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Excitation and ionization of freon molecules. III. Absolute oscillator strengths for the photoabsorption (8.5–200 eV) and the ionic photofragmentation (11.5–70 eV) of CF2Cl2
Chemical Physics 15 1 ( 199 1) 357-370 Noah-Ho~~nd
Excitation and ionization of freon molecules. III. Absolute oscillator strengths for the photoabso~tion (8.5-200 eV) and the ionic photofragmentation ( 11S-70 eV) of CF&12
Received 7 August 1990
Absolute photoabsorption oscillator strengths (cross sections) for CF;Cl2 (freon 12) have been measured using dipole (e, e) spectroscopy in the equivalent photon energy range 8.5-200 eV. The ionic photofragmentation branching ratios and the photoionization efR&ncy have baen determined from t~~f-~t mass spectra collected using dipole (e, e+ion) coincidence spectroscopy at equivalent Photon energies f%omthe first ionization thmsbold up to 70 eV. Absolute partial photoionization oscillator stmngtbs for dissociative photoionization of CF&is have bean obtained from the phot~~~ti~n and photoio~tio~ dam. A dipole induced breahdown scheme for CFzClz under WV energetic radiation is proposed.
1. Iu~u~on
In a continuing series of quanti~tive experimental studies investigating the photoabsorption, photoionization and ionic photofragmentation of freon type molecules under energetic el~~oma~etic radiation, we have earlier reported results for freon 14 (CF,) (11 and freon 13 (CF&l) 121. We now report results for freon 12 (CF&la f includ~g the absolute photoabsorption oscillator strength from 8.5-200 eV, the ionic photof~~en~tion bmncb~ ratios, photoio~tion efficiency and partial oscillator strengths for the dissociative photoionixation channels from 11S-70 eV. The absolute photoabsorption cross sections (oscillator strengths) of CF#?,lz have been measured previously in the photon energy region 6-69 eV [ 310] and also in the region 124-270 eV [ 111. The photoion~tion efhciency has been determined [ IO] using neon resonance lamp radiation ( ss 16.75 eV). The appearance potentials [ 3,12- 16 ] and pbotoion ’ Permanent address: Institute for Chemical Research Kyoto University, Uji, Kyoto 6 11, Japan. ~3O~~lO4/91/$03.50
yield spectra [ 3,12,13,16] of the photofm~ents CF,C12+, @Cl;, CF,Cl+, CFCl+, CF$, Ccl+, Cl+ and CF+ and the possible dissociative processes leading to their production [ 3,12,16] have been investigated.
2. ExperImentaI The dipole (e, e+ion) apparatus and experimental procedures for this work are the same as those used for CF, in part I f 1 ] of our series study. BriefIy, absolute photoabsorption oscillator strengths for C&Cl2 ( 8. S-200 eV ) were obtained by Rethe-Rorn conversion and TRK sum rule no~~ization of background subtracted dipole (e, e) electron energy loss spectra, The resulting measurements, made at zero degree scattering angle and high impact energy (8 keV), i.e. negligible momentum transfer, yield a spectrum governed by dipole selection rules that is entirely equivalent to an absolute optical absorption spectrum. The energy resoiution was 1 eV fwhm. The photoabsorption rn~su~rnen~ for CF$Zl, were performed at intervals of 0.5, 1 and 10 eV in tbe ~niv~ent photou energy ranges 8.5-40,40-100 and 100-200 eV re-
6 1991- ElseviersCience Pub1ish~~B.V. (Non-Hound)
358
W. Zhang et al. / Excitation and ionization offreon molecules. III
spectively. Photoionization mass spectra were collected by coincident detection of time~f-ant (TOF) mass analyzed ions with forward scattered electrons of a particular energy loss (photon energy) in the dipole ( e, e + ion ) spectrometer. The TOF spectrometer has efficient draw-out fields to ensure equally efficient collection of all ions with up to 20 eV excess kinetic energy of fragmentation [ 17-201. Photoion mass spectra were obtained at intervals of 0.5, 1, 2 and 5 eV in the energy ranges 11S-30,30-40,40-50 and SO-70 eV respectively. CF2C12of stated minimum purity 99.0 mole% was purchased from Matheson Chemicals Ltd. in a lecture bottle. No significant impurities were detected in the TOF mass spectra. The accuracy of the absolute oscillator strength scales is estimated to be better than + 5%. An indication of the random errors is given by the smoothness of the data in the various continuum regions.
3. Results and discussion 3. I. Electronic structure
ofCF&
CFzClz is of C,, symmetry and the inde~ndent particle electronic configuration of the ground state may be written [ 2 1,221 as inner shells: (Cl ~s)~(F ~s)~(C l~)~(C12s)~(C12p)‘~ valence shell: ( la1 )‘( lb1)2(2a1)2(1b2)2(3a1 )2(2b2)2 (4ar )2(2b1)2(5a, )2(Ia2)2(3br)2(3b2)2 (6a,)2(2a2)2{4b~)2(4b2)2:
‘A,.
The vertical ionization energies of the outer valence orbitals (4b2, 4b,, 2a2, 6a,, 3b2, 3b1, la2, 5ai ( 2b, + 4a, ) , 2b2 and 3ar ) have been determined from high resolution He1 and He11 photoelectron spectroscopy @ES) to be 12.26, 12.53, 13.11, 13.45, 14.36, 15.9, 16.30, 16.9, 19.3,20.4and22.4eVrespectively [ 211, corresponding to the ion states R ‘B2, A 2B1, B2A2, C’A,, fi2B2, E2B,, e2A2, Ti2A,, (~2B,+~ZA,),~2B2andK2A,.Theverticalionization potentials for the inner valence orbitals
(( lb2+2a, ), lb, and la,) have been measured by PES using synchrotron radiation [ 22,231 and are estimated [ 231 to be 27.2, 38.6 and 41.1 eV respectively, ion corresponding to the states (f.2B2+I$2A,),~2B, andC)‘A,. 3.2. Photoabsorption oscillator strengths (cross sections) The absolute photoabsorption oscillator strengths obtained in the present work for the valence shell of CF2C12are listed in table 1 and are shown in figs. la (8.5-100eV) and lb (70-2OOeV). Earlierphotoabsorption me~urements [4,11] obtained using synchrotron radiation light sources are also shown in fgs. la and lb for comparison. In fig. lc the present photoabsorption data are also compared in the low energy region (8.5-36 eV) with previously reported results obtained using synchrotron radiation [ 3,4] and the helium Hopfield continuum [ 6 ] as light sources. The previous photoabsorption data derived from small momentum transfer electron impact measurements [ 5 ] have been digitized from the reported diagram and are also shown in fig. 1c. The absolute photoab~~tion values presently reported were obtained by Bethe-Born conversion and TRK sum-rule normalization of dipole (e, e) electron energy loss measurements. The details of the procedure have been described in part I of our series study [ 11. In the present work a total oscillator strength of 33.17, corresponding to 32 valence electrons plus an estimated oscillator strength of 1.17 to correct for the Pauli excluded transitions from the inner shells to the already occupied valence shell orbitals [ 2,24,25], was used to normalize the relative valence shell photoabsorption spectrum. The relative area above 190 eV, the highest energy in the present work at which the cont~bution to the total photoabsorption is solely from valence excitation, was estimated from extrapolation by fitting a curve of the form
to the experimental data over the energy range 70190 eV (fig. lb) and then integrating the function from 190 eV to infinity Cf=oscillator strength, E = excitation energy and A, B and C are constants ) . The least-squares fit (see fig. 1b) was obtained with
W. Bang et al. / Excitation and ionization ofjkon molecules. III Table I Absolute oscillator strengths for the total photoabsoqtion Photon energy
Oscillator strength (lo-’
(eV)
photoabsorption
8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 31.0 32.0 33.0 34.0 35.0 36.0
0.17 6.08 19.22 26.20 30.45 38.00 42.48 46.44 52.02 55.32 53.68 56.98 63.48 66.49 68.95 71.20 72.36 72.47 74.37 76.63 79.73 81.06 81.55 80.31 77.94 75.44 73.03 71.08 69.58 68.21 66.63 65.08 63.23 61.86 59.95 57.02 54.65 52.10 49.61 47.39 45.25 43.28 40.16 39.60 35.26 32.82 29.71 27.11 25.86 24.68
eV-‘)
CFCl:
CF,cI+
0.14 0.62 1.05 2.47 4.81 6.66 7.98 8.93 10.10 10.32 10.19 9.82 9.41 8.67 8.18 8.16 7.82 7.44 7.14 6.94 6.79 6.52 6.64 6.61 6.60 6.68 6.41 6.34 6.30 6.36 6.21 5.93 6.06 5.82 5.54 5.34 4.99 4.85 4.61
8.33 20.84 32.09 39.06 39.32 49.20 55.84 59.70 62.66 65.83 64.16 63.80 65.12 65.52 66.72 65.39 63.33 59.31 56.46 53.74 50.78 48.02 46.18 44.43 42.18 40.07 38.45 37.02 35.53 33.23 31.42 29.48 27.50 25.92 24.09 22.54 20.49 19.81 16.55 14.74 12.62 10.71 9.63 8.71
359
and the dissociative photoionization of CF&
l)
CCI;
CFCI+
0.34 0.88 1.47 2.33 2.87 3.16 3.36 3.67 3.73 3.84 4.01 4.00 3.99 3.53 3.27 2.95 2.79 2.55 2.45 2.20 2.20 2.04 1.83 1.85 1.77 1.61 1.49 1.53 1.49 1.42
CF:
0.32 1.01 2.36 4.59 6.93 8.50 8.72 8.69 8.90 8.90 8.84 9.16 9.04 8.85 8.64 8.35 7.94 7.53 7.14 6.89 6.42 6.61 5.72 5.72 5.32 5.21 4.79 4.58 4.30 3.99 3.88 3.73
CCl+
Cl+
CF+
F+
C+
CF2C12+
Photoionization efficiency vi
0.20 0.45 0.62 0.71 0.73 0.87 0.89 0.91 0.94 0.99 0.98 0.99 l.Ob’
0.21 0.20 0.35 0.50 0.64 0.63 0.78 0.86 0.91 0.93 1.05 1.00 1.01 1.05 1.05 1.03 0.89 0.85 0.83
0.31 0.44 0.70 0.69 0.95 1.08 0.95 1.09 1.33 1.25 1.15 1.35 1.45 1.58 1.66 1.81 1.97 2.10 2.08 2.15 2.18 2.17 2.24 2.26 2.34 2.54 2.70
0.45 0.79 0.97 1.14 1.72 2.32 2.68 3.37 3.82 4.19 4.82 4.75 4.61 4.42 4.32 4.24 3.97 3.87 3.64 3.44 3.49 3.02 2.83 2.45 2.30 2.21 2.09
0.15
0.09 0.23 0.24 0.36 0.40 0.45
(continued on next page)
W. Zhang et al. / Excitation and ionization offreon molecules. III
360 Table 1 (continued) Photon energy
Oscillator
(eV)
photoabsorption
37.0 38.0 39.0 40.0 42.0 44.0 46.0 48.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 85.0 90.0 95.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0 180.0 190.0 200.0
24.81 23.71 22.84 22.27 21.20 19.84 18.49 17.36 16.73 14.91 13.83 12.97 11.75 10.51 9.94 8.92 8.32 7.71 6.91 6.22 5.41 4.70 4.07 3.60 3.25 2.76 2.63 2.32 4.08
strength
( 10e2 eV-‘)
CFCl:
4.58 4.42 4.30 4.04 3.78 3.38 3.04 2.81 2.63 2.21 1.85 1.71 1.54
‘) u(Mb)= 1.0975x 102(df/dE) b, The photoionization eficiency details.
”
CF,Cl +
8.23 7.49 7.03 6.59 6.07 5.49 5.12 4.79 4.66 4.00 3.69 3.31 2.95
(eV-I). is constant
Ccl:
CFCl+
CF:
Ccl+
Cl+
CF+
F+
C+
CF*Cl*+
0.02 0.00 0.02 0.01 0.02 0.02 0.02
1.59 1.54 1.50 1.62 1.56 1.48 1.35 1.25 1.19 0.93 0.90 0.80 0.72
3.77 3.60 3.51 3.43 3.23 3.06 2.77 2.62 2.54 2.21 2.04 1.91 1.66
0.80 0.71 0.70 0.59 0.56 0.55 0.52 0.52 0.55 0.53 0.50 0.46 0.44
3.03 3.10 3.16 3.26 3.21 3.15 3.00 2.84 2.74 2.69 2.73 2.67 2.42
2.17 2.07 1.96 2.01 2.14 2.14 2.10 1.99 1.87 1.69 1.52 1.39 1.25
0.14 0.16 0.14 0.17 0.19 0.18 0.17 0.18 0.20 0.21 0.23 0.28 0.31
0.51 0.57 0.51 0.47 0.37 0.33 0.30 0.25 0.26 0.34 0.30 0.36 0.34
0.05 0.04 0.08 0.09 0.09 0.09 0.10 0.08 0.09 0.05 0.06 0.09
and therefore
assumed
A=38.348,B=5.9654~ 1O’and C= -2.6863~105. The extrapolated portion of the oscillator strength from 190 eV to infinity was found to be 20.7% of the total valence shell oscillator strength. The dipole (e, e) technique has constant energy resolution (1 eV fwhm) at all photon energies, whereas photoabsorption techniques have an energy resolution which becomes lower with increasing photon energy [ 261. This feature, i.e. the large difference in energy resolution between the present measurements and photoabsorption measurements [ 3,6] of varying resolution, complicates comparison of data at lower energies, particularly in the region of discrete excitation. Similar difficulty exists in compar-
Photoionization efficiency vi
to be unity above 17.5 eV, see section 3.3 and the insert to tig. la for
ing the present results of 1 eV fwhm resolution in the discrete excitation region with the data [ 51 derived from intermediate impact energy EELS (electron energy loss spectroscopy) of ~0.05 eV resolution. In particular meaningful comparisons of such data with the present work are not possible below 12.5 eV and the higher resolution data from refs. [3,5,6] are omitted from fig. lc in this region. The shape of the presently reported photoabsorption spectrum in the energy region below 25 eV (fig. lc) is similar to the shapes of the spectra reported in refs. [ 3-5 1. However, considerable variations exist in the absolute magnitudes of the various data sets. In the structured low energy region below 15 eV Rydberg transitions
361
W. Zhang et al. / Excitation and ionization o/freon molecules. III
l
1
This work
I
I
80
60
1
l Ph Abs. dipole (c.c)
-
Polynomial l
Ph Abs.
o
Ph Abs
fit dipole
0 Ph Abs [dj
(e,e)
[ll]
81
I
10
PHOTON ENERGY
20
30
(eV)
Fig. 1. Absolute photo&sorption oscillator strengths for CFzC12. (a) 8.5-100 eV (insert shows ionization efficiency). (c) 8.5-36 eV (expanded scale).
have been assigned to the various features. In particular, the peak at 13eV just discernible in the present low resolution work has been attributed to the Rydberg transitions 4b2-+3d and/or 3a2+4s [ 5,27,28]. The Rydberg transitions 3az+ 3d and/or 6b, + 5p and (2a,+5b1)+4s are the assignments for the structures in the 15-17 eV region [ 5,271 (unresolved in the present work). The maximum at 19.5 eV has been
4(
(b) 70-200 eV.
assigned to the 7a, +4s Rydberg transition [ 5,27,28], while the broad shoulder at x 24.5 eV has been suggested by Robin [ 281 to be due to a Rydberg transition originating from the carbon 2s orbitals. A term value of = 3 eV can be derived for this transition from the recently reported ionization potential of the C2s orbitals (2a,+ lb*) of CF2C12 [23]. The magnitude of this term value is consistent with the assignment
362
W. Zhang et al. /Excitation and ionization offreon molecules. III
(2a,, lbz)-+3s or (2a,, lbz)+3p, since the typical magnitudes of Rydberg term values are in the range 2.8-5.0 eV for the lowest s orbital and 2.0-2.8 for the lowest p orbital [ 291. It should be noted that the Rydberg transition assigned to a particular structure depends largely on the ionization energies taken from PES measurements (i.e. different ionization energies give rise to different term values (term value = ionization energy-excitation energy ) for the same structure and therefore the assignment of the particular structure can be different in utilizing ionization energies from different PES measurements). The rise in photoabsorption oscillator strength at 200 eV in fig. 1b is caused by excitation of Cl 2p inner shell electrons [ 27 ] . Above a photon energy of 24 eV the present results are in excellent quantitative agreement with those reported by Wu et al. [ 41. However, in the 16-24 eV region the cross sections reported by Wu et al. [4] are = 10% higher than the present work (figs. 1a and 1c ) . Similar discrepancies in this energy region have been observed in earlier comparisons between our results [ 1,2] and those of Lee et al. [ 301 for the molecules CF4 and CF,Cl. As pointed out in part II [ 21 of our series study, the discrepancies in the 16-24 eV energy region are most likely due to a systematic error in the use of a Sn film in the work of Wu et al. [ 41 and Lee et al. [ 301. The data of King and McConkey [ 5 ] and of Jochims et al. [ 3 1, which were single point normalized to the photoabsorption measurements reported by Person et al. [ 91 and Rebbert and Ausloos [ lo] respectively, are of similar shape but proportionally higher than the present results below 24 eV (fig. lc). In the 24-30 eV energy region the data of King and McConkey [ 5 ] approach the present data with increasing photon energy, however, above 30 eV their data fall below ours. The agreement between the present results and those of Gilbert et al. [ 61 is good in the region 12- 16 eV, but above 16 eV their data diverge rather drastically from all other measurements (fig. 1c ) . The absolute photoabsorption cross sections of CFIClz from 124 to 270 eV have been measured by Cole and Dexter [ 111 using synchrotron radiation. Agreement with the present work is good below 130 eV but the measurements diverge at the higher energies and exhibit a 20% difference at 190 eV (fig. 1b ) . In comparing and assessing the various data sets it should be remembered that the pres-
ent dipole (e, e) measurements, unlike the other measurements, have the advantage of an absolute scale determined by the TRK sum rule and thus they are further constrained by the requirements of the total oscillator strength sum. A broad feature of low intensity (fig. 1a ) centered at x 40 eV is observed. Similar but more prominent broad features were also seen in the photoabsorption oscillator strength spectra of CF., [ 1 ] and CF$l [ 21. They were interpreted [ 21 as being associated with inner valence ionization or possibly scattering (diffraction) of the outgoing (outer valence) photoelectrons by the neighboring atoms in the molecules [ 3 1,321. The presently observed feature (fig. la) probably also has similar origins. Final assignment must await detailed PES experiments and theoretical calculations. An even weaker feature at x 37 eV is also present in the photoabsorption oscillator strength spectra of CFC& [ 33 1. 3.3. Molecular and dissociative photoionization Time-of-flight mass spectra of CFzClz have been measured in the equivalent photon energy range 11 S70 eV using the dipole (e, e+ion) spectrometer. A typical TOF mass spectrum, obtained at 50 eV, is shown in fig. 2. The positive fragment (dissociative) CFCl+, CF:, Ccl+, ions CFClZ , CF*Cl+, Ccl:, Cl+, CF+, F+, C+ and the doubly charged ion CF2C12+ were detected. The molecular ion CF2C1$ was not observed in contrast to the situation for CF&l [ 2 1. Photoion branching ratios determined by integrating the mass peaks in the TGF spectra are reported diagrammatically in fig. 3 and numerically in table 2. The relative photoionization efficiency of CF2C12 was obtained from the ratio of the total coincident ion signal to the forward scattered energy loss signal at each energy loss in the dipole (e, e+ion) experiments [ 201. The present photoionization efficiency (a) reaches an approximately constant value above 17.5 eV photon energy. Making the reasonable assumption that at sufficiently high energy the photoionization efficiency is unity [ 20 1, we have the result that vi reaches 1.O at = 17.5 eV. The insert to fig. 1a shows the photoionization efficiency curve up to 70 eV and table 1 gives the vi values numerically. The previously reported ionization efficiency obtained
363
W. Zhang et al. / Excitationand ionizationoffreon molecules.III
7
CF2CI+
2
4
3 TIME
OF FLIGHT
5
(psec)
Fig. 2. Time-of-flight mass spectrum of CF2C12at 50 eV equivalent photon energy.
with the neon resonance lines at = 16.75 eV [lo] is also shown in the insert to fig. la and the agreement with our result is very good. Fig. 4 shows the absolute oscillator strengths for production of the fragment (dissociative) ions which are obtained by taking the triple product of the photoabsorption oscillator strength, the photoionization efficiency and the photoion branching ratio at each photon energy. The results are also listed numerically in table 1. Table 3 presents the fragment ion appearance potentials ( * 1 eV) measured in the present work and appearance potentials for the various processes calculated from thermodynamic data [ 34-36 1, assuming zero kinetic energy of fragmentation. Previously reported values are also shown in table 3 for comparison. The calculated thresholds are denoted by arrows on figs. 3 and 4 together with the vertical ionization energies for production of the electronic states ofCF#l: [21,23]. A comparison will be made among the branching ratio spectra for various photoions produced by pho-
toionization of CF,, CF,Cl, CFZCIZand CFC13in part IV [ 33 ] of our series study on freon type molecules. 3.4. The dipole induced breakdown of CF2C12 In photoionization once the photon energy exceeds the upper limit of the Franck-Condon region, the internal energy of the molecular ion is independent of the photon energy for a given electronic state and the remainder of the energy is carried by the photoelectron according to Einstein’s photoelectric equation. On the assumption that fragmentation ratios for dissociative ionization from each electronic state of the ion are constant when the photon energy is above the Franck-Condon region [ 191, the partial oscillator strengths for the production of any singly charged stable molecular or dissociative (fragment) ion should be a fixed linear combination of electronic state partial photoionization oscillator strengths at all photon energies. This general approach and possible exceptions such as autoionization, internal eonversion to other electronic ion states and multiple ioni-
364
W. Zhang et al. / Excitation and ionization offreon molecules. III
El
CF2C12
20
.. . .. . *. . /”
s 0 F
0 100
-...
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:“cuc”
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.
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c+ 1 CF,CI++_ . . . .* *a * . . .. I I 1 ’ 80 40 60 1
I
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2 .O * 0 , 0
(eV)
Fig. 3. Branching ratios for dissociative photoionization of CF,Cl,. The vertical amws repmsent expected thermodynamic appearance potentials (see table 3) and the vertical lines indicate the vertical ionization energies [ 21,231 for production of the electronic states of CF,Cl: .
zation have been discussed earlier [ 19 1. The dipole induced breakdown patterns of many small molecules have been investigated with considerable SUG cess by this type of analysis, for example, see refs. [ 1,2,19,37]. Since the molecular ion CF, Cl,’ is not observed on the time scale of the TOF mass spectrometer and because CF,Cl’ is the only ion detected (see table 3 ) below the fi state ionization potential of CFICll [ 2 11, the %, A, B and C electronic states of the molecular
ion must exclusively lead to production of the CF,Cl+ ion. Considering the Franck-Condon region of the b state [ 211 and the appearance potential 6f CFClf , it appears that CFCl,+ is the fragment ion produced from the f3 electronic state of the molecular ion. Similarly the fragment ions Ccl,+, CF,+ , Ccl+, Cl+, F+ andC+canbeformedfromthe(~+6),~,~~,~ and (E+ G) electronic states [ 2 1,23 ] respectively, and the ions CFCl+ and CF+ from the (fi+T) states [ 2 11. With these considerations in mind the follow-
Table 2 Photoion branching ratios for CF$&
~--
.~~
Photon energy
Branching ratio (I)
(eW
CFCI:
CF@+
0.28 1.11 1.73 3.79 6.81 9.40 11.12 12.of. 13.18 12.94 12.57 12.04 11.72 11.12 10.85 11.18 11.00 10.69 10.47 10.42 10.43 10.31 10.73 11.03 11.58 12.23 12.31 12.78 13.30 14.05 14.34 14.76 15.29 16.50 16.88 17.98 18.41 18.74 18.67 18.46 f 8.66 18.80 18.13 17.83 17.05 16.46 16.16 15.73 14.85 13.39 f 3.20 13.10
100.00 100.00 f00.00 mo.00 100.00 99.72 98.89 98.27 96.21 93.19 90.60 88.88 87.56 85.50 83.67 80.67 77.66 73.84 72.44 71.23 69.53 67.56 66.37 65.13 63.31 61.58 60.80 59.84 59.26 58.29 57.49 56.57 55.43 54.70 53.23 52.07 51.02 50.03 46.94 44.91 42.47 39.49 37.26 35.29 33.15 31.59 30.77 29.61 28.64 27.67 27.70 27.62 27.82 26.80 26.70 25.54 25.13
11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5
18.0 18.5 19.0 13.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.f 30.0 31.0 32.0 33.0 34.0 35.0 36.0 37.0 38.0 39,O 40.0 42.0 44.0 46.0 48.0 50.0 55.0 60.0 65.0 70.0
CCl:
0.09 0.00 0.10 0.08 0.11 0.15 0.17
CFCl+
CF,+
0.42 1.09 1.80 2.90 3.68 4.19 4.60 5.16 5.35 5.63 6.01 6.15 6.31 5.71 5.45 5.17 5.10 4.90 4.93 4.64 4.85 4.72 4.57 4.67 5.01 4.89 5.00 5.66 5.78 5.75 6.41 6.51 6.55 7.26 7.37 7.44 7.31 7.22 7.12 6.27 6.53 6.18 6.14
0.44 1.31 2.96 5.67 8.49 10.58 11.19 11.52 12.18 12.52 12.71 13.44 13.57 13.60 13.66 13.50 13.24 13.20 13.06 13.23 12.94 12.90 12.63 13.23 13.24 13.15 13.58 13.96 14.46 14.72 15.01 15.10 15.20 15.16 15.36 f5.40 15.25 15.40 15.00 15.07 15.18 14.80 14.74 14.69 14.13
CCl+
0.32 0.32 0.56 0.83 1.13 1.15 1.50 1.72 1.93 2.05 2.43 2.50 2.54 2.99 3.21 3.45 3.28 3.30 3.36 3.21 2.98 3.06 2.64 2.62 2.78 2.81 3.02 3.29 3.58 3.62 3.56 3.75
Cl+
0.39 0.56 0.93 0.95 1.33 1.55 1.39 1.64 2.04 1.98 1.87 2.25 2.54 2.88 3.19 3.65 4.16 4.64 4.81 5.35 5.50 6.16 6.82 7.59 8.65 9.83 10.93 12.21 13.09 13.83 14.65 15.14 15.87 16.20 16.37 16.39 18.04 19.75 20.60 20.59
CF+
0.56 1.02 1.28 1.56 2.42 3.33 3.93 5.05 S.87 6.62 7.80 7.93 8.09 8.09 8.29 8.55 8.37 8.55 8.41 8.56 8.81 8.56 8.63 8.26 8.48 8.54 8.46 8.73 8.13 8.60 9.04 10.08 IO.80 11.35 11.47 11.15 11.33 11.00 10.70 10.65
F+
Cc
0.60 0.57 0.69 0.61 0.78 0.90 0.89 0.94 1.05 1.17 1.39 1.65 2.13 2.67
0.26 0.69 0.79 1.31 1.54 1.83 2.07 2.40 2.23 2.11 1.75 1.64 1.64 1.43 1.55 2.26 2.17 2.80 2.88
CF&l*+
0.19 0.18 0.38 0.42 0.46 0.49 0.59 0.51 0.60 0.33 0.47 0.79
1 0 4 0
2
60 40
ii
20 0
0
0.5 0 0.5 0 CF,GI++ **a.
**
*
I
0.1
l l
l*
0
Fig, 4. Absolute oscillator stnqths for dimtciative photoionization of CF& The vertical arrows indicate expected ~e~~~ic appearance potenti& (see table 3 ) and the vertical lines indicate the vertical i~u~tion energies { 2 1,231 for production of the electronic states of CF2Cl:.
ing relatio~ips between the partial oscillator the formation of CFCl$, strengths for CF&!l’,CCl,*,CFCl+,CF$, CCl+, Cl-+, CF’, F+ andC4,andtho~for~ep~ctionof~e (R-t-A}, (fs+i5), fi, &t-F+@, (%+l), J, il &+I@ and (R + is) electronic states are found to provide a reasonably consistent ratio~~tion of the dipole induced breakdown of C&Cl2 witbin the energy range of the present data:
(d~~)(CFCl~)=(d~~)(~+O.l~(~~~~~)), (dSf~)(~~Cl*)=(d~dE)((~+A) f (~+~)+0.47(~+~~~)~
,
(dfldE)(CCl~)=(dfldE)(O.OlS(~J+~)), (d~/~)(CFClf)=(d~/~)(0.38(ir+T)), (dfldE)(CF~)=(dfl~)(0.38(E~e+c>>,
W. Zhangetal. / Excitation and ionization offeon molecuks. III
367
Table 3 Calcolated and measured appearance potentials for the production of charged species from CF&I, ProcesS
Appfzanmx
potential (eV)
calculated‘)
experimental this work b,
11.8
(l)CF,W
14.0 11.5 46 18.5
(21 CFCI: (3) CF&l+ (4) ccl2+ (5) CFCI+ (6) CF,’ +Cl, (7) CF$ +2Cl (8) CCl+ (9) CI++CF~+Q (10) Cl++CF+FCI (11) Cl++CF+F+CI (12) CI++C+FCl+F (13)CI+sC+F2+CI (14) Cl++C+ZF+Ci (lS)CF++FCl+cI (16) CF*+F+Q (17) CF++F+2Cl (18) F++CF+Q (19) F+ +CF+2Cl (20) F++C+FCl+Q (21) F++C+F+Cl~ (22) F+fC+F+2Cl (23) C++2FCI (24) C*+F2+Cl,, (25) C++FCl+FSCI (26) C+f2FfClz (27) C++F2+2Cl (28) C++2F+2Cl (25) CF@++??
14.6 i7.L 18.5 21.2 23.8 26.8 27.7 29.3 17.3 17.4 19.9 25.7 28.2 31.2 31.3 33.8 20.5 23.5 25.1 25.1 26.0 27.6
‘) Using thc~~h~i~ bt + I eV.
17.5 24
ref. [3]
ref. f 121
ref.
11.75 14.15 12.10
11.75 13.81 11.99
12.31
17.76 17.22
ref. [ 151
ref. [ 131
ref. [Id]
11.87 12.55
13.30 11.96 18.60 14.90
16.98 21.60 18.76
20
20
17.65 20.20
17.35 19.84
36
31 38
data from refs. 134-361 assuming zero kinetic energy of ~en~tion.
f@i’~) KXJ+ ) = (d.d.W (0.36K:) , (dfldE)(c1+)=(dsldE;)(3+0.64if +0.485(~+~)), (dflti)(CF+)=(dfld.Y)(0.62(~+~) +o.z(FJ+b))
[ 141
,
(d~/~)(C+)=~d~/~)(~~+~)+O.l(~+~)). Pig. 5 shows these breakdown relationships as a function of photon energy. The electronic state partial os-
cillator strengths used in fig_ 5 were obtained from the products of the presently measured total photoabsorption oscillator strength? (section 3.2) and the eiectronic state branching ratios obtained from PES measurements in the 27-41 eV [38] and 41-70 eV [ 231 regions. The latter work [23] includes measurements of the inner-valence photoelectron bands. Due to the unresolvability of certain features in the phot~l~ron spectra 123,381 the b~ching ratios for the production of the relevant states Were reported as sums, i.e. @+A), (&t-C), (B+P+is), @+I), (E+k) and (11;$+6), therefore the corresponding partial oscillator strengths are presented in
368
W. zhang et ai. / excitation and ionization qffreoonmolecules. III
I
CFCl +
cc1
+
0.5-
0
PHOTON ENERGY (eV) Fig. 5. Absolute oscillator strengths for the proposed dipole induced breakdown scheme of CF#&. (0 ) Present dipole (e, e+ion) experimental data. (-) Sums of electronic state partial oscillator strengths using the presently determined photoabsorption oscillator strengths and PES branching ratio data in the 27-41 eV [ 381 and 41-70 eV [ 231 regions. See section 3.4 for details.
Fig. 6. Proposed dipole induced breakdown scheme for the ionic photofragmentation of CF2C12.See text for details.
similar form in the above breakdown relationships. In spite ofthe uncertainties involved in such a simple rationale the co~~~den~ of the proposed scheme with the partial photoionization oscillator strengths (fig. 5) is reasonably good in terms of both shape and magnitude. Fig. 6 shows the presently proposed dipole induced breakdown scheme for CF,Cl,, A more detailed analysis of the dipole induced breakdown scheme of CFzClz may be obtained from photoelectron-photoion coincidence studies as a function of photon energy.
This work has been financially supported by NSERC (Canada). In addition WZ acknowledges a University of British Columbia Graduate Fellowship, GC is grateful for receiving a SERC( UK)/ NATO postdoctoral fellowship and TI thanks the Yamada Science Foundation (Japan ) for support. We should also like to thank Brian Greene (Electronics shop, ~p~ment of Chemistry, UBC) for assistance with the maintenance of the dipole (e, e+ ion) spectrometer.
References [ I ] W. Zhang, G. Cooper, T. Ibuki and C.E. Brion, Chem. Phys. 137 (1989) 391. [ 2 J W. Zhang C. Cooper, T. Ibuki and C.E. Brian, Cbem. Phys. 151 (t991) 343. [ 3 ] H.W. Jochims, W. Lohr and H. Baumg&tel, Ber. BunsentIes. Physik. Chem. 80 ( 1976) 130. [4]C.Y.R. Wu, L.C. Lee and D.L. Judge, J. Chem. Phys. 71 (1979) 5221. [S]G.~.~ng~dJ.W.M~o~ey,J.Pb~.B ll(1978) 186t. [ 61 R. Gilbert, P. Sauvageau and C. Sandor& J. Cbem. Phys. 60 ( 1974) 4820. [ 7 ] J. Doucet, P. Sauvageau and C. Sandorfy, J. Chem. Phys. 58 (1973) 3708. [ 81 T. Ibuki, A. Hiraya and K. Shobatake, J. Chem. Phys. 90 ( 1989) 6290. [ 91 J.C. Person, D.E. FowIer and P.P. Nicole, Argonne National Laboratory Report ANL75-60, part 1q [IO] R.E. Rebbert and P. Ausmos, J. Res. Natl. Bur. Std. 75 A (1971) 481. [ II] B.E. Cole and R.N. Dexter, J. Quant. Spectry. Radiative Transfer 19 (1978) 303.
[ 121 J.M. Ajello, W.T. Huntress Jr. and P. Rayermann, J. Chem. Phys. 64 (1976) 4746. [ 131 EC.Y. WangandG.E. Len&Ann. IsraelPbys Sot 6 (1984) 210. [ 141 K. Watanabe, T. Nakayama and J. Mottl, J. Quant. Spectry. Radiative Transfer 2 (1962) 369. [IS] L.M. Leyhmd, J.R. Majer and J.C. Robb, Trans. Faraday Sot. 66 ( 1970) 898. f 16) H. Schenk, H. Oertel and H. BaumgSrtei, Ber. Bunsenges. Physik. Chem. 83 (1979) 683. [ 17 JC. Backx, G.R. Wig&, R.R. To1 and M.J. van der Wief, J. Phys. I3 8 (1975) 3007. [ I81 C. Backx and M.J. van der Wiel. J. Pbys. B 8 (1975) 3020. [ 191 F. Carnovale and C.E. Brian, Chem. Phys. 74 (1983) 253, and references therein. 1201 J. Gal&her, C.E. &ion, J.A.R. Samson and P.W. endow J. Phys. Chem. Ref. Data I7 (1988) 9. [ 211 T. Cvitas, H. Gusten and L. Klasinc, J. Chem. Phys. 67 (1977) 2687. [ 221 A.W. Potts, 1. Novak, F. Quinn, G.V. Marr, B. D&son, 1.H. Hillier and J.B. West, J. Phys. B 18 ( 1.985)3177. [23] G. Cooper, W. Zhang, C.E. Brion and K.H. Tan, C&em. Phys. 145 (1990) 117. [24] M. Inokuti, private communication; J.L. Dehmer, M. Inokuti and R.P. Saxon, Phys. Rev. A 12 (1975) 102; M. Inoknti, J.L. Dehmer, T. Baer and J.D. Hanson, Phys. Rev.A23(1981)95. [ZS] J.A. Wheeler and J.A. Bearden, Phys. Rev. 46 (1934) 755,
[ 261 C.E. Brion, ~mments At. Mol. Phys. 16 ( 1985 ) 249, and references therein; C.E. Brion and A. Hamnett, Advan. Chem. Phys. 45 ( 1981) 1. [27] W. Zhang, T. Ibuki and C.E. Brian, Chem. Phys., to be published. [28 ] M-B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 3 (Academic Press, New York, 1985) p. 133, 1291 M.B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 1 (Academic Press, New York, 1974). [30] L.C. Lee, E. Phillips and D.L. Judge, J. Chem. Phys. 67 (1977) 1237. 1311 G.M. Bsncroft, J.D. Boxek, J.N. Cutler and K.H. Tao, J. Electron Spectry. 47 ( 1988) 187; J.S. Tse, J. Chem. Phys. 89 ( 1988) 920; B.M. Addison-Jones, K.H. Tan, B.W. Yates, J.N. Cutler, G.M. Bancroft and J.S. Tse, J. Electron Spectry. 48 ( 1989) 155. [ 32 ] CR. Natoli, in: Springer Series in Chemical Physics, Vol. 27. EXAFS and Near Edge Structure, eds. A. Bianconi, L. Incouia and S. Striprich (Springer, Be&n, f983) p. 43; A. Bianconi, M. Dell-Ariccia, A. Gatgano and C.R. Natoh, in: Springer Series in Chemical Physics, Vol. 27. EXAFS and Near Edge S~ctu~, eds. 14. Bianconi, I,,. Incouia and S. Striprich (Springer, Berlin, 1983) p. 57.
370 [ 33
W. Zhang
et al. /Excitation
and ionization
] W. Zhang, G. Cooper, T. Ibuki and C.E. Brion, Chem. Phys.,
to be published. [34] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17 No. 1 (1988). [ 351 H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data 6 ( 1977 ) Suppl. 1.
offreon molecules. III
[36]D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, SM. Bailey and R.H. Schumm, NBS Tech. Note 270-3 (US Government Printing Office, Washington, 1968). [ 37 ] Y. Ii&, F. Carnovale, S. Daviel and C.E. Brion, Chem. Phys. 105 (1986) 21 I, and references therein. [38] I. Novak, J.M. Benson and A.W. Potts, J. Electron Spectry. 41 (1986) 175.
Excitation and ionization of freon molecules. III. Absolute oscillator strengths for the photoabso~tion (8.5-200 eV) and the ionic photofragmentation ( 11S-70 eV) of CF&12
Received 7 August 1990
Absolute photoabsorption oscillator strengths (cross sections) for CF;Cl2 (freon 12) have been measured using dipole (e, e) spectroscopy in the equivalent photon energy range 8.5-200 eV. The ionic photofragmentation branching ratios and the photoionization efR&ncy have baen determined from t~~f-~t mass spectra collected using dipole (e, e+ion) coincidence spectroscopy at equivalent Photon energies f%omthe first ionization thmsbold up to 70 eV. Absolute partial photoionization oscillator stmngtbs for dissociative photoionization of CF&is have bean obtained from the phot~~~ti~n and photoio~tio~ dam. A dipole induced breahdown scheme for CFzClz under WV energetic radiation is proposed.
1. Iu~u~on
In a continuing series of quanti~tive experimental studies investigating the photoabsorption, photoionization and ionic photofragmentation of freon type molecules under energetic el~~oma~etic radiation, we have earlier reported results for freon 14 (CF,) (11 and freon 13 (CF&l) 121. We now report results for freon 12 (CF&la f includ~g the absolute photoabsorption oscillator strength from 8.5-200 eV, the ionic photof~~en~tion bmncb~ ratios, photoio~tion efficiency and partial oscillator strengths for the dissociative photoionixation channels from 11S-70 eV. The absolute photoabsorption cross sections (oscillator strengths) of CF#?,lz have been measured previously in the photon energy region 6-69 eV [ 310] and also in the region 124-270 eV [ 111. The photoion~tion efhciency has been determined [ IO] using neon resonance lamp radiation ( ss 16.75 eV). The appearance potentials [ 3,12- 16 ] and pbotoion ’ Permanent address: Institute for Chemical Research Kyoto University, Uji, Kyoto 6 11, Japan. ~3O~~lO4/91/$03.50
yield spectra [ 3,12,13,16] of the photofm~ents CF,C12+, @Cl;, CF,Cl+, CFCl+, CF$, Ccl+, Cl+ and CF+ and the possible dissociative processes leading to their production [ 3,12,16] have been investigated.
2. ExperImentaI The dipole (e, e+ion) apparatus and experimental procedures for this work are the same as those used for CF, in part I f 1 ] of our series study. BriefIy, absolute photoabsorption oscillator strengths for C&Cl2 ( 8. S-200 eV ) were obtained by Rethe-Rorn conversion and TRK sum rule no~~ization of background subtracted dipole (e, e) electron energy loss spectra, The resulting measurements, made at zero degree scattering angle and high impact energy (8 keV), i.e. negligible momentum transfer, yield a spectrum governed by dipole selection rules that is entirely equivalent to an absolute optical absorption spectrum. The energy resoiution was 1 eV fwhm. The photoabsorption rn~su~rnen~ for CF$Zl, were performed at intervals of 0.5, 1 and 10 eV in tbe ~niv~ent photou energy ranges 8.5-40,40-100 and 100-200 eV re-
6 1991- ElseviersCience Pub1ish~~B.V. (Non-Hound)
358
W. Zhang et al. / Excitation and ionization offreon molecules. III
spectively. Photoionization mass spectra were collected by coincident detection of time~f-ant (TOF) mass analyzed ions with forward scattered electrons of a particular energy loss (photon energy) in the dipole ( e, e + ion ) spectrometer. The TOF spectrometer has efficient draw-out fields to ensure equally efficient collection of all ions with up to 20 eV excess kinetic energy of fragmentation [ 17-201. Photoion mass spectra were obtained at intervals of 0.5, 1, 2 and 5 eV in the energy ranges 11S-30,30-40,40-50 and SO-70 eV respectively. CF2C12of stated minimum purity 99.0 mole% was purchased from Matheson Chemicals Ltd. in a lecture bottle. No significant impurities were detected in the TOF mass spectra. The accuracy of the absolute oscillator strength scales is estimated to be better than + 5%. An indication of the random errors is given by the smoothness of the data in the various continuum regions.
3. Results and discussion 3. I. Electronic structure
ofCF&
CFzClz is of C,, symmetry and the inde~ndent particle electronic configuration of the ground state may be written [ 2 1,221 as inner shells: (Cl ~s)~(F ~s)~(C l~)~(C12s)~(C12p)‘~ valence shell: ( la1 )‘( lb1)2(2a1)2(1b2)2(3a1 )2(2b2)2 (4ar )2(2b1)2(5a, )2(Ia2)2(3br)2(3b2)2 (6a,)2(2a2)2{4b~)2(4b2)2:
‘A,.
The vertical ionization energies of the outer valence orbitals (4b2, 4b,, 2a2, 6a,, 3b2, 3b1, la2, 5ai ( 2b, + 4a, ) , 2b2 and 3ar ) have been determined from high resolution He1 and He11 photoelectron spectroscopy @ES) to be 12.26, 12.53, 13.11, 13.45, 14.36, 15.9, 16.30, 16.9, 19.3,20.4and22.4eVrespectively [ 211, corresponding to the ion states R ‘B2, A 2B1, B2A2, C’A,, fi2B2, E2B,, e2A2, Ti2A,, (~2B,+~ZA,),~2B2andK2A,.Theverticalionization potentials for the inner valence orbitals
(( lb2+2a, ), lb, and la,) have been measured by PES using synchrotron radiation [ 22,231 and are estimated [ 231 to be 27.2, 38.6 and 41.1 eV respectively, ion corresponding to the states (f.2B2+I$2A,),~2B, andC)‘A,. 3.2. Photoabsorption oscillator strengths (cross sections) The absolute photoabsorption oscillator strengths obtained in the present work for the valence shell of CF2C12are listed in table 1 and are shown in figs. la (8.5-100eV) and lb (70-2OOeV). Earlierphotoabsorption me~urements [4,11] obtained using synchrotron radiation light sources are also shown in fgs. la and lb for comparison. In fig. lc the present photoabsorption data are also compared in the low energy region (8.5-36 eV) with previously reported results obtained using synchrotron radiation [ 3,4] and the helium Hopfield continuum [ 6 ] as light sources. The previous photoabsorption data derived from small momentum transfer electron impact measurements [ 5 ] have been digitized from the reported diagram and are also shown in fig. 1c. The absolute photoab~~tion values presently reported were obtained by Bethe-Born conversion and TRK sum-rule normalization of dipole (e, e) electron energy loss measurements. The details of the procedure have been described in part I of our series study [ 11. In the present work a total oscillator strength of 33.17, corresponding to 32 valence electrons plus an estimated oscillator strength of 1.17 to correct for the Pauli excluded transitions from the inner shells to the already occupied valence shell orbitals [ 2,24,25], was used to normalize the relative valence shell photoabsorption spectrum. The relative area above 190 eV, the highest energy in the present work at which the cont~bution to the total photoabsorption is solely from valence excitation, was estimated from extrapolation by fitting a curve of the form
to the experimental data over the energy range 70190 eV (fig. lb) and then integrating the function from 190 eV to infinity Cf=oscillator strength, E = excitation energy and A, B and C are constants ) . The least-squares fit (see fig. 1b) was obtained with
W. Bang et al. / Excitation and ionization ofjkon molecules. III Table I Absolute oscillator strengths for the total photoabsoqtion Photon energy
Oscillator strength (lo-’
(eV)
photoabsorption
8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 31.0 32.0 33.0 34.0 35.0 36.0
0.17 6.08 19.22 26.20 30.45 38.00 42.48 46.44 52.02 55.32 53.68 56.98 63.48 66.49 68.95 71.20 72.36 72.47 74.37 76.63 79.73 81.06 81.55 80.31 77.94 75.44 73.03 71.08 69.58 68.21 66.63 65.08 63.23 61.86 59.95 57.02 54.65 52.10 49.61 47.39 45.25 43.28 40.16 39.60 35.26 32.82 29.71 27.11 25.86 24.68
eV-‘)
CFCl:
CF,cI+
0.14 0.62 1.05 2.47 4.81 6.66 7.98 8.93 10.10 10.32 10.19 9.82 9.41 8.67 8.18 8.16 7.82 7.44 7.14 6.94 6.79 6.52 6.64 6.61 6.60 6.68 6.41 6.34 6.30 6.36 6.21 5.93 6.06 5.82 5.54 5.34 4.99 4.85 4.61
8.33 20.84 32.09 39.06 39.32 49.20 55.84 59.70 62.66 65.83 64.16 63.80 65.12 65.52 66.72 65.39 63.33 59.31 56.46 53.74 50.78 48.02 46.18 44.43 42.18 40.07 38.45 37.02 35.53 33.23 31.42 29.48 27.50 25.92 24.09 22.54 20.49 19.81 16.55 14.74 12.62 10.71 9.63 8.71
359
and the dissociative photoionization of CF&
l)
CCI;
CFCI+
0.34 0.88 1.47 2.33 2.87 3.16 3.36 3.67 3.73 3.84 4.01 4.00 3.99 3.53 3.27 2.95 2.79 2.55 2.45 2.20 2.20 2.04 1.83 1.85 1.77 1.61 1.49 1.53 1.49 1.42
CF:
0.32 1.01 2.36 4.59 6.93 8.50 8.72 8.69 8.90 8.90 8.84 9.16 9.04 8.85 8.64 8.35 7.94 7.53 7.14 6.89 6.42 6.61 5.72 5.72 5.32 5.21 4.79 4.58 4.30 3.99 3.88 3.73
CCl+
Cl+
CF+
F+
C+
CF2C12+
Photoionization efficiency vi
0.20 0.45 0.62 0.71 0.73 0.87 0.89 0.91 0.94 0.99 0.98 0.99 l.Ob’
0.21 0.20 0.35 0.50 0.64 0.63 0.78 0.86 0.91 0.93 1.05 1.00 1.01 1.05 1.05 1.03 0.89 0.85 0.83
0.31 0.44 0.70 0.69 0.95 1.08 0.95 1.09 1.33 1.25 1.15 1.35 1.45 1.58 1.66 1.81 1.97 2.10 2.08 2.15 2.18 2.17 2.24 2.26 2.34 2.54 2.70
0.45 0.79 0.97 1.14 1.72 2.32 2.68 3.37 3.82 4.19 4.82 4.75 4.61 4.42 4.32 4.24 3.97 3.87 3.64 3.44 3.49 3.02 2.83 2.45 2.30 2.21 2.09
0.15
0.09 0.23 0.24 0.36 0.40 0.45
(continued on next page)
W. Zhang et al. / Excitation and ionization offreon molecules. III
360 Table 1 (continued) Photon energy
Oscillator
(eV)
photoabsorption
37.0 38.0 39.0 40.0 42.0 44.0 46.0 48.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 85.0 90.0 95.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0 180.0 190.0 200.0
24.81 23.71 22.84 22.27 21.20 19.84 18.49 17.36 16.73 14.91 13.83 12.97 11.75 10.51 9.94 8.92 8.32 7.71 6.91 6.22 5.41 4.70 4.07 3.60 3.25 2.76 2.63 2.32 4.08
strength
( 10e2 eV-‘)
CFCl:
4.58 4.42 4.30 4.04 3.78 3.38 3.04 2.81 2.63 2.21 1.85 1.71 1.54
‘) u(Mb)= 1.0975x 102(df/dE) b, The photoionization eficiency details.
”
CF,Cl +
8.23 7.49 7.03 6.59 6.07 5.49 5.12 4.79 4.66 4.00 3.69 3.31 2.95
(eV-I). is constant
Ccl:
CFCl+
CF:
Ccl+
Cl+
CF+
F+
C+
CF*Cl*+
0.02 0.00 0.02 0.01 0.02 0.02 0.02
1.59 1.54 1.50 1.62 1.56 1.48 1.35 1.25 1.19 0.93 0.90 0.80 0.72
3.77 3.60 3.51 3.43 3.23 3.06 2.77 2.62 2.54 2.21 2.04 1.91 1.66
0.80 0.71 0.70 0.59 0.56 0.55 0.52 0.52 0.55 0.53 0.50 0.46 0.44
3.03 3.10 3.16 3.26 3.21 3.15 3.00 2.84 2.74 2.69 2.73 2.67 2.42
2.17 2.07 1.96 2.01 2.14 2.14 2.10 1.99 1.87 1.69 1.52 1.39 1.25
0.14 0.16 0.14 0.17 0.19 0.18 0.17 0.18 0.20 0.21 0.23 0.28 0.31
0.51 0.57 0.51 0.47 0.37 0.33 0.30 0.25 0.26 0.34 0.30 0.36 0.34
0.05 0.04 0.08 0.09 0.09 0.09 0.10 0.08 0.09 0.05 0.06 0.09
and therefore
assumed
A=38.348,B=5.9654~ 1O’and C= -2.6863~105. The extrapolated portion of the oscillator strength from 190 eV to infinity was found to be 20.7% of the total valence shell oscillator strength. The dipole (e, e) technique has constant energy resolution (1 eV fwhm) at all photon energies, whereas photoabsorption techniques have an energy resolution which becomes lower with increasing photon energy [ 261. This feature, i.e. the large difference in energy resolution between the present measurements and photoabsorption measurements [ 3,6] of varying resolution, complicates comparison of data at lower energies, particularly in the region of discrete excitation. Similar difficulty exists in compar-
Photoionization efficiency vi
to be unity above 17.5 eV, see section 3.3 and the insert to tig. la for
ing the present results of 1 eV fwhm resolution in the discrete excitation region with the data [ 51 derived from intermediate impact energy EELS (electron energy loss spectroscopy) of ~0.05 eV resolution. In particular meaningful comparisons of such data with the present work are not possible below 12.5 eV and the higher resolution data from refs. [3,5,6] are omitted from fig. lc in this region. The shape of the presently reported photoabsorption spectrum in the energy region below 25 eV (fig. lc) is similar to the shapes of the spectra reported in refs. [ 3-5 1. However, considerable variations exist in the absolute magnitudes of the various data sets. In the structured low energy region below 15 eV Rydberg transitions
361
W. Zhang et al. / Excitation and ionization o/freon molecules. III
l
1
This work
I
I
80
60
1
l Ph Abs. dipole (c.c)
-
Polynomial l
Ph Abs.
o
Ph Abs
fit dipole
0 Ph Abs [dj
(e,e)
[ll]
81
I
10
PHOTON ENERGY
20
30
(eV)
Fig. 1. Absolute photo&sorption oscillator strengths for CFzC12. (a) 8.5-100 eV (insert shows ionization efficiency). (c) 8.5-36 eV (expanded scale).
have been assigned to the various features. In particular, the peak at 13eV just discernible in the present low resolution work has been attributed to the Rydberg transitions 4b2-+3d and/or 3a2+4s [ 5,27,28]. The Rydberg transitions 3az+ 3d and/or 6b, + 5p and (2a,+5b1)+4s are the assignments for the structures in the 15-17 eV region [ 5,271 (unresolved in the present work). The maximum at 19.5 eV has been
4(
(b) 70-200 eV.
assigned to the 7a, +4s Rydberg transition [ 5,27,28], while the broad shoulder at x 24.5 eV has been suggested by Robin [ 281 to be due to a Rydberg transition originating from the carbon 2s orbitals. A term value of = 3 eV can be derived for this transition from the recently reported ionization potential of the C2s orbitals (2a,+ lb*) of CF2C12 [23]. The magnitude of this term value is consistent with the assignment
362
W. Zhang et al. /Excitation and ionization offreon molecules. III
(2a,, lbz)-+3s or (2a,, lbz)+3p, since the typical magnitudes of Rydberg term values are in the range 2.8-5.0 eV for the lowest s orbital and 2.0-2.8 for the lowest p orbital [ 291. It should be noted that the Rydberg transition assigned to a particular structure depends largely on the ionization energies taken from PES measurements (i.e. different ionization energies give rise to different term values (term value = ionization energy-excitation energy ) for the same structure and therefore the assignment of the particular structure can be different in utilizing ionization energies from different PES measurements). The rise in photoabsorption oscillator strength at 200 eV in fig. 1b is caused by excitation of Cl 2p inner shell electrons [ 27 ] . Above a photon energy of 24 eV the present results are in excellent quantitative agreement with those reported by Wu et al. [ 41. However, in the 16-24 eV region the cross sections reported by Wu et al. [4] are = 10% higher than the present work (figs. 1a and 1c ) . Similar discrepancies in this energy region have been observed in earlier comparisons between our results [ 1,2] and those of Lee et al. [ 301 for the molecules CF4 and CF,Cl. As pointed out in part II [ 21 of our series study, the discrepancies in the 16-24 eV energy region are most likely due to a systematic error in the use of a Sn film in the work of Wu et al. [ 41 and Lee et al. [ 301. The data of King and McConkey [ 5 ] and of Jochims et al. [ 3 1, which were single point normalized to the photoabsorption measurements reported by Person et al. [ 91 and Rebbert and Ausloos [ lo] respectively, are of similar shape but proportionally higher than the present results below 24 eV (fig. lc). In the 24-30 eV energy region the data of King and McConkey [ 5 ] approach the present data with increasing photon energy, however, above 30 eV their data fall below ours. The agreement between the present results and those of Gilbert et al. [ 61 is good in the region 12- 16 eV, but above 16 eV their data diverge rather drastically from all other measurements (fig. 1c ) . The absolute photoabsorption cross sections of CFIClz from 124 to 270 eV have been measured by Cole and Dexter [ 111 using synchrotron radiation. Agreement with the present work is good below 130 eV but the measurements diverge at the higher energies and exhibit a 20% difference at 190 eV (fig. 1b ) . In comparing and assessing the various data sets it should be remembered that the pres-
ent dipole (e, e) measurements, unlike the other measurements, have the advantage of an absolute scale determined by the TRK sum rule and thus they are further constrained by the requirements of the total oscillator strength sum. A broad feature of low intensity (fig. 1a ) centered at x 40 eV is observed. Similar but more prominent broad features were also seen in the photoabsorption oscillator strength spectra of CF., [ 1 ] and CF$l [ 21. They were interpreted [ 21 as being associated with inner valence ionization or possibly scattering (diffraction) of the outgoing (outer valence) photoelectrons by the neighboring atoms in the molecules [ 3 1,321. The presently observed feature (fig. la) probably also has similar origins. Final assignment must await detailed PES experiments and theoretical calculations. An even weaker feature at x 37 eV is also present in the photoabsorption oscillator strength spectra of CFC& [ 33 1. 3.3. Molecular and dissociative photoionization Time-of-flight mass spectra of CFzClz have been measured in the equivalent photon energy range 11 S70 eV using the dipole (e, e+ion) spectrometer. A typical TOF mass spectrum, obtained at 50 eV, is shown in fig. 2. The positive fragment (dissociative) CFCl+, CF:, Ccl+, ions CFClZ , CF*Cl+, Ccl:, Cl+, CF+, F+, C+ and the doubly charged ion CF2C12+ were detected. The molecular ion CF2C1$ was not observed in contrast to the situation for CF&l [ 2 1. Photoion branching ratios determined by integrating the mass peaks in the TGF spectra are reported diagrammatically in fig. 3 and numerically in table 2. The relative photoionization efficiency of CF2C12 was obtained from the ratio of the total coincident ion signal to the forward scattered energy loss signal at each energy loss in the dipole (e, e+ion) experiments [ 201. The present photoionization efficiency (a) reaches an approximately constant value above 17.5 eV photon energy. Making the reasonable assumption that at sufficiently high energy the photoionization efficiency is unity [ 20 1, we have the result that vi reaches 1.O at = 17.5 eV. The insert to fig. 1a shows the photoionization efficiency curve up to 70 eV and table 1 gives the vi values numerically. The previously reported ionization efficiency obtained
363
W. Zhang et al. / Excitationand ionizationoffreon molecules.III
7
CF2CI+
2
4
3 TIME
OF FLIGHT
5
(psec)
Fig. 2. Time-of-flight mass spectrum of CF2C12at 50 eV equivalent photon energy.
with the neon resonance lines at = 16.75 eV [lo] is also shown in the insert to fig. la and the agreement with our result is very good. Fig. 4 shows the absolute oscillator strengths for production of the fragment (dissociative) ions which are obtained by taking the triple product of the photoabsorption oscillator strength, the photoionization efficiency and the photoion branching ratio at each photon energy. The results are also listed numerically in table 1. Table 3 presents the fragment ion appearance potentials ( * 1 eV) measured in the present work and appearance potentials for the various processes calculated from thermodynamic data [ 34-36 1, assuming zero kinetic energy of fragmentation. Previously reported values are also shown in table 3 for comparison. The calculated thresholds are denoted by arrows on figs. 3 and 4 together with the vertical ionization energies for production of the electronic states ofCF#l: [21,23]. A comparison will be made among the branching ratio spectra for various photoions produced by pho-
toionization of CF,, CF,Cl, CFZCIZand CFC13in part IV [ 33 ] of our series study on freon type molecules. 3.4. The dipole induced breakdown of CF2C12 In photoionization once the photon energy exceeds the upper limit of the Franck-Condon region, the internal energy of the molecular ion is independent of the photon energy for a given electronic state and the remainder of the energy is carried by the photoelectron according to Einstein’s photoelectric equation. On the assumption that fragmentation ratios for dissociative ionization from each electronic state of the ion are constant when the photon energy is above the Franck-Condon region [ 191, the partial oscillator strengths for the production of any singly charged stable molecular or dissociative (fragment) ion should be a fixed linear combination of electronic state partial photoionization oscillator strengths at all photon energies. This general approach and possible exceptions such as autoionization, internal eonversion to other electronic ion states and multiple ioni-
364
W. Zhang et al. / Excitation and ionization offreon molecules. III
El
CF2C12
20
.. . .. . *. . /”
s 0 F
0 100
-...
.
:“cuc”
10
fpf”i”
CFQ+ .
.
.
. +
,’ I
I
I
I
I
: w .
I
I
iz g
c
. :..*._*..--
..
.
.
.
-4
*
-2
I
I
I
CF2CI+ .
2 ii? I
f
ccl+
I I ira
11)/I! I [ I I rEEBi if ti.P)
. . . *f
.’ .*
I
.
.
.
I
.O
*
- 20 cl+ -
II I.....
5o
-*.,
-...
tpf lip
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.
.
,
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I
I
m
.-.
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.
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1
1
CFCI+
:
0
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.
.
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... 1
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. I
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:
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.# I
I
.
.
1 I’
...
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0
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.
*
ccl*+
1
I
I
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..
I
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.
i? 5
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81 ENERGY
I
*?
.....
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.
.
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I
.
.
.
, .
F+
c+ 1 CF,CI++_ . . . .* *a * . . .. I I 1 ’ 80 40 60 1
I
I
2 .O * 0 , 0
(eV)
Fig. 3. Branching ratios for dissociative photoionization of CF,Cl,. The vertical amws repmsent expected thermodynamic appearance potentials (see table 3) and the vertical lines indicate the vertical ionization energies [ 21,231 for production of the electronic states of CF,Cl: .
zation have been discussed earlier [ 19 1. The dipole induced breakdown patterns of many small molecules have been investigated with considerable SUG cess by this type of analysis, for example, see refs. [ 1,2,19,37]. Since the molecular ion CF, Cl,’ is not observed on the time scale of the TOF mass spectrometer and because CF,Cl’ is the only ion detected (see table 3 ) below the fi state ionization potential of CFICll [ 2 11, the %, A, B and C electronic states of the molecular
ion must exclusively lead to production of the CF,Cl+ ion. Considering the Franck-Condon region of the b state [ 211 and the appearance potential 6f CFClf , it appears that CFCl,+ is the fragment ion produced from the f3 electronic state of the molecular ion. Similarly the fragment ions Ccl,+, CF,+ , Ccl+, Cl+, F+ andC+canbeformedfromthe(~+6),~,~~,~ and (E+ G) electronic states [ 2 1,23 ] respectively, and the ions CFCl+ and CF+ from the (fi+T) states [ 2 11. With these considerations in mind the follow-
Table 2 Photoion branching ratios for CF$&
~--
.~~
Photon energy
Branching ratio (I)
(eW
CFCI:
CF@+
0.28 1.11 1.73 3.79 6.81 9.40 11.12 12.of. 13.18 12.94 12.57 12.04 11.72 11.12 10.85 11.18 11.00 10.69 10.47 10.42 10.43 10.31 10.73 11.03 11.58 12.23 12.31 12.78 13.30 14.05 14.34 14.76 15.29 16.50 16.88 17.98 18.41 18.74 18.67 18.46 f 8.66 18.80 18.13 17.83 17.05 16.46 16.16 15.73 14.85 13.39 f 3.20 13.10
100.00 100.00 f00.00 mo.00 100.00 99.72 98.89 98.27 96.21 93.19 90.60 88.88 87.56 85.50 83.67 80.67 77.66 73.84 72.44 71.23 69.53 67.56 66.37 65.13 63.31 61.58 60.80 59.84 59.26 58.29 57.49 56.57 55.43 54.70 53.23 52.07 51.02 50.03 46.94 44.91 42.47 39.49 37.26 35.29 33.15 31.59 30.77 29.61 28.64 27.67 27.70 27.62 27.82 26.80 26.70 25.54 25.13
11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5
18.0 18.5 19.0 13.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.f 30.0 31.0 32.0 33.0 34.0 35.0 36.0 37.0 38.0 39,O 40.0 42.0 44.0 46.0 48.0 50.0 55.0 60.0 65.0 70.0
CCl:
0.09 0.00 0.10 0.08 0.11 0.15 0.17
CFCl+
CF,+
0.42 1.09 1.80 2.90 3.68 4.19 4.60 5.16 5.35 5.63 6.01 6.15 6.31 5.71 5.45 5.17 5.10 4.90 4.93 4.64 4.85 4.72 4.57 4.67 5.01 4.89 5.00 5.66 5.78 5.75 6.41 6.51 6.55 7.26 7.37 7.44 7.31 7.22 7.12 6.27 6.53 6.18 6.14
0.44 1.31 2.96 5.67 8.49 10.58 11.19 11.52 12.18 12.52 12.71 13.44 13.57 13.60 13.66 13.50 13.24 13.20 13.06 13.23 12.94 12.90 12.63 13.23 13.24 13.15 13.58 13.96 14.46 14.72 15.01 15.10 15.20 15.16 15.36 f5.40 15.25 15.40 15.00 15.07 15.18 14.80 14.74 14.69 14.13
CCl+
0.32 0.32 0.56 0.83 1.13 1.15 1.50 1.72 1.93 2.05 2.43 2.50 2.54 2.99 3.21 3.45 3.28 3.30 3.36 3.21 2.98 3.06 2.64 2.62 2.78 2.81 3.02 3.29 3.58 3.62 3.56 3.75
Cl+
0.39 0.56 0.93 0.95 1.33 1.55 1.39 1.64 2.04 1.98 1.87 2.25 2.54 2.88 3.19 3.65 4.16 4.64 4.81 5.35 5.50 6.16 6.82 7.59 8.65 9.83 10.93 12.21 13.09 13.83 14.65 15.14 15.87 16.20 16.37 16.39 18.04 19.75 20.60 20.59
CF+
0.56 1.02 1.28 1.56 2.42 3.33 3.93 5.05 S.87 6.62 7.80 7.93 8.09 8.09 8.29 8.55 8.37 8.55 8.41 8.56 8.81 8.56 8.63 8.26 8.48 8.54 8.46 8.73 8.13 8.60 9.04 10.08 IO.80 11.35 11.47 11.15 11.33 11.00 10.70 10.65
F+
Cc
0.60 0.57 0.69 0.61 0.78 0.90 0.89 0.94 1.05 1.17 1.39 1.65 2.13 2.67
0.26 0.69 0.79 1.31 1.54 1.83 2.07 2.40 2.23 2.11 1.75 1.64 1.64 1.43 1.55 2.26 2.17 2.80 2.88
CF&l*+
0.19 0.18 0.38 0.42 0.46 0.49 0.59 0.51 0.60 0.33 0.47 0.79
1 0 4 0
2
60 40
ii
20 0
0
0.5 0 0.5 0 CF,GI++ **a.
**
*
I
0.1
l l
l*
0
Fig, 4. Absolute oscillator stnqths for dimtciative photoionization of CF& The vertical arrows indicate expected ~e~~~ic appearance potenti& (see table 3 ) and the vertical lines indicate the vertical i~u~tion energies { 2 1,231 for production of the electronic states of CF2Cl:.
ing relatio~ips between the partial oscillator the formation of CFCl$, strengths for CF&!l’,CCl,*,CFCl+,CF$, CCl+, Cl-+, CF’, F+ andC4,andtho~for~ep~ctionof~e (R-t-A}, (fs+i5), fi, &t-F+@, (%+l), J, il &+I@ and (R + is) electronic states are found to provide a reasonably consistent ratio~~tion of the dipole induced breakdown of C&Cl2 witbin the energy range of the present data:
(d~~)(CFCl~)=(d~~)(~+O.l~(~~~~~)), (dSf~)(~~Cl*)=(d~dE)((~+A) f (~+~)+0.47(~+~~~)~
,
(dfldE)(CCl~)=(dfldE)(O.OlS(~J+~)), (d~/~)(CFClf)=(d~/~)(0.38(ir+T)), (dfldE)(CF~)=(dfl~)(0.38(E~e+c>>,
W. Zhangetal. / Excitation and ionization offeon molecuks. III
367
Table 3 Calcolated and measured appearance potentials for the production of charged species from CF&I, ProcesS
Appfzanmx
potential (eV)
calculated‘)
experimental this work b,
11.8
(l)CF,W
14.0 11.5 46 18.5
(21 CFCI: (3) CF&l+ (4) ccl2+ (5) CFCI+ (6) CF,’ +Cl, (7) CF$ +2Cl (8) CCl+ (9) CI++CF~+Q (10) Cl++CF+FCI (11) Cl++CF+F+CI (12) CI++C+FCl+F (13)CI+sC+F2+CI (14) Cl++C+ZF+Ci (lS)CF++FCl+cI (16) CF*+F+Q (17) CF++F+2Cl (18) F++CF+Q (19) F+ +CF+2Cl (20) F++C+FCl+Q (21) F++C+F+Cl~ (22) F+fC+F+2Cl (23) C++2FCI (24) C*+F2+Cl,, (25) C++FCl+FSCI (26) C+f2FfClz (27) C++F2+2Cl (28) C++2F+2Cl (25) CF@++??
14.6 i7.L 18.5 21.2 23.8 26.8 27.7 29.3 17.3 17.4 19.9 25.7 28.2 31.2 31.3 33.8 20.5 23.5 25.1 25.1 26.0 27.6
‘) Using thc~~h~i~ bt + I eV.
17.5 24
ref. [3]
ref. f 121
ref.
11.75 14.15 12.10
11.75 13.81 11.99
12.31
17.76 17.22
ref. [ 151
ref. [ 131
ref. [Id]
11.87 12.55
13.30 11.96 18.60 14.90
16.98 21.60 18.76
20
20
17.65 20.20
17.35 19.84
36
31 38
data from refs. 134-361 assuming zero kinetic energy of ~en~tion.
f@i’~) KXJ+ ) = (d.d.W (0.36K:) , (dfldE)(c1+)=(dsldE;)(3+0.64if +0.485(~+~)), (dflti)(CF+)=(dfld.Y)(0.62(~+~) +o.z(FJ+b))
[ 141
,
(d~/~)(C+)=~d~/~)(~~+~)+O.l(~+~)). Pig. 5 shows these breakdown relationships as a function of photon energy. The electronic state partial os-
cillator strengths used in fig_ 5 were obtained from the products of the presently measured total photoabsorption oscillator strength? (section 3.2) and the eiectronic state branching ratios obtained from PES measurements in the 27-41 eV [38] and 41-70 eV [ 231 regions. The latter work [23] includes measurements of the inner-valence photoelectron bands. Due to the unresolvability of certain features in the phot~l~ron spectra 123,381 the b~ching ratios for the production of the relevant states Were reported as sums, i.e. @+A), (&t-C), (B+P+is), @+I), (E+k) and (11;$+6), therefore the corresponding partial oscillator strengths are presented in
368
W. zhang et ai. / excitation and ionization qffreoonmolecules. III
I
CFCl +
cc1
+
0.5-
0
PHOTON ENERGY (eV) Fig. 5. Absolute oscillator strengths for the proposed dipole induced breakdown scheme of CF#&. (0 ) Present dipole (e, e+ion) experimental data. (-) Sums of electronic state partial oscillator strengths using the presently determined photoabsorption oscillator strengths and PES branching ratio data in the 27-41 eV [ 381 and 41-70 eV [ 231 regions. See section 3.4 for details.
Fig. 6. Proposed dipole induced breakdown scheme for the ionic photofragmentation of CF2C12.See text for details.
similar form in the above breakdown relationships. In spite ofthe uncertainties involved in such a simple rationale the co~~~den~ of the proposed scheme with the partial photoionization oscillator strengths (fig. 5) is reasonably good in terms of both shape and magnitude. Fig. 6 shows the presently proposed dipole induced breakdown scheme for CF,Cl,, A more detailed analysis of the dipole induced breakdown scheme of CFzClz may be obtained from photoelectron-photoion coincidence studies as a function of photon energy.
This work has been financially supported by NSERC (Canada). In addition WZ acknowledges a University of British Columbia Graduate Fellowship, GC is grateful for receiving a SERC( UK)/ NATO postdoctoral fellowship and TI thanks the Yamada Science Foundation (Japan ) for support. We should also like to thank Brian Greene (Electronics shop, ~p~ment of Chemistry, UBC) for assistance with the maintenance of the dipole (e, e+ ion) spectrometer.
References [ I ] W. Zhang, G. Cooper, T. Ibuki and C.E. Brion, Chem. Phys. 137 (1989) 391. [ 2 J W. Zhang C. Cooper, T. Ibuki and C.E. Brian, Cbem. Phys. 151 (t991) 343. [ 3 ] H.W. Jochims, W. Lohr and H. Baumg&tel, Ber. BunsentIes. Physik. Chem. 80 ( 1976) 130. [4]C.Y.R. Wu, L.C. Lee and D.L. Judge, J. Chem. Phys. 71 (1979) 5221. [S]G.~.~ng~dJ.W.M~o~ey,J.Pb~.B ll(1978) 186t. [ 61 R. Gilbert, P. Sauvageau and C. Sandor& J. Cbem. Phys. 60 ( 1974) 4820. [ 7 ] J. Doucet, P. Sauvageau and C. Sandorfy, J. Chem. Phys. 58 (1973) 3708. [ 81 T. Ibuki, A. Hiraya and K. Shobatake, J. Chem. Phys. 90 ( 1989) 6290. [ 91 J.C. Person, D.E. FowIer and P.P. Nicole, Argonne National Laboratory Report ANL75-60, part 1q [IO] R.E. Rebbert and P. Ausmos, J. Res. Natl. Bur. Std. 75 A (1971) 481. [ II] B.E. Cole and R.N. Dexter, J. Quant. Spectry. Radiative Transfer 19 (1978) 303.
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[ 261 C.E. Brion, ~mments At. Mol. Phys. 16 ( 1985 ) 249, and references therein; C.E. Brion and A. Hamnett, Advan. Chem. Phys. 45 ( 1981) 1. [27] W. Zhang, T. Ibuki and C.E. Brian, Chem. Phys., to be published. [28 ] M-B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 3 (Academic Press, New York, 1985) p. 133, 1291 M.B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 1 (Academic Press, New York, 1974). [30] L.C. Lee, E. Phillips and D.L. Judge, J. Chem. Phys. 67 (1977) 1237. 1311 G.M. Bsncroft, J.D. Boxek, J.N. Cutler and K.H. Tao, J. Electron Spectry. 47 ( 1988) 187; J.S. Tse, J. Chem. Phys. 89 ( 1988) 920; B.M. Addison-Jones, K.H. Tan, B.W. Yates, J.N. Cutler, G.M. Bancroft and J.S. Tse, J. Electron Spectry. 48 ( 1989) 155. [ 32 ] CR. Natoli, in: Springer Series in Chemical Physics, Vol. 27. EXAFS and Near Edge Structure, eds. A. Bianconi, L. Incouia and S. Striprich (Springer, Be&n, f983) p. 43; A. Bianconi, M. Dell-Ariccia, A. Gatgano and C.R. Natoh, in: Springer Series in Chemical Physics, Vol. 27. EXAFS and Near Edge S~ctu~, eds. 14. Bianconi, I,,. Incouia and S. Striprich (Springer, Berlin, 1983) p. 57.
370 [ 33
W. Zhang
et al. /Excitation
and ionization
] W. Zhang, G. Cooper, T. Ibuki and C.E. Brion, Chem. Phys.,
to be published. [34] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17 No. 1 (1988). [ 351 H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data 6 ( 1977 ) Suppl. 1.
offreon molecules. III
[36]D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, SM. Bailey and R.H. Schumm, NBS Tech. Note 270-3 (US Government Printing Office, Washington, 1968). [ 37 ] Y. Ii&, F. Carnovale, S. Daviel and C.E. Brion, Chem. Phys. 105 (1986) 21 I, and references therein. [38] I. Novak, J.M. Benson and A.W. Potts, J. Electron Spectry. 41 (1986) 175.