A mass spectrometry study of n-octane: Electron impact ionization and ion-molecule reactions

Electron impact ionization of n-octane over an energy range of 10-70 eV and the subsequent ion-molecule reactions with the parent molecule have been s tudied using Fourier-transform mass spectrometry. Molecular ion and fragmen t ions C-1(+)-C-6(+) are produced from the electron impact with a total ion ization cross section of 1.4 +/-0.2x10(-15) cm(2) between 60 and 70 eV. C3H 7+ is the most abundant ion at most of the ionizing energies with the excep tion for E less than or equal to 16 eV where C6H13+ and C6H12+ are the most abundant. Among the fragment ions only C4H7+ and smaller ions react readil y with the parent molecule, primarily producing C5H11+ and C4H9+, with rate coefficients of 0.32-2.4x10(-9) cm(3) s(-1). Essentially all of the ions, including the molecular ion and the large fragment ions, undergo decomposit ion upon collision with neutral molecules after they are kinetically excite d to an energy range of 1-5 eV, forming a variety of small hydrocarbon ions . Many of the decomposition product ions in turn are capable of further rea cting with n-octane. Isotope reagents have been utilized in experiments to probe the type of the ion-molecule reactions studied. (C) 2001 American Ins titute of Physics.