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JOURNAL OF RESEAR CH of the Notional Bureau of Standards - A Ph ysics and Chemistry Vol. 75A, No. 6, November-December 1971

The Photochemistry of Propane at High Photon Energies (8.4-21.2 eV) *

R. E. Rebbert, S. G. Lias, and P. Ausloos

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(June 28,1971)

Neon and helium resonance lamps, whic h delive r photons of 16.7- 16.8 eV and 21.2 eV e nergy, resp ecti ve ly, have been used to photolyze C:, HH, C:,DH, C:,HWC:,OH(l: 1) mixtures, and CD"CH,CD" and the re sults obta in ed at th e two e nergies are compared . Ln partic ul ar, it is noted that although the quantum yie ld of ionization in propane is unit y at 16.7- 16 .8 e V. when the ene rgy is raised still further to 21.2 eV , the probabil it y of ionization apparentl y diminishes to 0.93, a n o bservation whie h s uggests that at 21.2 eV, superexc ited s tates may be reached whose di ssociation into neutral frag ments competes with ioni zati on. The quantum yield s of th e lower hydrocarbon products formed in the prese nce of a radical scav­ enger in C:, HHa nd C:,OHa re reported, and a re compared with quantum yield s of products formed in the vacuum ultrav iolet photolysis at lower energies. (Quantum yields of produc ts formed at 8.4 eV and 10.0 eV are reported here fo r th e first time.) Acetylene is form ed as a product in the decomposition of the neutral excit ed propane mol ecul e. and it s yi eld in creases in im port ance with in creasin g e ne rgy; at 16.7- 16.8 e V. wh ere a ll product formati on ca n be traced to ioni c processes. acetylene is formed in ne gli gible yie lds . Lt is co nc luded th at ioni c processes in propane do not lead to th e formati on of acetylene, and the observation of this product in radiolyti c s yste ms may be a re li a ble indicat or of th e relative importance of ne utra l exc it ed molecul e decompositio n processes. From the results obta in ed with the C:,HH' C:,OH( 1: 1) mixture, and with CO"CH,CO", detai ls of th e ion-molecul e reacti on mechanisms a nd th e unimolec ular decomposition of the propa ne ion a re de rived .

Key words : Ion-m olecul e reactio n; photoionization; propane ; q uantum yields; ra re-gas reso nance radiation; unim o l e ~ ul ar dissocia ti on.

1 . Introduction scope, of the chemi cal effects brought about in meth­ ane [2- 4] and in argo n-propane mi xtures [5] by helium In the past, most photolysis studies in the vacuum resonance radiation have been published. One of th ese ultraviolet region have been carried out at energies be­ studies [2] was carried out with a differentially pumped low 11.8 eV (104.8 nm). The lamps commonly used in windowless helium lamp; under such conditions, the this energy region are rare gas resonance lamps, which investigator is restricted to a low pressure range. No deliver the resonance lines of xenon (8 .4 eV, 147.0 nm), studies at pressures above the millitorr range utilizing krypton (10.0 eV, 123.6 nm), or argo n (11.6- 11.8 eV, neon resonance radiation have so far appeared in the 106.7- 104.8 nm). Higher energy resonance lamps literature. could not be made because of the lack of suitable This paper reports the results of a study where, for windows which would transmit photons above 11.8 eV the first time, a compound is exposed to both neon and in energy. Recently, in this laboratory, enclosed neon helium resonance radiation , and th e chemical effe cts and helium lamps have been fabricated; their opera­ at the two energies are compared. Propane was chosen tional characteristi cs have been described in detail as the subject compound because the unimolecular [1]. t These lamps, which deliver the neon and helium and bimolecular (ionic and free radical) processes oc­ resonance lines (16.7- 16.8 e V and 21.2 e V, respec­ curring in this system have been extensively investi­ tively), are fitted with windows made of thin (2000 - 4000 gated [6] and are well understood. In the high energy A) film s of aluminum. region used in this study, chemical effects will be Photolysis studies in this high energy region are, of brought about nearly exclusively through ionic proces­ course, still very rare. Only a few studies, limited in ses. A comparison of the effects brought about by photons of two different energies may highlight subtle *S upporlcd in part by the U.S. At omi c Energy Commi ssion , Washington, D. C. 20545 I Fi gures in brackets indicate the lit erature references at the end of thi s paper. differences, and, therefore, increase our understanding 607 of the activation processes in the high energy range. composition in the system under a particular set of con­ These results should also be of value in interpreting ditions. In photoionization experiments, where some results obtained in systems exposed to high energy quanta produce ions, it has been found convenient in radiation (x rays, gamma rays, energetic electrons, the past [7] to use two systems of units depending on etc.) where end product formation is brought about not whether a process involving a neutral excited molecule only through ionic processes similar to those observed or an ionic process is being considered. Thus, the here, but also through the unimolecular decomposition yields of products resulting fron nonionic processes of superexcited molecules. were expressed in units of M(X)/Nex' or molecules M of product X formed per neutral excited molecule dis­ 2. Experimental Procedure sociating in the system, Nex; for photolysis in the sub­ ionization region M(X)/Nex is simply the quantum The reaction vessel and the resonance lamps used in yield. Ionic product yields, on the other hand, have this study have been described before [lJ. Both the generally been expressed in ion pair yield units, that neon and helium resonance lamps were provided is, as M(X)/N + , or molecules M of product X formed with 2000 A aluminum windows which were entirely per positive ion formed in the system, N + _ leak·free, and could withstand a pressure differential Experimentally, an ion-pair yield is easily measured between the reaction vessel and the interior of the since N + is directly determined by measuring the resonance lamp of 100 torr or more. At the start of saturation ion current: the study the photon flux of the helium and neon resonance lamps was, respectively, 5 X 1013 and N+ = 1 X t(s)X6.24xlOI8 ions/C 7 X 1012 quanta/so It was ascertained that the helium and neon lamps were essentially monochromatic throughout the study by introducing neon and helium, (where 1 is the saturation ion current in amps and C is respectively, into the first compartment of a double coulombs). If the quantum yield of ionization, <1> +, is cell arrangement described before [1]. It was ascer­ known, then M(X)/Nex can be determined from the tained that neon gas placed in the sample cell absorbed relationship: the neon resonance radiation and was transparent to the radiation emanating from the helium lamp, while helium was trans parent to the photons from the neon lamp and absorbed those from the helium lamp. In the course of an experiment, either at the neon or the helium line, the decay in the light flux was no more The overall quantum yield is thus: than five percent from beginning to end. The analytical procedures, as well as the purifica­ tion of the materials used in this study, have been described [7J. In most experiments, quantum yield determinations In this paper, we are concerned mainly with results of the end product were made which were based on obtained at very high energies where all, or almost saturation ion currents measured periodically during all, product formation results from ionic processes; the course of an irradiation [1, 7]. In these experiments, under these conditions the quantum yields and ion approximately 15 torr of a nonabsorbing inert gas pair yields are essentially identical. The products (helium in the neon resonance line experiments, and formed at these energies are, however, compared with vice versa) was added; it has been shown that the products observed in the photolysis at lower energies. addition of such an inert will generally improve the All product yields are expressed in quantum yield definition of the plateau of the saturation ion current units. Thus, in the 8.4 and 10.0 e V experiments, no [1 , 81, and therefore lead to a more accurate determi­ ionization occurs, and there is no complication about nation of the quantum yields of the end products the yield units. At 11.6-11.8 eV, on the other hand, formed in the photolysis. only 73 percent (table 1) of the quanta absorbed lead Allene and methylacetylene were noted as products to neutral excited molecule formation. Therefore, an at 8.4 to 11.8 e V, but their quantum yields were not determined. TABLE 1. Ionization quantum yields and extinction 3. Results and Discussion coefficients of propane a 3.1. Units Photon energy E cm-1atm- 1 cf> + In a photolysis experiment carried out at an energy 11.6-11.8 e V n.d. 0.27 b below the ionization energy of the compound of inter­ 16.7-16.8 e V 2950 1.00 est, a quantum yield of a given product, <1>, is simply 21.2 eV 2240 0.93 the probability that the particular product will result " Arg-o n lamp: 70 percent 11.6 eV, 30 percent 11.8 eV. when an excited molecule undergoes unimolecular de- b C. E. Ki ois. to be published. 608 estimation of the probability that the decomposition --'> Hz+ Ca Hli (3) of the superexcite d propane molecule formed at this (4) energy would lead to the forma tion of a giv en product --'> CRI+ C2 H ~ re quires that th e overall quantum yie ld s be divided --'> H+ CaH7 (5) by 0.73; th e quantum yields can be trans lated into ion pair yie ld s, if the product of interest is of ionic ori gin , i l--r !Tl llTl"lT b y dividing by 0.27. 2.0 Hel ium Lamp (584i1) 3.2. Ionization Quantum Yields

In tabl e 1 are given the extinc ti on coeffi c ie nts and Amp s ionization quantum yields of propane at th e argon, X 10 8 neon, and helium resonance lines, 11.6- 11.8 e V 1.2 (106.7-104.8 nm), 16.7- 16.8 eV (74.4-73.6 nm) and 21.2 eV (58.4 nm), respective ly [8]. It is noteworthy that wh en the photon ene rgy is in c reased fro m 11.6- 11.8 e V to 16.6- 16.7 e V, the quantum yield of ionization increases from 0.27 to a value of unity, but whe n the photon energy is further incre ased to 21.2 e V, the 40 120 200 280 360 440 520 60 0 quantum yield of ionization seems to decrease to a VOLTS value lower tha n unity. The value re ported for 21.2 F«;UHE 1. Saturation ion carrellts 1lIf'!lSI.II·('(1 ill S torr of h ydrugen e V is based on the saturation ion c urrent measure· Ollrl ill 0.2 torr ofC, HHill the presence 20 torr of helium , irradi!l /ed me nt s show n in fi gure 1. At thi s wavele ngth , hydrogen with th e 21.2 eV heliullI re SO ll!ln ce line. has an ioniza ti on quantum yield of unit y [8- 10]; be· cause at the helium resonance line, hydrogen has a low extincti on coeffi cie nt [8, 9, 11] (€= 185), the sat­ uration current measured in hydrogen has a well de· de fin ed plateau extending over a ra nge of several EFFECT OF CONVERSION hundred volts (fi g. 1). Therefore, the hydrogen satura­ 160 tion ion c urre nt is an ideal standa rd again st whic h NEON LAMP to co mpare saturation ion curre nts measure d in other 140 .." co mpounds at the same li ght Aux. Unde r pressure con­ '2 120 ditions where all incide nt photons are absorbed , the x Ul 100 w qu antum yield of ionization of the unknown is given ..J ::J 80 simply by the ratio of the two pl atue u values of th e 0 W ..J 60 saturation ion c urre nt measure ments. As th e fi gure 0 :;; clearly s hows, the plateau of the satura ti on ion c urrent 40 me asured in propane at this e ne rgy is not as we ll defined as th at for hyd rogen, but seems to fall below the plateau measured in hydroge n. 24 32 A recent study from this laboratory [8] re ported QUANTA AB SORBED X 10. 15 that th e ioniza tion quantum yields of C I to C alka nes at the neon resonance lines (16. 7-16.8 e V) are a ll F IGU RE 2. Molecules of ethane, methane, propylene, ond eth ylell e form.ed in the 16.7- 16.8 eV /Ih otolysis oI (/ Ilropane·oxy{!en-he IiLLIII unity. The appare nt drop in the importance of ioniza­ ( / .0 :0.03:8.3 ) m ixtllre at a total presSlI re oI28/orr, as ofllll ctio/l tion in propane as th e energy is raised to 21.2 ~ V of th e /lllmber oIquol/to absorbed by th e propane. is also observed in these other alkanes [12 , 13J. This I is an interesting observation whi ch, if correct, s uggests that in alkanes whic h have absorbed a 21.2 eV photon, superexcited s ta tes may be reached which dissociate 3.3. Photolytic Product Yields so rapidl y that decomposition can compete with ioni ­ zation. Over all product distributions s uch as th ose Figure 2 shows the number of molecul es of vari ous shown in the table do not, of course, in the mselves lower hydrocarbon products form ed as a fun cti on give us mu c h informati on about photolytic mechan­ of th e n umbe r of 16.7- 16.8 e V quant a absorbed isms. Additional informati on, such as that obta ined in a propane-oxygen-helium (l.0 :0.03 :8.3) mixture from de uterium lab e lin g experime nts is required in at a propane pressure of 3 torr. The amounts of the order to trace the modes of formation of a give n prod­ metha ne, etha ne, and e th yle ne products formed uct. S uc h e xperime nts have de monstrated [17- 19] in crease linearly with the n umber of photons absorbed. that ne utral excited propane molecul es undergo the Propylene, on the other hand, does not show suc h a following pri mary processes: linear relationship. This behavior may be accounte d for by several fac tors. For instance, it has been s hown before [5] that one precursor of propylene in propane Cl H ~ --'> Ct HI; + CHt (1) is the propyl ion, and this ion may re act with accumu­ --'> CH4 + C2H4 (2) lated products at high conversions. Another plausible 609 TABLE 2. Quantum yields of end products in the photolysis of C3 Hs

Photon energy Propane Scavenger Methane Acetylene Ethylene Ethane Propylene

8.4 eV C3 H, 0, 0.040 0.028 0.080 0.015 0.23

10 eV C;jH, 0, .115 .073 .22 .043 .17

11.6-11.8 eV C3H, 0, .22 .22 .27 .0585 .13

16.7-16.8 eV C3 H, 0, .27 .001 .084 .46 .32 NO .29 .001 .072 .42 .36

C3 D, 0, .29 .001 .052 .43 .14 NO .27 .001 .060 .48 .23

21.2eV C:,H, 0, .29 .018 .113 .37 .405 NO .27 .018 .13 .37 .40 C"D, 0, .28 .023 .092 .41 .23 NO .31 .020 .095 .40 .24

Press ure = 3.0 torr.

explanation of the fact that the propylene yield drops rather than ions, as precursor. The estimate that the off at high conversions is that propylene product may ratio of neutral excited molecule formation to ioniza­ be removed by reaction with ions or H atoms in the tion is 0.4 in the radiolysis of propane [18], based system. It has been shown [15, 16] that H atoms add on this assumption is, thus, probably approximately to propylene faster than to ethylene or acetylene. correct. Apparently, at the relatively low pressures at which Considering that acetylene formation can apparently the experiments are carried out, oxygen is not a very all be ascribed to a neutral excited propane precursor, effective interceptor of hydrogen atoms. it is interesting that the yield of acetylene increases In table 2, the experiments were carried out under ten-fold when the photon energy is raised from 16.7- conditions where no more than 3 X 10 15 quanta were 16.8 eV to 21.2 eV (table 3). This is in agreement with absorbed by propane at a pressure of 3 torr in the the tentative conclusion reached above, that at 21.2 reaction cell. Thus, the yields of propylene shown in eV as many as 7 percent of the activated species may table 2 have not been much affected by these second­ dissociate as neutral excited molecules before an ary processes. electron can be ejected. Table 2 shows the quantum yields of products It is of interest to note that according to the quantum formed in C3 Hs and C3 Ds irradiated with 8.4, 10.0, yield data given in table 2, the probability of the 11.6-11.8, 16.7-16.8, and 21.2 eV photons in the elimination of an alkane from neutral excited propane presence of 5 percent O 2 or NO added as a radical (processes (1) and (2)) increases with the energy of the scavenger. As the photon energy is increased the photon [28]. The increase probably occurs at the amount of energy to be distributed among the frag­ expense of the H2 elimination process (3). In a recent ments will be larger and secondary decompositions study on the far ultraviolet photolysis of ethane [29] will become more prevalent. For instance the forma­ a similar trend was seen. In that case, however, all tion of acetylene, whose quantum yield of production molecular elimination processes decrease at the ex­ is seen to increase with energy from 8.4 to 11.6-11.8 pense of direct bond cleavage processes as the energy e V, can be attributed to the decomposition of C2 H4 is increased further. Similar trends can be expected and C:JH6 formed in primary processes 2 and 3 respec­ for propane. The data given in table 2 do not allow tively. It is of interest that at 16.7-16.8 e V, where a more detailed analysis of the primary processes only ionic processes lead to end product formation, occurring in propane in the 8.4-11.8 e V energy range. the quantum yield of acetylene is negligible (-0.001). Quantum yields of all free radicals have to be known This observation is not unexpected because as shown as well. A partial analysis of the role of free radicals by Cermak and Herman [20] none of the fragmentation in the decomposition of neutral excited propane IS processes of the C4Ht ion formed by collision with given elsewhere [18,19,30,31]. metastable neon atoms yields C2 H2 as a neutral prod­ 3.4. Isotopic Labeling Experiments uct. Furthermore, there are no known ion-molecule reactions involving fragments from C;jHt which would In table 3 are given the isotopic distributions of the ultimately result in the formation of C2 H2 [21-27]. major lower hydrocarbon products formed in the Thus, the result that acetylene formation from ionic photolysis of a CIHs·C3 DH (1: 1) mixture in the processes (i.e., in the 16.7-16.8 eV photolysis) in presence of a radical scavenger with 16.7-16.8 and propane is negligible, confirms the assumption made 21.2 e V photons. earlier [18], that all acetylene formed in the radiolysis The product ethane is known [5, 27, 32] to be of propane has neutral excited propane molecules, formed in the reactions of ethyl ions with propane: 610 (6) of H- /H2- transfer reacti ons for ethylene ions reactin g with C3HS was 1.15.) On this basis, we can estimate and in the ethylene ion reaction: ion pair yields for the eth ylene ion generated in the 16.7-16.8 eV photolysis or the 21.2 eV photol ysis of C2 D1 + CIHH ---'> C2 D4 H2 + CIHij. (7) propane of about 0.22 and 0.08, respectively. The corresponding ion pair yields of ethylene ions gener· It can be estimated from the relative amounts of C2D5H ated in a mass s pectrom eter by colli sion with meta­ and C2 D.,H 2 formed· in these ex periments at 16.7- ;; tahle neon and helium atoms are 0.20 and 0.17. 16.8 eV, that about 75 percent of the ethane resulted Because the meth ane formed in the photolysis of from th e ethyl ion reacti on, and about 25 percent C3Hs-ClDs (1:1) mixture contains very little partially from the ethylene ion reaction. On this basis, since the deuterated product, it must be formed almost exclu­ e th yl ion do es not undergo any alternate reactions sively in the unimolecul ar decompositions: with propane [27J we can estimate roughly that the ion pair yield of the ethyl ion in the 16.7-16.8 eV (10) photolysis of propane is about 0. 32-0.34; the ion pair yield of this ion observed in the mass s pectrometric (11 ) study in which propane ions were generated by colli· sion with metastable neon atoms [20] was 0.37. Process (11) and/or further decomposition of C2H! Usin g the same reasoning, it can be estimated that at formed in process (10) must be of importance in vi e w 21.2 e V, 89 percent of the ethane originates from the of the fact that th e estimated quantum yields of reaction of an ethyl ion precursor. This gives an CzH! are lower than the quantum yield of molecular approximate ion pair yield for the ethyl ion in the photo· methane (table 2). lysis at 21.2 eV of - 0. 33; when CaHt ion s are From studies on CD3 CH2CD3 , the fragmentation gene rated in a mass spectrometer through collision of the propane ion to form an ethylene ion (process with metastable helium atoms [20], th e ion pair 10) is known to occur through two mechanis ms. The yield of ethyl ions observed was 0.25. lower energy process (A.P. = 11.8 eV) [34J is a 1,3 elimination of methane from the propane ion: TABLE 3. Photolysis of C3H8-C3 DS (1 :1); isotopic compositions of products (12)

Photon Ethanes Methanes Propyienes Ethyienes Energy and the hi gher energy process (A.P. = 12.2 eV) [34] C,D. C, D,H C,D4 H, CD"H/CD. C:, D, H/C, D. C,D"H/C, D is a 1,2 elimination: 16.7·16.8eV l.00 0. 75 0.25 0.057 0.32 0.63

21.2 eV l.00 .95 .1 2 .051 .33 .46 It is of interest then that the observed ratio CD3 H/CD4 Prescure = 3 tor r. (table 4) indeed in creases from 0.69 at 16.7-16.8 eV S perc ent O2 added. to 0.76 at 21.2 e V. In an earlier stud y [34] , it was noted The yields of ethanes form ed in reacti ons s uch as 7 that CD3 CH2 CD! ion s generated by charge exchange from ethylene ion precursors do not directly give us with Xe+ ions (12.1-13.4 eV) underwent processes 12 the yield of the ethylene ion , sin ce this ion can also and 13 to gi ve a CD:lH/ CD4 ratio of 0.35. undergo an H- (or D- ) transfer reaction with propane: Because the ethylene and propylene contain large fractions of partially deuterated products (table 3), (8) it is evident tha t an important mode of formation of these products is bimolecular ion-molecul e reactions to form an ethyl "radi cal, which in these experim ents such as, for instance: will be scavenged. However, in one experiment H2 S was added to ClDH irradiated with 21.2 eV photons. In this system, the d euterated ethyl radical formed in TABLE 4. Photolysis ofCD3CH2CD:l ; isotopic reaction 8 will react with H2S to form CZD5H: composition of methane

(9) Photon Energy CD"H/C D,

while all other ethane-forming reactions will give fully 16.7- 16 .8 e V 0.69 deuterated ethane, C 2D6 . From the observed yield of C2 D5H, it could be deduced that for an ethylene ion 21.2 eV .76 r'eacting with propane-ds, the ratio of D- /D2- transfer Pressure = 3 torr. reactions is 0.9. This value is in reasonably good agree­ 5 percent O ~ added. ment with the results of Sieck and Searles [33], who recently ascribed a value of 0.8 to this ratio on the basis C2D ~ + CIHH ---'> C2 D:IH + CIH~ (14) of results obtained in the NBS high pressure photoioni­ zation mass spectrometer. (They found that the ratio CIDt + CIHH ~ C3DSH + C3Ht· (15) 611 Reaction 14 is thought to proceed through formation [161 Oaby, E. E. , Niki , H., and Weinstock, B., J. Phys . Che rn. 75 , 1601 (1971 ) and references cited therein. of a condensation ion, C5D3Ht, whi ch will di ssociate to [1 7] McNesby, ./ . R .. and Okabe. H .. Advan. Photochem. 3, 157 form other partially deute rated ethylenes besides (1964). C2 D:! H [271. Reaction 15 is a simple hydride transfer [1 8 1 Ausloos, P., and Li as, S. G., J. Che rn . Phys. 44,521 (1 966). reaction. [1 91 Au sloos, P. , and Li as, S. G., Be r. Bun. Phys ik. Chem. 72, 187 For a more detailed di scussion on the neutral end· (1968). [201 Cermak, V. and Herm an , Z., Coil. Czechoslov. Che rn . Commun. product resulting from ion· molecul e reactions we refer 30,169 (1965). the reader to earlier studies [5, 18, 27 ,32,35]. 1211 Ryan, K. R. . and Futre ll , J. H .. J. Che rn . Phys . 42, 819 (1965). [22] P etterso n, E. , and Lindholm . E .. Arkiv . F ys ik 24, 49 (1963). 123] Munson, M. S. B., F ranklin. J L. , and Fi eld. F. H .. J . Phys . Chem. 68, 3098 (1 964). 4. References [24] Oerwi sh, G. A. W., Galli , A.. Giardini·Guidoni , A. , and Volpi. G. G .. J. Chem. Phys. 41,2998 (1964). 125] Aquilanti, V. . and Volpi . G. C.. .1. Che m. Phys . 44, 2307 (1966). [II Gorden, R. Jr. , Rebbert, R. E .. and Au sloos, P., Nat. Bur. Stand. 126] Sieck. L. W .. and Futre ll . J H. , J. Chem. Ph ys . 45,560 (1 966). (U.S.), Tech. Note 496, 55 pages (Oct. 1969). 127] Bone . L. I. , and Futrell . J. H. , J. Chem. Phys. 46, 4084 (196 7). [21 Back, R. A., and Wa lk er, D. C. , J. Che rn . Phys. 37, 2348 l28] At 11.6- 11.8 eV fragment ati on of the C, Ht ion may co ntribute (1962). to the forma'ti on of CHI . However at th e pressures used in [31 Jensen, C. A., and Libby, W. J., J . Che rn. Phys. 9,2831 (1968). thi s study. th e llnimoleclli ar decompositi on of C, Hii is [41 Rebbert, R. E. , and Ausloos, P., Am. Che rn . Soc. 90,7370 .J. entirely quenched. (1968). 129] Li as , S. G., Collin. G. J. , Rebbert. R. E., and Ausloos. P ., .J. [51 Lias, S. G., Rebbert , R. E .. a nd Au sloos, P., J. Chern. Phys . Che rn. Phys . 52, 1841 (1 970). 52, 773 (1970). [30] Vo rachek . ./. H .. and Koob. R. D. . J. Phys. C he m. 74,4455 [61 Fundamental Processes in R adiation Chemi stry, P. Ausloos, (1 970). I<:d ., (Interscience Publishers, New York , N. Y., 1968). [31] Ohin gra. A. K., and Koob. R. 0 .. J. Phys. Che m. 74, 4490 [71 Ausloos, P., Rebbert , R. E., and Li as, S. G., J. Phys. Che rn . (1 971 ). 72 , 3904 (1968). [32] Ausloos, P. , Li as. S. G. , and Sandoval. I. B. . Di sc. Faraday [81 Rebbert... R. E. , and Ausloos, P., J. Res. Nat. Bur. Stand. (U .S.), Soc. 36, 66 (1963). 75A (t'hys . and Che rn .), No.5, 481- 486 (Sept.·Oct. 1971 ). [33] Sieck, L. W., and Searl es. S. K. , .1. Am. Chem. Soc. 92, 2937 191 Bennett , S. W. , Tellinghui se n, J. B. Phillips. L. F .. J. Phys. (1 970). Che m. 75 , 71 9 (1971). [34] Futrell. J. H., and Ti e rn an. T. 0., J . Chern. Ph ys. 39, 2539 [101 Cook, C. R. , and Metzger, P. H. , .J. Opt. Soc. Am. 54, 968 (1 963). (1 964). [35] Bone . L. I., Sieck. L. W .. and Futre ll , J. 1-1. , The Chemi stry [I I] Cook, G. R., Metzge r. P. H .. Oga wa, M. , Becker. R. A .. and of Ioni za ti on and Exc itati on. G. R. A. John son and G. Scholer. Ching, B. K. . Aero space Corp. Report No. TOR- 469 (926()- Eds .. Taylor and Fran cis Ltd .. London (196 7). 01 )-4 (1965). [121 Rebbert, R. E., and Ausloos, P ., un published results. [13J Metzger, P. H. , and Cook, C. R., J. Chem. Phys. 41, 642 (1964). [141 Schoen, R. 1. , J. Chem. Phys. 37,2032 (1962). [1 51 Cowfer, J. A. , Keil , D. K., Mi chael, J. V. , and Yeh, c. , J. Phys. Che rn. 75 , 1584 (197 1) and references cited therein. (Paper 75A6- 692)

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