N. Demirdöven

Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.

Coupling and orientation between anharmonic vibrations characterized with two-dimensional infrared vibrational echo spectroscopy

Two-dimensional vibrational spectroscopy has been used to characterize transient molecular structure by measuring the couplings and projection angles between two strongly coupled anharmonic vibrations. Two-dimensional Fourier-transform infrared spectra of the coupled carbonyl stretches of Rh(CO)2(C5H7O2) in hexane have been obtained from femtosecond vibrational echo signals detected with spectral interferometry. The eight resonances in the two-dimensional spectrum can be interpreted as two diagonal peaks and two cross peaks, each split into a pair. The splitting between the peak pairs is directly related to the diagonal and off-diagonal anharmonicity of the symmetric and asymmetric carbonyl stretches. The ratio of the amplitude of the cross peaks for two different polarization geometries determines the projection angle between the coupled transition dipoles. The experimental characterization of the vibrational eigenstates allows the local carbonyl structure to be modeled as bilinearly coupled cubic anharm...

Separation of cascaded and direct fifth-order Raman signals using phase-sensitive intrinsic heterodyne detection

Cascaded third-order Raman signals can be separated from direct two-dimensional fifth-order Raman signals using heterodyne detection with a local oscillator that is cycled from in-phase to out-of-phase with the direct signal. The difference of these scans gives the direct signal, while the sum gives the in-quadrature cascaded signal.

Dispersion compensation with optical materials for compression of intense sub-100-fs mid-infrared pulses

We report on the generation of essentially transform-limited mid-infrared pulses as short as 75 fs centered at 5mum with 6muJ of energy. Infrared materials with group-velocity dispersion values of opposite sign are used in combination to null second-order dispersion and compress the mid-infrared pulses.