Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of gas phase C60

摘要

This thesis research details the development of a quantum cascade laser (QCL) based continuous-wave cavity ringdown spectrometer (cw-CRDS) coupled to an oven supersonic expansion source. This is the first such work that uses a QCL in conjunction with the cw-CRD technique and a supersonic expansion source. The primary goal of the research is to acquire a rotationally-resolved, cold, gas-phase spectrum of an infraredband of C60 around 8.5 μm. A high-resolution spectrum will be valuable from an applied astronomy and fundamental spectroscopy perspective. While there have been astronomical detections of C60 emission from thermal emission or UV-excitation, a gas phase laboratory spectrum would enable astronomical searches for gas-phase C60 in different astronomical environments. Collection of such a spectrum would representa significant technical achievement, as it would be the largest and most symmetric molecule to have itsrotationally-resolved spectrum collected via gas-phase absorption spectroscopy.To test the performance of the instrument with the supersonic oven expansion source at room temperature,the v8 band of methylene bromide (CH2Br2) has been studied using supersonic expansions generatedfrom pinhole and slit nozzle geometries. In total, 297 transitions have been assigned for the three dominantmethylene bromide isotopologues with a fit standard deviation of 0.00024 cm−1 (7 MHz). Though methylenebromide is only a test molecule for the system, it is still the only work where a QCL was used to resolve therotational structure of a vibrational band that previously was unresolved in other studies.As an intermediate challenge between that offered by methylene bromide and C60, the first high-resolutionspectrum of a bending mode of pyrene (C16H10) around 1184 cm−1 has been collected. 464 transitions havebeen assigned, and a fit standard deviation of 0.00036 cm−1 (11 MHz) has been achieved using an asymmetrictop Hamiltonian without the inclusion of quartic distortion constants. Successful assignment of thevibrational band enabled analysis of the intensity of selected lines measured in the slit expansion, providinga gross estimate of the vibrational temperature between 60 - 90 K. This indicates efficient vibrational coolingin the supersonic expansion with a slit nozzle expansion at an initial oven temperature of 430 K. Thoughpyrene is not as large as C60, it still represents the largest such molecule to be rotational-resolved usinginfrared absorption spectroscopy.Spectral searches for C60 from 1184 – 1186 cm−1 have all resulted in non-detections despite favorablecalculated signal-to-noise values for single rovibrational transitions. The possible cause for lack of signalcould be insufficient vibrational cooling in the supersonic expansion, greatly reducing the population of C60molecules in their ground vibrational state. There are still combinations of carrier gas backing pressure andnozzle geometries that should be explored. The recent development and implementation of a larger boreoven will enable future spectroscopic searches with higher number densities of C60 seeded in the expansion.Outside of the laboratory work discussed in the thesis, a permutation inversion (PI) group theory analysishas been carried out on the fluxional benzenium ion (C6H7+). The benzenium ion is a proposed intermediatein the chemical pathway for the synthesis of benzene in dense interstellar clouds and protoplanetary nebulae.As a molecule of astrophysical insterest, it presents a future target for high-resolution spectroscopy in theMcCall group using the sensitive cooled resolved ion beam spectroscopy technique. A full PI group theoreticaltreatment of the benzenium ion may provide a useful first step in understanding future high resolutionmicrowave or infrared spectra, where tunneling splittings from the proton “ring-walk” motion may be seen.The PI treatment carried out in this work provides information on the number of expected tunnellingsplittings and spin statistical weights for the C6H7+ and C6D6H+ molecular ions. The linear combinationof localized wavefunctions (LCLW) method has been been successfully applied to C6D6H+ to provide aquantitative estimate of the splitting pattern for rotational levels in the ground state.