Feynman diagram description of 2D-Raman-THz spectroscopy applied to water

Abstract

2D-Raman-THz spectroscopy of liquid water, which has been presented recently (Proc. Natl. Acad. Sci. USA 110, 20402 (2013)), directly probes the intermolecular degrees of freedom of the hydrogen-bond network. However, being a relatively new technique, its information content is not fully explored as to date. While the spectroscopic signal can be simulated based on molecular dynamics simulation in connection with a water force field, it is difficult to relate spectroscopic signatures to the underlying microscopic features of the force field. Here, a completely different approach is taken that starts from an as simple as possible model, i.e., a single vibrational mode with electrical and mechanical anharmonicity augmented with homogeneous and inhomogeneous broadening. An intuitive Feynman diagram picture is developed for all possible pulse sequences of hybrid 2D-Raman-THz spectroscopy. It is shown that the model can explain the experimental data essentially quantitatively with a very small set of parameters, and it is tentatively concluded that the experimental signal originates from the hydrogen-bond stretching vibration around 170 cm −1 . Furthermore, the echo observed in the experimental data can be quantified by fitting the model. A dominant fraction of its linewidth is attributed to quasi-inhomogeneous broadening in the slowmodulation limit with a correlation time of 370 fs, reflecting the lifetime of the hydrogen-bond networks giving rise the absorption band.

Impact of nuclear quantum effects on the structural inhomogeneity of liquid water

“…2 A and B serves to demonstrate the qualitative differences in the 2D response at the extreme temperatures considered in this study. As has been K K 3 3 3 3 3 3 3 3 3 3 9 9 9 9 9 9 9 9 9 2 2 2 2 2 2 2 2 2 2 2 O O O O O O O K 6K 6K 76 76 27 27 27 discussed previously (50,52,55), the measured signals in 2D Raman-THz spectroscopy are governed by the quite evolved IRF, which significantly smears out the real molecular signatures. We will focus our analysis on those parts of the 2D response that are less susceptible to such contaminations from the IRF, that is, the diagonal t1 = t2 in the upper right quadrant of Fig.…”

“…In pure water, we found that the signal is indeed slightly extended in the echo direction t1 = t2 (50). Modelling the data with a very simple model (a single anharmonic oscillator), we suggested that the echo originates mainly from the hydrogen bond bend vibration at ≈200 cm −1 and that a large fraction of its linewidth is attributed to quasiinhomogeneous broadening in the slow modulation limit with a correlation time of 370 fs (52). However, the echo is masked to a significant extent by the instrument response function (IRF); hence, we set out in a subsequent publication (53) to artificially increase the amount of inhomogeneity by adding salts to the solution.…”

“…This coherence evolves as a function of time t2 and emits a THz field that is detected. Among the many possible coherence pathways, there is a rephasing pathway that switches the sign of the coherence, which requires that the first Raman interaction induces a singlequantum transition, while the second THz interaction induces a two-quantum transition (51,52). If the mode under consideration is inhomogeneously broadened on the timescale of the pulse sequence, that coherence pathway will result in an echo, which peaks at a time t2 that equals the time separation t1 between the two excitation pulses.…”

Towards Intense THz Spectroscopy on Water: Characterization of Optical Rectification by GaP, OH1, and DSTMS at OPA Wavelengths

“…The collective transient dynamics of the hydrogen-bonded network dictate the unique solvation properties of water, and are at the origin of the puzzling physical and chemical properties of this special liquid [10][11][12]. However, pivotal issues remain about the interpretation of these THz features [13]. For example, mapping how the energy dissipates upon excitation of a low frequency mode could allow a deeper understanding and, possibly, engineering of solvation processes [14,15].…”

Interpretation of the THz-THz-Raman Spectrum of Bromoform

“…However, this information is encoded in complex, threepoint correlation functions that must be disentangled with the aid of theoretical and computational methods. 30,31,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] This problem of interpretation is further complicated by the number of vibrational states that need to be considered. Whereas infrared-active modes are usually in their vibrational ground state at room temperature and typically only involve single excitations upon illumination, THz-active modes can be thermally excited at room temperature, and multiple transitions between the different states are possible.…”