Precision Spectroscopy on OH

Abstract

This thesis describes the implementation of a high precision laser system which, as a first demonstration of its capabilities, has been used to measure electronic transitions from the X²Π₃⸝₂, v''=0, J''=3/2 rovibronic ground state to the 12 lowest levels of the A²Σ⁺, v'=0 vibronic state in the hydroxyl radical (OH) and the 16 lowest levels of the same vibronic state in the deuterated hydroxyl radical (OD). The relative uncertainty of the absolute frequency measurements is within a few parts in 10¹¹. These electronic transition frequencies are determined by comparing the spectroscopy laser with reference frequency standards using an optical frequency comb (OFC). The OFC transfers the high short term stability of a narrow-linewidth I₂ stabilized referenced laser onto the spectroscopy laser around 308 nm. The second reference used with the OFC is an atomic clock, which provides an absolute accuracy of the measured transitions frequencies. The OH and the OD molecules are inside a highly collimated molecular beam, with the ultraviolet (UV) laser beam propagating perpendicular to it. This setup reduces possible pressure shifts and Doppler-broadening. Additionally, the laser beam is retroreflected to reduce Doppler-shifts. Shifts due to Zeeman-, AC-Stark- and saturation-effects are also considered in the analysis, in an effort to determine the zero-field transition frequencies. Previous studies determined the absolute A ← X transition frequencies with an accuracy of approximately 100 MHz, based on rich Fourier-transform spectra. In contrast, this thesis supplies absolute electronic transition frequencies with an uncertainty of less than 100 kHz. These new measurements of the optical transition frequencies were combined with existing data for fine and hyperfine splittings in the A state and used to fit the parameters of an effective Hamiltonian model of the A²Σ⁺, v'=0 state of each isotopologue. Some of these newly-determined spectroscopic constants, are orders of magnitude more precise than the previous values. Future experiments will benefit from the improved accuracy of the electronic excitation frequencies determined in this experiment. As a next step, a new mid-infrared laser will be used to probe the vibrational excitation frequencies of OH. This OFC-stabilized mid-infrared optical parametric oscillator (OPO), which provides a narrow linewidth and wide tuning range, is also described in this thesis.