Ultrafast mid-infrared laser spectroscopy : applications and technique developments

• Main Author: Yan, Suxia
• Other Authors: Tan Howe Siang
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Science::Chemistry::Physical chemistry::Reactions and kinetics
• Online Access: https://hdl.handle.net/10356/48029
• Institution: Nanyang Technological University

This thesis discusses applications and technique development in ultrafast mid infrared nonlinear spectroscopies. After a brief introduction in Chapter 1, the vibrational dynamics of a metal carbonyl compound Os3(CO)12 are investigated with mid-infrared (MIR) pump-probe spectroscopy in Chapter 2. Specifically the vibrational relaxation dynamics of the four infrared active carbonyl stretching normal modes of Os3(CO)12 were measured using broad-band frequency resolved MIR pump-probe spectroscopy. The frequency resolved pump-probe traces measured at the fundamental absorptions exhibit marked differences: The two axial modes at frequencies of 2068 cm-1 and 2034 cm-1 yield similar bi-exponential decay traces, while the two equatorial modes at 2014 cm-1 and 2002 cm-1 show an extra rising component. The axial-equatorial combination anharmonicity constants are found to be near zero. This results in the appearance of the pump-probe signals of these combination bands at the same frequencies as the fundamental transitions, leading to interference and the resultant anomalous rising features. If unaccounted for, these interferences may lead to erroneous conclusions about the dynamics of these vibrational stretches. To avoid such pitfalls, it is therefore imperative to resolve such ambiguities. No obvious direct vibrational energy transfer between the axial and equatorial CO stretching modes was observed. Since perturbed free induction decay was observed in the MIR studies on Os3(CO)12, this phenomena was studied in further detail for a simpler system, namely W(CO)6, which is presented in Chapter 3. Perturbation theory in the interaction picture was used to model this coherent signal for the fundamental and overtone transitions, respectively. Broadband MIR pump-probe experiments were carried out to yield transient absorption spectra and kinetic signals at negative time delays. The experimental measurements were compared and fitted with the presented modeling, showing excellent agreement. In the later part of this thesis, we explore the technical improvements towards pulse-shaping assisted two-dimensional (2D) spectroscopies in a pump-probe geometry with a phase cycling scheme. Chapter 4 outlines the development of pulse shaping technique, which produces shaped pulse trains for 2D spectroscopies in a pump-probe geometry. The generation of amplitude, phase, and polarization controlled pulses in the MIR tunable around 3.5 µm is demonstrated. Two temporally separated sets of individually phase and amplitude shaped pulse profiles in the near-infrared are transferred into the MIR via two independent optical parametric amplification processes in two perpendicularly oriented nonlinear crystals in a common-path geometry. The resulting two shaped MIR light fields of orthogonal polarizations are temporally recombined in a birefringent material. In Chapter 5, the necessary phase cycling schemes for 2D optical spectroscopy in a pump-probe beam geometry are presented.