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The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance R(c) is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for CO(2), by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry ≈ 2.1 Å and <θ(OCO)> ≈ 163° (equilibrium CO(2) geometry is ≈ 1.16 Å and <θ(OCO)> ≈ 172°). The CO(2)(3+) molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO(2) and identify the electronic states responsible for CREI.

The Coulomb explosion of carbon dioxide in a 55 fs laser pulse of intensity 1.5-3 × 10 16 W cm −2 has been studied using a variety of techniques based on time-of-flight mass spectroscopy. Covariance mapping has been used to identify Coulomb explosion channels and to measure the associated kinetic energy release. By comparing time-of-flight spectra taken with linearly and circularly polarized light, a clear signature of laser-induced reorientation is found, which is strongest for the lowest Coulombic channel. Ion-momentum imaging coupled with Monte Carlo simulation shows that the zero-point bend-angle distribution is better preserved than for longer laser pulses. However, some residue of the sequential processes dominant in much longer pulses is found.

A fast beam of H2+ ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused laser intensity and angular distributions of neutral dissociation products have been measured. Significant differences are observed between the present results and those obtained from experiments involving neutral H2 molecules. These differences are indicative of the precursor H2 molecule playing an important and hitherto neglected formative role in the laser-induced fragmentation processes.

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