Strong Field and X-ray Interactions with Aligned Molecules PDF Download

Author: James Patrick Cryan
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Languages : en
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Book Description
Atomic and molecular photo-reactions are best described in a reference frame which is fixed with respect to the atom or molecule. In general, this molecular frame does not coincide with the laboratory frame where measurements of these reactions are made. The random orientation of the molecules in the laboratory frame leads to diminished resolution in studying chemical reactions. Although a wealth of theoretical and experimental effort has been devoted to measuring the orientation of the molecule during and following the photo-reaction, an alternative approach would be to know the orientation of the molecular frame \textit{a priori}. Here I describe a method to bring the molecular frame into a predefined orientation with respect to the laboratory. I also present preliminary measurements of molecular processes made in this molecular frame. My discussion of molecular frame measurement begins with a description of strong-field laser-induced molecular alignment. I apply laser induced molecular alignment to make angle-resolved molecular frame measurements of x-ray excited molecules. I present a technique which uses a train of laser pulses, which take advantage of the periodic nature of rotational wavepackets, to create a highly aligned ensemble of molecules. Besides being useful for making molecular frame measurements, these highly aligned ensembles are a novel medium for studying wavepacket decoherence. Using my multiple pulse technique I am able to measure the population relaxation of highly non-thermal rotational wavepackets, and show that the population lifetime increases with $J$-state. The alignment technique described in this thesis produces a window~($\sim200$~fs) of field-free alignment in a molecular ensemble. The Linac Coherent Light Source~(LCLS), a high brightness, x-ray free electron laser (xFEL), can be focused to produce extremely high x-ray power densities. These high x-ray power densities saturate $K$-shell absorption in low-$Z$ atoms inside this alignment window. Given this impressive source I am able to measure the angle-resolved Auger electron emission pattern from diatomic nitrogen. The capability to resolve Auger emission angular distributions in the molecular frame provides a new tool for spectral assignments in the congested normal $KLL$-Auger electron spectra and takes advantage of the symmetries of the final dication states. The high brightness x-rays from the LCLS, in addition to providing sufficient flux to perform experiments on pre-aligned molecular ensembles, are capable of producing double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. I observed a rich single-site double core vacancy Auger electron spectrum in good agreement with ab initio calculations, and I measured the corresponding Auger electron angle dependence in the molecular frame.