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Detection and characterization of unidirectional molecular rotation Bloomquist, Casey


The main goal of this work is the detection of the directionality of molecular rotation and the characterization of two experimental approaches to controlling the directionality of molecular rotation with ultrashort pulses. Control of the directionality of molecular rotation is desired in order to learn more about the internal properties of molecular systems as well as for studying and controlling molecular interactions. Further, the techniques for generating unidirectional molecular rotation must be studied to understand the properties of the molecular ensembles that are generated. In order to detect the directionality of molecular rotation, we use circular polarization sensitive resonance-enhanced multiphoton ionization spectroscopy to allow state-selective directionality detection. In this work we explain this technique and demonstrate its ability to measure the directionality of individual rotational states. The two methods for controlling the directionality of molecular rotation are based on the molecular interaction with either a pair of pulses (a “double-kick” scheme) or a larger sequence of pulses (a “chiral pulse train” scheme). In both cases, rotational control is achieved by varying the polarization of and the time delay between consecutive laser pulses. The double-kick and chiral train methods have demonstrated the ability to control the directionality of molecular rotation but have not been extensively studied. In this work, we perform experiments with both the double-kick and chiral train techniques for thorough comparison and characterization of both methods. We show that both methods produce significant rotational directionality. We also demonstrate that increasing the number of excitation pulses enables one to control the sense of molecular rotation and predominately excite a single rotational state, i.e. quantum state selectivity. To further explore the capabilities of both techniques we perform experiments on selectivity in mixtures of spin isomers and molecular isotoplogues. We demonstrate the ability of both techniques to generate counter-rotation of molecular nuclear spin isomers (here, ortho- and para-nitrogen) and molecular isotopologues.

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