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. 2016 Jun 21;113(25):6857-61.
doi: 10.1073/pnas.1605631113. Epub 2016 Jun 6.

Coherent two-dimensional terahertz-terahertz-Raman spectroscopy

Ian A Finneran  1 , Ralph Welsch  1 , Marco A Allodi  1 , Thomas F Miller 3rd  1 , Geoffrey A Blake  2
Affiliations

Coherent two-dimensional terahertz-terahertz-Raman spectroscopy

Ian A Finneran et al. Proc Natl Acad Sci U S A. .

Abstract

We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic media, and is sensitive to the coupling and anharmonicity of thermally activated THz modes that play a central role in liquid-phase chemistry. Specifically, by varying the timing between two intense THz pulses, we control the orientational alignment of molecules in a liquid, and nonlinearly excite vibrational coherences. A comparison of experimental and simulated 2D TTR spectra of bromoform (CHBr3), carbon tetrachloride (CCl4), and dibromodichloromethane (CBr2Cl2) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes.

Keywords: coherent multidimensional spectroscopy; terahertz; ultrafast dynamics.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
An overview of the 2D TTR experiment. (A) The pulse sequence used in this work includes two intense THz pulses separated by time delay t1, followed by a weak NIR probe pulse at delay t2. The signal is measured as a birefringence of the NIR probe pulse and is sensitive to changes in molecular polarizability. The polarizations of the light fields are shown in the circles above the pulses. (B) By controlling the phase overlap of two high-field THz pulses in liquid CHBr3, a positive (t1 = 0 fs) or negative (t1 = 250 fs) signal corresponding to the birefringence can be generated. Multiexponential contributions from intermolecular vibrations and librations are present near t2 = 1 ps. At other times (t1 = 150), the orientational birefringence is dampened. (C) The proposed molecular mechanism causing a sign change in the birefringence is due to control of the electronic polarizibility and orientational alignment of the molecules in the sample.
Fig. 2.
Fig. 2.
Representative time domain 2D TTR signals. (A) The 2D time domain response of liquid bromoform. (B) Cutting the response in A beyond t2 = 1 ps and detrending fits isolates the vibrational coherences.
Fig. 3.
Fig. 3.
A quantum mechanical description of the measured signals. (A) Two of 24 Liouville pathways in the 2D TTR experiment for generalized eigenstates |a〉, |b〉, and |c〉. (B and C) The energy levels and observed couplings in (B) CHBr3 and (C) CCl4. THz excitations are shown in black, Raman in red.
Fig. 4.
Fig. 4.
Experimental (Left) and RDM simulated (Right) 2D TTR spectra of (A) CHBr3, (B) CCl4, and (C) CBr2Cl2. The f1 axis corresponds to the THz pump, and the f2 axis corresponds to the optical Raman probe.
Fig. 5.
Fig. 5.
The vibrational energy levels of CBr2Cl2. Observed THz excitations are shown as black arrows, and those for Raman excitations are in red.
See this image and copyright information in PMC

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