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.