title: Molecular ion-electron recombination in an expanding ultracold neutral plasma of NO+.
abstract: Using state-selected double-resonant excitation, we create a Rydberg gas of NO molecules excited to the principal quantum number n = 50 of the f-series converging to the ion rotational level, N(+) = 2. This gas evolves to form an ultracold plasma, which expands under the thermal pressure of its electrons, and dissipates by electron-ion recombination. Under conditions chosen for this experiment, the observed rates of expansion vary with selected plasma density. Electron temperatures derived from these expansion rates vary from T(e) = 12 K for the highest density up to 16 K at four-fold lower density. Over this range, the apparent electron coupling parameter, defined as Γ(e) = e(2)/4πε(0)ak(B)T(e), falls from nearly three to about one. The decay of charged-particle density fits with a kinetic model that includes parallel paths of direct two-body and stepwise three-body dissociative recombination. The overall recombinative decay follows a second-order rate law, with an observed rate constant that fits with established scattering-theory estimates for elementary two-body dissociative recombination. A small residual increase in this rate constant with decreasing charged-particle density suggests a growing importance of the three-body recombination channel under conditions of decreasing electron correlation.
authors.affiliation: Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.
authors.forename: Hossein
authors.initials: H
authors.lastname: Sadeghi
authors.forename: Markus
authors.initials: M
authors.lastname: Schulz-Weiling
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Morrison
authors.forename: Julian C H
authors.initials: JC
authors.lastname: Yiu
authors.forename: Nicolas
authors.initials: N
authors.lastname: Saquet
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Rennick
authors.forename: Edward
authors.initials: E
authors.lastname: Grant