PHYSICAL REVIEW A 82, 062715 (2010) Electron-impact dissociation of ozone cations O 3 + S. H. M. Deng, C. R. Vane, and M. E. Bannister Physics division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6372, USA M. Fogle Physics Department, Auburn University, Auburn, Alabama 36849, USA (Received 3 November 2010; published 30 December 2010) Absolute cross sections for electron-impact dissociation of O 3 + ions yielding O + and O 2 + fragment ions have been measured using a crossed electron-ion beams method for energies from about 3 eV to 100 eV. While the O 2 + channel dominates the dissociation cross section over the measured energy range, a strong enhancement is observed in the O + channel at low energy. DOI: 10.1103/PhysRevA.82.062715 PACS number(s): 34.80.Ht I. INTRODUCTION Ozone constitutes only a relatively small concentration in the atmosphere but it plays an important role for human beings. On one hand, in the stratosphere layer ozone absorbs energetic ultraviolet light (UV) that is responsible for skin cancer, cataracts, and depression of the human immune system [1]. On the other hand, ozone damages the eyes and the respiratory systems of humans and other mammals if the concentration is too high [2–4]. Since its discovery by. Sch¨ onbein in 1840, ozone has been extensively studied for over 100 years. Spurred by the ozone deficit problem, especially the discovery of the Antarctic ozone hole [5–7], more and more research has been focused on the study of various destruction mechanisms of the ozone molecule and the ozone cation. In the atmosphere, ozone is created from oxygen by UV radiation or lightning, and decomposes by absorbing UV or colliding with other densely populated radicals or molecules, such as NO, NO 2 , HO, H 2 O. At the earth’s surface these processes maintained a relatively stable dynamic equilibrium between ozone and oxygen, until the industrial age and massive introduction of pollutants modified these processes. Ozone is depleted efficiently by chlorine Cl, ClO, bromine Br, and BrO, which act as catalysts for ozone consumption processes [6,8]. Atomic chlorine, bromine, and their monoxides come mainly from pollutants such as the chlorofluorocarbon (CFC) and bromofluorocarbon (halon) compounds, which have small concentrations but long lifetimes in the stratosphere. Even though most of these compounds have been banned during the last decades, the released pollutants will continue to deplete the ozone layer for the next several decades. However, this is not the only channel for ozone depletion. The reactions of the ozone molecule and the ozone cation with other atoms, molecules, radicals, ions, and electrons contribute to the destruction of ozone as well. The ozone cation exists naturally and is important for our understanding of ozone depletion [9]. The ozone molecule has been studied extensively both experimentally and theoretically, but its positive ion has received little attention. The main problem for experimentalists is the lack of sources of pure O 3 + ions. Most O 3 + ion sources studied are mixed with excited states or/and oxygen ions (O + and O 2 + ). The theoretical calculation for the electronic states of the ozone cation is difficult due to strong electron correlation. So far, there are no detailed potential energy curves available for the ozone cation. The O 3 + ion has a nonlinear structure and the geometry including the bond distance and angle is very different for each available electronic state. The first photoelectron spectra of O 3 + were obtained in 1974 [10], but the assignment of the states was controversial and was not clarified until 2005 [11–15]. The three lowest doublet states of O 3 + are assigned as ˜ X 2 A 1 , ˜ A 2 B 2 , ˜ B 2 A 2 and their energies are within 1 eV of each other. Recently, Willitsch et al. measured accurately that the energy difference between 2 A 1 and 2 B 2 is only 0.135 eV [13]. Most of the theoretical treatments compute the doublet states and only a few calculations have been done for the quartet states [14,15]. The calculation in Ref. [15] shows that the quartet states, 4 A 1 , 4 A 2 , and 4 B 2 , are close in energy to the fourth lowest doublet state 2 B 1 , which is about 2 eV higher than the ground state of O 3 + . The O 3 + ion is weakly bound and will dissociate by photon absorption, collision, or recombination. The disso- ciative recombination of ozone cation with free electrons has been investigated using the ion storage-ring method at CRYRING [16,17] detecting neutral fragments, O or O 2 , and is dominated at 0 eV by three-body (O + O + O) dissociation. In the present work we are only concerned with dissociative processes yielding O + or O 2 + fragments. The photodissoci- ation of O 3 + yielding these two fragments has been studied [18–20]. Vestal and Mauclaire [18] reported that the cross section of the O + channel is predominant in the visible light range. In this paper, we report the measured absolute cross sections for electron-impact dissociation of O 3 + producing ion fragments. Possible channels for electron-impact dissociation of O 3 + include e − + O 3 + → e − + O + O 2 + (α) 0.64 eV, → e − + O + + O 2 (β ) 2.19 eV, → e − + O + + O + O (γ ) 7.35 eV, → 2e − + O + + O 2 + (δ) 14.26 eV, → 2e − + O + + O + + O (ε) 20.97 eV, (1) where the energies given are the threshold for each channel, respectively, for the 2 A 1 ground state of O 3 + [21]. The first three channels are dissociative excitation (DE) processes 1050-2947/2010/82(6)/062715(4) 062715-1 ©2010 The American Physical Society