El-Zein, Ali Abed Ali; (2001) Electron and photon interactions with molecules. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Electron interaction with H2O molecules (chapters 1 to 4) are studied to determine the absolute differential (DCS) and integral (ICS) cross sections for the electron impact excitation of the vibrational symmetric bending (010) and unresolved symmetric + asymmetric stretching (100+001) modes respectively. The energy of the incident electron beam ranges from 6 to 20 eV with the scattering angle varying from 10° to 135°. The scattered electrons show significant angular structure in both the (010) and the (100+001) DCSs due to decay from Feshbach resonances in H2O- with a B2 symmetry state. This observation agrees for the first time with theoretical studies, and the H2O grand total cross section (GTS) results confirm the earlier experimental findings. Femtosecond photon interactions with xenon atoms (chapters 5 to 7) are studied by scanning a focused laser spot along the direction of propagation and using a sub-millimetre aperture detector that exposes regions of different laser intensities to the detector. The ion signal from different regions of the laser focus varies as a function of intensity within the confocal volume. Fitting the ion signal to the intensity volume shell allows detailed deductions to be made of the laser molecule interactions as a function of laser intensity. This has shown ionisation suppression in high intensity short laser pulses. Photon interactions with H2O molecules (chapters 8 to 10) are investigated using time of flight mass spectrometer (TOFMS) and a femtosecond polarised laser beam. The high intensity laser spot (2x1016 W/cm2) results in Coulomb explosion of the molecular ion at a high level of ionisation. The rotation of the laser polarisation enables the construction of a TOF matrix of the spectra. For the first time the data are corrected for the experimental acceptance angle that allows a full study of the fragmented ions. The detected energy and angular distributions of the fragmented ions show that the H2O molecule undergoes geometrical stretching, straightening and reorientation modifications. The Monte Carlo simulation of the dissociative process and the 2D fit give the geometrical bond length, bend angle and alignment angle distributions.

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