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 as number density range 1.23 x 10(17) less than or equal to n less than or equal to 3.61x 10(18) cm(-3). We found a strong density dependence of the ''zero-field'' density-norma lized mobility (mu n). Only about 10% of the observed density variatio n can be qualitatively explained by coherent and incoherent multiple s cattering effects. With increasing gas density an increasing number of linear HCN dimers is formed which due to the high dipole moment (D = 6.552 D) represent much stronger electron scatterers than the HCN mono mers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport proce ss where short-lived dipole-bound electron ground states (lifetime gre ater than or equal to 12 ps) as quasilocalized states are involved. Fo r comparison the electron mobility in saturated 2-aminoethanol vapor w ith a dipole moment of similar size (D = 3.05 D) does not show any ano malous density behavior in the temperature range 298 less than or equa l to T less than or equal to 435 K, In contrast to this the electron m obility in saturated but also in nonsaturated CH3CN gas (D = 3.925 D) shows a density behavior similar to that in HCN. (C) 1998 American ins titute of Physics. [S0021-9606(98)01626-2].