Valence- and inner-shell (Cl 2p, 2s; C 1s) photoabsorption and photoionization of carbon tetrachloride. Absolute oscillator strength (5–400 eV) and dipole-induced breakdown pathways

ChemicalPhysics North-Holland

181 (1994) 147-172

Valence- and inner-shell (Cl 2p, 2s; C 1s) photoabsorption and photoionization of carbon tetrachloride. Absolute oscillator strengths (5-400 eV) and dipole-induced breakdown pathways Gordon R. Burton, Wing Fat Chan, Glyn Cooper and C.E. Brion Department of Chemistty, The University ofBritish Columbia. Vancouver, BC, Canada V6T IZI

Received 26 July 1993

Electronic excitation spectra and absolute oscillator strengths (cross sections) have been measured in the vacuum ultraviolet and soft X-ray regions for the valence- and inner-shell (Cl Zp, 2s; C 1s) photoabsorption of CC4 from 5.5 to 400 eV, using lowresolution (1 eV fwhm) dipole (e, e) spectroscopy. High-resolution (0.048 eV fwhm) dipole (e, e) spectroscopy has been used to study the valence region from 5.0 to 30 eV in greater detail. These data are compared with previously reported optical measurements in those limited energy regions where such data exist. Dipole (e, e+ ion) spectroscopy has been used at a resolution of 1 eV fwhm to measure the photoionizarion efficiency and ionic photofragmentation branching ratios for Ccl., from 11.0 to 80.0 eV in the valence-shell region and from 190.5 to 220.5 eV in the vicinity of the Cl 2p inner-shell edge. Absolute partial oscillator strengths for dissociative photoionization channels of Ccl, have been derived. Using these and other spectroscopic data, a dipoleinduced breakdown scheme for Ccl, in both the valence-shell and Cl 2p inner-shell regions is proposed.

1. Introduction

Following the initial photographic work of Leifson [ 11, a range of measurements of the absolute photoabsorption oscillator strength (cross section) distribution of carbon tetrachloride have been reported in the energy region below 25 eV in the valence shell [2-131 and in the vicinity of the Cl 2p absorption edge [ 141. The discrete region below the Cl 2p edge has been studied by O’Sullivan [ 151 from 199 to 208 eV. In other work, small momentum transfer electron energy-loss spectroscopy (EELS) has been used throughout the 100-400 eV region to study the electronic transitions associated with the Cl 2p, 2s and C 1s inner-shell excitations [ 16 1, but no absolute data were reported. More recently using empirical procedures, approximate values of absolute differential oscillator strengths have been estimated [ 171 for the inner-shell regions (Cl 2p, 2s; C 1s ) of Ccl,, from the EELS spectra reported earlier by Hitchcock and Brion [ 161. However, these absolute measurements [ 214,17 ] have been made over only limited energy regions, and there is no comprehensive absolute pho0301-0104/94/$07.00 SD1

toabsorption data set which spans the valence- and inner-shell regions. The valence-shell ionization energies of CCL have been extensively studied by photoelectron spectroscopy (PES) [ 18-241, and by angle-resolved PES [ 25-281 in the gas phase, and also by tunable photon energy PES of a solid film of carbon tetrachloride [ 291. These latter measurements [ 25-291 were made using either line sources [ 28 ] or synchrotron radiation [25-27,291 to determine the variation of the asymmetry parameter [ 25-28 1, and absolute [ 25 ] or relative [ 291 electronic state partial cross sections, as a function of photon energy. In other work, the technique of X-ray photoelectron spectroscopy (XPS) has been used to determine the ionization energies for the inner shells of CC& [ 30-37 1. Photoionization and electron-impact methods [ 381, [ 391 #I, [40-521 have provided information about the relative photoionization yield curves [ 38,42,45,46,49] and the appearance potentials [ 38-41,43-45,47,50-521 for x1 As referenced by Farmer et al. [40] and Reed and Snedden [411.

0 1994 Elsevier Science B.V. All rights reserved.

0301-0104(93)E0396-D

148

G.R. Burton et al. /Chemical Physics 181 (1994) 147-172

the major ionic photofragments of carbon tetrachloride. Absolute oscillator strength data for electronic excitation and ionization in the VUV and soft X-ray regions are useful in a number of areas of science and technology including aeronomy, astrophysics, and radiation chemistry and physics [ 53 1. Wide-ranging absolute oscillator strength data for atoms and molecules are also required for the construction of dipole oscillator strength distributions, which cover the entire photoabsorption spectrum, and for calculating a wide range of dipole properties [ 541. More specifically, carbon tetrachloride has been found to contribute to the “greenhouse effect”, and is a trace gas component and harmful pollutant of the atmosphere whose amount is increasing significantly [ 55 1. Carbon tetrachloride is a useful electron scavenger [ 561, is a source of Ccl3 and Ccl2 radicals by photodecomposition [ 12,571 and is also used in the production of chlorofluorocarbons by catalyzed fluorination with HF [ 58 1. The absolute inner-shell [ 17 ] and valenceshell [ 59-621 photoabsorption oscillator strengths for the chlorofluoromethanes (i.e. the freons CF,C14_x, x= l-4) have been reported previously by Zhang et al. and the present work extends these results to the related molecule, CCL,. Studies of such a series of molecules as the chlorofluorocarbons allow for the investigation and evaluation of systematic trends and additivity concepts [ 631 in the photoabsorption spectra of these molecules, and larger systems containing similar functional groups. The techniques of dipole (e, e) and dipole (e, e+ion) spectroscopy used in the present work involve the forward scattering of high-energy electrons off gaseous target molecules. These methods provide accurate oscillator strength data over wide equivalent photon energy ranges and have been shown [ 5 3 ] to produce results which are entirely equivalent to those that would be obtainable by the techniques of absolute photoabsorption, and photoionization mass spectroscopy, respectively, using direct optical methods with tunable light sources. These electron-impact techniques are ,non-resonant and since they do not depend on the logarithmic transformation required for obtaining absolute oscillator strength data using the Beer-Lambert law, they do not suffer from “linesaturation” (bandwidth) effects which can result in large errors in cross sections obtained by optical mea-

surements in the discrete excitation region [ 64-691. The presently used electron-impact methods also avoid contributions from higher order radiation which are often present when using monochromated synchrotron radiation [ 701. In the present work, we now report measurements of the absolute photoabsorption oscillator strength distribution of CCL, from 5.5 to 400 eV, obtained USing low-resolution ( E 1 eV fwhm) dipole (e, e) spectroscopy. These low-resolution photoabsorption data span the visible through to the soft X-ray photon energy region of the electromagnetic spectrum. Highresolution dipole (e, e) spectroscopy has also been used in the present work to measure oscillator strength data in the low-energy valence-shell region from 5.0 to 30 eV, where sharp discrete excitation features exist. In addition to the photoabsorption studies, the technique of dipole (e, e + ion) spectroscopy has been used in the present work at a resolution of 1 eV fwhm to measure the photoionization efftciency and the photoionization branching ratios for CC& in the valence shell from 11.0 to 80 eV and also in the Cl 2p ionization region from 195.5 to 220.5 eV. Absolute partial photoionization oscillator strengths for the dissociative photoionization channels of CCL, have been obtained as a function of photon energy from the triple product of the presently measured photoionization efficiencies, photoionization branching photoabsorption oscillator ratios, and absolute strength data. The presently obtained results have been used along with other spectroscopic information to determine quantitative and qualitative information concerning the dipole-induced breakdown of CCL, caused by WV and soft X-ray radiation in both the valence- and Cl 2p inner-shell regions.

2. Experimental methods The experimental methods used to obtain absolute photoabsorption oscillator strengths and partial photoionization oscillator strengths with the low-resolution dipole (e, e+ion) spectrometer have been reported previously [ 7 l-74 1. In brief, the relative lowresolution ( 1 eV fwhm) oscillator strength spectrum (5.5-400 eV) was obtained from the backgroundsubtracted, Bethe-Born converted electron energyloss spectrum measured at an incident beam energy

G.R. Burton et al. /Chemical Physics181(1994) 147-172

of 8000 eV and a mean scattering angle of zero degrees. The high electron-impact energy, small scattering angle about the forward direction, and small energy losses (relative to the incident electron beam energy) used in the present work ensure that the momentum transferred to the target molecule is negligible, so that essentially only dipole-allowed transitions are excited. The relative oscillator strength spectrum was placed on an absolute scale using the partial Thomas-Reiche-Kuhn (TRK) sum rule 1751. The high-resolution dipole (e, e) spectrometer and experimental procedures used to obtain the oscillator strength data at a resolution of 0.048 eV fwhm have been described previously [ 64,65,76]. The high-resolution, background-subtracted, electron energy-loss spectrum (5.0-30 eV) of CCL, was multiplied by the Bethe-Born factor for the spectrometer [ 641 to give the relative oscillator strength spectrum. This highresolution spectrum was then placed on an absolute scale by normalization to the presently measured lowresolution spectrum in the smooth continuum region where no sharp structures exist. The high-resolution techniques have recently been used in the discrete excitation region to measure absolute oscillator strength spectra of He [ 641 and Hz [ 67 1, and the results are in excellent agreement with high-level ab initio calculations. This lends strong support for the use of the present method for the accurate determination of high-resolution absolute oscillator strengths in the discrete excitation region of atoms and molecules. It should also be noted that such high-resolution dipole (e, e) measurements are free of the “line-saturation” (bandwidth) effects which can cause serious errors in cross-section determinations when Beer-Lambert photoabsorption methods are used [ 64,681. The photoionization mass spectra for CC& were obtained using the low-resolution ( 1 eV fwhm) dipole (e, e+ion) spectrometer. The forward scattered electrons at a particular energy loss are collected in coincidence with time-of-flight (TOF) mass analyzed cations. The flight time is proportional to the square root of the m/e for each ion and is determined from a single-stop time-to-amplitude converter using the electron signal as the “start” and the ion signal as the “stop”. The TOF mass spectrometer was designed with ion extraction fields which ensure uniform collection of energetic ions with excess kinetic

149

energies of fragmentation up to 20 eV. The branching ratios for the individual ions formed from CC& were obtained by integrating the baseline-subtracted TOF mass spectra which had been corrected for the response function of the ion multiplier as a function of m/e [ 771. The baseline of random coincidences for such an experimental arrangement decays exponentially with increasing time [ 781. Carbon tetrachloride was obtained from BDH (minimum purity 99%) and was used without any further purification. The liquid was degassed by several freeze-pump-thaw cycles before being admitted into the spectrometer in order to remove any dissolved gases in the sample. No impurities were detected in either the TOF mass spectra or the high-resolution electron energy-loss spectra of Ccl,.

3. Results and discussion 3.1. Electronic structure

Carbon tetrachloride has tetrahedral symmetry ( Td) and the molecular orbital configuration of the ground electronic state in an independent particle model is:

Cl 1s

c 1s

cizs

(3t2)6(4a~)2(le)4(lt~)6(4t2)a

c12P

valence shell

The vertical ionization potentials (VIPs) for the five outermost occupied valence orbitals have been measured to be 11.69 (2ti’); 12.44, 12.65, 12.78 (7ty’); 13.37, 13.50 (2e-I); 16.58 (6tF1);and20.0 eV (6ai’) using He(I) and He(I1) PES [20]. The inner-valence and the inner-shell VIPs, as determined using XPS [36,37], are as follows: 24.8 (5ti’), 28.0 (sail), 207.04 2p,,2,208.73 2pi,2 (Cl 2p), 278.0 (Cl 2s), and 296.3 (C 1s).

G.R. Burton et al. /Chemical Physics 181(1994) 147-172

150

3.2. Photoabsorption oscillator strengths

j+$ =AE-‘.5+BE-2.5+CE--3.S

3.2.1. Low-resolution measurements was used to fit the high-energy portion of the valenceshell oscillator strength spectrum from 98-198 eV. Here dfldE is the differential oscillator strength, E is the energy loss (or equivalent photon energy) and A, B, and C are best fit parameters which were deter-

The low-resolution ( z 1 eV fwhm) relative oscillator strength spectrum, obtained from the BetheBorn converted EELS spectrum (see section 2)) was placed on an absolute scale by valence-shell TRK sum rule normalization. A curve of the form

250

200 150

150 100

100 .

50

LR dipok

(.,a)

Thie Work

50

.

0

r*

I 50

I 150

100

X

.

4

I 200

0

(b)

0

4

2

2

0

0 80

120

PHOTON

160

ENERGY

200

(eV)

oscillator strengths for CC4 obtained using low-resolution ( 1 eV fwhm) dipole (e, e) spectroscopy: (a) The valence-shell region from 5.5 to 198 eV. (b) The high-energy portion of the valence-shell region from 82 to 198 eV. Also shown are previously published photoabsorption data [ 141, and summed theoretical [ 8 1] and experimental [ 821 atomic oscillator strengths for the constituent atoms of CCL. The insert to (a) shows the photoionization efftciency measured in the present work from 1l-80 eV (see section 3.3).

Fig. 1. Absolute photo&sorption

G.R. Burton et al. /Chemical Physics 181(1994) 147-172

151

Table 1 Absolute oscillator strengths for the total photoabsorption (5.5-195 eV) and the dissociative photoionization ( 1 l-80 eV) of Ccl, Photon energy

Oscillator strength (lo-*

(eV)

Photoabsorption

5.5 6.0 6.5 7.0 7.5 8.0 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

1.65 1.76 3.53 5.74 7.02 11.64 26.87 60.08 75.10 53.94 49.35 63.39 74.17 87.67 104.5 133.8 153.8 165.9 170.7 167.8 164.5 166.0 155.2 151.9 144.7 131.0 128.6 128.1 126.7 125.6 124.3 121.1 119.7 116.5 110.3 104.8 99.56 98.07 88.48 83.73 79.32 73.53 69.29 65.78 62.06 58.10 53.52 48.63 46.45 40.79

C+

0.08 0.10 0.15 0.23 0.31 0.44 0.53 0.58 0.67 0.62

eV-‘)

*)

cl+

0.12 0.13 0.18 0.37 0.46 0.61 0.43 0.66 0.78 0.85 1.13 1.34 1.54 1.81 1.97 2.16 2.27 2.38 2.49 2.63 2.58 2.78 2.56

Photoionization efficiency ccl+

0.02 0.12 0.10 0.32 0.91 1.80 3.16 4.87 7.06 8.44 8.93 9.63 9.80 9.96 10.92 10.29 10.20 10.07 9.48 9.20 a.93 8.46 7.92 7.24 6.45 6.12 5.36

ccl:+

a:

0.04 0.00 0.08 0.07 0.08 0.09 0.12 0.15 0.08 0.15 0.15 0.15 0.20 0.23 0.16

ccl:

Ccl:

rli

0.13 0.03 0.20 0.21 0.68 3.30 5.76 11.10 14.87 18.73 21.25 23.80 24.97 25.51 25.50 24.89 24.28 23.39 21.95 20.44 19.40 19.04 17.42 16.86 16.34 15.39 14.87 14.40 13.73 13.06 12.03 11.07 10.56 9.31

5.06 14.76 33.02 59.52 96.77 116.4 129.1 137.9 141.7 144.3 140.8 140.0 127.9 119.4 112.2 107.1 103.3 99.82 96.74 93.53 88.72 86.40 83.73 78.04 73.78 69.35 66.90 59.36 55.05 50.92 46.46 42.76 39.87 37.01 34.05 30.94 27.75 26.09 22.78

0.08 0.20 0.38 0.57 0.72 0.76 0.78 0.81 0.85 0.88 0.87 0.94 0.92 0.93 1.00 b)

G.R. Burton et al. /Chemical Physics 181(1994) 147-172

152 Table 1 (continued) Photon energy

Oscillator

strength

(10-2eV-L)

(eV)

Photoabsorption

C+

cl+

32.0 34.0 36.0 38.0 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 95.0 100.0 105.0 110.0 115.0 120.0 125.0 130.0 135.0 140.0 145.0 150.0 155.0 160.0 165.0 170.0 175.0 180.0 185.0 190.0 195.0

26.37 18.07 12.49 8.96 7.56 6.02 5.71 5.62 5.56 5.60 5.34 5.29 5.05 4.86 4.69 4.44 4.19 3.91 3.67 3.49 3.32 3.19 3.02 2.92 2.72 2.57 2.48 2.24 2.16 2.01 1.96 1.87 1.79 1.71 1.62 1.55 1.47 1.41 1.34

0.55 0.43 0.29 0.23 0.19 0.15 0.13 0.12 0.15 0.15 0.14 0.15 0.15 0.14 0.15 0.13

2.78 2.53 2.14 1.87 1.82 1.58 1.57 1.58 1.58 1.55 1.53 1.51 1.48 1.45 1.42 1.35

Photoionization efficiency

‘)

‘) o(h4b)=109.75df/d.E(eV-t). b, The photoionization effticiency is normalized

ccl+

3.21 2.27 1.58 1.20 1.06 0.94 0.93 0.92 0.92 0.94 0.85 0.84 0.76 0.74 0.69 0.68

ccl:+

a:

0.05 0.06 0.06 0.05 0.06 0.03 0.04 0.03 0.02 0.03 0.01 0.04 0.03 0.03 0.04 0.03

0.15 0.11 0.12 0.07 0.06 0.04 0.03 0.04 0.03 0.06 0.03 0.05 0.05 0.04 0.02 0.03

ccl:

6.09 4.25 3.04

-2.22 1.86 1.43 1.31 1.25 1.20 1.14 0.98 0.91 0.83 0.77 0.71 0.66

ccl:

1,

13.54 8.43 5.27 3.32 2.50 1.85 1.69 1.68 1.66 1.73 1.78 1.80 1.75 1.69 1.66 1.57

to unity above 18.0 eV. See text for details.

mined to be 21.099 eV0.5, 4829.4 eW5, and - 3.4397 x lo5 eV2,5, respectively. This particular functional form was chosen because of the good quality of the fit to the present data and because of its effectiveness in fitting the high-energy portion of the valence shell of the chlorofluoromethanes [ 59-621.

Using this fitted curve the portion of the valence-shell oscillator strength above 198 eV is estimated to be 11.8%. The total area under the presently measured data from 5.5-l 98 eV plus the contribution above 198 eV (to infinity), estimated from the extrapolation of the fitted polynomial, was set to an integrated oscil-

G.R. Burton et al. /Chemical Physics 181(1994) 147-I 72

6

150

(b) . A

: -

Z

0

tT

100

CL 0 g

2 c

50

153

0 . . . -----. ---

HR dipole (0.0) PhAbs [a] PhAbs [lo] PhAbs [l?.] PhAbs [13] PhAbs [7] PhAba [3] PhAba [a] PhAba [ll]

Thi8

b

x4 0

0 T

CL

0

Fig. 2. Absolute photoabsorption oscillator strengths measured in the present work at high resolution (0.048 eV t?vhm) and at low resolution (1 eV tihm) using dipole (e, e) spectroscopy (solid dots and open squares, respectively): (a) The low-energy valence-shell region from 5 to 32 eV. (b) The valence-shell discrete region from 5 to 12 eV, below the first IP. Previously reported absolute photoabsorption measurements are also shown for comparison [ 3-131. The vertical lines in (a) and (b) indicate the positions of the valenceshell VIPs [ 20,361.

lator strength value of 33.14. This corresponds to the 32 valence-shell electrons of CC4 plus an estimated contribution of 1.14 to account for Pauli-excluded transitions from the inner-shell orbitals to the already occupied valence-shell orbitals [ 7579,801. The absolute photoabsorption oscillator strengths for the valence shell of Ccl, obtained in the present work at a resolution of 1 eV fwhm are shown in fig. la from 5.5 to 198 eV, and are given numerically in

table 1 from 5.5 to 195 eV. The peak observed at 9.5 eV in this spectrum corresponds to contributions from the two strongest transitions in the discrete region of Ccl4 below the first IP. However, due to the limited resolution only one peak is observed. A high-resolution spectrum has been measured in the present work in the discrete region below the first IP and will be discussed in section 3.2.2 below. In the vicinity of x 40 eV the photoabsorption spectrum of CC& pro-

154

G.R. Burton et al. /Chemical Physics 181(1994)

147-172

Table 2 Absolute oscillator strengths for the total photoabsorption in the inner-shell region (Cl 2p, 2s; C 1s) of Ccl, from 195 to 400 eV Photon energy

Photoabsorption oscillator strength (lo-* eV-I) *)

Photoabsorption oscillator strength ( 10m2eV-‘) *)

(eV)

(eV)

195.0 196.0 197.0 198.0 198.5 199.0 199.5 200.0 200.5 201.0 201.5 202.0 202.5 203.0 203.5 204.0 204.5 205.0 205.5 206.0 206.5 207.0 207.5 208.0 208.5 209.0 209.5 210.0 210.5 211.0 211.5 212.0 212.5 213.0 213.5 214.0 214.5 215.0 215.5 216.0 216.5 217.0 217.5 218.0 218.5 219.0 219.5 220.0 220.5

Photon energy

total

valence

1.34 1.33 1.33 1.34 1.35 1.47 1.68 2.34 2.72 2.82 2.67 2.71 2.77 2.98 3.13 3.21 3.10 2.82 3.11 3.27 3.57 4.02 4.92 7.11 8.33 8.86 9.38 9.76 9.98 9.98 9.96 10.08 10.27 10.41 10.56 10.68 11.01 11.21 11.48 11.65 11.81 11.97 12.00 11.87 11.75 11.80 11.77 11.55 11.38

1.34 1.33 1.33 1.32 1.31 1.31 1.30 1.30 1.29 1.28 1.28 1.21 1.27 1.26 1.26 1.25 1.25 1.24 1.24 1.23 1.23 1.22 1.22 1.21 1.21 1.20 1.20 1.19 1.19 1.18 1.18 1.17 1.17 1.16 1.16 1.15 1.15 1.15 1.14 1.14 1.13 1.13 1.12 1.12 1.11 1.11 1.11 1.10 1.10

Cl 2p+c12s +c 1s

0.02 b’ 0.03 0.17 0.37 1.05 1.43 1.53 1.39 1.43 1.50 1.72 1.88 1.95 1.86 1.58 1.87 2.04 2.34 2.80 3.71 5.90 7.12 7.66 8.18 8.57 8.79 8.80 8.78 8.91 9.11 9.24 9.40 9.53 9.86 10.07 10.34 10.52 10.68 10.84 10.88 10.76 10.63 10.69 10.66 10.45 10.28

221.0 221.5 222.0 222.5 223.0 223.5 224.0 224.5 225.0 225.5 226.0 228.0 230.0 232.0 234.0 236.0 238.0 240.0 242.0 244.0 246.0 248.0 250.0 252.0 254.0 256.0 258.0 260.0 262.0 264.0 266.0 268.0 270.0 272.0 274.0 276.0 278.0 280.0 282.0 284.0 286.0 288.0 288.5 289.0 289.5 290.0 290.5 291.0 291.5

total

valence

Cl 2p+c12s +c 1s

11.28 11.38 11.24 11.31 11.22 11.12 11.16 11.08 11.02 10.91 10.92 10.68 10.58 10.52 10.46 10.26 10.10 10.00 9.84 9.67 9.42 9.34 9.27 9.16 9.17 9.20 9.17 9.26 9.17 9.28 9.35 9.43 9.68 9.75 9.17 9.26 9.28 9.15 8.96 8.87 8.78 8.74 8.67 9.50 11.38 13.15 15.36 14.33 12.49

1.09 1.09 1.08 1.08

10.18 10.29 10.15 10.23 10.15 10.04 10.09 10.02 9.96 9.86 9.87 9.64 9.56 9.52 9.47 9.29 9.14 9.05 8.90 8.75 8.51 8.45 8.39 8.28 8.31 8.35 8.34 8.43 8.36 8.48 8.55 8.65 8.91 8.98 8.42 8.52 8.54 8.43 8.24 8.17 8.08 8.05 7.98 8.82 10.70 12.47 14.68 13.65 11.81

1.08 1.07

1.07 1.06 1.06 1.06 1.05 1.04 1.02 1.01 0.99 0.98 0.96 0.95 0.93 0.92 0.91 0.90 0.88 0.87 0.86 0.85 0.84 0.82 0.81 0.80 0.79 0.78 0.77 0.76 0.75 0.74 0.73 0.72 0.72 0.71 0.70 0.69 0.69 0.69 0.68 0.68 0.68 0.68 0.68

G.R. Burton et al. /Chemical

Physics 181(1994)

147-172

155

Table 2 (continued) Photon energy

Photoabsorption oscillator ( lOa eV-‘) a)

strength

(eV)

Photoabsorption ( 10-2eV-1) ‘)

oscillator

strength

(eV) total

292.0 292.5 293.0 293.5 294.0 294.5 295.0 295.5 296.0 296.5 297.0 297.5 298.0 298.5 299.0 299.5 300.0 301.0 302.0 303.0 304.0 305.0 306.0 307.0 308.0 309.0 310.0

Photon energy

10.02 9.24 8.98 8.63 8.60 8.63 8.82 8.81 9.07 9.02 8.95 9.03 9.19 9.10 9.12 9.16 8.80 9.10 9.05 9.17 9.09 8.99 8.92 8.70 8.56 8.29 8.38

valence

0.67 0.67 0.67 0.67 0.67 0.66 0.66 0.66 0.66 0.66 0.65 0.65 0.65 0.65 0.65 0.64 0.64 0.64 0.63 0.63 0.63 0.62 0.62 0.62 0.61 0.61 0.61

Cl 2p+c12s +c 1s 9.35 8.57 8.31 7.96 7.93 7.97 8.16 8.15 8.41 8.36 8.30 8.38 8.54 8.45 8.47 8.52 8.16 8.46 8.42 8.54 8.46 8.37 8.30 8.09 7.94 7.68 7.77

total

valence

Cl 2p+c12s +cDlS

311.0 312.0 313.0 314.0 315.0 316.0 317.0 318.0 319.0 320.0 325.0 330.0 335.0 340.0 345.0 350.0 355.0 360.0 365.0 370.0 375.0 380.0 385.0 390.0 395.0 400.0

8.36 8.38 8.26 8.29 8.24 8.16 8.13 7.96 7.91 7.89 7.61 7.28 7.15 6.91 6.76 6.68 6.48 6.40 6.33 6.08 6.00 5.80 5.57 5.45 5.25 5.15

0.60 0.60 0.60 0.59 0.59 0.59 0.58 0.58 0.58 0.57 0.56 0.54 0.53 0.52 0.50 0.49 0.48 0.47 0.46 0.44 0.43 0.42 0.41 0.41 0.40 0.39

7.76 7.78 7.66 7.70 7.65 7.58 7.55 7.38 7.33 7.32 7.05 6.73 6.62 6.39 6.26 6.19 6.00 5.94 5.88 5.64 5.56 5.38 5.15 5.04 4.85 4.76

a) a(Mb)=109.75df/d.E (eV-I). b, The valence-shell contribution above 198 eV was obtained by extrapolating the fit to the valence-shell spectrum (98-l 98 eV) to higher photon energies. The inner-shell (Cl 2p+Cl2s+C 1s) contribution was obtained by subtracting the valence-shell contribution from the total (valence plus inner-shell) photoabsorption oscillator strength.

gresses through a local minimum. This so-called “Cooper minimum” has been observed previously [ 251 in several of the electronic state partial photoionization cross sections of CC& and is due to the fact that the 3p wavefunction of atomic Cl has a radial node. Fig. lb shows the low-resolution absolute oscillator strength data for Ccl4 from 82-198 eV along with previously measured absolute photoabsorption data [ 141 and both theoretical [ 8 1 ] and experimental [ 821 atomic oscillator strength sums for the constituent atoms (C + 4Cl). From fig. lb it can be seen that there is quite good agreement from 14219 1 eV between the presently measured dipole (e, e)

data and the previously reported photoabsorption measurements of Cole and Dexter [ 141. However, the latter data [ 141 are higher than the present results between 124 and 142 eV. In addition, it can be seen that although the summed atomic oscillator strength data sets [ 81,821 are consistent with each other, they are x 25% higher than the present experimental results from 90 to 200 eV. 3.2.2. High-resolution measurements The high-resolution (0.048 eV fwhm) photoabsorption spectrum of CC& measured in the present work is shown in fig. 2. This spectrum was placed on

156

G.R. Burton et al. /Chemical Physics 181(1994)

an absolute scale by one-point normalization to the present absolute low-resolution photoabsorption spectrum (see section 3.2.1) at 25 eV in the smooth continuum region. The assignments of the spectral features in the discrete region below the first IP have been reported previously [2,3,5,7,9,12,13,83]. These high-resolution data are shown in fig. 2a along with the presently measured low-resolution (Z 1 eV fwhm) data and previously reported photoabsorption data [ 4-61. The lower-resolution data reported by Sowers et al. [ 51 below x 11 eV are reasonably consistent with the present work. The data reported by Person and Nicole [ 61 in the 13-22 eV region and the single data point reported by Rebbert and Ausloos [ 41 at = 17 eV are about 16% higher than the present work. The low-energy portion of the presently measured high-resolution spectrum is shown in fig. 2b from 5-12 eV, along with previously reported photoabsorption data for comparison [3,7-l 31. The present high-resolution absolute data are in good agreement with the measurements reported by Tsubomura et al. [ 3 1, Robbins [ 8 1, and Hubrich and Stuhl [ lo] in the vicinity of the broad absorption peak at z 7 eV. In the same region the data of Russell et al. [ 7 1, Causley and Russell [ 9 1, Roxlo and Mandl [ ill, and Ibuki et al. [ 121 are x 90%, z 40%, and x40% lower and 17% higher, respectively, than the present work. From 8-l 2 eV the present results agree best with the recently reported optical work of Lee and Suto [ 13 1. Most of the photoabsorption oscillator strength data obtained using optical methods are higher than the present work at absorption peak maxima in the region of sharp structure (8.5-l 1 eV) because of the much higher resolution in the optical work [7,9,12,13]. 3.2.3. Inner-shell (Cl 2p, 2s; C Is) measurements Absolute photoabsorption oscillator strengths have been measured at a resolution of z 1 eV fwhm for the inner shells (Cl 2p, 2s; C 1s) of Ccl4 from 195-400 eV. These data are given numerically in table 2. The absolute scale was obtained by normalization to the high-energy tail of the absolute valence-shell oscillator strength distribution determined as in section 3.2.1. The assignments of the spectral features in these inner-shell regions (Cl 2p, 2s; C Is) have been reported previously [ 15,16 1. The complete photoabsorption spectrum (valence plus inner shells) mea-

147-I 72

sured in the present work is shown in fig. 3a from 5.5-400 eV while the inner-shell regions are shown in greater detail in fig. 3b. The direct optical photoabsorption measurements reported by Cole and Dexter [ 141 shown in fig. 3b are in very good agreement with the present work below the Cl 2p absorption threshold at z 198 eV but are z 12% higher above 198 eV to the limit of their data at 270 eV. The theoretical [ 8 1 ] and experimental [ 82 ] atomic oscillator strength sums for the total (valence plus inner shells) photoabsorption of Ccl, from = 190-400 eV are x lo%-20% higher than the present measurements. However, the agreement improves above 300 eV. Although summed atomic oscillator strength data for the constituent atoms of a molecule have been used at 20-30 eV above an absorption edge to place inner-shell electron energy-loss spectra on an absolute scale (for examples, see refs. [ 17,841) such a normalization assumes that there are no further molecular effects (e.g. shape resonances) beyond this energy range. However, it can be seen from a comparison of the oscillator strength scales of the present work (fig. 3b) and fig. 6 of ref. [ 171 that such a procedure is not particularly satisfactory for CC&. The discrepancies are due to the relatively close proximity of the Cl 2s and C 1s excitation features to the Cl 2p edge and to the presence of oscillating extended fine structure (EXELFS) in the molecular case [ 171. Similar findings and conclusions have resulted from other recent inner-shell oscillator strength studies of SO2 [85], SiF, [86], SiH4 [87], PH3 [88], and CH30H [ 891. Consideration of these results and the present work suggests that absolute scales determined for molecular inner-shell spectra using atomic oscillator strength data are likely to be subject to appreciable errors and that such calibrations are at best only approximate. From fig. 3a it can be seen that the valence-shell region oscillator strength distribution is strongly peaked in the low-energy region due to the large contribution from the 3p and 3s orbitals of the third-row chlorine atoms [ 901. The Cl 2p inner-shell oscillator strength distribution, on the other hand, involves second-row orbitals and therefore is much less peaked and does not fall off as quickly with increasing photon energy. The valence-shell effects are well illustrated by a consideration of the photoabsorption spectra shown in fig. 4, where the presently reported

157

G.R. Burton et al. /Chemical Physics 181(1994) 147-172

(a>

160

Valence

Cl 2p

i

A

Cl 2s c 1s

I

I I 80

(b)

Cl 2p II

CI2s

I

2

-

Cls

I

1 x Atomic Sum (C+4CV ls21 -------------________-__________________

I

I

I

I

I

200

250

300

350

400

PHOTON

ENERGY

(eV)

Fig. 3. Absolute photoabsorption oscillator strengths obtained at a resolution of 1 eV fwhm: (a) The measured low-resolution photoabsorption spectrum from 5.5-400 eV. (b) The inner-shell (Cl 2p, 2s; C 1s) region from 150 to 400 eV on an expanded scale. The Cl 2p absolute inner-shell photoabsorption measurements of Cole and Dexter [ 141 are shown along with theoretical [ 8 1 ] and experimental [ 821 atomic oscillator strength sums for the constituent atoms of CCl+ The vertical lines represent the positions of the indicated innershell VIPs [ 36,37 1. The &shed line in (b ) is the extrapolation of the polynomial which was fitted to the high-energy valence-shell region from 98-l 98 eV. This curve was used to estimate the contribution of the valence-shell oscillator strength above 198 eV.

low-resolution absolute oscillator strength data for Ccl, are compared with corresponding valence-shell data measured earlier in this laboratory for the chlorofluoromethanes [ 59-621. Since these molecules are all valence-shell isoelectronic, the absolute oscillator strength scale has been obtained in all cases by normalization to 33.1 electrons. It can be seen from fig.

4 that with a systematic successive replacement of fluorine by chlorine the oscillator strength distribution of the chlorofluoromethanes becomes increasingly peaked at lower photon energies and drops off much more rapidly with increasing photon energy as the number of chlorine atoms is increased: The outermost electrons in a chlorine atom are in the 3p or-

G.R. Burtaa et al. /Chemical Physics 181(1994) 147-172

r zz a

0

50

150

100

PHOTON

ENERGY

200

(eV)

Fig. 4. Absolute photoabsorption oscillator strengths for the chlorofluoromethanes in the vtience-shell repjon from S-190 eV. The presently measured low-resolution photoabsorption spectm of CC\, is shown in comparison with those of the freons, CCl,_,F, (x- l-4), which have been measured previously at the %ameresol\ltion ( 1 eV fwhm) using dipole (e, e) spectroscopy [ 59-621.

bitaI, whereas they are in a 2p orbital in a fluorine atom. The 3p orbital is more diffuse and therefore the 3p electron is on average further away from the nucleus than the electron in a 2p orbital. As the number of electrons further from the nucleus is increased the valence-shell oscillator strength distribution is seen to peak at lower photon energies with correspondingly lower oscillator strength values at higher energies. 3.3. Dissociative photoionization in the valence-shell region The technique of dipole (e, e-t ion) spectroscopy has been used to provide information on the dissociative photoionization of CCI, in the valence-shell region. Time-of-flight (TOF) mass spectra have been measured as a function of energy loss (equivalent photon energy) from 11.Q-80.0 eV. Fig. 5 shows the

TOF mass spectrum obtained in the present work at 50 eV. The cations produced from the dissociative photoionization of CCL observed in the present work are: Ccl:, Ccl:, Cl;, CCl$+ , Ccl+, Cl+, and C+. The chlorine-containing peaks show structure reflecting the 35CIand “Cl isotopic composition. The Nz and 0: peaks shown on fig. 5 (and the N+ and Of peaks at m/e 14 and 16, respectively) are due to photoionization events involving the background gases in the spectrometer. The base pressure of the dipole (e, e-t ion) spectrometer used in the present work is NN 1x 10L7 Tot-r, and the TOF mass spectral measurements were made at a total pressure of 6 x lo- 6 Torr . The background gas peaks become appreciable in TOF mass spectra where the photoabsorption cross section of the target being studied is very low, as in the present case of CC& above about 40 eV (see fig,. 1) . Tt skauld be noted that the background contributions to photoabsorption have been

G.R. Burton et al. /Chemical Physics181(1994) 147-172

159

1500

P 5

hv=SOeV 1000

8 8 Z

ki 0

500

Z

E 0 1

2

TIME

3

OF FLIGHT

4

5

(psec)

Fig. 5. The time-of-flight mass spectrum of CCl, measured at an equivalent photon energy of 50 eV. The peaks associated with foment ions containing chlorine atoms are broadened or multiply peaked because of isotopic contributions from “Cl and 37C1.

eliminated by background subtraction (see above). No Ccl: molecular ion was observed in the present work which is consistent with previous studies [ 501 which have indicated that the carbon tetrahalide ions, CX: (X =F, Cl, Br), are thermodynamically unstable and dissociate exothermically to CXT + X. The only stable multiply charged cation observed in the present work was Ccl:+. Branching ratios for the various dissociative photoionization channels were obtained by first integrating the peaks of the baseline subtracted TOF mass spectra. These branching ratio data were then corrected using the recently determined [ 771 ion detector response function for ions of different m/e. The valence-shell branching ratios are shown in fig. 6 and given numerically in table 3. The mass resolution of the TOF mass spectrometer used in the present study was sufficient to resolve all the fragment ions of a particular stoichiometry and charge, but was insuffrcient to resolve the individual peaks for all the chlorine-containing isotopomers for each ion. Therefore,

the reported branching ratios for the fragment ions from CC& include the contributions from both the “Cl and “Cl isotopes. The relative photoionization efficiency in dipole (e, e + ion) spectroscopy is given by the ratio of the total coincident ion signal to the number of forward scattered electrons as a function of energy loss. The presently measured ratio for carbon tetrachloride is found to be essentially constant above 18.0 eV, within experimental error. Making the reasonable assumption that the absolute photoionization efficiency (vi) is unity at higher energies, we therefore conclude that q reaches 1 at 18.0 eV. The absolute photoionization effticiency data from 1l-80 eV are shown as an insert in fig. 1 and are given numerically in the last column of table 1. Absolute partial photoionization oscillator strengths for the production of fragment ions from Ccl, were obtained from the triple product of the absolute photoabsorption oscillator strength, the absolute photoionization efficiency, and the branching

160

G.R. Burtonetal.

/ChemicalPhysics

181(1994)

//

100

50

0

1-L..

20 IO 0

//

IccI,J l. . . ., . .

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147-172

8 I ,/**'""".

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cc13+ .

.

.

.

1 . . . ,

I

.:**...*. ' I I

l

l

. ------__ // I ,,I

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I 1. ccl,+

l

'--------v /1 1 ,,I I I

I

___;________ $2 . /I 1 ,,I I I

I

I

2

_*

.

l**.**** . . , . .-----_-_ cc1,2+ ,, P . .' I 1 1 ,,I I I I 20

0 60 40

l,,.*, . . . . .-------<+ ..-' - lllllJ_ 11 I I I ,,I , I - 111 I I II Valence IPS l

20 0 a 4

l_**.

l

l*..

l

.

.

.

’

I

i

I

I

20

40

60

80

PHOTON

ENERGY

m+

.--------

Ill,

0

,,I

I

Cl 2p

/l

I

190

200

IPS I

210

I

220

(eV)

Fig. 6. The branching ratios for the dissociative photoionization of Ccl, in the valence-shell region from 11 to 80 eV and in the vicinity of the Cl 2p edge from 195.5 to 220.5 eV. The vertical arrows indicate the position of calculated appearance potentials for the various fragment ions (see table 4). The vertical lines represent the valence-shell and Cl 2p VIPs [20,36,37].

ratio for each ion, as a function of photon energy. The absolute partial photoionization oscillator strength data thus obtained for the valence shell of Ccl4 from 11 to 80 eV are shown in fig. 7 and given numerically in table 1. The molecular and fragment ion appearance potentials from Ccl, measured in the present work are compared in table 4 along with literature values

[38,39,41-45,501. It can be seen that there is excellent agreement (within the present experimental uncertainty of k 1 eV) between the present ion appearante potentials and those reported previously [ 38,39,4 l-45,50] using electron-impact and ph& toionization methods. Table 4 also includes the calculated appearance potentials for all possible fragmentation processes leading to the production of a

G.R. Burton et al. /Chemical Physics 181(1994) 147-172

161

Table 3 Branching ratios for the dissociative photoionization of CCL, from 11.0 to 80.0 eV Photon energy (eV)

Photoionization branching ratio (%) C+

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

Cl+

0.10 0.10 0.14 0.30 0.38 0.51 0.37 0.60 0.75 0.85 1.15 1.52

ccl+

0.02 0.09 0.08 0.25 0.7 1 1.42 2.52 3.92 5.83 7.05 7.66 a.73 9.35 10.01 11.13 11.63

ccl:+

a:

0.04 0.00 0.08 0.08

ccl:

ccl:

0.11 0.02 0.14 0.15 0.47 2.29 3.95 7.98 11.07 14.30 16.52 18.58 19.71 20.31 20.52 20.55 20.28 20.08 19.91 19.50 19.49 19.41 19.69

100.0 100.0 100.0 100.0 100.0 99.89 99.98 99.86 99.85 99.53 97.71 96.05 92.00 88.84 85.63 83.23 80.61 78.77 77.03 75.26 73.24 72.16 71.88 70.76 70.37 69.65 68.22 67.09

particular singly charged cation of CCL. These values have been calculated from thermodynamic data [ 921, assuming zero kinetic energy of fragmentation, and are indicated by vertical arrows in figs. 6 and 7. 3.4. The dipole-induced breakdown of carbon tetrachloride The total absolute photoionization (i.e. absolute photoabsorption x photoionization efficiency ) oscillator strength distribution for an atom or molecule can, in general, be partitioned in two different but complementary manners. By measuring the photoionization branching ratios, as in the present work for Ccl, using dipole (e, e+ion) spectroscopy, absolute partial oscillator strengths for molecular and dissociative photoionization can be determined.

Photon energy (eV)

Photoionization branching ratio (%) C+

Cl+

Ccl+

25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0 48.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0

0.10 0.14 0.21 0.35 0.51 0.75 0.99 1.19 1.43 1.51 2.07 2.36 2.32 2.53 2.55 2.50 2.35 2.22 2.66 2.74 2.58 2.87 2.97 2.84 3.28 3.01

1.84 2.29 2.68 3.11 3.46 3.84 4.28 4.92 5.31 5.98 6.28 10.55 13.98 17.12 20.90 24.15 26.27 27.60 28.07 28.35 27.67 28.69 28.54 29.40 29.89 30.30 30.30

12.19 12.70 12.89 13.28 13.57 13.64 13.63 13.52 13.27 13.18 13.15 12.18 12.55 12.62 13.41 14.09 15.57 16.21 16.32 16.53 16.78 15.99 15.86 15.04 15.26 14.67 15.23

ca:+

cl:

ccl:

0.18 0.35 0.49 0.54 0.77 0.56 0.62 0.56 0.40 0.52 0.26 0.67 0.59 0.56 0.82 0.65

0.09 0.12 0.16 0.22 0.13 0.25 0.26 0.29 0.41 0.50 0.40 0.58 0.58 0.95 0.80 0.79 0.63 0.57 0.69 0.56 1.05 0.62 0.89 0.90 0.87 0.38 0.65

20.14 20.60 20.94 21.47 21.88 22.12 22.48 22.47 22.77 22.73 22.82 23.11 23.52 24.32 24.78 24.60 23.77 23.03 22.28 21.60 20.44 18.41 17.15 16.38 15.90 15.07 14.87

ccl: 65.74 64.19 63.19 61.70 60.60 59.65 58.60 57.80 57.06 56.17 55.84 51.34 46.65 42.18 37.05 33.07 30.69 29.61 29.87 29.90 30.81 33.45 34.02 34.73 34.69 35.48 35.31

However, it is also possible to partition the total absolute photoionization oscillator strength distribution into the partial electronic ion state oscillator strengths by measuring the branching ratios for the production of the different electronic states of the molecular ion. The latter data can be obtained using tunable energy PES or the equivalent electron-impact technique of dipole (e, 2e) spectroscopy [ 531. Details of the dipole-induced breakdown scheme for a molecule can be deduced from these two sets of data since the partial oscillator strength for a specific molecular or dissociative ion will involve contributions from the breakdown of the various electronic states of the molecular ion [ 53,73,89,93]. Once the photon energy exceeds the upper limit of the Franck- Condon region for the production of a given electronic state of the molecular ion, its internal energy distribution

162

G.R. Burton et al. /Chemical

150 100

5

50

147-172

150

“\ ICC

A

I

Physics lSl(l994)

- . _

1: .

ca3+ - 50 l**_______ ______ 0

\

.

100

I

I

I

I

30

l

. *.

20

\ . : 1: **a.....

cm,+ -

.

l

-

10

l

.

.

.

.

I

I

.

. I

0.2

0

0.2

0.0 0.1

0.0 12

0.0 12

8 4 0 4

0 0.8

f -

11; I

8

\

l**. . . . . . .

. . . .

III I I I I

cc!+ 4 0

+

- ,,&pY.

. .. l

I

I

l cl.

4

I

0

0.8 0.4 0.0 20

40

PHOTON

60

ENERGY

80

(eV)

Fig. 7. Absolute partial photoionization oscillator strengths (cross sections) for the dissociative photoionization of Ccl, in the valenceshell region from 1 l-80 eV. The vertical arrows indicate the positions of calculated appearance potentials for the various fragment ions (see table 4). The vertical lines represent the valence-shell VIPs [ 20,361. The contribution from the (6a, ) - ’ ion state to the production of Cl+, estimated in the present work, is indicated by the dashed line in the POS for the Cl+ cation.

is independent of the photon energy and the remainder of the energy is carried away by the ejected photoelectron. Thus the branching ratios for ionic photofragmentation into stable molecular and/or fragment ions are also determined for each electronic state of the molecular ion. Thereafter only the overall probability of direct ionization to that state will vary with increase in photon energy for each electronic state.

Thus the partial oscillator strengths for the production of a given molecular or fragment ion are expetted to be given by a fixed linear combination of partial electronic ion state oscillator strengths (or vice versa). It should be noted however that local departures from the above simple breakdown model, based on linear combinations of oscillator strengths, are to be expected at lower photon energies where autoion-

32.0

Cl++cCls Cl++cClr+Cl Cl++cCl+c1r CI++CCl+2CI c1++c+c1+c1* Cl++C+3Cl

19.62 22.09 24.57

16.01 18.12 19.17 21.65 23.80 26.28

25.5

23.5

19.0

PhIon [44,91] =’

PhIon 1451

EI 1501

23.5kO.2

19.1 to.2

34.0* 1.0

17.1f0.2 19.5f0.2

16.4f0.5

19.3kO.02

16.10+0.01

11.83+0.05

11.67kO.l

23.05 f 0.07

19.35f0.05

16.10f0.02

11.90f0.07

16.10f0.2

11.65f0.10

11.47+0.01

*) Values calculated using thermodynamic data for the enthalpy of formation of ions and neutrals (taken from Lias et al. [ 92]), and assuming zero kinetic energy of fragmentation. t-bMeasured using electron-impact (EI) or photoionization (PhIon) methods. c) Reported as IP of CCC.

(19) c++2Cl* (20) c++Cl,+zc1 (21) c++4Cl

(13) (14) (15) (16) (17) (18)

(12) ca:+

(10) cCl++ciz+c1 (ll)cCl++3Cl 17.0 19.0

31.8f 1.0

23.0

14.16 17.69 19.84 22.32

(5)Cl:+cCl, (6) Cl: +CCl+Cl (7) cl: +c+c12 (8) Cl: +C+ZCl

15.11 17.59

23.0+ 1.0

13.5 16.0

13.04 15.52

(3) cCl: +Cl* (4) ccl: +2Cl 16.0f0.2

11.0

10.84

(9) ccl:+

EI 1431

11.28~bO.03 11.6f0.3

EI 1411

12.2f0.2

EI ]401

11.5fO.l

EI ]391

11.0fl.O

EI [381

experimental b,

(2) cCl: +c1

this work +leV

11.47

calculated a)

Appearance potential (eV)

(l)Ccq’

Products

Table 4 Calculated and measured appearance potentials for the production of positive ions from CCL

164

G.R. Burton et al. /Chemical Physics181(1994) 147-l 72

ization occurs, and also at higher photon energies if significant multiple photoionization processes occur [ 73 1. A series of studies of molecular photoabsorption and photoionization (see, for example, refs. [ 5962,74,89] ) have shown that useful quantitative information on the dipole-induced breakdown pathways of a molecule can be deduced on the basis of the simple breakdown model outlined here. 3.4.1. The proposed dipole-induced breakdown scheme for the low-energy valence region The various types of theoretical and experimental data which can assist in the elucidation of the dipoleinduced breakdown pathways of a molecule have been summarized previously [ 891. Of these, the present work only provides branching ratios (fig. 6 and table 3 ) and partial oscillator strength curves for dissociative photoionization (fig. 7 and table 1 ), as well as the appearance potentials (table 4) for the many fragment ions formed from Ccl+ Additional insights can be gained from the photoelectron-photoion coincidence (PEPICO) work of Kischlat and Morgner [ 491, which lists the electronic states that contribute to the production of the CCI: (12= l-3) cations for photon energies below 21 eV. Although PES studies of the photoionization cross sections for production of the electronic states of Ccl: have been reported on an absolute scale by Carlson et al. [ 25 ] in the gas phase, and on a relative scale for solid films by Fock and Koch [ 29 ] neither of these data sets included the contributions from the inner-valence region. As such, the reported [25,29] electronic ion state branching ratios are in error above z 30 eV. Furthermore, the sum of the absolute partial cross sections reported by Carlson et al. [ 251 in the region below 30 eV is not consistent with the TRK sum rule since it is only x 60% of the total photoabsorption cross section determined in the present work (fig. 1, table 1). Therefore, we have recalculated the electronic state partial oscillator strengths from both [ 25,291 photoelectron studies as follows. Firstly, the contribution from the inner-valence ( 5t2 and Sa, ) orbitals was estimated from the sum of the presently measured partial oscillator strengths for Cl:, C+, and part of Cl+ (see discussion below) and was subtracted from the total photoionization oscillator strength distribution determined in the present work. In addition the Ccl:+ yield was subtracted since doubly charged ions are not

expected to arise from the decomposition of singly charged ions. The total photoionization sum, corrected in this fashion, was then multiplied by the respective electronic ion state branching ratios for the outer valence orbitals, according to the data reported by Carlson [ 25 ] and by Fock and Koch [ 29 1. In this way, new partial oscillator strengths for the production of each of the five outer valence states of Ccl,’ were derived which are consistent with the present total photoionization results. From an examination of the shape of the branching ratio curves (fig. 6, table 3), and the shapes and magnitudes of the partial oscillator strength curves (fig. 7, table 1) for the dissociative photoionization of CCL,, as well as the VIPs [ 20,361 and the FranckCondon widths [ 361 of the electronic ion states, certain aspects of the dipole-induced breakdown scheme for CCL, may be deduced as follows. Firstly, the molecular ion CC12 is unstable [ 501 since it is not observed on the time scale of the TOF mass spectrometer used in the present work. The branching ratio (fig. 6 and table 3 ) of the Ccl: cation is 100% from 11.0-l 3.0 eV indicating that ionization from the 2t; ’ and 7t; ’ states yields only this ion. The branching ratio remains greater than 97% up to 16.0 eV suggesting a further major contribution from the 2e-’ state. The change in slope of the partial oscillator strength (POS) curve for Ccl: (fig. 7) at z 20 eV corresponds closely to the energy positions of additional peaks observed in the relative partial electronic ion state oscillator strength spectra for the 2t; I and 7t; ’ states in solid CCL, [ 29 1. These maxima have been assigned [26] to shape resonance channels. The present observations concerning the production of Ccl: are in good agreement with PEPICO studies [ 491, where it was found that the Ccl: cation was produced by ionization from the 2t, ’ , 7t, I, and 2e-’ states. Contributions to the Ccl: cation from the 2t, ’ , ?t, ’ , and 2e- * states have also been suggested by charge-exchange mass spectroscopy experiments [ 941. The appearance potential (AP) of the Ccl:+ dication is x 32 eV, which is above the highest valence-shell VIP of CCL,. It is reasonable to assume that doubly charged ions are formed directly by singlephoton absorption and not via singly ionized electronic states. Therefore the production of the Ccl:+

G.R. Burton et al, /Chemical Physics l&(1994)

0 cc14

147-l 72

165

Molecule in Ground State

Im~fl/~//I’ Dipole (e,e+iou) Fragmentation (%)

Fig. 8. The principal dipole-induced breakdown pathways following valence-shell photoionization of CC&below 80 eV.

dication has not been attributed to ionization from any of the valence-shell states. The Ccl,’ cation has an AP of 13.5 eV, which, along with the shape of the partial oscillator strength curve for this ion (fig. 7 ), indicates a very small contribution from the 2e-’ state. The major increase in the POS curve of CClf at z 16 eV suggests a dominant contribution to the Ccl,’ POS from the 6tF’ state. From an examination of the branching ratio curve for the Ccl,’ cation, and the small shoulder in the POS curve for Ccl,’ at z 25 eV, these features are perhaps attributable to a contribution from the 5t11 state and possibly also from the 5ai’ state. The PEPICO experiments [ 49 ] confirm a contribution to the production of CClf from the 6tF ’ state. The AP of the Ccl+ cation at = 17 eV, and the shape of the POS curve for this ion, suggest that a small portion of Ccl+ comes from the 6ty1 state. However, the steep rise in the POS at = 20 eV clearly indicates that the dominant contribution to the Ccl+ POS comes from the 6ai’ state. The PEPICO experiments [ 49 ] confirm that the Ccl+ cation is formed dominantly from the 6ai1 state, and the relative in-

tensity of this contribution is quoted [ 49 ] as about one half of the 6tF 1 state contribution to the POS for Ccl,‘, which is in good agreement with the present work (see fig. 7). The Cl+ cation has an AP of 19 eV which suggests a contribution to the production of this ion from the 6a;-’ state. The POS curve (fig. 7) for this ion shows a change in slope at z 25 eV, which suggests a further major contribution from the 5tF1 state. The Cl; and C+ cations have very similar APs ( x 24 eV), which suggests contributions to the production of these ions from the 5t, ’ state although contributions from the 5ai’ state (which is close in energy) cannot be discounted. It should be noted that the shapes of the POS curves for these two ions are somewhat different on their leading edges. The shape of the POS curve for the Cl,’ cation suggests a contribution from each of the 5tg ’ and 5ai * states, whereas the dominant contribution to the POS for C+ apparently comes from the 5ai ’ . This is logical since the AP of the C+ cation is at the high-energy side of the Franck-Condon region of the 5t11 state, and thus any contribution from this electronic ion state will likely be small.

G.R. Burton et al. /Chemical Physics 181 (1994) 147-172

166

150 -

cz

(a>

CCI,

::h , ‘.

‘; ,100 al

(b) G 30 -I 6 ^I

/L\

cc>

m

:*fg*

6

0

.:

0

. . . . .

.

.

f-2 !

-

l oO-**

4

I

I

I

(d) l

C1,++0.92CI++C+

This

Work

2 0

I 10

I 20

.**

I 30

PHOTON

I 40

ENERGY

I 50

I 6C

(eV)

Fig. 9. Oscillator strength sums determined from the proposed dipole-induced breakdown scheme for CCL below 80 eV (see fig. 8 and section 3.4 for details). The presently measured absolute partial photoionization oscillator strength sums are compared with electronic ion state oscillator strength sums determined from the PES measurements reported by Fock and Koch [29] (solid line) for a solid fdm of Ccl4 and by Carlson et al. [ 2.51 (open circles) for gas phase Ccl,. It should be noted that the electronic ion state data of references [25,29] have been reanalyzed as branching ratios including the inner-valence contributions as derived from the present measurements. These revised electronic ion state branching ratio data have then been placed on an absolute photoionization oscillator strength scale by using the presently measured absolute photoabsorption data (see text for details).

The principal contributions from the different electronic ion states to the various ionic photofragmentation products of CCL, suggested by the above analysis are summarized in the proposed dipole breakdown scheme shown in fig. 8. In summary, the above considerations suggest that (1) The (2t, )-I, (7&)-l, and essentially all ofthe

(2e) - l electronic ion states of Ccl: dissociate to give Ccl: as the only charged product. (2 ) Almost all of the Ccl: is formed from the (6t2)-i state. (3 ) The (6a, ) - ’ state dissociates to give essentially all the Ccl+ and part ( ~OAI) of the Cl+. (4) The Cl: and C+ cations and the remaining

G.R. Burton et al. /Chemical Physics181(1994) 147-I 72

167

VALENCE

(a >

hv=

195.5

- 100

e\, Cl 2p+Cont. hv=210.5

Cl 2p+-Cont. hu=215.5 eV

(4 hv=200.5

eV

eV n

(C>

Cl Zp+Rydberg Cl+ 1

hv=205.5

hv=220.5

eV

e

-0

TIME

OF FLIGHT

(,usec)

Fig. 10. Timeof-tlight mass spectra measured in the vicinity of the Cl 2p edge of CCL,:(a) in the valence-shell region at 195.5 eV, (b) in the region ofdiscrete excitation at 200.5 eV, (c) in the region ofdiscrete (Rydberg) excitation at 205.5 eV, (d), (e), and (f) at 210.5, 2 15.5 and 220.5 eV, respectively, in the ionization continuum region above the Cl 2p edge at w 208 eV.

portion (92%) of the Cl+ cation are formed from processes involving the breakdown of the (5t,) -’ and (5ar )-’ states. Therefore this oscillator strength sum (Cl,+ +C+ +0.92Cl+) provides an estimate of the summed inner-valence ( 5tl’ + 5ai ’ ) electronic ion state partial oscillator strengths. This estimate has been used (see above) in the calculation of the electronic ion state partial oscillator strengths from the previously published PES data [ 25,291. These fmdings are illustrated by the oscillator strength sum comparisons shown in fig. 9. It can be seen that quite good agreement exists for Ccl,’ and Ccl: with the respective electronic ion state oscillator strength sums. The situation is less satisfactory in the case of (6a, ) -’ (see fig. 9). In the case of the

present estimate for the electronic ion state partial oscillator strength sum ( 5tF1 + 5ai’) it should be noted that the resulting shape is consistent with expectations for inner-valence orbitals having large Cl 3s and C 2s contributions. In making these comparisons it should be noted that the electronic ion state partial oscillator strengths from Fock and Koch [ 291 are for solid and not gaseous CCL and thus some shape differences might be expected. In addition, the proposed dipole breakdown scheme should only be regarded as a first approximation reflecting the major breakdown processes (see ref. [ 591 and references therein for a discussion of limitations to this approach and likely sources of error). Undoubtedly there are other smaller contributions to the various

168

G.R. Burton et al. /Chemical Physics lSl(l994)

147-172

20

A ‘; > aI

lo

N I 0 t

0

0

1

’

2

0

-

2s Eo

Z

6 LT

b-l

1

1

0

0

g

0.2

2

0.1

2

0.0

E

0.4

d

-

(x2+ .-c

I

I

0 t

z: 0

0.2

z

0.1

g

0.0

0

0.4

z

0

0 c

z 0.0 2 0 F 5 Fi 0 b I

n-

1 O

10

5 0

-

v------

cl+

.-•

.__.----.-----.

I

I

I

PHOTON

220

210

200

0

ENERGY

(eV)

Pig. 11. Absolute partial photoionization oscillator strengths (cross sections) for the dissociative photoionization of CCL, in the CI 2p region from 195.5-220.5 eV (see table 6). The top panel shows the total (valence plus Cl 2p) absolute oscillator strength spectrum from 194-224 eV. The solid line in the top panel represents the tit to the valence-shell region from 98-198 and gives an indication of the valence-shell contribution in the 195-220 eV region. The vertical lines in the top panel indicate the positions of the Cl 2p VIPs [ 371.

breakdown channels and a more detailed understanding can onhi be provided by high-resolution photoelectron-photoion and photoion-photoion coincidence experiments.

3.4.2. The dissociativephotoionization of Ccl, in the Cl 2p inner-shell region Time-of-flight mass spectra (fig. 10 ) have been recorded at 195.5 eV (valence-shell continuum, just below the Cl 2p edge), 200.5 eV (Cl 2p-+o* excitation), 205.5 eV (Zp-*Rydberg excitation), and at 210.5, 215.5, and 220.5 eV in the (Cl 2p)-’ ioniza-

G.R. Burton et al. /Chemical Physics181(1994) 147-I 72

169

Table 5 Branching ratios for the dissociative photoionization of CC& from 195.5 to 220.5 eV Photon energy (eV)

Photoionization branching ratio (%) C+

cl+

ccl+

ccl;’

cl:

El:

CCl:

195.5 200.5 205.5 210.5 215.5 220.5

3.32 6.62 8.08 6.74 6.29 7.44

32.38 40.32 50.51 57.37 59.02 57.55

16.18 23.34 17.64 11.91 13.53 13.25

0.53 0.75 1.70 1.89 1.43 2.11

1.09 0.23 1.93 1.25 1.oo 1.14

13.57 10.79 5.87 11.73 10.84 9.94

32.94 17.95 14.28 9.11 7.90 8.57

Table 6 Absolute oscillator strengths for the total photoabsorption and the dissociative photoionization of CCL from 195.5 to 220.5 eV a) Photon energy (eV)

195.5 200.5 205.5 210.5 215.5 220.5

Oscillator strength (lo-’ eV-‘) b, Photoabsorption

C+

cl+

ccl+

ccl:+

a:

ccl:

ccl:

1.36 2.72 3.11 9.98 11.48 11.38

0.05 0.18 0.25 0.67 0.72 0.85

0.44 1.10 1.57 5.72 6.78 6.55

0.22 0.64 0.55 1.19 1.55 1.51

0.01 0.02 0.05 0.19 0.16 0.24

0.01 0.01 0.06 0.12 0.11 0.13

0.18 0.29 0.18 1.17 1.24 1.13

0.45 0.49 0.44 0.91 0.91 0.98

‘) Assuming a photoionization efficiency of unity. ‘) o(h4b) = 109.75 df/dE (eV-I).

tion continuum, in order to investigate the changes in the dipole-induced breakdown in going from valence-shell ionization to Cl 2p excitation and ionization. It can be seen (fig. 11) that significant changes in the fragmentation pattern occur particularly with regard to the increased relative yields (see also the branching ratios in fig. 6) of Cl+ and C+ and also in the significant increase in the yield of Ccl:+ in the inner-shell region. The branching ratio (fig. 6, table 5 ) and partial oscillator strength (fig. 11, table 6 ) data for the ionic photofragmentation products of CCL, measured in the vicinity of the Cl 2p edge provide information on the dipole-induced breakdown of CC& in the Cl 2p inner-shell energy region. It has been assumed that the absolute photoionization efficiency is 1.O throughout the 195-220 eV energy region. From fig. 6 it can be noted that the branching ratio curves from 195-220 eV for the dissociative fragment ions can be grouped into three classes, depending on their trend as the photon energy increases: ( 1) ions whose

branching ratio curves markedly decrease relative to the valence-shell contribution (Ccl,’ ), (2) ions whose branching ratio curves remain approximately Cl;, and Ccl’), and (3) ions constant (Ccl:, whose branching ratio curves increase (Ccl:+, Cl+, and C+ ). These trends indicate that in general, as the photon energy is increased to access the Cl 2p innershell excitation and ionization regions, the production of low m/e and multiply charged cationic fragments is favoured over the production of larger, higher m/e cations. For example, in the 1 l-40 eV region (see fig. 6 ) the dominant ion is Ccl:, however in the 195-220 eV region the dominant ion is Cl+. When analyzing the partial oscillator strength data from 200-220 eV it should be remembered that the ions detected in the TOF mass spectrum can be produced by excitations in both the Cl 2p region and the underlying valence-shell continuum region. However, the contribution from the underlying valenceshell continuum is quite small and, as shown in the

170

G.R. Burton et al. /Chemical Physics I81 (1994) 147-172

top panel of fig. 11 (and fig. 3b), comprises only z 101 of the total photoabsorption at 220 eV. The partial oscillator strength curves for all the fragment ions show an increase over the 195.5-220.5 eV region. However, the Ccl: cation shows the smallest relative increase with x 50% of the ion intensity at higher energies still coming from the underlying valence-shell continuum. The POS distributions for the other ions (Ccl:, Cl:, Ccl:+, Ccl+, Cl+, and C’ ) each show much more pronounced increases as the photon energy increases through the Cl 2p region. At 200.5 eV, in the region of the Cl 2p+o* virtual valence orbital excitation [ 15,16 ] local increases in ion yield are observed for all ions except Cl: and possibly Ccl: with local enhancements due to fragmentation from the CT*excited state being particularly noticeable for Ccl: and Ccl+. Whereas only minor further differences in fragmentation are observed in the region of Cl 2p-+Rydberg excitation at 205.5 eV, very significant changes occur above the (Cl 2p) --I ionization edges as can be seen from the data points (fig. 11) at 210.5, 215.5, and 220.5 eV. In considering both the POS (fig. 11) and branching ratio (fig. 6 ) curves above the (Cl 2p) - ’ edge it can be seen that the dominant product consists of Cl+ atomic cations which are (table 6) more than 15 times as intense as in the valence-shell region at 195.5 eV. While the absolute intensities are lower, similar large relative increases in intensity are also observed above210eVforCCl~,C12+,CCl~+,CCl+,andC+. The large yield of Cl+ reflects the strong tendency for the decomposition of the Cl 2p hole to involve further carbon-chlorine bond breaking since Ccl: is the most abundant species (in the absence of the molecular ion Ccl: ) throughout the valence-shell region. The large relative increase in the yield of Ccl:+ above the (Cl 2p)- ’ edge (fig. 11) can be attributed to Auger decay processes. Detailed information concerning the contributions from double dissociative photoionization to the singly charged ion yields could be obtained from photoion-photoion coincidence (PIPICO) experiments. The much lower yield of doubly charged ions (see also fig. 11) from the Cl 2p+dc excited state at 205.5 eV is consistent with earlier findings for core excited molecules which have been found to preferentially decay to singly charged products [ 951 via participator or spectator resonance Auger processes [ 9 1,96,97 1.

Acknowledgement Financial support for this work was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada and by the Canadian National Networks of Centres of Excellence (Centres of Excellence in Molecular and Interfacial Dynamics). A Graduate Fellowship from the University of British Columbia (WFC) and a NSERC Postgraduate Scholarship (GRB) are gratefully acknowledged.

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