VUV photo-absorption cross-section for CCl2F2

16 October 2002

Chemical Physics Letters 364 (2002) 535–541 www.elsevier.com/locate/cplett

VUV photo-absorption cross-section for CCl2F2 P. Lim~ ao Vieira a

a,1

, S. Eden a, P.A. Kendall a, N.J. Mason

a,*

, S.V. Hoffmann

b

Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK Institute for Storage Ring Facilities, University of Aarhus, Ny Munkgade, DK-8000 Aarhus C, Denmark

b

Received 31 May 2002; in final form 7 August 2002

Abstract The photo-absorption spectrum of CCl2 F2 has been measured using synchrotron radiation in the range 5.5–11 eV (225 > k > 110 nm). Electronic state assignments have been suggested for each of the observed absorption bands incorporating both valence and Rydberg transitions. The high resolution achieved has allowed vibrational series in one of these bands to be assigned for the first time. The measured VUV cross-sections may be used to derive the photolysis rate of CCl2 F2 in the terrestrial atmosphere. Ó 2002 Elsevier Science B.V. All rights reserved.

1. Introduction Dichlorodifluoromethane (CCl2 F2 or CFC-12) is a halogenated hydrocarbon extensively used in refrigeration systems, as a foam blowing agent and as an aerosol propellant. It also plays an important role in the plasma etching industry [1,2] for reactive ion etching (RIE) of GaSb as it can be readily dissociated to produce Cl and F radicals [3]. Halogenated hydrocarbons or halocarbons such as CCl2 F2 are (under photolysis) a source of atmospheric radicals and therefore are widely recognised to contribute significantly to stratospheric ozone depletion [4,5] by Cl or Br atoms released by *

Corresponding author. Fax: +44-20-7679-3460. E-mail address: [email protected] (N.J. Mason). 1 Also at Departamento de Fı´sica, FCT – Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal and Centro de Fı´sica Molecular, Complexo I, IST, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal.

photolysis. Halogenated hydrocarbons are also strong greenhouse gases, CCl2 F2 has a residence time in the atmosphere of about 100 years and an estimated global warming potential of 8500 in a 100-year period [6]. Thus, under the regulations of the Montreal Protocol and its amendments, the use of CFC-12 in industry must be phased out and alternatives found within the next decade. In this paper we report new data on the photoabsorption cross-section and electronic spectroscopy of CCl2 F2 . The electronic structure of this molecule has been studied previously [7–13] as have its photo-absorption, photo-ionisation and photo-fragmentation cross-sections [4,8,10,11,14]. The vacuum-ultraviolet (VUV) absorption spectrum of CCl2 F2 in the wavelength range 120–200 nm (10–6 eV) was first measured during the 1950s by Zobel and Duncan [15] and subsequently discussed by Doucet et al. [10] together with a comparison with the photo-electron spectrum. An energy loss spectrum using 500 eV electrons was

0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 3 0 4 - 0

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reported by King and McConkey [12] under experimental conditions that simulated optical absorption. Zhang and coworkers [7,13] reported the absolute photo-absorption oscillator strengths using dipole spectroscopy between 8.5 and 200 eV, ionic photo-fragmentation branching ratios and the photo-ionisation efficiency at energies from the first ionisation threshold up to 70 eV. Recently, Au et al. [16] refined the data of Zhang et al. [7] reporting the absolute photo-absorption spectra from 5 to 60 eV by high-resolution dipole (e; e) spectroscopy. Photo-absorption cross-sections have been compared with cross-sections obtained from differential oscillator strength measurements using electrons with 100 eV and 8 keV, by Christophorou et al. [14]. A comprehensive study of the ultraviolet and visible emissions produced by dissociative electron impact on CCl2 F2 was reported by Jabbour and Becker [17] with absolute photoemission cross-sections reported for a variety of neutral, ionic fluorine and chlorine features as well as for diatomic CCl and CClþ bands. Mann and Linder [18] measured differential cross-sections for both elastic and inelastic scattering from CCl2 F2 in the electron energy range 0.5–10 eV and reported an excitation of the infra-red active CF2 and CCl2 stretching modes below 1 eV. However, a complete high-resolution VUV photo-absorption spectrum of CCl2 F2 has yet to be recorded. Nor have the absorption bands been conclusively assigned to discrete valence and Rydberg transitions. We have therefore made a definitive series of measurements to determine both the absolute photo-absorption cross-section of CCl2 F2 and to probe its electronic spectroscopy over the energy range from 5.5 to 11 eV (110 > k > 225 nm).

absorption path length of 25 cm and the transmitted intensity, It measured at 0.05 nm intervals using a photo-multiplier. The typical resolution was 0.075 nm. As well as the transmitted light intensity, the sample pressure (measured on an MKS 390HA Baratron capacitance manometer, 0.1%) and the synchrotron beam ring current were monitored at each wavelength. The sample cell is then evacuated and the radiation transmitted through the empty cell to record the background intensity, I0 . The absolute photo-absorption cross-section may then be calculated using the Beer–Lambert law I ¼ I0 expðnrxÞ;

ð1Þ

where n is the target gas number density, r is the absorption cross-section, and x is the path length [21]. The minimum wavelength for which data can be collected is determined by the entrance and exit windows in the gas cell. The present configuration uses an LiF entrance window and a CaF2 exit window in front of the photo-multiplier tube. Hence, the lowest wavelength at which reliable data can be collected is 115 nm. The cross-section was measured over a wide range of pressures to ensure that the data was free from any saturation effects [22]. The cross-section was measured over the pressure range 0.015–0.525 Torr with typical attenuation of less than 10%. Precautions were also taken in both experiments to avoid any second-order light effects. The CCl2 F2 gas was purchased from Fluorochem and has a purity of greater than 99%. The gas was used without further purification or treatment.

3. Results and discussion 2. Experimental apparatus

3.1. Spectroscopy of CCl2 F2

The photo-absorption spectra presented in this paper were recorded using synchrotron radiation at the ultraviolet vacuum line (UV1) of the Astrid facility [19] at Aarhus University, Denmark. The apparatus used has been described before [20] so only a brief description is given here. The beam is passed through a simple static gas cell with an

The total photo-absorption cross-section spectrum is shown in Fig. 1. The spectrum is composed of two absorption bands (each with low crosssections) between 6.0 and 8.5 eV and a series of sharp, more intense peaks above 9.5 eV. The feature close to 11.0 eV is only partially recorded due to the cut-off in the transmission of the CaF2

P. Lim~ao Vieira et al. / Chemical Physics Letters 364 (2002) 535–541

(a)

537

(b)

Fig. 1. The high-resolution total photo-absorption spectrum of CCl2 F2 recorded at the ASTRID Synchrotron ring facility. (a) Inset spectrum in the 5.5–9.0 eV absorption band of CCl2 F2 . (b) Inset spectrum in the 8.50–9.25 eV absorption band of CCl2 F2 .

window. The energy regions 5.5–9.0 eV and 8.50– 9.25 eV are displayed as inserts in Figs. 1a and b, respectively, the latter region shows a strong vibrational series. A brief review of the structure, geometry and properties of CCl2 F2 is helpful in the analysis and interpretation of the present data. The CCl2 F2 molecule has C2v symmetry with twofold axis and consequently only non-degenerate symmetry. The nine non-degenerate fundamental vibrational modes are classified in the symmetry types Cvib ¼ 4A1 þ A2 þ 2B1 þ 2B2 . The fundamental vibrational energies were summarised by Mann and Linder [18]; briefly m1 ¼ 0:1361 eV, m2 ¼ 0:0827 eV, m3 ¼ 0:0566 eV, m4 ¼ 0:0325 eV, m5 ¼ 0:0399 eV, m6 ¼ 0:1447 eV, m7 ¼ 0:0553 eV, m8 ¼ 0:1144 eV and m9 ¼ 0:0539 eV. The electronic configuration of the outer shells 2 2 of CCl2 F2 may be written as [23]: ð3a1 Þ ð3a2 Þ 2 2 2 2 2 2 ð3b1 Þ ð3b2 Þ ð4a1 Þ ð4a2 Þ ð4b1 Þ ð4b2 Þ , giving a valence shell independent-particle configuration for the ground state as 1 A1 . The four lowest ionisation potentials are 12.26, 12.53, 13.11 and 13.45 eV, for 2 B2 , 2 B1 , 2 A2 and 2 A1 , respectively, and the highest ionisation potentials are due to bonding orbitals of mainly C–Cl character [8]. For this molecule, the lowest ionisation potentials are related to the molecular orbitals

formed by the chlorine lone-pair character (n) [11]. The VUV absorption spectrum up to 9.92 eV is characterised by transitions from such orbitals [11]. The lowest absorption features observed in Fig. 1, in the range 6.0–8.5 eV, may be assigned to transitions involving valence shell type orbitals in the C–Cl bond, and may be characterised as ðr 4b2 Þ. Features between 8.5 and 11 eV have been interpreted as belonging to a Rydberg series converging to the CCl2 Fþ 2 ground state [10,15], arising from the transition of a lone-pair chlorine electron to the Rydberg orbitals 4s and 4p. In general, the higher energy range band (>10.2 eV) features exhibit no vibrational fine structure and are broadened due to predissociation and mutual overlap between bands. Each of these bands will be discussed in more detail below. 3.2. The 5.5–8.5 eV region Special attention has been devoted to VUV photo-dissociation studies of CCl2 F2 because excitation in this region results in prompt dissociation along the C–Cl bond due to the strong repulsive nature of the r excited states. Thus, Cl atoms that damage the ozone layer are released through solar photolysis of these electronic states (see Section 3.4).

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The broad and weak continuous absorption feature centred at 6.98 eV (178 nm) with a maximum cross-section of 1.90 Mb is a result of the excitation to an antibonding orbital along the C–Cl bond ðr 4b2 Þ of the CCl2 F2 molecule. At 8.14 eV (152 nm) there is a similar structure with a maximum cross-section of 1.06 Mb. This band has been assigned as one of the intervalence excitations ðr 4b2 Þ [23]. In agreement with Au et al. [16] our data show the 8.14 eV band with lower intensity than the 6.98 eV band, this in contradiction with the results of Ibuki et al. [23]. These weak absorptions can be described as C–Cl (n ! r ) transitions within the valence shell, where n is the outermost lone-pair orbital of Cl and r is the anti-bonding carbon chlorine r molecular orbital [11].

Table 1 Vibrational progressions in the 8.50–9.25 eV absorption band of CCl2 F2 (energies in eV) n

n0

nm9

DE ðm9 Þ

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

–

8.574 8.607 8.640 8.673 8.704 8.734 8.768 8.800 8.831 8.862 8.894 8.926 8.955 8.988 9.024 9.060 9.090

– 0.033 0.033 0.033 0.031 0.030 0.034 0.032 0.031 0.031 0.032 0.032 0.029 0.033 0.036 0.036 0.030

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

3.3. The 8.5–11.0 eV region Molecular excitation in this region arises from (n ! 4s) and (n ! 4p) Rydberg transitions of the Cl lone-pair electrons relating to the four lowest ionisation potentials found in accordance with the molecules symmetry. However it is shown in Table 2 that for the feature at 10.472 eV, a transition to a 3d orbital is possible and is in agreement with the calculated quantum defect (d). A strong vibrational structure is observed between 8.50 and 9.25 eV superimposed upon a broader band repulsive state. Such structure was first reported by Zobel and Duncan [15] albeit at a lower resolution. The higher resolution in the present results allows us to assign the structure to a vibrational progression in the CCl stretching mode, m9 ðb2 Þ, with a frequency 0.0323 eV of the excited state, in agreement with reference [22]. The present data also reveals a longer progression with 16 peaks being clearly identified with the lowest being assigned to n ¼ 0 (Table 1). The weak feature at 8.574 eV may however be ascribed to a hot band and in this case an alternative labelling, n0 may be given with the first term in the series allocated to 8.607 eV. This band is attributed to the ð4s 4b2 Þ. Rydberg transition corresponding to the 12.26 eV ionisation potential [23] (see Table 2).

A set of two broad bands appear at 9.360 and 9.641 eV. These features may be assigned as 4s Rydberg transitions, with ionisation potentials of 12.53 and 13.11 eV, respectively (see Table 2). The sharp peak at 9.801 eV (with a local maximum cross-section of 116 Mb) has been assigned to a 4p Rydberg transition (ionisation potential of 12.26 eV) and is in good agreement to the absolute differential oscillator strengths reported by Au et al. [16], the photo-absorption data of Ibuki et al. [23] and Jochims et al. [24] all of which are higher than the data of Doucet et al. [10]; while the features at 9.871 and 9.935 eV have been assigned as the 4p and 4s transitions, (ionisation potentials of 12.53 and 12.26 eV), respectively [23] (Table 2). The broad features at 10.472 and 10.781 eV may be interpreted as the 3d and 4p bands related to the 12.26 and 13.45 eV ionisation potentials, respectively. However, due to the cut-off in the transmission of the CaF2 window the position of the latter featureÕs maximum is unclear. In addition, two weak shoulders at 10.01 and 10.07–10.09 eV are observed here for the first time. These electronic transitions are accompanied by the vibrational excitation of the m2 normal mode with a frequency of 0.075 eV, which is quite similar to that observed in the ionic electronic ground state

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Table 2 Peak energy values, derived quantum defects and assignment of the Rydberg series converging to the fourth lowest ionisation potentials of CCl2 F2 (energies in eV) Quantum defect ðdÞ

Energy

Assignment

This work

[23]

This work

[23]

This work

[23]

8.926 9.801 10.472

8.940 9.802 10.473

1.98 1.65 0.24

1.98 1.65 0.24

4sr 4pk 3dp

4s 4p 3d

4b2 4b2 4b2

9.360 9.871

9.380 9.904

1.93 1.94

1.92 1.72

4sr 4pk

4s 4p

4b1 4b1

9.641 9.717 10.472

9.612 – 10.473

2.02 – 1.73

2.03 – 1.73

4sr 4sr þ m2 4pk

4s – 4p

4a2

9.935 10.010 10.080 10.781

9.857 – – 10.783

2.03 – – 1.74

2.05 – – 1.74

4sr 4sr þ m2 4sr þ 2m2 4pk

4s – – 4p

4a1

(600 cm1 ) [8], and for the latter (10.08 eV) tentatively assigned as 4sr þ 2m2 (see Table 2). Again the current absolute cross-section values are in good agreement with those derived by Au et al. [16], Ibuki et al. [23] and Gilbert et al. [11], but lower than Jochims et al. [24].

4a2

4a1

While there is a fairly substantial photolysis rate at the higher altitudes leading to a correspondingly short lifetime of a few days, at lower altitudes the

3.4. Cross-section and photolysis rate Absolute photo-absorption cross-sections have been reported by several groups, these have been reviewed and recommended values listed in the NASA atmospheric data base for k P 170 nm (e.g. [4]) [25]. The NASA recommended values are plotted in Fig. 2 and compared to the present results. The two data sets are in excellent agreement, thus we can have confidence in our VUV crosssectional values measured at higher energies. The photolysis rates of CCl2 F2 may then be evaluated as the product of the solar actinic flux [25] and molecular photo-absorption cross-section at different altitudes and wavelengths. The total rates shown in Fig. 3 are the summation over the wavelength range of these partial rates assuming that the quantum yield for photo-dissociation is assumed to be unity. The local lifetime to photolysis at a given altitude, also shown in Fig. 3, is thence simply reciprocal of the total photolysis rate. The lifetimes calculated are for a molecule with fixed altitude in a sunlit, clear sky atmosphere.

Fig. 2. The photo-absorption cross-section of the lowest band of CCl2 F2 showing data recorded at ASTRID compared with previous data from the NASA data book [24] (j).

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(a)

(b) Fig. 3. The photolysis rate and lifetime for CCl2 F2 as a function of altitude in the EarthÕs atmosphere.

lack of solar actinic flux at the absorption wavelengths of this molecule lead to an extremely long lifetime. It is this long lifetime at low altitudes coupled with its strong infrared absorption properties that make CCl2 F2 such a strong greenhouse gas. However the fairly short lifetimes at 40 and 50 km indicate that the molecule can be broken up fairly easily leading to liberation of the constituent halogens, which can then participate in ozone destruction in the stratospheric regions.

absorption spectrum of CCl2 F2 in the range 5.5–11 eV (225–110 nm) yet reported. The cross-sectional values measured at the lower end of the energy range studied are in excellent agreement with the existing data. Vibrational features at the higher energies are assigned for the first time. Using this data refined photolysis rates and local lifetimes of the molecule have been calculated for various altitudes in the atmosphere.

Acknowledgements 4. Conclusions The experimental results presented in this paper provide the highest resolution VUV photo-

P.V.L. acknowledges the Portuguese ÔFundacß~ao para a Ci^encia e a TecnologiaÕ for a Postgraduate Scholarship; S.E., P.K. and N.J.M. acknowledge

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