Ionization potentials of carbon tetraiodide and tetraiodoethylene in the solid state

Chemical Physics 113 (1987) 445-451

North-Holland, Amsterdam

445

IONIZATION POTENTIALS OF CARBON TJWRAIODIDE AND TETRAIODOETHYLENE IN THE SOLID STATE

Naoki SAT0 ’

Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami, Kumamoto 860, Japan

and

Hiroo INOKUCHI

Institute for Molecular Science, Myodaiji, Okazaki 444, Japan

Received 17 November 1986

Ultraviolet photoelectron spectra measured for vapor-deposited thin films of carbon tetraiodide and tetraiodoethylene allowed to determine the threshold ionization potentials in the solid state to be 5.5a and 5.0, eV, respectively. The spectral

features do not necessarily correspond to each other between the gaseous and solid states; the energy differences between the

adiabatic ionization potentials in the gas phase and the threshold ionization potentials are obtained to be 3.5, and 3.5, eV for

the respective compounds above. These values are much larger than the polarization energies of any other organic solids

reported so far. The large energy differences are explained by a strong intermolecular interaction working in those solids, extra

to the van der Waals force; this is supported by the results of their crystal structure analyses.

1. Introduction

The ionization potential of an organic molecular solid is generally smaller than that of its con- stituent molecule in the gas phase. The difference corresponds to a stabilization energy for a molecule ionized in the solid, resulting from electrostatic polarization on the surrounding molecules induced by the ionic charge; the energy is termed polarization energy [l]. The polarization energy makes clear its own physical meaning, as the energy difference caused only by the electrostatic polarization, for the molecular aggregate, bound by a weak intermolecular interaction, in which the molecular identity still persists, in particular, with respect to the electronic structure. Such a condi- tion is realized in the van der Waals solids.

We have hitherto studied the electronic structure in organic solids using ultraviolet photoelectron spectroscopy (UPS) (cf. for example, refs.

’ Present address: Department of Chemistry, College of Gen-

eral Education, The University of Tokyo, Meguro, Tokyo

153, Japan.

[2-41). The photoelectron spectral features between the gaseous and the solid states for most organic compounds correspond to each other, ex- cept for an energy shift on the ionization potential scale as well as a peak-broadening for the solid spectra. This observation is corrected with the polarization energy. Previous studies have derived an expression of the polarization energy P, as the difference between the adiabatic ionization potential of a molecule in the gas phase 1: and the threshold ionization potential of its solid lih, principally by examining the line shape for solid- state photoelectron spectra [3]. Furthermore, they have determined polarization energies of a good many organic solids, most of which belong to organic semiconductors. As an example, it was clearly recognized that the polarization energies of condensed polycyclic aromatic hydrocarbons with a planar molecular structure are nearly constant, = 1.7 eV, in spite of the difference in molecular sizes as well as crystal forms [4].

In such a series of studies, p-iodanil has been found to show the largest P, value (3.0 eV) in the polarization energies of organic solids determined

0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

446 M. &to, H. Znokucki / Z~~~~~t~~n pote~tja~ of solid C14 and C,I,

so far 121. Furthermore, an UPS study of solid iodine has recently reported a drastic lowering (A1 = 2.92 eV) of ionization potential from the value for the gaseous molecule [5]. These values are nearly twice as large as the polarization energy for most of organic solids. This behavior appears to be peculiar to iodine-containing compounds.

In the present study, we have selected a few polyiodo carbon compounds which have simple molecular structures, i.e. carbon tetraiodide (CI,) and tetraiodoethyl~e (C,I,), in order to confirm this and to explain the origin for the large values above. Thus, the measurements of ultraviolet photoelectron spectra of these solids have permitted a comparison with their gas-phase spectra already published [6,7], which determines energy difference corresponding to the polarization energies. The results will be discussed under consideration of intermolecular interaction in view of the crystal structures.

2. Ex~rimen~

Carbon tetraiodide and tetraiodoethylene were commercially obtained from Aldrich Chemical Co., Inc. and Pfaltz & Bauer, Inc., respectively. Each company supplied also bromine derivatives of the respective materials; the bromine compounds allowed an examination of the effect to be expected on the electronic structure from the substitution by different halogen atoms. Sublimation under a vacuum of ( 3 x 10p4 Pa purified the materials. The heating temperature needed careful control, since the iodine compounds, in particular carbon tetraiodide [6,8,9], are very sensitive to thermal decomposition.

A thin film vapor-deposited upon the surface of a small copper disc permitted in situ measurements of ultraviolet photoelectron (UP) spectra for each compound. For the iodine compounds, initial thicknesses of the films, i.e. = 100 nm estimated from the frequency shifts gauged on a quartz-oscillator monitor just after deposition, decreased gradually; this did not affect the UP spectra as long as the arranged me~urements were completed. On the other hand, in the case of the bromine compounds, it was difficult to keep films

from subliming out for a long time, since they have rather high vapor pressures and the apparatus used in the present work was not able to provide the substrate with a low temperature to cool it. Hence, much thicker films deposited from their ethanol solutions outside of the vacuum chamber were useful for measuring UP spectra only to determine threshold ionization potentials of the compounds; when they decreased their thickness in a vacuum as far as they were still thick enough to suppress electron emission from the substrate through them and at the same time thin enough to get rid of the surface charging effect [lo], they disclosed reliable UP spectra at a few longer wavelengths in the vacuum ultraviolet region.

Solid-state UP spectra were obtained for the above films with an apparatus which was a slightly improved version of that reported in detail previously [ll]. The light source, a water-cooled capillary hydrogen discharge tube with a 0.5 m Seya-Namioka mon~hromator, was used in the photon energy range from 6 to 10 eV. The electron energy was analyzed with a spherical retarding- field analyzer with an ac modulation method. This permitted experimental errors in the observed energy values of < 0.2 eV.

3. Results and discussion

Figs. 1 and 2 depict UP spectra observed for carbon tetraiodide and tetr~odoethylene thin films, respectively, prepared from vapor deposition in the same vacuum chamber as these measurements were made. The abscissa indicates ionization potential in the solid state determined from original spectra as described afterward. The ordinate represents the photocurrent differenti- ated with respect to the retarding voltage, and this quantity is equivalent to number intensity of the, electrons emitted. However, the scale of the ordinate is arbitrary for each spectrum, since the intensity is not normalized with, e.g., quantum yield for phot~~ssion.

The spectra for both compounds are of unre- solved broad features, in the photon energy range from 6 to 10 eV. This is not due to insufficiently

M. Sate, H. Inokuchi / Ionization potentials of solid Cl, and C, I4 447

r

10 8 6 L

Is /eV

Fig. 1. Ultraviolet photoelectron spectra of a carbon tetraiodide vapor-deposited thin film. Incident photon energy is

written beside each spectrum.

high resolution of the analyzer, since well-resolved spectra can in practice be obtained for other organic compounds, e.g., naphthacene, using the

9 76

86 -

10 a 6 L I, /eV

Fig. 2. Ultraviolet photoelectron spectra of a tetraiodoethylene vapor-deposited thin film. Incident photon energy is written

beside each spectrum.

same apparatus. An only peak observed in each spectrum not always allows to be uniquely assigned to the occupied electronic structure of the solid, but often includes much more contribution from electrons travelling up to the surface with suffering energy loss by the secondary electron scattering [12]. Accordingly, spectral features observed on the scale of energy lower than the peak energy contain the occupied electronic structure.

Carbon tetraiodide shows a few spectral features to be barely resolved as indicated by arrows in fig. 1. These positions were determined from a plot of peak and shoulder positions on the retarding voltage scale for original spectra versus incident photon energy, as shown in fig. 3; such a graphical method is also useful to determine the threshold ionization potential I,‘” by plotting the stopping voltage V,, i.e. a low-energy threshold, and the saturation voltage V,, i.e. a high-energy cutoff, in the same way as above, where the relation among these quantities is as follows:

Z,” = hv - e(V, - V,). (1)

This was in practice used for the determination of I,‘” also for the other compounds.

On the other hand, the molecular UP spectrum of carbon tetraiodide in the gas phase [6] exhibits the first three bands to be compared with the corresponding features in the solid-state spectra, as shown in fig. 4. These bands comprise seven sub-bands located at 9.10, 9.19, 9.58, 9.78, 9.89, 10.76, and 10.75 eV on the gas-phase ionization

I I 5 6 7 8 9 10

hv/eV

Fig. 3. Graphical determination of the threshold ionization potential and peak positions for the photoelectron spectra of solid carbon tetraiodide. 0, stopping voltage, A, peak or

shoulder position, and 0, saturation voltage.

448 M. Sato, H. Inokuchi / Ionization potentials of solid CI, and C,I,

Is /eV 9 7 5

9.18

I,/eV

Fig. 4. Comparison of the first three features in the photoelectron spectra of carbon tetraiodide between the solid state and the gas phase. The gas phase spectrum is reproduced from ref.

tel.

potential (Is) scale. The first three sub-bands are assigned to the orbital which is solely located on the iodine atom (especially I 5p), with the spin-orbit split between the first two sub-bands and the other, and also with the Jahr-Teller splitting for the first two. The next three sub-bands are assigned to the orbital dominated by iodine lone pair contributions, with the spin-orbit split between the first two of them and the other (the sharp component of the third band), and again with the Jahn-Teller splitting for the two. The last sub-band (the broader feature for the third band) is assigned to the iodine lone pair orbital.

The first three features in the solid-state spectra at 6.1, 6.9, and 7.7 eV do not entirely coincide with the first three bands in the gas-phase spectrum, no matter how energy scales are shifted each other. When the first and the third bands are nearly matched to each other for both phases, the second bands obviously do not, as shown in fig. 4. Such a discrepancy is uncommon for most organic compounds: in the case of polycyclic aromatic hydrocarbons, e.g., naphthacene [13], peak posi-

-I

tions in the UP spectra for both phases coincide with each other, as described above. This coinci- dence originates in the weak intermolecular interaction, i.e. van der Waals force, binding those solids. Hence, the discrepancy observed for carbon tetraiodide suggests the possible existence of an additional intermolecular interaction extra to the van der Waals force in the solid, and some mixing of molecular orbitals between molecules through the interaction.

The above disaccord of the first three features between the gas-phase and the solid-state UP spectra of carbon tetraiodide can qualitatively be understood by an increase of the spin-orbit splitting on going from the gas to the solid, enhanced by the additional intermolecular interaction: the increase of the spin-orbit splitting involved in the first three sub-bands leads to the lowering of the weighted center of the second band relative to the first band, if the highest orbital located solely on the iodine atom suffers the intermolecular interaction more efficiently than the next orbitals dominated by iodine lone pair contributions.

In this connection, UP spectroscopic studies on crystalline iodine films [5] have also reported such discrepancies. These are explained with the mixing of molecular orbitals between neighboring molecules, i.e. an additional intermolecular interaction extra to the van der Waals force. This is supported by the crystal structure [14], which shows that the nearest interatomic distance for two adjacent iodine molecules in the UC plane (356 pm) is smaller than twice the van der Waals radius of iodine [15] (2r,,, = 396 pm), and also by results from theoretical calculations for the electronic structure of iodine crystals. Crystal structures are therefore instructive for intermolecular interac- tions in these solids.

In addition, UP spectra of solid p-iodanil were not able to be compared with the gas-phase spectrum in the previous work [2], since they are much less in feature just like UP spectra of solid tetraiodoethylene as shown in fig. 2. The broad features peculiar to the UP spectra of solid p-iodanil, however, may be affected by an additional intermolecular interaction extra to the van der Waals force, other than by the interaction between photoinduced cations and the lattice vibrations, the

M. Suto, ZZ. Znokuchi / Ionization ~oie~fj~~ of solid CZ4 and C,Z, 449

cation-site i~omogeneity, and so on [l&17] as is the case in the common molecular solids. Here again, the crystal structure supports the existence of an extra intermolecular interaction. The crystal structure of p-iodanil has reported an unusually short contact between oxygen and iodine atoms for a set of adjacent molecules in the crystal [18]. The contact length (323 pm) is shorter than the sum of the van der Waals radii for oxygen and iodine atoms (350 pm) [15]. A notable decrease in electrical resistivity of polyc~stal~n~ p-iodanil under high pressures (from 10” Q m at atmo- spheric pressure down to 5 x 10m4 $2 m at 50 GPa) has been connected with an increase in intermolecular orbital mixing through the extra interaction, regarded as a kind of charge transfer interaction [19]. Thus, an interatomic contact distance involving an iodine atom for adjacent molecules can reveal characteristics in the electronic properties of these solids.

Accordingly, this paper also requires the examination of crystal structures for the two iodine compounds above. Both compounds exhibit respective crystal structures settled at the present stage, although they appear to still have problems to be solved in terms of crystallography in much higher degree of accuracy. The latest paper on the crystal structure of carbon tetraiodide reports that whereas the structure is closely related to a face- centered cubic cell, it is not isostructural with carbon tetrabromide [20]. This is ascribed to a weak intermolecular iodine-iodine interaction additional to the simple intermolecular force, on account of the ~termolec~ar iodine-iodine contact being slightly but si~fic~tly smaller than twice the van der Waals radius of an iodine atom. On the other hand, a settled crystal structure of tetraiodoethylene at room temperature has pointed out that three values out of fourteen independent intermolecular iodine-iodine distances are smaller than twice the van der Waals radius of an iodine atom [21]. This also indicates that tetraiodoethylene molecules interact with one another in the crystal by a force stronger than the van der Waals force.

The ~termolecular iodine-iodine distances for the two compounds noted above are much closer to twice the van der Waals radius of an iodine

atom than those in an iodine crystal. However, the short intermolecular iodine-iodine contacts are distributed three-dimensionally everywhere in the crystal for these, much more than for an iodine crystal. This may allow the lowering of valence electronic levels with an increasing possibility of electron exchange leading to electron reorganiza- tion in the solid, through the additional intermolecular interaction for these iodine-containing compounds. This prediction appears to match the results on the 1,” values as follows.

The 1,” values of carbon tetraiodide and tetraiodoethylene are determined to be 5.5, and 5.0, eV, respectively, as listed in table 1. These are rather small values for simple organic compounds [3]; this also holds true in the case of the two bromine analogues (while the value of carbon tetrabromide has an ambiguity based on experimental difficulties}. As described above, previous studies have shown that the difference of ionization potentials 1: - 1,‘” stands for the polarization energy P, in the organic solid state, in which the molecular identity is preserved, or molecules are bound by a weak intermolecular interaction, i.e. the van der Waals force. Here the two iodine-containing solids do not fall under the category of such a solid state, as their crystal structures indicate. The energy difference AI = It - lib, however, can be determined for these compounds since the Ii values are obtained from the results of their gas-phase UP spectra [6,7]. The values thus determined are also listed in table 1: A1 = 3.5, and 3.5, eV for carbon tetraiodide and tetraiodoethylene, respectively. These values are much larger than the polarization energy of any other organic solid, and still larger than AI of solid iodine (2.92 eV) or p-iodanil(3.0 eV) above.

In the previous studies [3,23], an approximate

Table 1 Ionization potentials and the energy difference for the gaseous and solid states (all values in eV)

Compound Zp” I; Z,” AI

carbon tetraiodide 9.00 3 9.10 a> 5.5, 3.50 carbon tetrabromide 10.2 b, 10.40 bf 7.3 2.9 tetr~~~thylene 8.57 =) 8.65 ‘) 5.0, 3.5, tetrabromoethylene 9.105 d) 9.264 d, 6.1, 2.q

‘) Ref. 161. b, Ref. 1221. ‘) Ref. [7]. d, Ref. 191.

450 M. Sato, H. Inokuchi / Ionization potentials of solid CI, and C, I,

expression of the polarization energy in an organic solid is derived from an evaluation of the interaction energy between a photoinduced cation and the dipoles induced on all the molecules surrounding the ion; the equation in SI units is introduced as follows:

P,+ = 6.99(e/‘4~e~)~Zp~‘~, (2)

where e and E,-, are fundamental physical con- stants, i.e. the elementary charge and the permit- tivity of the vacuum, respectively; furthermore, Z is the average molecular polarizability and p is the molecular packing density, which is identical to the number density of molecules in the crystal, defined as Z/V with the crystallographic repre- sentation.

Theoretical calculations using a-function potential model have reported the values of average molecular polarizabilities for the above two compounds and their bromine analogues [24,25]. The values are cited in the third column of table 2; they are reduced from the values in original units to those in SI units. No experimental values to be compared with these values have been avail- able so far. For some compounds which present both calculated and experimental values of polarizabilities in refs. [24,25], the agreement between both values is largely good, while the experimental value appears to be slightly larger than the calculated one for most halogen-containing molecules; the largest ratio for the experimental value to the calculated one stands for cis-di- iodoethylene giving 1.16, which considerably ex- ceeds the ratios for the other (more than twenty) compounds. As a result, the molecular polarizabilities estimated from the calculation above and

Table 2

Comparison between the differential ionization potential and the polarization energy calculated from reported parameters using eq. (2)

Compound AI PC” 103% lOP’Z/V

(ev) (ev) (F m2) (me3)

carbon tetraiodide 3.5, 1.63 2.05835 ‘) 5.094 ‘) carbon tetrabromide 2.9 1.70 1.62806 =) 6.2757 d, tetraiodoethylene 3.5, 1.68 2.1605 b, 5.015 ‘) tetrabromoethylene 2.9, 1.57 1.5781 b, 6.051 0

‘) Ref. [25]. b, Ref. [24]. ‘) Ref. [20]. d, Ref. [26]. ‘) Ref. [27]. 0 Ref. [28].

listed in table 2 are nearly reliable for the present compounds.

Accordingly, the values of P, can be estimated using eq. (2) from the molecular polarizabilities above together with the values of Z/V, which are calculated from the crystal data in literature [20,26-281, for the compounds of notice, as also listed in table 2. These values are somewhat unre- alistic, since they correspond to the values estimated for such crystalline systems that admit the contribution from the extra intermolecular interaction only to the molecular packing density, but not to the molecular polarizability. However, they can serve as upper limits for the polarization energies to be estimated; the molecular packing density for a respective compound among those should be smaller than the value applied in the above calculations, so that the value of P, should become smaller than the value listed in table 1, if there are no intermolecular interaction extra to the van der Waals force in those solids.

The P, values thus estimated for the four compounds appear to be nearly constant around 1.6, eV, which coincides fortuitously with the common value of P, (= 1.7 eV) observed for polycyclic aromatic hydrocarbons as mentioned above. In comparison with these values, the energy differences AI again listed in table 2 are much larger - around twice the P, values. This indicates that the extra intermolecular interaction, presumably rather strong in particular for the two iodine-containing compounds, contributes to the large AI values as follows: the strong intermolecular interaction enhances electron exchange between molecules in the solid and, furthermore, causes electron delocalization almost everywhere in the crystal, then the polarizability of the crystal when an electric charge is induced in it receives a great increase in contrast to the case for a typical molecular crystal in which the molecular identity is preserved completely.

With this in mind, the difference between AI observed and P, estimated appears to indicate a magnitude of the intermolecular interaction extra to the van der Waals force in an organic solid like the present compounds. The values for the two iodine-containing compounds are much larger than those for the two bromine-containing ones. Be- sides, crystal structure studies of carbon tetra-

M. Sato, H. Inokuchi / Ionization potentials of solid Cl, and C,I, 451

halides [26,29] and tetrahalogenoethylene [28] im- ply that the extra intermolecular interaction in the crystal of the bromine-containing compounds is much weaker, than that of the iodine-containing ones. This corresponds well to the above observa- tions. Furthermore, the remarkable increase of AI with the increasing size of the atoms on descend- ing the halogen group in the periodic table of the elements, in particular very large value for each iodine-containing compound, can be connected with the results for tetrahalogeno-p-benzo- quinones [ 21.

4. summary

Ultraviolet photoelectron spectra of carbon tetraiodide and tetraiodoethylene, and also their bromine analogues, in the solid state were measured to determine their threshold ionization potentials I,‘“. Spectral features in the photoelectron spectra of the iodine compounds do not correspond to each other between the gaseous and solid states; this is unusual to most organic solids bound by the weak van der Waals force. The energy differences AI between the I,‘” values obtained and the adiabatic ionization potentials Ii, determined also from ultraviolet photoelectron spectroscopy in literature, are about twice the polarization energies P, of most organic solids measured previously, and are also much larger than P, values estimated from a typical ap- proximated relation for the polarization energy using the average molecular polarizability and the molecular packing density in the solid state. In particular, the two iodine compounds show very large AI values around 3.5 eV, which is larger than AI or P, values of any other organic compounds reported so far. The unusually large AI values indicate a strong intermolecular interaction extra to the van der Waals force, a kind of electron exchange interaction or charge-transfer interaction, working in these solids; this is consistent with the results on their crystal structure analyses already published.

References

[l] L.E. Lyons, J. Chem. Sot. (1957) 5001; F. Gutmann and L.E. Lyons, Organic semiconductors

(Wiley, New York, 1967).

PI

131

t41

]51

161

(71

]81

191

1101

[ill

]121

1131

N. Sato, K. Seki and H. Inokuchi, J. Chem. Sot. Faraday

Trans. II 77 (1981) 47.

N. Sato, K. Seki and H. Inokuchi, J. Chem. Sot. Faraday

Trans. II 77 (1981) 1621.

N. Sato, K. Inokuchi, B.M. S&mid and N. Karl, J. Chem.

Phys. 83 (1983) 5413.

H. Yamamoto, K. Seki and H. Inokuchi, J. Chem. Phys.,

to be published.

G. Jonkers, C.A. de Lange and J.G. Snijders, Chem. Phys.

69 (1982) 109.

G. Jonkers, S.M. van der Kerk, R. Mooyman, CA. de

Lange and J.G. Snijders, Chem. Phys. 69 (1982) 115.

I. Novak, T. CvitaH and L. Klasinc, Chem. Phys. Letters

79 (1981) 154.

A.W. Potts, I. Novak and M.L. Lyus, J. Electron Spectry.

31 (1983) 57.

W.D. Grobman and E.E. Koch, in: Photoemission in

solids, Vol. 2, eds. L. Ley and M. Cardona (Springer,

Berlin, 1979) pp. 262-263.

T. Hirooka, K. Tanaka, K. Kuchitsu, M. Fujihira, H.

Inokuchi and Y. Harada, Chem. Phys. Letters 18 (1973)

390.

W.E. Spicer, in: Optical properties of solids, ed. F. Abel&s

(North-Holland, Amsterdam, 1972) ch. 10.

K. Seki, H. Inokuchi and Y. Harada, Chem. Phys. Letters

20 (1973) 197.

[14] A.I. Kitaigorodskii, T.L. Khotsyanova and Yu.T.

1151

1161

]171

]181

1191

1201

]211

]221

Struchkov, Zh. Fiz. Khim. 27 (1953)-780.

A. Bondi, J. Phys. Chem. 68 (1964) 441.

W.R. .%&neck, C.B. Duke, W. Eberhardt, E.W. Plummer

and H.J. Freund, Phys. Rev. Letters 45 (1980) 280.

N. Sato, K. Seki, H. Inokuchi, Y. Harada and T.

Takahashi, Solid State Commun. 41 (1982) 759.

H. Kobayashi, T. Danno and I. Shirotani, Bull. Chem.

Sot. Japan 47 (1974) 2333.

I. Shirotani, A. Or&era, Y. Kamura, H. Inokuchi and N.

Kawai, J. Solid State Chem. 18 (1976) 235.

S. Pohl, Z. Krist. 159 (1982) 211.

B.C. Haywood and R. Shirley, Acta Cryst. B33 (1977)

1765.

J.C. Green, M.L.H. Green, P.J. Joachim, A.F. Orchard

and D.W. Turner, Phil. Trans. Roy. Sot. A 268 (1970)

111.

1231

1241

~251

WI

N. Sato, G. Saito and H. Inokuchi, Chem. Phys. 76 (1983)

79.

E.R. Lippincott, G. Nagarajan and J.M. Stutman, J. Phys.

Chem. 70 (1966) 78.

A.N. Pandey, D.K. Sharma, S.L. Gupta, V. Kumar and K.

Balasubramanyan, Indian J. Phys. 51B (1977) 251.

M. More, F. Baert and J. Lefebvre, Acta Cryst. B 33

(1977) 3681.

[27] M. Kipps, M.Sc. Dissertation, University of Surrey,

Guildford, UK (1973);

D.P.C. Thackeray, R. Shirley, C. Orahatmanee, M. Kipps

and B.C. State, J. Mol. Struct. 20 (1974) 293.

[28] T. DahI and 0. Hassel, Acta Chem. Stand. 22 (1968)

2851.

[29] R. Powers and R. Rudman, J. Chem. Phys. 72 (1980) 1629.

Documents

Ionization potentials of carbon tetraiodide and tetraiodoethylene in the solid state