Molecules in a seeded supersonic beam are aligned by the interaction between an intense nonresonant linearly polarized laser field and the molecular polarizability. We demonstrate the general applicability of the scheme by aligning I2, ICl, CS2, CH3I, and C6H5I molecules. The alignment is probed by mass selective two dimensional imaging of the photofragment ions produced by femtosecond laser pulses. Calculations on the degree of alignment of I2 are in good agreement with the experiments. We discuss some future applications of laser aligned molecules.

A strong nonresonant nanosecond laser pulse is used to align neutral iodine molecules. The technique, applicable to both polar and nonpolar molecules, relies on the interaction between the strong laser field and the induced dipole moment of the molecules. The degree of alignment is enhanced by lowering the initial rotational energy of the molecules or by increasing the laser intensity. The alignment is measured by photodissociating the molecules with a femtosecond laser pulse and detecting the direction of the photofragments by imaging techniques. The strongest degree of alignment observed is 〈cos2 θ〉=0.81.

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Dissociative recombination and excitation of H 2 O ϩ and HDO ϩ in the vibrational ground state have been studied at the heavy-ion storage ring ASTRID. Absolute cross sections have been measured in the energy range from ϳ0.1 meV to ϳ50 eV. The total cross sections for dissociative recombination are essentially the same for the two molecular ions. Complete branching ratios for all possible product channels in dissociative recombination at Eϭ0 have been determined. Three-particle breakup accounts for ϳ60% of the recombination events. With HDO ϩ , recombination into ODϩH is twice as probable as recombination into OHϩD. An isotope effect is also evident in the cross sections for dissociative excitation of HDO ϩ , where H ϩ production is more likely than D ϩ production.

Lifetimes of doubly charged diatomic and triatomic molecules have been measured by monitoring the decay curves of such ions in a heavy-ion storage ring. CO2+, N22+, CO22+, CS22+ and SH2+ are all found to possess long-lived components which survive for time periods greater than a few seconds. All these dications are found to be essentially stable and their ultimate destruction is due to interactions with residual gases in the ring. CO2+ possesses many more lifetime components in the millisecond range than the isoelectronic N22+ ion. Translational energy spectrometry experiments on the latter species also fail to reveal any short-lived (microsecond) components. Ab initio configuration interaction calculations have been carried out and the potential energy curve for the lowest-energy metastable state of N22+ (1 Sigma g) has been determined, along with Franck-Condon factors for vertical transitions to different vibrational levels from the ground state of neutral N2; tunnelling times of each vibrational level have been computed.

Using the ASTRID heavy-ion storage ring, absolute cross sections for dissociative recombination and dissociative excitation of vibrationally cold H 3 ϩ have been measured as a function of energy in the region Eϭ0Ϫ34 eV. A thermal rate coefficient for dissociative recombination of (1.0Ϯ0.2)ϫ10 Ϫ7 cm 3 s Ϫ1 at 300 K was obtained from cross sections at low energy. At high energy, capture into a group of doubly excited states of H 3 gives rise to resonances in the dissociative recombination as well as in the dissociative excitation cross sections.

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