Interpretation of the THz-THz-Raman Spectrum of Bromoform

Abstract

Nonlinear THz-THz-Raman (TTR) liquid spectroscopy offers new possibilities for studying and understanding condensed-phase chemical dynamics. Although TTR spectra carry rich information about the systems under study, the response is encoded in a three-point correlation function comprising of both dipole and polarizability elements. Theoretical methods are necessary for the interpretation of the experimental results. In this work, we study the liquid-phase dynamics of bromoform, a polarizable molecule with a strong TTR response. Previous work based on reduced density matrix (RDM) simulations suggests that unusually large multi-quanta dipole matrix elements are needed to understand the measured spectrum of bromoform. Here, we demonstrate that a self-consistent definition of the time coordinates with respect to the reference pulse leads to a simplified experimental spectrum. Furthermore, we analytically derive a parametrization for the RDM model by integrating the dipole and polarizability elements to the 4 th order in the normal modes, and we enforce inversion symmetry in the calculations by numerically cancelling the components of the response that are even with respect to the field. The resulting analysis eliminates the need to invoke large multi-quanta dipole matrix elements to fit the experimental spectrum; instead, the experimental spectrum is recovered using RDM simulations with dipole matrix parameters that are in agreement with independent ab initio calculations. The fundamental interpretation of the TTR signatures in terms of coupled intramolecular vibrational modes remains unchanged from the previous work.

2D Raman–THz Spectroscopy of Binary CHBr3–MeOH Solvent Mixture

“…A deeper understanding of these effects will require molecular dynamics simulations, in connection with procedures to calculate the 2D-Raman THz response. [40][41][42][43] Such MD simulations will be demanding, since they will have to build on a polarizable and flexible force field, and since convergence of the required three-point correlation functions scales very unfavorably with the coherence time of the system (which is long in the case of the intermolecular mode of CHBr 3 ). We nevertheless hope that our work triggers effort in that direction.…”

Sum-Frequency Signals in 2D-Terahertz-Terahertz-Raman Spectroscopy

“…With 2D-TTR spectroscopy, complex spectra have been observed in several halogenated methane (HM) liquids, and were interpreted as signatures of coherent energy transfer pathways between intra-molecular vibrational modes. [13][14][15][16] . In this work, we present a thorough re-investigation of two HM liquids -bromoform (CHBr 3 ) and chloroform (CHCl 3 ) -which casts doubt on this original interpretation.…”

2D Raman-THz spectroscopy of binary CHBr3-MeOH solvent mixture

“…A deeper understanding of these effects will require molecular dynamics simulations, in connection with procedures to calculate the 2D-Raman THz response. [38][39][40][41] Furthermore, the double peak structure of the cross peak, or correspondingly, the beating of the time-domain data in the t 2 -direction, emphasize that the response is not instantaneous in the THz dimension in our RTT pulse sequence, unlike for the TTR pulse sequence. 23 The demonstrated sensitivity of 2D Raman-THz spectroscopy toward lowfrequency intra/intermolecular vibrational couplings can be utilized further to study structural characteristics of hydrogen bond network forming systems.…”