Photochemistry of methane in the photoionization region

Abstract

Methane was irradiated with microwave operated helium (21.2 eV) and neon (16.7-16.8 eV) resonance lamps whi ch were separated from the reaction vessel by an aluminum window. The quantum yields of the stable end products hav e been dete rmin ed at methane pressures ranging from 1 to 20 torr. Over thi s pressure ran ge the ab und ances of the primary ions, dete rmined throu gh various diagnostic expe riments, are within experim ental error the sa me at 21.2 eV as at 16_7 -16.8 eV ItC H; = CH t = 0.5, CH:; ~ 0.02), and are in good agree ment with th e primary mass s pectra obtained in a photoionization mass s pectrometer und er collision free condition s (P < 10-5 torr). The C,Ht whi ch is form ed by the reaction: CHt + CH. ~ C,Ht + Hz loses a proton by an undetermined mec hanism to give C,H. as a product.There is no evid ence for the formation of neutral fragm ents such as H2 , C, CH or CH, at 16.7-U,,8 eV. The fact that the ionization quantum is equal to unity in thi s e nergy range accounts for the absence of these intermediates. At 21.2 eV wh ere (<P ,onizali on = 0.95) there is co ncrete e vidence for the formation of carbon atom s (<P(C ) "" 0.002).Tn an attempt to de mon strate the usefulne ss of enclosed neon and helium resonance li ght sources in the ion-molecule kin eti c studies, the relative probabilities of tran sfer of H-over D-from various partially de uterium labeled hydrocarbons to C,Ht (or C,Dt). has been determin ed_ The res ults presented in this article resolve the existing disagreements be tween previous helium resonance photolysis studi es on CH4•

Pulse radiolysis of methane

“…H -H reco mbination will be ver y slow because of the third body requirement and will not effectively co mpete [19]_ That reaction (13) is important in thi s syste m is qualitatively confirmed by the results of experiments in which the yield of H atoms is redu ced_ It has recently been shown [7] that the addition of SFs as an electron scavenger in the pulse radiolysis of me thane eliminates reaction (4) as a source of ethyle ne a nd H atoms, probably through the occurrence of a reaction such as: (15) Thus, in the presence of SF 6, a major source of H atom formation is eliminated. Under these conditions, th e yield of ethane is inc reased (table 2) b ecause of the inc reased importance of methyl-methyl combination:…”

“…As discussed above , it is expected that e thylene and acetylene should be formed in the radiolysis of metha ne under high dose rate co nditions [7] from the neutralization of the e thyl ion according to reaction (4) , as well as from reaction (11) (ethylene formati on) and reaction (12) (acetyle ne formation). Also, it is possible that other sources of these products exist , for in stance, reaction s of excited CH2 s pecies with methane, or reactions of triplet CH 2 with other radic als.…”

“…The experimental procedure was identical to that described in two previous publications from this laboratory [7][8]. Two field emission sources (Febetron 705 and 706) [9] were used in this investigation.…”

“…Finally, the C atoms may be in the unreactive 3p state, or in the IS or 'D states. It has recentl y been demonstrated that the in sertion of C atoms into methane res ults in the formation of acetylene [7]:…”

“…At the high dose rates attained in pulse radiolysis experiments, on the other hand, the C2H~ and CH~ ions will have a relatively short lifetime with respect to neutralization, and therefore are less likely to be intercepted by impurities or product molecules. The present investigation is essentially a continu ation of an earlier study [7] which examined in detail the neutralization me chanism of the C2Ht ion in methane. The present investigation examines in more detail the effects of dose, dose rate, electron scavengers, and positive ion scavengers on the yields of the radiolytic products formed in methane under pulse radiolysis (high dose rate) conditions.…”