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Photochemistry of Methane in the Photoionization Region*

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Photochemistry of Methane in the Photoionization Region* JOURNAL OF RESEARCH of the Notional Bureau of Standards - A. Physics and Chemistry Vol. 77A, No. 1, January-February 1973 Photochemistry of Methane in the Photoionization Region* R. E. Rebbert and P. Ausloos Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (October 4, 1972) Methane was irradiated with microwave operated helium (21.2 eV) and neon (16.7-16.8 eV) res­ onance lamps whi ch were separated from the reaction vessel by an aluminum window. The quantum yields of the stable end products have been determined at methane pressures ranging from 1 to 20 torr. Over this pressure range the abund ances of the primary ions, determined through various diagnostic expe riments, are within experimental error the same at 21.2 eV as at 16_7 -16.8 eV ItC H; = CH t = 0.5, CH:; ~ 0.02), and are in good agreement with the primary mass spectra obtained in a photoioniza­ tion mass spectrometer under collision free conditions (P < 10- 5 torr). The C,Ht whi ch is formed by the reaction: CHt + CH. ~ C,Ht + Hz loses a proton by an undetermined mechanism to give C,H. as a product. There is no evid ence for the formation of neutral fragments such as H2 , C, CH or CH, at 16.7- U,,8 eV. The fact that the ionization quantum is equal to unity in this energy range accounts for the absence of these intermediates. At 21.2 eV wh ere (<P ,onizali on = 0.95) there is concrete evidence for the formation of carbon atoms (<P(C ) "" 0.002). Tn an attempt to de mon strate the usefulness of enclosed neon and helium resonance li ght sources in the ion-molecule kineti c studies, the relative probabilities of transfer of H- over D- from various partially deuterium labeled hydrocarbons to C,Ht (or C,Dt). has been determined_ The results presented in this article resolve the existing disagreements between previous helium resonance photolysis studies on CH4 • Key words: Ion-molecule reacti on; ion pair yield; methane; photochemistry; photoionizati on; quantum yields. 1. Introduction resonance (A = 74.37 -73.59 nm) light source. An attempt is made to resolve the existing disagreements in the yields of the chemical products formed in the In a pioneering study, Back and Walker [lJ I carried out the first photochemistry study on lower hydro­ windowless apparatus used by Back and Walker [1] carbons in the photoionization region using the self­ as compared to the enclosed light source apparatus reversed 58.4 nm (21.2 e V) helium resonance line. employed by Jensen and Libby [2]. Finally, reactions The apparatus used was windowless and the photolysis of the major ions, CHt and C 2Ht, formed in the vessel was separated from the light source (microwave photoionization of methane with various hydrocarbons discharge through helium) by a fast flow separation are explored in order to assess the feasibility of using system. Significant experimental results were obtained enclosed helium and neon resonance light sources in even though such an open device limited the reactant the study of ionic reactions. pressure range in the reaction vessel « 0.3 torr) and resulted in a certain degree of uncertainty concerning 2. Experimental Procedure the monochromaticity of the incident radiation. More recently, thin (1500 to 4000 A thick) aluminum films 2.1. Apparatus supported on a metal screen have been used to sepa­ The design of the titanium-gettered light sources and rate the light source from the reaction vessel and to of the reaction vessel has been described in a recent filter out light at wavelengths > 90 nm [2-6]. 'Such report from this laboratory [7]. The aluminum windows devices were first used in 1968 to reexamine certain (0.61 in in diameter) were supplied by the Sigmatron aspects of the photolysis of methane at 58.4 nm at Co., Goleta, California [8]. They consisted of a 1500 ± reactant pressures up to 15 torr. 100 A film supported on a nickel screen. The windows The present paper describes a more detailed study were examined for pin-holes under a microscope and of the photochemistry of CH4 in the photoionization were tested for their pressure resistance. The approxi­ region using an enclosed helium as well as a neon mately 50 photolysis experiments carried out in the course of this investigation at differential pressure ·Sponsored in part by th e U.S. Atomic Energy Commi ssion. readings up to 60 torr, were carried out by using one I Figures in brackets ind icate the literature references at the end of this paper. and the same enclosed light source. Transformation 101 of the helium lamp to a neon resonance light source, analysis experiments were carried out at relatively and vice versa, was accomplished by interchanging the high conversion (1.5 X 1017 photons absorbed by 10 torr rare gases in the body of the enclosed lamp. The of methane in the 220 cm3 reaction vessel). The iso­ pressure in the helium lamp was 1.6 torr and that in topic composition of the ethylene produced in the the neon lamp was 1.2 torr. In a recent detailed ex· 58.4 nm photolysis of the CH4-CD4-NO (1:1:0.01) mix­ amination [9] of the output of a helium resonance lamp ture is as follows: C2D4-14 percent, C2D3H-16 percent, it was shown that at a helium pressure of 1.2 to 2 torr C2D2H2-22 percent, C 2DH3-16 percent, C2H4-32 per­ the 53.7 nm line transmitted through a 1500 Awindow cent. Photolysis of an equimolar CH4-CD4 in the is approximately 2 percent of the intensity of the 58.4 presence of 1 mole percent i-C 4HIO yielded: C2D4- nm line. The total output of the helium lamp used in 16 percent, C2D3H-17 percent, C2D2H2-21.5 percent, this study was approximately 2 X 1013 photons/so The C 2DH3-13.5 percent, and C2H4-32 percent. same lamp filled with neon had a light flux of 0.8 X 10 13 The isotopic analysis of the ethane formed in the photons/so The output of the lamp, which was mono neon resonance photolysis of a CD4-C 3Hs-02 (1:0.01:0.1) itored by measuring the saturation ion current in CH4 mixture at a total pressure of 20 torr, was as follows: showed only very minor fluctuations (2 to 5%) from C 2DsH-47.2 perce nt, C 2 D4H 2-O.5 percent, and C 2H 6- one experiment to the next. The volume of the reaction 52.3 percent. The ethane formed in the helium and vessel equipped with nickel electrodes was 220 cm3. neon resonance photolysis of CD4-i-C4HIO-02 (1:0.01: 0.01) mixtures consisted nearly entirely (> 97 percent) 2.2. Materials of C2D5H. All chemicals used in this study were purified by 4. Discussion gas chromatography. The methane-d4 contained 5 per­ 4.1. General Comments cent CD3H. The cyclohexane,1,1,2,2,3,3-d6 sample used in some experiments contained 10.3 percent cyclo­ Recent determinations of the photoionization quan­ hexane-d5, 2.4 percent cyclohexane-d7 and 0.3 percent tum yield (<1> +) of CH4 at 58.4 nm and at 74.37-73.59 cyclohexane-ds. Propane-l,1,1,3,3,3-d6 contained 6 nm indicate that approximately one ion pair is pro­ percent CD2HCH2CD3 and 1.7 percent CD3CHDCD3. duced for each photon absorbed by methane [l0-11]. More than 93 percent of the isobutane-dl consisted of It follows that in this wavelength region there is one (CH3hCD. The calculated isotopic compositions of predomin-ant primary process the products reported in this paper have been corrected statistically to bring the data up to 100 percent deute­ CH4 + hv ~ (CHt )* + e. (1) rium purity. The ion pair formation process 3. Results All product yields are expressed in terms of quantum (2) yields (<1». Because the ionization quantum yield of methane is close to unity in the wavelength region is apparently of minor importance [12-131. The helium covered in this study [10-11], quantum yields are and neon resonance photoelectron spectrum shows one essentially eqUIvalent to ion pair yields (number of broad band over the range 12.7 to 16-17 e V, exhibiting product molecules per positively charged ion produced three maxima associated with the ionization of the t·) in the system). The absolute accuracy of the quantum orbital [14-15]. Besides the ionization potential at yield values given in tables 1 and 2 are estimated to be 12.7 e V, there is a second I.P. near 23 e V, ascribed to ± 15 percent. Besides the products listed in tables 1 ionization from the al orbital, which is however of no and 2, hydrogen and several higher molecular weight concern in this study. A fraction of the vibrationally hydrocarbon products such as i-C4Hlo and i-CsH12 excited (CHt)* formed in process 1 can dissociate to were observed. The quantum yields of i-C 4H 10 and produce CHt (appearance potential 14.2 eV) and CHt i-CsH12 were seen to increase with the number of (appearance potential 15.2 eV) [12-13] photons absorbed in the system and were therefore ascribed to secondary reactions involving accumulated (CH t )*~ CHt+H (3) products. The values of <I>(i-C 4H IO ) and <I>(i-C sHI2) for the lowest conversion experiment listed in table 1 (4) were 0.003 and 0.002 respectively. Hydrocarbon products with more than five carbon The abundances of CHt, CHt , and CHt observed in atoms were not observed at low conversions (<I> < a photoionization mass spectrometer [121 using an 0.005).
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