ELECTRON AND ANION MOBILITY IN LOW-DENSITY HYDROGEN-CYANIDE GAS - I -DIPOLE-BOUND ELECTRON GROUND-STATES

We measured the mobility of excess electrons in the polar hydrogen cya nide gas (D = 2.985 D) at low densities as a function of density and t emperature by the so-called pulsed Townsend method. Experiments were p erformed at 294 and 333 K in the gas number density range 1.23x10(17)l ess than or equal to n less than or equal to 3.61x10(18) cm(-3). We fo und a strong density dependence of the ''zero-field'' density-normaliz ed mobility (mu n). Only about 10% of the observed density variation c an be qualitatively explained by coherent and incoherent multiple scat tering effects. With increasing gas density an increasing number of li near HCN dimers is formed which due to the high dipole moment (D=6.552 D) represent much stronger electron scatterers than the HCN monomers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport process wh ere short-lived dipole-bound electron ground states (lifetime greater than or equal to 12 ps) as quasilocalized states are involved. For com parison the electron mobility in saturated 2-aminoethanol vapor with a dipole moment of similar size (D = 3.05 D) does not show any anomalou s density behavior in the temperature range 298 less than or equal to T less than or equal to 435 K. In contrast to this the electron mobili ty in saturated but also in nonsaturated CH3CN gas (D = 3.925 D) shows a density behavior similar to that in HCN. (C) 1998 American Institut e of Physics.