Laser Ablation of Nanostructured Matrices for Spectroscopy

Microwave spectroscopy is a spectroscopic technique that employs microwave radiation to investigate rotational transitions of molecules in the gas phase. When microwave radiation is absorbed by a molecule, transitions between rotational energy levels are induced. The microwave spectrum obtained can lead to information about the structure of molecules or complexes under investigation such as molecular geometry, bond distances and conformational preferences.

A Chirped Pulse Fourier Transform Microwave spectrometer will be employed to record rotational spectra in the frequency range 7.0 to 18.5 GHz. The rotational spectra of solid compounds and complexes with solid precursor molecules can be recorded. This is achieved through vaporisation of the solid sample into the gas phase using a bespoke laser ablation source situated within the spectrometer. The current experimental methodology requires approximately 100 mg of solid sample to obtain a microwave spectrum. The amount of sample required means that microwave spectra are often not obtained for novel and newly synthesised organic compounds. This project aims to reduce the amount of material required to obtain a spectrum, allowing structural information of a newly synthesised compound to be obtained more feasibly using rotational spectroscopy. This will be achieved by the introduction of nanomaterials into the experimental methodology. Metallic nanoparticles such as gold, silver or copper are metal structures whose dimensions are on the nanoscale. They have size dependent optical properties and a high absorption coefficient. The latter allows nanoparticles to efficiently absorb laser light. When exposed to light of a specific wavelength, nanoparticles experience an oscillation of the metal's valence electrons, this is known as a surface plasmon resonance. The energy gained by absorption of laser light is rapidly dissipated as heat to the surrounding environment. The addition of metallic nanoparticles to the organic sample should lead to efficient vaporisation into the gas phase- a crucial experimental pre-requisite for their study by microwave spectroscopy. Experiments and computational studies will be conducted with a variety of nanoparticles of differing composition, sizes and shapes in order to investigate which type of nanoparticle induces vaporisation of organic samples most efficiently. It is possible that the heat dissipated through nanoparticle heating will be conducted away from the laser focus too quickly before vaporisation of the sample is achieved. It is then proposed to produce composites of metal nanoparticles and the organic sample held within a porous silicon host. Porous silicon has a low thermal conductivity. This feature of porous silicon should allow the laser heating to be confined and lead to vaporisation of the organic material.

Once the nanomaterials have been successfully incorporated into the experimental methodology, the aim will be to record and analyse rotational spectra of newly synthesised organic compounds. Analysis of these compounds will be conducted using the spectroscopic fitting software, PGOPHER, to assign rotational transitions and determine parameters such as rotational constants. Computational studies will also take place alongside spectroscopic studies, to allow a comparison to be made with the structural information obtained by experimental means. Ab initio and density functional theory calculations will be used to perform geometry optimisation, calculate rotational constants, bond lengths and angles.