Elsevier

Vacuum

Volume 78, Issues 2–4, 30 May 2005, Pages 187-191
Vacuum

Coulomb explosion of C7H72+ ions produced by electron impact ionization of toluene

https://doi.org/10.1016/j.vacuum.2005.01.024Get rights and content

Abstract

Doubly charged C7H72+ ions produced by electron impact ionization of toluene (C7H8) show metastable unimolecular dissociation into two singly charged fragments. A high-resolution double focusing mass spectrometer (reversed Nier–Johnson type BE geometry) was used to monitor these decomposition reactions. We have identified four competing charge separation channels. The mass-analyzed ion kinetic energy (MIKE) technique was used to determine mean values of the kinetic energy release (KER). We obtained values of the average KER for the charge separation reactions of C7H72+ ranging from 2.5 to 3.5 eV.

Introduction

Recent interest in the dynamics and decay mechanism of doubly charged hydrocarbon ions has been stimulated not only because of importance in atmospheric chemistry [1], [2], but also by the development of new experimental techniques like photoelectron–ion–ion triple coincidence spectra [3], [4], or an approach based on the combination of coincidence techniques with position-sensitive detectors that provide images of the molecular break-up [5].

The majority of small doubly charged molecules are metastable, and charge separation may occur by tunneling, thermal activation, curve crossing or other mechanisms, resulting in two fragment ions that are accelerated to several eV in their mutual electrostatic field (Coulomb explosion). The stability of aromatic compounds is a well-established concept in organic chemistry and the decomposition of alkylbenzenes and derivatives induced by electron impact has been generally interpreted as proceeding through benzyl or substituted benzyl ions [6], [7]. These cyclic molecules turn out to resist Coulomb explosion much better than many other small molecules [8], [9], [10]. Moreover, it has been suggested that these systems may be used for energy storage [11].

In the present work, we analyze spontaneous metastable Coulomb explosion reactions of doubly charged C7H72+ ions produced by electron impact ionization of toluene via reaction (1a), (1b), (1c), (1d), i.e.,C7H8+e→+3eC7H72+→C4H4++C3H3+→C5H3++C2H4+→C5H4++C2H3+→C5H5++C2H2+.With the help of a double focusing sector field mass spectrometer of reversed geometry (BE geometry) we have analyzed metastable decomposition reactions of C7H72+. For an unambiguous identification of the fragment ions and in order to obtain the kinetic energy release of ions formed via metastable Coulomb explosion, the mass-analyzed ion kinetic energy (MIKE) technique was used. This method has been applied to several systems that are unstable with respect to Coulomb explosion, like metastable decay of doubly charged molecules [12], [13], [14], multiply charged fullerenes [15], [16] or multiply charged ions prepared by charge stripping, photon or electron impact ionization of singly charged precursors [17].

For all the reactions specified in (1), extremely dished metastable peaks were measured. The average kinetic energy released upon Coulomb explosion is deduced from these peaks.

Section snippets

Experimental

Details of the experimental setup and data analysis have been published elsewhere [14], [18], [19]. In short, the apparatus consists of a high-resolution double focusing mass spectrometer (Varian MAT CH5-DF) of reversed Nier–Johnson type BE geometry. The vapor of toluene is introduced via a capillary leak gas inlet system into the collision chamber of a Nier type ion source, where it is ionized by an electron beam of 100 eV energy and 500 μA electron current. Ensuing cations are extracted by a

Results and discussion

Fig. 1 shows a mass spectrum of primary ions produced by electron impact ionization of toluene. In agreement with the earlier work of Rylander et al. [7], the most abundant peak is at a mass to charge ratio of 91 Thomson that corresponds to the C7H7+ ion. In the present mass spectrum, the abundance of the parent ion of toluene is about 78% of the abundance of the C7H7+ fragment ion. In the mass to charge range from 42 up to 46 Thomson, we can observe the presence of doubly charge ions. The signal

Acknowledgment

This work partly supported by the FWF, ÖNB and ÖAW, Wien, Austria, and the European Commission, Brussels (network program). Moreover, this work has been carried out within the Association EUROATOM-ÖAW. The content of the publication is the sole responsibility of its publishers and it does not necessarily represent the views of the EU Commission or its services.

References (25)

  • J.H.D. Eland et al.

    Chem Phys Lett

    (1994)
  • M. Rabrenovic et al.

    Int J Mass Spectrom Ion Proc

    (1983)
  • A. Vincze et al.

    Int J Mass Spectrom

    (1999)
  • D. Mathur

    Phys Rep

    (1993)
  • S. Matt et al.

    Int J Mass Spectrom

    (1999)
  • J.L. Holmes et al.

    Int J Mass Spectrom Ion Phys

    (1977)
  • K. Vekey et al.

    Int J Mass Spectrom Ion Proc

    (1997)
  • E.E. Ferguson et al.

    Acc Chem Res

    (1981)
  • D.M. Golden

    Chemical kinetics and photochemical data for use in stratospheric modeling, eval #8

    (1987)
  • M. Lundqvist et al.

    Phys Rev Lett

    (1995)
  • B. Siegmann et al.

    J Phys B

    (2001)
  • D.O. Sichssler et al.

    J Chem Phys

    (1954)
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    1

    Permanent address: Institute of Mathematics, Physics and Computer Sciences, Maria Curie–Sklodowka University, 20-031 Lublin, Poland.

    2

    Guest professor at the University of Innsbruck. Permanent address: Department of Physics, University of New Hampshire, Durham, NH 03824, USA.

    3

    Also adjunct professor at: Department Plasma Physics, Comenius University, 84248 Bratislava, Slovakia.

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