Bryan, William Alexander; (2001) Ultrafast processes in small molecules. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The interaction of ultrafast (6 x 10⁻¹⁴ s) intense (~10¹⁵ - 10¹⁶ Wcm⁻²) laser pulses with a number of small molecules is investigated. When focussed, the Ti:sapphire laser used in this work causes multiple ionization, resulting initially in dissociation, followed by Coulomb explosion into two or more energetic ionized particles. Ions generated by the Coulomb explosion are detected in a time-of-flight mass spectrometer. Computer controlled data acquisition and laser parameter control is effected through a graphical user interface. Covariance mapping is employed to identify ionization channels, a polarization comparison technique is used to quantify molecular reorientation, and an ion momentum imaging technique is developed, coupled with Monte Carlo simulations to measure the molecular geometry. In water, the internuclear bonds double in length, in accordance with enhanced ionization theory. The molecule is also observed to straighten considerably, and the signature of laser-induced reorientation is observed. The geometry modification is interpreted in terms of bend angle softening, and comparisons with theoretically predicted reorientation rates are made. In carbon dioxide, the ionization channels are identified, and a comparison is made with the results of other experiments. Laser-induced reorientation is observed, and quantified as a function of ionization channel. The bend angle distribution observed is compared with the expected zero-point distribution of the neutral molecule. Geometry modification in terms of light-dressed molecular potentials and the significance of stable molecular ions is discussed. The Coulomb explosion of sulphur hexaflouride is investigated, and a comparison made with enhanced ionization theory. The explosion of this molecule is consistent with a single critical distance. The dissociation and Coulomb explosion of the hydrogen molecular ion is investigated for the first time. Comparisons are made with previous experimental and theoretical results, and significant differences are observed, indicating the importance of the neutral molecule in the fragmentation process.

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