A Feynman diagram description of the 2D-Raman-THz response of amorphous ice

Abstract

The 2D-Raman-THz response in all possible time-orderings (Raman-THz-THz, THz-Raman-THz, and THz-THz-Raman) of amorphous water ice is calculated in two ways: from atomistic molecular dynamics simulations and with the help of a Feynman diagram model, the latter of which power-expands the potential energy surface and the dipole and polarizability surfaces up to leading order. Comparing both results allows one to dissect the 2D-Raman-THz response into contributions from mechanical anharmonicity, as well as electrical dipole and polarizability anharmonicities. Mechanical anharmonicity dominates the 2D-Raman-THz response of the hydrogen-bond stretching and hydrogen-bond bending bands of water, and dipole anharmonicity dominates that of the librational band, while the contribution of polarizability anharmonicity is comparably weak. A distinct echo of the hydrogen-bond stretching band is observed for the THz-Raman-THz pulse sequence, again dominated by mechanical anharmonicity. A peculiar mechanism is discussed, which is based on the coupling between the many normal modes within the hydrogen-bond stretching band and which will inevitably generate such an echo for an amorphous structure.

Equilibrium–nonequilibrium ring-polymer molecular dynamics for nonlinear spectroscopy

“…13,14 However, the interpretation of such spectra is still an open question and requires adequate simulation methods. 12,[15][16][17] Several computational methods have been proposed for simulating two-dimensional offresonant Raman and hybrid terhertz-Raman spectra. Model-based approaches aim at constructing simplified, few-dimensional model systems that can be solved fully quantum mechanically in the presence of a harmonic oscillator bath.…”

Simulating two-dimensional correlation spectroscopies with third-order infrared and fifth-order infrared--Raman processes of liquid water

“…The vibrational modes interact through the mechanical anharmonic coupling (MAHC) described by g s 2 s and g ss 2 and the electric anharmonic coupling (EAHC) described by µ ss and Π ss . Although the effects of MAHC and EAHC in 2D spectra have been discussed analytically, 38,39,[69][70][71][72] no quantum theory exists for complex systems involving complex system-bath interactions, such as those in this study.…”

“…In the present case, the difference between the fundamental frequencies of the intermolecular and intramolecular modes is very large, and we can excite each mode separately by choosing the light sources: a highfrequency intramolecular mode is solely excited by an IR pulse, whereas a low-frequency intermolecular mode is solely excited by a Raman or THz pulse. Note that, although the contribution from EAHC arises from nonlinear dipole elements or nonlinear polarizability elements are not important for a spectroscopic approach based on the third-order response functions for s = 1 and 2 and they can be ignored, 38,39 we used the same parameter values presented in Ref. 37 as given in Tables I and II because the computational cost in our approach is the same.…”

“…[34][35][36][37] The cross peaks predicted from the 2D THz-IR-visible spectra arise from both MAHC and EAHC contributions of the stretching-translational modes, but, in the 2D IR-IR-Raman-Raman spectra, the cross peaks always arise from MAHC, whereas those from EHAC appear at different positions or do not appear, as can be seen from the analytical expressions of the response functions. 38,39,[69][70][71] Moreover, although 2D THz-IR-visible measurement is designed to detect the coherence between the two different modes through the mode-mode interactions, the present 2D IR-IR-Raman-Raman spectroscopy is designed to detect the population transfer among modes under dephasing and relaxation through the use of period t 2 . Thus, the information obtained from the two approaches is different and complimentary.…”

“…], μ(0)]] / 3 because the contributions from the nonlinear polarizability and dipole moments involved in the laser interactions give rise to ad-ditional peaks, and nondiagonal spectral peaks are not necessary to represent mode-mode coupling peaks, as in the case of 2D IR spectra. 38,39 In this study, we extend our analysis of 2D THz-Raman [40][41][42][43] and 2D IR-Raman spectroscopies 36,37 by including an additional time duration for laser excitation and calculate the fifth-order 2D IR-IR-Raman-Raman spectrum defined as R…”