorption feature extending to the lowest ionization energies (IE1 and IE2). The proposed Rydberg structures are labeled in Figure 1 and presented in Table 1. The peak positions of the Rydberg states, En, have been identified using the Rydberg formula:

En ¼ Ei  R=ðn  dÞ2

ð1Þ

where Ei is the ionization energy (10.4 eV for (20a)1 and 11.4 eV for (19b)1), n is the principal quantum number of the Rydberg

orbital of energy En, R is the Rydberg constant (13.61 eV), and d the quantum defect resulting from the penetration of the Rydberg orbital into the core. Quantum defects in the range 0.9–1.2, 0.7, and 0–0.3 are expected for ns, np, and nd transitions, respectively [30], where the values of the lowest experimental terms of these series show, generally speaking, good agreement. The Rydberg series assigned in Table 1 converging to the ionic electronic ground and first ionic electronic excited states, are associated with the vacation of pa(20a) and pb(19b) orbitals. The lowest Rydberg transition at 7.57(7) eV is assigned to (3sa pb(19b)), with a quantum defect d = 0.80 (Table 1). The n = 4 member of this series has a value for d = 0.79. The weak feature at 9.64(6) eV is interpreted as n = 5 term with a quantum defect

F. Ferreira da Silva et al. / Chemical Physics Letters 550 (2012) 62–66 Table 1 Energies (eV), quantum defects, and assignments of the ns, np and nd Rydberg series converging to the ionic electronic ground (20a)1 and first (19b)1 excited states of hexafluoro-1,3-butadiene (1,3-C4F6). Vertical energy

Quantum defect (d)

Assignment

IE1 = 10.4 eVa 7.57(7) (b) 9.112 9.64(6) (w) 8.26(3) (s) 9.304 8.66(3) (s) 9.498

0.80 0.79 0.75 0.48 0.48 0.20 0.12

3sa 4sa 5sa 3pa 4pa 3da 4da

IE2 = 11.4 eVa 8.66(3) (s) 9.290

0.77 0.47

3sb 3pb

(b) Indicates a broad structure; (w) weak structure; (s) a shoulder (the last decimal of the energy value is given in brackets for these less-resolved features). a Vertical value.

Table 2 Proposed vibrational assignments in the 8.0–9.5 eV absorption bands of hexafluoro1,3-butadiene (1,3-C4F6). Energy (eV)

Assignment

DE (t01 ) (eV)

8.057 (s) 8.246 8.455 8.663 8.863 8.933 (s) 9.112 9.304 9.498

1t1 2t1 3t1 4t1 5t1 1t1 2t1 3t1 4t1

– 0.189 0.209 0.208 0.200 – 0.179 0.192 0.194

(s) Indicates a shoulder structure (the last decimal of the energy value is given in brackets for these less-resolved features).

d = 0.75. The rather low quantum defects for the ns orbital, may indicate the mixed valence nature of the transitions. The first members of the np and nd series are associated with the shoulder structures at 8.26(3) eV (d = 0.48) and 8.66(3) eV (d = 0.20) (Table 1) and have been assigned to the (51A 11A, 3pa pa(20a)) and 1 1 (6 A 1 A, 3da pa(20a)) transitions, respectively. The peak at 8.66(3) eV has been assigned tentatively to the n = 3 term of an ns series whereas the feature at 9.290 eV is assigned to n = 3p, both converging to the ionic electronic first excited sate. In the 8.0–9.5 eV energy region Rydberg transitions are accompanied by vibronic structure which is tentatively attributed to excitation of the t01 (a) C@C stretching mode (Table 2). The 9.290 eV feature has been previously assigned in the EEL spectrum at 9.27 eV to the (71A 11A, 3pb pb(19b)) transition with d = 0.47. Note that this feature being attributed to a Rydberg transition is in agreement with the early analysis of Pottier et al. [17] far UV data. 5. Absolute photoabsorption cross sections and atmospheric photolysis Previous absolute VUV photoabsorption cross sections of hexafluoro-1,3-butadiene are only available in the wavelength ranges 115–227 nm (5.46–10.78 eV) [17] and 200–228 nm (6.20– 5.44 eV) [16]. Pottier et al. [17] reported cross section values at 162 and 201 nm of 11.3 and 5.2 Mb, respectively, lower than the present respective values of 32.8 and 19.2 Mb. Rutner and Bauer [16] reported cross sections of 8.3 Mb (200 nm, 6.2 eV) considerably lower than the present respective values of 19.3 Mb. The agreement of previous cross sections measured at the Daresbury

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beamline with the most precise data available in the literature (see e.g. Eden et al. [31] and references therein), suggests that the present 1,3-C4F6 cross sections can be relied upon across the range studied. These absolute cross sections can be used in combination with solar actinic flux [32] measurements from the literature to estimate the photolysis rates of 1,3-C4F6 in the atmosphere from an altitude of 20 km to the stratopause at 50 km. Details of the programme are presented in a previous publication by Limão-Vieira et al. [33]. As the p⁄ MO in these transitions has C@C anti-bonding character the quantum yield for dissociation following absorption is assumed to be unity. The reciprocal of the photolysis rate at a given altitude corresponds to the local photolysis lifetime. Photolysis lifetimes of less than one sunlit hour were calculated at altitudes above 25 km. This indicates that hexafluoro-1,3-butadiene molecules can be broken up quite efficiently by VUV absorption at these altitudes. At 15 km the photolysis lifetimes increase to 103 sunlit hours and will be considerably longer at lower altitudes. The work of Acerboni et al. [14], reports a comprehensive study on the reactions of hexafluoro-1,3-butadiene with OH, O3 and NO3. Hexafluoro-1,3-butadiene is highly reactive with OH radicals and the work of Acerboni et al. [14] report a rate coefficient at room temperature of (1.1 ± 0.3)  1011 cm3 molecule1 s1, which may provide one of the main reactive sink mechanism in the Earth’s atmosphere. The rate coefficients for O3 and NO3 were considered negligible, i.e. (6.5 ± 0.2)  1021 cm3 molecule1 s1 and <3  1015 cm3 molecule1 s1, respectively. Therefore, compared with radical reactions, UV photolysis is not expected to play a significant role in the tropospheric removal of these molecules. 6. Conclusions This Letter provides the first complete optical electronic spectrum of hexafluoro-1,3-butadiene (1,3-C4F6) and the most reliable set of absolute photoabsorption cross sections available from 5.0 to 11.0 eV. The observed photoabsorption structure has been assigned to contain both valence and Rydberg transitions. Fine structure has been assigned to observed vibrational series, and are predominantly due to excitation of the t01 (a) C@C stretching mode. Our previous theoretical results are in good agreement with the experiments and predict significant mixing of Rydberg and p⁄ states. Photolysis lifetimes of 1,3-C4F6 have been derived for the Earth’s troposphere and stratosphere. Acknowledgements FFS and DA acknowledge the Portuguese Foundation for Science and Technology (FCT–MES) through post-graduate and post-doctoral scholarships SFRH/BPD/68 979/2010 and SFRH/BD/32 271/ 2006, respectively. PLV acknowledges the PEst-OE/FIS/UI0068/ 2011 grant as well as the visiting professor position in the Molecular Physics group, Open University, UK. PLV and NJM acknowledge the support from the British Council for the Portuguese– English joint collaboration. ED recognizes support from NERC and EV The Open University for support of their PhD studentships. This Letter forms part of the EU COST Action CM0805 Programme ‘The Chemical Cosmos’, respectively. References [1] I. Rozum, P. Limão-Vieira, S. Eden, J. Tennyson, N.J. Mason, J. Phys. Chem. Ref. Data 35 (2006) 267. [2] N.J. Mason et al., Int. J. Mass Spectrom. 223–224 (2003) 647. [3] N.J. Mason, A. Dawes, R. Mukerji, E.A. Drage, E. Vasekova, S.M. Webb, P. LimãoVieira, J. Phys. 38 (2005) S893. [4] P. Limão-Vieira, S. Eden, N.J. Mason, Radiat. Phys. Chem. 68 (2003) 187.

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