Drift and clustering of daughter negative ions of H 2 O in parent gas

• Published in: Journal of physics. D, Applied physics Vol. 46; no. 3; p. 35201
• Main Authors: de Urquijo, J, Bekstein, A, Ruiz-Vargas, G, Gordillo-Vázquez, F J
• Format: Journal Article
• Language: English
• Published: 23.01.2013
• ISSN: 0022-3727; 1361-6463
• DOI: 10.1088/0022-3727/46/3/035201

Abstract The mobility of daughter negative ions of H 2 O in parent gas has been measured with a pulsed Townsend technique over the density-reduced field strength, E / N , range 9–100 Td and a pressure range 2–16 Torr. It has been found that the mobility of the anions is dependent on the gas pressure. Using a transport theory considering the influence of the permanent dipole field of H 2 O, we have found that the pressure-dependent mobilities can be associated with a series of cluster ions of the type OH − (H 2 O) n ( n  = 1–3), with the mass of the cluster species increasing with the total gas pressure. Also, the mobility of H − and OH − could be estimated. Using a Townsend avalanche simulator we have been able to explain the measured ionic currents in terms of an ion–molecule reaction scheme with a single set of swarm and reaction coefficients for each value of the density-reduced field strength, E / N , at several pressures. Regarding the positive ions, the only drifting ion is H 3 O + , the mobility of which could be estimated. The rate constants relative to the formation of the OH − (H 2 O) n ( n  = 1–3) species were also derived from this study.