ION-MOLECULE REACTIONS OF ARN2-HYDROCARBONS AT THERMAL-ENERGY( WITH SIMPLE ALIPHATIC)

The product ion distributions and rate constants are determined for io n-molecule reactions of ArN2+ with C2Hn (n=2,4,6) and C3Hn (12=6,8) by using a thermal ion-beam apparatus. Although charge-transfer channels leading to parent ions and/or fragment ions are found, no displacemen t reaction leading to ArCmHn+ and N2CmHn+ is detected. A comparison of the product ion distributions with breakdown patterns of the parent i ons suggests that fragment ions, formed through cleavage of C-H and/or C-C bonds, are produced via near-resonant ionic states in the 13.1-13 .4 eV range. The branching ratios of parent ions for C2H4 (68%) and C3 H6 (20%) are larger than those for C2H6 (5%) and C3H8 (5%). The large branching ratios of the parent ions for the unsaturated hydrocarbons t are explained as due to a strong interaction of a vacant orbital of Ar N2+ with the highest occupied pi(C=C) orbital of the unsaturated hydro carbons which induces nonresonant charge transfer. The total rate cons tant for C2H2 is 6.8x10(-10) cm(3) s(-1) while those for C2Hn (n=4,6) and C3Hn (n=6,8) are in the range (8.5-9.8)X10(-10) cm(3) s(-1). The f ormer and the latter values correspond to 69% and 77%-90% of the calcu lated values from Langevin or average dipole orientation (ADO) theory. The smaller k(obs)/k(calc) ratio for C2H2 is attributed to the lack o f near-resonant ionic states with favorable Franck-Condon factors for ionization.