Chemical Physics 19 (1977) 233-240 Q North-Holland Publishing Company EMISSION CROSS SECTlONS OF Ha AND CH(A*A --f X2H) RADIATIONS FOR ELECTRON IMPACT ON HYDROCARBONS G.R. MOHLMANN* and F.J. DE HEER F0.WInsC~ut YOOI Atoom- EII.Molecuulfysica, Kruislnon 407. Amricrdam/IV~m., 771~Nerherhds Rcceivcd 26 July 1976 Absolule emis%on cross sections for II, and CH(A*A - X’n) radtitions have bcon determined for elecuons (0-)_OOO eV) incident on hydrourbons. The hydrocarbons consisted of alkancs from C& up to C H 8 lB, the cyclic zdipharic compounds CJH~, CsH10 and C6H12, and the uns&urated compounds CIHzand C21&. The experimental results show tiat the emis- sion cross sections for H, 3s well 3s For CH(A’A * X’fI) radiations decrexe remarlwbly with increasing number oi atoms in the parent molecule. 1. Introduction The aim of this study is to obtain information about the emission cross sections, oem, of H, and CH(A’n+ X’ll) radiations for electron impact on hydrocarbons. Especially, the dependence of uem on some molecular properties, such as the number of atoms and the ar- rangement of atoms in the molecules, has been investi- gated. Values for uem of H and CH(A2A -+ X’lI) radi. ations have been published for electrons incident on the smaller hydrocarbons (up to butane) and for ben- zene [l-7]_ The results in refs. [l-7] show that the a cm’s for H(~I’ + n ,,) are roughly independent of the parent hydrocarbon molecule except for benzene. In the case of CH(A2A + X211) radiation the results in refs. [5-81 show that the u,,‘s are not independent of the parent molecule, but that uem decreases with increasing number of atoms in the molecule. In the present work we have extended the measurements of (T em for Ha and CH(A2A * X’H) radiations to larger hydrocarbons ta see the dependence of uem over a wider range of number of atoms in the parent molecule. An abstract of the present work is given in ref. [Sl. Recently uqublished results came to our knowl- edge concerning radiation from excited fragments for * Department ofTheoreticalOr@nic olemistry, University of Leyden, The Netherlands electrons on alkanes up to heptane [9]. A comparison between our experimental results and those of refs. [l-7,9] is made in the present work. 2. Experimental The apparatus used for the ucm measurements has been described in some detail by Beenakker and de Heer [lo]. It basically consists of a vacuum chamber in which an electron gun is mounted, connected with a collision chamber via a differential pumping hole through which the electron beam is directed. For the measurements in the electron energy range O-100 eV a coaxial magnetic tield of about 50 gauss has been used to confine the electron beam. In the energy range 10062000 eV no magnetic field was used to avoid contributions of pro- cesses due to secondary electrons. The electron beam current has been kept below 50 pA. The pressures of the target gases never exceeded 3 X 10L4 torr and were measured by means of a capacitance manometet (MKS Baratron). The emitted radiation was observed at 90” to the eIectron beam by means of a Leiss mono- chromator. For the detection of HU radiation a RCA 31034 A photomultiplier, and for the CH(A’A -+ X*I’l) radiation an EMI 6256 A photomultiplier were used. The multiplier output pulses were discriminated, amplified and fed into a pulse counter (SSR model