J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 4615–4626. Printed in the UK PII: S0953-4075(00)51029-3 Dissociative ionization of benzene in intense ultra-fast laser pulses A Talebpour†‡, A D Bandrauk‡, K Vijayalakshmi† and S L Chin† † Centre d’Optique, Photonique, et Laser (COPL) and Dept de Physique, Universit´ e Laval, Qu´ ebec, Qc, Canada G1K 7P4 ‡ Laboratoire de Chimie th´ eorique, Facult´ e des Sciences, Universit´ e de Sherbrooke, Sherbrooke, Qc, Canada J1K 2R1 Received 12 June 2000, in final form 25 August 2000 Abstract. Based on the possibility of multiphoton ionization of the inner valence electrons in polyatomic molecules, a model was proposed to predict the abundance of different fragments resulting from dissociative ionization of the polyatomic molecules interacting with short laser pulses. According to this model the fragmentation of these molecules occurs through multiphoton ionization of the inner valence electrons, which results in a molecular ion in an excited state. Through radiationless transition the population is transferred to the highly excited levels of the ground electronic state. The molecular ion created in this manner is not stable and dissociates rapidly. To test the applicability of the model, the fragmentation of a benzene molecule interacting with linearly and circularly polarized Ti:sapphire laser pulses was studied. The model was tested by comparing the measured laser-intensity-dependent abundance of C 4 H + 4 fragment ions with the calculated ones and excellent agreement was obtained with the experimental results. (Some figures in this article are in colour only in the electronic version; see www.iop.org) 1. Introduction Interaction of molecules with radiation in low light intensity conditions has been well understood from the single-photon and low-order multiphoton processes. However, when higher laser intensities are used, where the corresponding instantaneous electric fields generated are comparable in magnitude to the intramolecular Coulomb fields, a host of new phenomena start appearing that could influence the dynamics of molecules which are yet to be elucidated. The perturbative treatment of laser–molecule interaction which could explain the dynamics of molecules at low light intensities is no longer applicable at laser intensities exceeding 10 12 W cm -2 which are sufficient to drive higher-order processes. On the other hand, for accurate, time-dependent, quantum mechanical calculations current computational power is sufficient only to investigate higher-order processes for the simplest single-electron systems such as H + 2 ,H + 3 , etc [1–3]. However, for multielectron systems, such as diatomic and polyatomic molecules, the complexity of the system prohibits detailed calculations of field–molecule interactions at high laser intensities. Among the polyatomic molecules, the interaction of hydrocarbons with strong ultra-fast laser pulses is promising in terms of the potential applications of multiphoton processes in organic chemistry. This importance requires a detailed study of the multiphoton ionization (MPI; tunnelling ionization is included in this general appellation) and of the fragmentation of some simpler molecules such as benzene which are considered as the building blocks of many more complicated hydrocarbons. In the recent 0953-4075/00/214615+12$30.00 © 2000 IOP Publishing Ltd 4615