Chemical Physics Letters 374 (2003) 334–340 www.elsevier.com/locate/cplett

Dissociative photoionization dynamics in ethane studied by velocity map imaging Wen Li

a,b

, Lionel Poisson c, Darcy S. Peterka c, Musahid Ahmed c, Robert R. Lucchese d, Arthur G. Suits a,b,c,*

a

c

Department of Chemistry, Stony Brook University, Stony Brook, NY 11793-3400, USA b Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA d Department of Chemistry, Texas A&M University, College Station, TX 77642, USA Received 19 February 2003; in final form 18 April 2003

Abstract We have studied the dissociative ionization of ethane from 12.0 to 12.8 eV using velocity map imaging with synchrotron radiation. We obtained translational energy release distributions and angular distributions for the C2 Hþ 4 product. The translational energy distributions are consistent with a 0.6 eV barrier to H2 elimination in the ethane cation, while the angular distributions are characterized by a limiting anisotropy parameter of b ¼ 0:3. Theoretical calculations predict a lower anisotropy; possible reasons for the discrepancy are considered. Ó 2003 Published by Elsevier Science B.V.

1. Introduction The elimination of molecular hydrogen from the vibrationally excited ethane cation represents a classic problem in unimolecular dissociation, studied both from a theoretical and experimental perspective. Early work by Chupka and Berkowitz [1] using ethane and other alkane ions provided some of the most precise evaluations of rate theories at the time. This work led to the first threshold photoelectron–photoion coincidence (TPEPICO) studies [2], which were applied to

*

Corresponding author. Fax: +1-631-632-7960. E-mail address: [email protected] (A.G. Suits).

methane, ethane and isotopomers, with an interest in precisely defining the internal energy in the ion for the benefit of the refined treatment of the implications for the theory of unimolecular dissociation. In recent years, interest in C2 Hþ 6 has persisted. Owing to the presence of a tight transition state but a relatively small barrier to H2 loss and the possible importance of tunneling, it has become benchmark system with which to appraise theoretical descriptions of these phenomena. In recent years, several groups have combined diverse experimental techniques and ab initio calculations in an effort to probe the detailed kinetics and underlying dynamics of these dissociation processes. Br aten et al. [3] used two-sector and four-sector mass spectrometers to study metastable

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W. Li et al. / Chemical Physics Letters 374 (2003) 334–340

decay of excited ethane cations prepared by electron impact. They were able to measure kinetic energy distributions for C2 Hþ 4 arising from nonenergy-selected ethane ions and isotopic variants and found distributions for H2 elimination peaking around 160 meV. These studies were augmented by ab initio calculations showing a tight transition state involving 1,2 hydrogen elimination. Weitzel st al. [4] have studied unimolecular dissociation in ethane extensively using high-resolution TPEPICO techniques. They compared the rotational energy dependence of the dissociation rate for methane and ethane in a form of Ôtransition state spectroscopyÕ, where methane and ethane were chosen to illustrate loose and tight transition state behavior, respectively [5]. Weitzel subsequently reported an extensive ab initio study of the ethane system in which he explored the reaction coordinate for H2 loss [6]. He identified several minima and transition states for the reaction, placing the barrier at about 1.0 eV. More recently, Guthe and Weitzel [7] applied high-resolution TPEPICO methods in a reflectron mass spectrometer to study the rate constant for dissociation of energy-selected ethane. Their approach allowed determination of this rate with an energy resolution of 10 meV and they found rates ranging from 103 to 107 s1 , resulting from an increase in the cation internal energy of less than 0.1 eV. These rates were well reproduced by tunneling RRKM calculations with a transition state energy of 860 meV and an imaginary frequency of 500 cm1 . Finally, Kurosaki and Takayanagi [8] have reported ab initio calculations in which they have found a transition state considerably lower than that reported by Weitzel. They performed RRKM calculations using the ab initio results and also obtained good agreement with the rate measurements, but concluded that tunneling had only a modest influence on the effective barrier height. Ion imaging techniques have emerged in recent years as a powerful means of studying photodissociation and reactive scattering processes [9,10]. Application of these and related methods to dissociative ionization processes is surprisingly less developed despite the advantages offered by the detailed vector correlations that can be observed [11]. Although less widely applied, it must be noted

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that there have been some sophisticated coincidence studies applied to dissociative ionization and dissociative photodetachment processes, and some of these rely on imaging and position sensitive detection [12–16]. It is clear that these are likely to grow in importance with the advent of velocity map imaging and the availability of intense vacuum ultraviolet (VUV) light sources. In this Letter we present a velocity map imaging study of the dissociative ionization of ethane in a supersonic molecular beam. Because this is not a coincidence study, the precise energy of the parent ion is not known. Nevertheless detailed dynamical information is apparent in the results providing new insight into the dissociation process.

2. Experimental The experiments were performed at Endstation 3 of the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Briefly, it employed a continuous molecular beam source of ethane seeded 15% in argon, which was skimmed once before entering the photoionization chamber. Product ions are accelerated by repeller and focused by velocity mapping ion optics into a 1.0 m flight tube perpendicular to the plane of the beams. Product ions strike a position sensitive detector, which is 40-mm-diameter dual microchannel plate (MCP) coupled to a phosphor screen. The images were recorded using the McLaren TM1000CV camera system employing centroiding. Typical accumulation time was 10 min for each image. In order to preserve pure velocity mapping conditions and to match the duty cycle of the photon source, no pulsed fields were applied to the ion optics. However, because the light and the molecular beam are continuous, no TOF mass selection is obtained under these conditions. Nevertheless, since it is known that the dominant ions at these energies are the parent ethane and the product of interest, ethylene, interference from C2 Hþ 5 (