High-sensitivity measurement of ultrafast transient infrared spectra based on optically heterodyned detection of absorption anisotropy

A method for high-sensitivity measurement of ultrafast transient infrared spectra is proposed. It is based upon optically heterodyned detection (OHD) of transient absorption anisotropy, and its principle is similar to OHD-RIKES (Raman-induced Kerr-effect spectroscopy). The usefulness of this method is examined on a transient infrared spectrum of electronically excited 4-dimethylamino-4′-nitrostilbene in a benzonitrile solution. It is found that the present method is much more sensitive than the conventional pump–probe measurement of a transient infrared absorption. The advantages and drawbacks of this method are discussed.