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Title: New techniques and applications for high resolution laser spectroscopy of molecules in the gas phase
Author: Mohr, Christian
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2012
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Spectroscopy is at the heart of physical chemistry. In this thesis: several new spectroscopic experiments are introduced which have been used to study fundamental and analytical topics. The implementation of a cost-effective Raman spectrometer for undergraduate and graduate exper- iments and research is presented. The spectrometer features a modular and compact design including the ability to run solely from batteries. Parts cost are below GBP 3000. The spectral resolution is 20 cm -I and the accuracy to determine peak positions is 2 cm -.1. Spectra of liquid H20: HDO: and D20 as well as temperature dependent spectra (10 "C ... 70 QC) of H20 are recorded. It is shown that. the H20 and D20 spectra can be qualitatively well described by assuming a Fermi resonance between the //1 symmetric OH-stretching vibration and the 21/2 bending overtone vibration. Previous theories that the band contours and split rings are caused by different hydrogen bonding environments could not be continued. Values for the Fermi constant of 140 cm -1 (H20) and 1OOcm -1 (D20) are retrieved but must. be considered qualitative in nature at present. Stimulated Raman spectroscopy (SRS) emitted from H20 microdroplets are analysed. The energy build-up of light trapped inside microdroplets leads to a much lowered threshold for many nonlinear processes including SRS. The contour of the OH-stretching band around 3400cm-1 changes with increasing pump power from a broad feature known from bulk water to one narrow peak, and finally to two distinct narrow peaks. It is shown that these changes can be qualitatively described by standard Raman theory if higher order Stokes generation is included. The theory also successfully describes SRS from CCl4 and D20 microdroplets. A photoacoustic Raman spectroscopy (PARS) spectrometer is set up and characterised. It features a resolution of 0.05 cm "! and absolute wavenumber accuracy of 0.03 cm - I It is used in the analysis of the Fermi resonance system in cyclopropane involving the VI: 2/12: v2 + 2//14: 4//14: and 2119 vibrations. For the first. time v2 + 2V14, 4v14: and 2V9 vibrations are reported at high resolution and included in a Fermi resonance analysis. However, the retrieved values for unperturbed band positions and Fermi constants must be treated as fit parameters only at present since additional perturbations are identified which could nor yet he included ill a globa1 analysis. The PARS spectrometer is extended to measure corrosive gases by replacing the electret micro- phone with a custom build optical microphone. Different optical microphones based on deflection and interferometric detection are evaluated and they show the same linearity and at least comparable sensitivity with respect to the normally used microphone. The extended PARS spectrometer is used to measure the CH-stretching vibration of formic acid monomers and cyclic dimers. A blue-shift of 7 cm -I is observed and attributed to a secondary effect or the two hydrogen bondings building the cyclic dimer. The newly developed optical feedback cavity-enhanced resonant photoacoustic spectroscopy (OF- CERPAS) scheme uses power build-up in an optical cavity: optical feed hack stabilisation, and resonant photoacoustic detection. A single-mode cw diode laser is stabiliscd to a high-finesse linear cavity by optical feedback. Inside the cavity: a build-up of laser power occurs. Absorbing gas phase species are detected using the photoacoustic (PA) technique. Light coupled into the cavity is modulated corre- sponding to a longitudinal resonance of an organ pipe acoustic resonator U = 1.35 kllz, Q ~ 10). OF- CERPAS has been characterised by measuring very weak rovibrational water overtone transitions near 6:3011111. Normalised noise-equivalent absorption coefficients are determined as IY ~ 4.4 .10-9 cm "! SI/2 and 2.6 . 10-11 cm -I SI/2 W. Due to the optical feedback: the linewidth of the diode laser is nar- rowed to the cavity bandwidth which has a FWH:M of 16 kHz. Key advantages of OF-CERPAS are: zero-background method: little requirements on electronics and data acquisition: simple set-up. In conclusion, several new spectroscopic experiments were set up successfully and applied to scientific problem in thc field of analysis, dynamics and structure of molecules.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available