ELECTRON AND ANION MOBILITY IN LOW-DENSITY HYDROGEN-CYANIDE GAS - II - EVIDENCE FOR THE FORMATION OF ANIONIC CLUSTERS

We have measured the drift mobility of anions in low-density hydrogen cyanide (HCN) gas. They are formed by attachment of photoinjected exce ss electrons to HCN molecules. Mobility isotherms for the temperatures 294, 333, and 373 K were determined in the HCN number density range 9 .36x10(17)less than or equal to n less than or equal to 1.21x10(20) cm (-3). The density-normalized mobility (mu(anion)n)of the anions shows a rather strong density dependence. At least in the limit of very low density the value (mu(anion)n)(0) of some postulated anions can be tes ted by the statistical adiabatic channel model (SACM) for ion-dipole m olecule capture processes published recently by Tree [J. Chem. Phys. 1 05, 6249 (1996)]. The observed density variation can be explained by c luster anion formation. But additionally one has to consider the influ ence of HCN dimer formation in the drift gas on the anion mobility. It is further shown that the experimental mobilities recently obtained f or cluster ions like NO+(CH3CN)(2) and NO+(CH3CN)(3) in acetonitrile a re in total contradiction not only to our experimental results but als o to theory. (C) 1998 American Institute of Physics.