ABSTRACT
In condensed molecular matter, low-frequency modes (LFMs) associated with specific molecular motions are excited at room temperature and determine essential physical and chemical properties of materials. LFMs, with typical mode energies of up to â¼500 cm-1 (62 meV), contribute significantly to thermodynamic parameters and functions (e.g., heat capacity and entropy) and constitute the basis for room temperature molecular dynamics (e.g., conformational fluctuations and change). LFMs are often analyzed indirectly by the measurement of their effect on specific high-frequency modes (HFMs); the LFM-HFM coupling is reflected in the lineshape, as well as in the spectral and angular diffusion of the HFM. Two-dimensional terahertz-infrared-visible (2D TIRV) spectroscopy allows measuring the LFM-HFM coupling directly and can thereby provide new insights into the strength and nature of the coupling and the character of LFMs. However, the interference between the different signals generated by different excitation pathways can complicate 2D TIRV spectra, preventing a straightforward analysis. Here, we develop an experimental method to distinguish different excitation pathways in 2D TIRV spectroscopy and plot them separately in different quadrants of a 2D spectrum. We validate this method by measuring the spectra of CaF2 and nitrogen gas. For CaF2, only sum-frequency mixing between infrared and terahertz fields generates the signal. In contrast, for N2, only difference-frequency mixing is observed. We then use this method to separate sum- and difference-frequency pathways in the 2D TIRV spectrum of liquid water, verifying the previous interpretation of the lineshape of the 2D TIRV spectrum of water.
ABSTRACT
The interplay of electronic and nuclear degrees of freedom in semiconductor hybrid organic-inorganic perovskites determines many of their fundamental photophysical properties. For instance, charge carriers are dressed with phonons, that is, form polarons, and combination modes composed of strongly mixed localized vibrations and delocalized phonons can provide pathways for electronic energy relaxation and dissipation. Mixing of the different types of nuclear motion in vibrational combination modes requires their strong coupling. The direct measurement of coupling between the high-frequency N-H stretch modes of the organic methylammonium and formamidinium ions and low-frequency Pb-I phonon modes of the inorganic sub-lattice in hybrid organic-inorganic perovskites is presented. The results reveal direct and substantial coupling between the non-covalently interacting organic and inorganic sub-lattices.
