Two-dimensional terahertz rotational spectroscopy in the gas phase

Abstract

Two-dimensional (2D) coherent spectroscopy has been developed to study molecular dynamics and structures for decades, but its extension into the terahertz (THz) regime remains rare. In this thesis, I describe several experiments using two-dimensional terahertz rotational spectroscopy. Employing intense THz electromagnetic fields and the differential chopping technique, we have extended multi-dimensional coherent spectroscopy into the THz regime. We have observed rotational dynamics of linear, symmetric-top, and asymmetric-top molecular species, indicating that 2D THz spectroscopy is an incisive tool for investigating collective quantum effects of the rotational degree of freedom. Based on the quantum mechanical rigid rotor model, we have developed simulation and calculation approaches to disentangling spectroscopic signals from molecular rotations.

We have shown ultrafast 2D THz photon echo spectroscopy of gaseous acetonitrile samples, revealing J-state-resolved rotational dynamics in symmetric-top molecular rotors. We have revealed nonlinear rotational couplings and many-body interactions in water vapor, uncovering the strongly correlated nature of rotational quantum states in water molecules. Additionally, experimental evidence of linear and nonlinear THz spectroscopy of stable water dimers in the vicinity of atmospheric conditions has been observed. We have reported dual-type rotational couplings and a propensity for the K-state-dependent cross-peaks in sulfur dioxide, highlighting distinct rotational properties in slightly asymmetric-top molecules. We have measured the quartic THz effect using two-dimensional THz-Raman hybrid spectroscopy, opening the way for understanding and applications of higher-even-order THz-matter coherences beyond the linear and quadratic THz field effects.

Utilizing the density matrix and time propagation approaches, we have developed a set of simulation and calculation methodologies to characterize rotational dynamics in the gas phase based on the quantum mechanical rigid-rotor model. Our work shows the remarkable capability of 2D THz spectroscopy to interrogate rotational dynamics in the gas phase, laying a foundation for understanding and manipulation of nonlinear light-molecule interactions via multi-dimensional coherent THz spectroscopy.