Objective

• To separate homogeneous and inhomogenous contributions to vibrational linewidth.

• To probe couplings between nuclear motions.

• To reveal anharmonic potential surface of liquid motions.

• To extend the technique, via heterodyne detection, to a variety of systems.

Approach

An electronically nonresonant Raman excitation of a system by two ultrafast laser pulses leaves the system in a superposition state of two vibrational levels, a vibrational coherence.

After a time delay, a second pair of ultrafast pulses can transfer the system to a second superposition state or to a population state.

After a second time delay, a final pulse stimulates a Raman scattering event in a particular phase matched direction.

The existence of a fifth-order signal is predicated on the presence of a nonlinearity, either a nonlinearity in the polarizability or an anharmonicity in the liquid potential. Recent simulations suggest that anharmonicity is the nonlinearity that dominates¹; thus, the fifth-order experiment can reveal details of a liquid's potential that lower order spectroscopies can not.

Further, the fact that the two states induced by the first two sets of pulses may be superposition states that have nearly equal and opposite time-evolution (as determined by the frequency differences between the vibrational levels involved in the two states) allows for rephasing. Rephasing effects removal of inhomogeneous contributions to the line shapes for certain values of the two time-delays involved.

1. The molecular origins of the two-dimensional Raman spectrum of an atomic liquid. II. Instantaneous-normal mode theory, A. Ma, R.M. Stratt, submitted.

Results

• Demonstrated that 5th order 2-D Raman signals are dominated by 3rd order cascades.

• Obtained signals dominated by the direct 5th order response (minimizing the cascades) by adjusting the phase-matching geometry and polarization.

• Demonstrated the polarization selectivity of the signals and obtained several tensor elements of the 5th order 2-D Raman signal.

• Employed a heterodyne detection and phase-locking scheme that amplifies the small measured signal and allows for phase- selective detection.

Contour plots of the magic angle tensor elements of the two-dimensional fifth-order signal of room temperature CS2: (a) Rmmzzzz, (b) Rzzmmzz, and (c) Rzzzzmm

References

• Fifth-order electronically non-resonant Raman scattering: two-dimensional Fourier deconvolution, L.J. Kaufman, J. Heo, G.R. Fleming, J. Sung, M. Cho. Chem. Phys. 266, 251-271 (2001)

• Polarization selectivity in fifth-order electronically nonresonant Raman scattering from CS_2, L.J. Kaufman, D.A. Blank, G.R. Fleming. J. Chem. Phys. 114, 2312-2331 (2001).

• Direct Fifth-Order Electronically Non-Resonant Raman Scattering from CS2 at Room Temperature, D. A. Blank, L. J. Kaufman, and G. R. Fleming. J. Chem. Phys. 113, 771-778 (2000).

• Intrinsic Cascading Contributions to the Fifth- and Seventh-Order Electronically Off-Resonant Raman Spectroscopies, M. Cho, D. A. Blank, J. Sung, K. Park, S. Hahn, and G. R. Fleming. J Chem Phys, 112, 2082-2094 (2000).

• Fifth Order Two Dimensional Raman Spectra of CS2 Are Dominated by Third Order Cascades, D. A. Blank, L. J. Kaufman and G. R. Fleming. J. Chem. Phys. 111, 3105-3114 (1999).

• Two-dimensional femtosecond vibrational spectroscopy of liquids, Y.Tanimura, S. Mukamel, J. Chem. Phys, 99, 9496 (1993).

• Nonresonant intermolecular spectroscopy beyond the Placzek approximation. II. Fifth-order spectroscopy, R.L. Murry, J.T. Fourkas, T. Keyes, J.Chem. Phys. 109, 7913 (1998).

• Diffractive optics implementation of six-wave mixing, V. Astinov, K.J. Kubarych, C.J. Milne, R.J. Dwayne Miller, Optics Letters, 25, 853 (2000).

• Separation of cascaded and direct fifth-order Raman signals using phase-sensitive intrinsic heterodyne detection, O.Golonzka, N.Demirdoeven, M.Khalil, A. Tokmakoff. J. Chem. Phys. 113, 9893 (2000).

• Mode-coupling theory of the fifth-order Raman spectrum of an atomic liquid, R.A. Denny, D.R. Reichman, Phys Rev. E, 63, 065101 (2000).