Development of Ultrafast Time-Resolved Chiral Infrared Spectroscopy

Abstract

Among the different techniques available to study the molecular structure, chiral spectroscopy is a fast, reliable method, where molecules can be investigated in the liquid phase. Chiral spectroscopy is based on optical activity. A molecule is said to be optically active if it interacts differently with left- and right-circular polarised light. The difference of absorption between the two circular states is referred to as circular dichroism (CD), whereas the difference of refraction is known as optical rotatory dispersion (ORD). Because the optical activity finds its origin in asymmetry, it is directly dependent of the molecular geometry. Probing optically active vibrational transitions allows to retrieve even more structural information as infrared spectra are usually more resolved than electronics ones. Extension of this technique to the recording of time-resolved chiral vibrational signals may enable the dynamics of conformation changes in biomolecules such as peptides and proteins to be followed with unprecedented details. Toward this goal, we report the first pulsed laser set-up capable of recording both static infrared CD and ORD spectra and photo-induced changes in vibrational circular dichroism (VCD) with picosecond time resolution. A femtosecond laser system is synchronized to a photo elastic modulator to produce alternating left- and right-circular polarised mid-IR pulses. Transient changes in vibrational circular dichroism of the CH-stretch vibrations of the cobalt-sparteine complex Co(sp)Cl2 are presented in a first proof-of-principle experiment. Both static and transient vibrational chiral spectroscopy suffer two important drawbacks: Chiral signals are usually small and sensitive to polarisation-based artefacts, which mainly originate from the interaction between an imperfect probe beam polarisation and a non isotropic sample. We report on a new scheme for synchronizing the laser system and the photo elastic modulator which generates almost perfect probe polarisation states. The technique reduces possible polarisation-based artefacts and allows multichannel detection of the chiral signals normally obscured by polarisation sensitive optics of the monochromator. To increase signal size, a self-heterodyning configuration is implemented where a part of the probe pulse acts as a phase-locked local oscillator heterodyning the chiral signal. The technical improvements presented in this thesis should open the door to measurements of transient vibrational chiral spectra of biomolecules