Infrared frequency combs for high resolution
spectroscopy of large buffer gas cooled molecules
Infrared spectroscopy is a primary tool for investigating molecular structure and nuclear motion
dynamics. Extraordinarily precise and detailed insights are achievable when experiments reach
"rotational resolution," i.e. when transition frequencies between internal rotation-vibration (and even
hyperfine) states of a molecule are individually resolved, permitting the determination of the effective
molecular Hamiltonian. The experimental feasibility of this task depends strongly on molecule size and
atom number: large molecules have numerous degrees of freedom, and thermally access many internal
quantum states at standard conditions – all resulting in dense, congested spectra.
Addressing these issues requires both a sensitive, high resolution spectrometer and a method to prepare
cold molecular gases of sufficient number density. In our group, the first need is met with infrared
frequency combs coupled to high finesse enhancement cavities for direct broadband absorption
spectroscopy. These combs utilize a variety of technologies, including optical parametric oscillator and
difference frequency generation based sources. In particular, I will discuss the current development of
long wavelength comb systems extending to and beyond 10 μm. Secondly, we prepare cold gas-phase
molecules using the technique of buffer gas collisional cooling, wherein a cyrogenically cooled inert
buffer gas is used to thermalize hot molecular samples to lower than 10 K. I will present several
applications of this method to a variety of complex molecular systems, culminating in our most recent
results on the cooling and first rotationally resolved spectrum of buckminsterfullerene, C