Procino, I.; (2011) Laser induced molecular axis alignment: measurement and applications in attosecond science. Doctoral thesis , UCL (University College London).

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Abstract

This thesis reports the measurement and applications of molecular axis alignment induced by strong non-resonant linearly polarised laser fields. The spatial alignment of gas phase molecules overcomes the loss of information that results from averaging angle-dependent quantities over all the possible orientations of an isotropic sample. Therefore, laser-induced molecular alignment techniques are an essential component in new experiments aimed at measuring the structure of molecules with attosecond time resolution. In the first part of this thesis an experiment to measure molecular axis alignment is described. This experiment is based on the velocity map imaging technique in conjunction with time-resolved femtosecond laser Coulomb explosion of the molecular sample by an intense circularly polarised laser beam. A circularly polarised beam is needed to ensure a uniform detection efficiency for each possible orientation of the molecular axis in the polarisation plane. However, such a polarisation produces ion distributions that are not cylindrically symmetric, preventing the use of the standard Abel inversion technique to retrieve the three-dimensional ion distributions from the detected two-dimensional images. A new inversion algorithm is presented that allows the retrieval of molecular axis distributions from angular distributions of ions without cylindrical symmetry. The second part of the thesis focuses on the application of laser-induced molecular alignment to retrieve molecular structure and dynamics from high-order harmonic generation (HHG) experiments. HHG with a mid-infrared laser source (1300 nm) from aligned molecular samples of CO2, N2, C2H2, and N2O are presented. The use of a laser source with a wavelength longer than that used in previous experiments (800 nm) has increased the amount of information obtainable from such experiments. These experiments have provided insight into the hole dynamics of CO2 following ionisation, and reveal for the first time structural features in the HHG spectra of molecules with low ionisation potentials such as C2H2.

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