The National Bureau of Standards' was established by an act of Congress March 3. 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics-Electricity-Heat-Mechanics-Optical Physics-Linac JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry Vol. 75A, No. 3, May-June 1971 The Solid Phase Photolysis and Radiolysis of Ethylene at 20 to 77 K* R. Gorden, Jr., and P. Ausloos Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (January 11, 1971) Films of ethylene condensed onto a cold finger maintained at 20 K were irradiated with photons whose energy ranged from 8.4 to 21.2 eV. At the higher photon energies the relative yields of products compare well with those seen in the radiolysis of solid ethylene. Experiments on CH2CD2 demonstrate that in the photolysis hydrogen is mainly formed by the elimination processes CH2CD* →H2(D2) +C2D2(C2H2) and CH2CD* → HD+ C2HD. The relative probabilities of these three processes are independent of the energy of the incident photons from 8.4 to 11.6 eV and are within experimental error identical to those observed in earlier gas phase photolysis experiments. Relative to acetylene, cyclobutane is a minor product at 8.4 eV but increases by an order of magnitude at higher energies where ions play a role. Cyclobutane, 1-butene and methylcyclopropane formed upon irradiation of frozen C2H1— CD mixtures consisted mainly of C4D8, C4D4H4, and C4H8. Plausible mechanisms which may account for the formation of the latter products are examined. In the solid phase as in the gas phase the relative importance of H-atom production is seen to increase with increasing photon energy. Cyclopropane, apparently formed by insertion of CH into C2H4, is observed as a product at all wavelengths in the photolysis, and in the radiolysis. Key words: Ethylene; free radical reactions; ion-molecule reactions; photolysis; polymerization; The condensed phase photolysis of simple olefins has not as yet been investigated in detail. Only one experiment dealing with ethylene (8.4 eV photons, 36 K) has been discussed [1] in the literature. It was suggested that the products (1-C4H8 and methylcyclopropane) observed in the latter experiment were formed by the addition of ethylidene radicals to ethylene. The condensed phase radiolysis of olefins, on the other hand, has been investigated extensively [2, 3]. In the radiolysis studies on ethylene, 1-butene was noted as major product and was thought to be produced by addition of an ethylene ion to ethylene. Hexenes, octenes, and decenes which were also observed were suggested to be formed by subsequent additions of product ions to ethylene. Isotopic analysis of the C4, C6, and Cs products formed in the radiolysis of C2H4-C2D4 did reveal [3c] that these products are indeed mainly formed by consecutive additions to form polymer molecules containing deuterium in multiples of four. The purpose of the study reported here is to explore the processes occurring in the solid phase irradiation of ethylene more fully. The photolysis has been in vestigated at four different energies. At the lowest *This research was supported by the Atomic Energy Commission. > Figures in brackets indicate the literature references at the end of this paper. of these energies (8.4 eV) ionic processes can be assumed to be relatively unimportant. At the highest energy (21.2 eV) it is likely that a large fraction of photons absorbed lead to the formation of ethylene parent ions. Therefore, this series of experiments may allow us to contrast the product formation which follows ionization with that associated with neutral excited molecule formation. In addition, certain aspects of the solid phase radiolysis have been reinvestigated. Finally deuterium labeling has been utilized in several experiments in order to examine in more detail the modes of formation of certain products. 2. Experimental Procedure The apparatus and procedure for the solid phase photolysis and radiolysis experiments have been described previously [4, 5]. A detailed description of the rare gas resonance light sources has also been given in an earlier report [6]. The NBS 20,000 Curie cobalt-60 source was used for the gamma radiolysis experiments. One radiation experiment with 21.2 eV photons was carried out with a helium resonance lamp provided with an aluminum window [6]. Isotopic analyses of hydrocarbon products were carried out on a high resolution mass spectrometer using a low energy (12-15 eV) electron beam. 3. Results The relative yields of the major products which were measured are given in table 1. Only the sum of the hexene yields is given in the last column. At least eight different C6H12 isomers are formed in the radiolysis as well as in the photolysis at all wavelengths. No attempt was made to analyze for products with molecular weights higher than those of the Co products. In the photolysis of C2H, C2D, mixtures at 11.6-11.8 eV, 70 percent of the hexene mixture consisted of C6D12, C6D8H4, C6D4H8, and C6H12. Two products, cyclopropene and methylcyclopropene which were previously reported to be formed in the 8.4 eV photolysis of C2H4 at 36 K were not observed in our study. In accord with the observation made in the previous investigation, there was a compound a compound which eluted from a squalane column between i-C4H10 and 1-C4H8. However, mass spectrometric analysis indicated it to be a C4H, isomer rather than methylcyclopropene (C4H6) as suggested previously. The yield of the CH, product was seen to increase relative to that of the other products when the irradiation time was increased. It may, therefore, be ascribed to secondary photolysis of a product, probably acetylene. Two other products, cyclobutene and an unknown C4H6 product were also seen to increase with the percent conversion of ethylene. At the lowest percent. decomposition (0.02%), the yields of C4H, and cyclobutene were less than 1 percent of that of acetylene. It is of interest that the cyclobutene product formed in the irradiation of a C2H-C2D, mixture with 8.4 eV photons consisted mainly (~90%) of C4D6, C4D4 H2, C4D2H4, and C4H6. Such a distribution is consistent with a mechanism involving the addition of an acetylene molecule to ethylene. In pure ethylene, the relative yields of all products listed in table 1 showed only minor (~ 10%) variations when the percent conversion was varied over a 10-fold range (from 0.02 to 0.2%). The precision with which the relative yields are measured is approximately 10 percent for products whose yields are 1 percent or more of that of acetylene. For the other products the precision is estimated at 10 to 20 percent. Quantum yields were not determined in any of the photolysis experiments. However, at any particular wavelength the yield of acetylene per unit time was seen to change by not more than 20 percent from one experiment to the next. Besides the isotopic analyses referred to above and in tables 2, 3, and 4, several other products have been analyzed isotopically. Cyclopropane in the 11.6-11.8 eV photolysis of C2H4-C2D, (1:1) at 20 K: C3H6-100; C3DH5-25, C3D2H4-98; C3D3H3-16; C3D4H2-97; C3D5H-11 and C3D6-89. Cyclobutane in the 10.0 eV photolysis of C2H4-C2D4 (1:1) at 20 K: C4H8-120; CDH9-6; C‚Ð3⁄4H5-15; C,D,H,—140; C1DH-10; C4D6H2-16; C4D7H-11, and C4D8-110. 4. Discussion Table 1 shows the relative yields of products formed in the photolysis of ethylene at 20 K with 8.4, 10.0, 11.6-11.8, and 21.2 eV photons, as well as in the gamma-radiolysis at 77 K. In the gas phase, the ionization energy of ethylene is 10.5 eV; [7] the ionization energy in the solid phase is unknown, and may be 1-2 TABLE 3. Acetylene and hydrogen in the photolysis and radiolysis (table 2) that in the solid phase essentially all the hydro of CH2CD2 C2H2 C2HD C2D2 D2 HD H2 Percent distribution gen and acetylene are produced in molecular elimination processes or by geminate disproportionation reactions (which in such a mixture lead to the formation of C2H2 and C2D2, D2, and H2, exclusively). Although in the gas phase radiolysis C2H2 may be formed by charge transfer [10] from C2H2 to C2H4 such a mechanism can be discounted in the condensed phase because of the reduced fragmentation of parent ions with increase in density [11]. Therefore, in the condensed phase photolysis and radiolysis, acetylene is tentatively assumed to be formed via decomposition of neutral excited ethylene molecules formed by direct excitation and by neutralization of the parent ion. In the gas phase, the excited ethylene formed by absorption of 8.4 to 11.8 eV photons, dissociates as follows [9, 12]: (1) 11.1 62.2 26.7 16.7 41.5 41.8 14.1 53.4 32.5 16.8 41.0 42.2 eV lower than the gas phase value [8]. Most likely, ionization is unimportant in the solid phase photolysis at 8.4 eV, which is more than 2 eV below the gas phase ionization energy, but in all other experiments shown in table 1, ionic processes undoubtedly play a major role. In the gas phase the photoionization quantum yield of C2H4 at pressures around 20 torr is approximately 0.2 and 0.9 at 11.6 and 16.8 eV respectively [6, 9]. The photoionization quantum yield of C2H4 in the solid phase is not known and may be expected to differ from the gas phase values. Keeping in mind the lack of knowledge concerning the ionization processes (2) (3) C2HCHCH+H2 →CH2C+H2 →C2H2+2H At atmospheric pressure the excited vinyl radicals formed as intermediates in process 3 dissociate at wavelengths below 147 nm (8.4 eV). Only at wavelengths above 155 nm, have stable CH2CH radicals been noted [13]. In the condensed phase photolysis and radiolysis all, or at least a considerable fraction, of the vinyl radicals may be expected to be stabilized. Actually vinyl radicals have been observed by ESR in the liquid phase radiolysis of ethylene [3f]. The occurrence of process 2 has been established from the isotopic distributions of the hydrogen products [12] formed in the photolysis of CD2CH2. The relative importance of processes 4 through 6 (i.e., 1 and 2) are approximately 0.41:0.42:0.17 in the gas phase, invariant with energy: (4) CD,CH* → CDCH+HD → CD2C+H2 →CH2C+D2 (5) (6) The gas phase results are compared with the analogous results obtained in the solid phase experiments in table 3. It is seen that the isotopic composition of the hydrogen fractions in the solid phase photolysis experiments closely resemble those observed in the gas phase. Furthermore, in the solid phase photolysis experiments at 8.4 and 10.0 eV, there is a near equality between the yields of hydrogen and acetylene (table 1); |