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The initial structural evolution of carboxymyoglobin (MbCO) following photodissociation of CO is measured using a recently developed method of heterodyne detection. This method increases the sensitivity, generates tilted pulse fronts which exactly cancel temporal broadening in noncollinear beam geometries, and provides an inherent acoustic reference that enables unambiguous separation of the real and imaginary components to the nonlinear susceptibility. With the latter advance, both the absorption anisotropy and its real counterpart, the phase anisotropy, can be measured with this technique. Access to the real part of the material anisotropy provides new information that can be correlated to the bath dynamics. In the MbCO studies, the phase anisotropy is found to develop on picosecond time scales and is much greater than can be attributed to the symmetry of the heme dipole transition. This provides direct evidence that the shape of the protein changes in the first few picoseconds following photodissociation. The asymmetric nature of the protein structure presumably plays an integral role in the development of the nonuniform displacements. The magnitude of the phase anisotropy and observed dynamics give strong evidence that the low-frequency collective modes of the protein are involved in transducing the reaction forces of the Fe ligation site into directed motions associated with the initial oxy to deoxy tertiary structure change.

The initial structural evolution of carboxymyoglobin (MbCO) following photodissociation of CO is studied using optically heterodyne-detected (OHD) transient grating (TG) spectroscopy. This method provides detailed dynamical information on the electronic and structural states of the heme protein following photoexcitation. The phase anisotropy of MbCO is found to develop on subpicosecond to picosecond time scales and is much greater than can be attributed to the symmetry of the heme dipole transition. Control studies of carboxyprotoheme and deoxymyoglobin were used to identify the components due to protein structural relaxation and thermal relaxation, respectively. A geometric decomposition of the MbCO grating signals into contributions relative to the molecular axes provides evidence that the protein effectively changes its shape within 500 fs following ligand dissociation. These anisotropic mass displacements are a signature of functionally important motions since they imply a certain degree of directionality or mode selective coupling to the response. The anisotropic relaxation and observed dynamics provide further evidence that the low-frequency collective modes of proteins play an important role in transducing reaction forces into functions.

A scheme satisfying CARS phase matching condition in dispersive media at every laser shot in a broadband wavelength region is suggested and is experimentally verified for liquid water.