Two-dimensional fifth-order Raman spectroscopy of liquid formamide: Experiment and Theory

Abstract

The fifth-order two-dimensional (2D) Raman spectrum of liquid formamide has been obtained. The absolute signal levels, qualitative features, as well as quantitative aspects of the 2D spectrum are found to be in good agreement with recent molecular dynamics calculations. The most important singular feature is the relatively strong rephasing signal observed along the diagonal. This finding illustrates the more structured nature of the hydrogen bond network of liquid formamide in comparison with simple liquids as exemplified by CS(2). The theoretical calculations have been extended to include comparisons of different potentials that illustrate the sensitivity of the experiment to the anharmonic motions in the liquid state. The theoretical results point out the key features in the 2D spectrum that probe the essential details in the intermolecular potential. The experiment has been demonstrated to provide new insight into collective effects operating in hydrogen bonded liquids and opens up the exploration of other liquids with this approach.

Discriminating cascading processes in nonlinear optics: A QED analysis based on their molecular and geometric origin

“…Such signals scale as N 2 and are known as cascading. The cascading signals have the same power dependence in the incoming fields and the same phase matching and are thus hard to distinguish from the direct signal [3][4][5][6][7][8][9]. In neat liquids, cascading dominates the direct signal due to the quadratic vs. linear scaling in molecular density.…”

“…An important example is the direct 5th-order Raman χ (5) process which is accompanied by a product of two 3rd-order χ (3) signals. Separating the two had drawn considerable attention and took several years to recognize [3,4,6,7,[10][11][12][13][14][15]. This has been the main obstacle for multidimensional Raman spectroscopy (5th-and-higher orders) [8].…”

“…We will derive a more general, geometrydependent result and show under what conditions it reduces to this simple macroscopic relation. The extra emission event (from the source molecule) in a cascading process thus renders the cascading signal out of phase relative to the direct nonlinear signal (E ∝ P − iP P ) [1,7,16]. This property has been parlayed into a number of successfullyimplemented techniques to select for the direct 5th-order Raman signal and eliminate the cascading contributions [3,4,16,17] (techniques based on polarization-sensitive measurements have also been pursued [18]).…”

Ultrafast dynamics of solvation: the story so far

“…131 The homodyne-detected fifth-order Raman signal of formamide for the R (5) zzzz tensor element is shown in Figure 5. 131 The experimental results shown in Figure 5 have three distinctive features: (i) an absence of signal along t 2 , the time delay between the two "pump" pulses (four Raman pulses).…”

“…131 The homodyne-detected fifth-order Raman signal of formamide for the R (5) zzzz tensor element is shown in Figure 5. 131 The experimental results shown in Figure 5 have three distinctive features: (i) an absence of signal along t 2 , the time delay between the two "pump" pulses (four Raman pulses). Previously, it had been shown that four-wave mixing signals along t 2 may dominate the overall response of the 2D Raman experiment on carbon disulfide thereby masking the information content of the fifth-order data.…”