Techniques in Molecular Spectroscopy: From Broad Bandwidth to High Resolution

Cossel, Kevin C.

Graduate Thesis Or Dissertation

Techniques in Molecular Spectroscopy: From Broad Bandwidth to High Resolution Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/rv042t08j

Abstract

This thesis presents a range of different experiments all seeking to extended the capabilities of molecular spectroscopy and enable new applications. The new technique of cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) provides a unique combination of broad bandwidth, high resolution, and high sensitivity that can be useful for a wide range of applications. Previous demonstrations of CE-DFCS were confined to the visible or near-infrared and operated over a limited bandwidth: for many applications it is desirable to increase the spectral coverage and to extend to the mid-infrared where strong, fundamental vibrational modes of molecules occur. There are several key requirements for CE-DFCS: a frequency comb source that provides broad bandwidth and high resolution, an optical cavity for high sensitivity, and a detection system capable of multiplex detection of the comb spectrum transmitted through the cavity. We first discuss comb sources with emphasis on the coherence properties of spectral broadening in nonlinear fiber and the development of a high-power frequency comb source in the mid-infrared based on an optical-parametric oscillator (OPO). To take advantage of this new mid-infrared comb source for spectroscopy, we also discuss the development of a rapid-scan Fourier-transform spectrometer (FTS). We then discuss the first demonstration of CE-DFCS with spectrally broadened light from a highly nonlinear fiber with the application to measurements of impurities in semiconductor manufacturing gases. We also cover our efforts towards extending CE-DFCS to the mid-infrared using the mid-infrared OPO and FTS to measure ppb levels of various gases important for breath analysis and atmospheric chemistry and highlight some future applications of this system.

In addition to the study of neutral molecules, broad-bandwidth and high-resolution spectra of molecular ions are useful for astrochemistry where many of the observed molecules are ionic, for studying molecules such as CH₅⁺ with highly non-classical behavior, and for tests of fundamental physics. We have developed a new technique – frequency comb velocity-modulation spectroscopy – that is the first system to enable rapid, broadband spectroscopy of molecular ions with high resolution. We have demonstrated the ability to record 150 cm^-1 of spectra consisting of 45,000 points in 30 minutes and have used this system to record over 1000 cm^-1 of spectra of HfF⁺ in the near-infrared around 800 nm. After improvements, the system can now cover more than 3250 cm⁻¹ (700-900 nm). We have combined this with standard velocity-modulation spectroscopy to measure and analyze 19 ro-vibronic bands of HfF⁺.

These measurements enabled precision spectroscopy of trapped HfF⁺ for testing time-reversal symmetry. For this experiment, we perform Ramsey spectroscopy between spin states in the metastable ³∆₁ level to look for a permanent electric dipole moment of the electron with what we believe is the narrowest line observed in a molecular system (Fourier limited with 500 ms of coherence time). The long coherence time is a major advantage of using ions, but there are also some added complexities. We discuss various aspects metastable state preparation, state detection, and spectroscopy in a rotating frame (due to the necessary rotating electric bias field) that were particular challenging. In addition, we discuss limits to the coherence time – in particular, ion-ion collisions – as well as the sensitivity of the current measurements and provide a path towards a new limit on the electric dipole moment of the electron.

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