Curve-crossing quantum wavepacket dynamics - Experiment and theory

• Main Author: Gador, Niklas
• Format: Doctoral Thesis
• Language: English
• Published: KTH, Fysik 2004
• Subjects: physical chemistry; molecular
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3754; http://nbn-resolving.de/urn:isbn:91-7283-776-4

In this thesis, I present experimental and theoretical workon quantum wavepacket dynamics in potential curve-crossings,using gas-phase Rb2 as working media. Particularly, we havefocused on curve-crossing cases with intermediate strengthcoupling, which leads to complicated wavepacket motion withe.g. large splittings and interference. Previous experiments onsuch systems are scarce. Experimentally, femto-second pump-probe spectroscopy wasperformed using two independent optical parametric amplifiers.A near-effusive Rb2molecular beam source was developed to produce astable, high density and collision-free beam. Pump-probefluorescence was detected using an optical assembly designedfor good collection efficiency. Theoretically, analysis of experimental data was aided byquantum dynamical calculations. The used numerical simulationprogram is powerful in its ability to include any number ofstates with coupling elements, together with a fully timepropagated pump pulse-molecule interaction. It was furtherdeveloped to include molecular rotation as a centrifugalcorrection term to the potential curves, and to do statisticalthermal averaging to permit direct comparison withexperiment. Our work on the Rb2A-state system is a pioneering femto-secondexperimental curve-crossing study of a system of twointermediately coupled bound electronic states. The wavepacketfragments, following different roads, meet and interfere attheir return to the crossing. Thus, new results on theinterference properties of wavepacket dynamics in such a systemwere obtained, such as the existence of two hybrid diabatic/adiabatic trajectories, robust towards thermal averaging.Further, we show that certain scanning possibility existbetween relative contents of these two trajectories at elevatedtemperature by scanning the pump wavelength. The systemrepresents a quantum matter-wave close analogue to an opticalpulsed Michelson interferometer. The dynamics of the A-statesystem was also investigated by anisotropy measurements. Thehigh degree of signal to noise ratio obtained, revealed a newtype of small oscillatory structure, which the analysis showsoriginates from coupling between all degrees of freedom of theRb2molecule, namely electronic, vibrational androtational motion. The results of the work on the higher lying D-state systemconsist of the determination of a parallel excitationmechanism, where two wavepackets are simultaneously created intwo different electronic states. Further analysis showed thattheir future dynamics proceed essentially independently. Oneperforms adiabatic dynamics in a single‘shelf-shaped’state, while the other goes throughcurve-crossings of somewhat weaker coupling strength thanintermediate. We propose the shape of the final, unknown,pump-probe states, guided by the quantum dynamical simulationstogether with the experimental data.