International Journal of Mass Spectrometry 248 (2006) 1–8 Molecular hydrogen ion elimination from alkyl iodides under strong laser beam irradiation C. Kosmidis a,∗ , S. Kaziannis a , P. Siozos a , A. Lyras a , L. Robson b , K.W.D. Ledingham b,c , P. McKenna b , D.A. Jaroszynski b a Department of Physics, University of Ioannina, 45110 Ioannina, Greece b Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 0NG, Scotland, UK c AWE Ltd., Aldermaston, Reading RG7 4PR, UK Received 5 September 2005; received in revised form 12 October 2005; accepted 12 October 2005 Available online 16 November 2005 Abstract The elimination of H 2 + from alkyl iodides under strong (up to 5 × 10 15 W cm -2 ) laser irradiation is studied by means of time-of-flight mass spectrometry. The study has been performed by using 60 fs (λ = 800 nm) and 35 ps (λ = 1064, 532, 355 and 266 nm) laser pulses. It is concluded that the H 2 + ions are ejected from ionic states via Coulomb explosion processes. The molecular rearrangement leading to H 2 + formation is attributed to a tunneling process through a H transfer barrier. For the case of methyl iodide, about 10% of the doubly charged parent ions undergo molecular rearrangement. From a comparison of the H 2 + /H + ion yield ratio of the studied molecules, it turns out that the H 2 + formation from H atoms bonded to a terminal carbon atom is more efficient than that arising from H atoms bonded to central C atoms of the molecular chain. © 2005 Elsevier B.V. All rights reserved. Keywords: Mass spectrometry; Molecular hydrogen elimination; Strong laser field; Alkyl iodide 1. Introduction The detection of molecular hydrogen ion (H 2 + ) in the mass spectra of many organic compounds under strong laser irradia- tion has been reported on many occasions [1–7]. Recently, we have studied the ionization/dissociation pro- cesses of some alkyl iodides by means of time-of-flight (TOF) mass spectrometry under strong (∼10 16 W cm -2 ) femtosecond [8] and picosecond [9,10] laser irradiation. In both cases, an intense ionic signal corresponding to atomic and molecular hydrogen ions has been recorded. The H 2 + elimination process is a complicated one since, at least, the rupture of two bonds and the formation of a new one are needed. To the best of our knowledge, the appearance of the H 2 + ion in the mass spectra of organic compounds induced by strong lasers has not been studied extensively in the past [7]. On the other hand, molecular hydrogen elimination, among the other dissociation channels, from small alkanes (methane, ∗ Corresponding author. Fax: +30 26510 98695. E-mail address: kkosmid@cc.uoi.gr (C. Kosmidis). ethane, propane, etc.) has been studied extensively [11,12]. The conclusions of these studies are of importance because the alkyl iodides are the products of the substitution of a hydrogen atom by an iodine one in the molecular skeleton. For the case of propane, Tonokura et al. [13] have shown that the atomic hydrogen elim- ination channel exhibits a site specificity. The hydrogen atoms are produced mostly by a C H bond rupture from the terminal carbon atoms of the alkyl chain. On the other hand, other groups [14–16] have shown that for the case of small alkanes the vast majority of the released molecular hydrogen is formed from hydrogen atoms bonded to carbons in the middle of the alkyl chain. Molecular hydrogen formation from the other hydrogen atoms of the skeleton (i.e., from the terminal CH 3 groups) is also possible and it is found that they are released with higher kinetic energies (∼3.3 eV) compared to those generated from the middle of the alkyl chain (∼1.5 eV). It should be noted that these experiments on alkanes have been performed using a laser beam at λ = 157 nm, i.e., below the ionization energy and for this case the efficiency of the channels leading to H 2 elimination is higher than that leading to atomic hydrogen formation. In addition, H 2 elimination has also been reported from ionic species and for the case of C 2 H 4 + , using a photoelectron– 1387-3806/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ijms.2005.10.004