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A novel femtosecond IR method is introduced and used to study the third-order response of liquids. This method, based on the gated detection of a CW probe beam, requires only one ultrafast pulse and a CW probe beam. This opens the possibility for the study of the two-color third-order response in a much wider spectral range. It is demonstrated here that this detection method is sensitive to the change in both the amplitude and phase of the probe field. In transparent liquids, the third order susceptibility affects the geometry, polarization, and frequency of the probe IR beam, all of which can be measured by this method. Examples of study with optical femtosecond pump and CW IR probes on CS2 and other liquids are presented. The measured thirdorder susceptibilities are compared to those measured in the optical region. At 1843 cm-' for CS2 the linear dipole polarizability correlation function adequately describes the nuclear contribution to the third order susceptibilities; for benzene and benzene derivatives, the linear polarizability dominates the nuclear response, but contributions from the dipolar and hyperpolarizability may need to be considered.

We have undertaken ultrafast infrared (IR) spectroscopic studies in order to elucidate the geometry of bound CO in the alpha and beta subunits of hemoglobin (Hb) M Boston 13CO. Hb M Boston is a mutant human Hb in which the distal histidine in the alpha subunits is replaced by a tyrosine. The IR absorptions of bound 13CO fall at 1925 cm-1 for the alpha subunits and 1907 cm-1 for the beta subunits. Despite a difference of nearly 20 cm-1 in these peaks, the measured anisotropies of the bound 13CO depletions following 30% photolysis are nearly identical, with values of -0.142 +/- 0.002 obtained for the alpha subunits and -0.140 +/- 0.003 obtained for the beta subunits. These translate to values of 20 degrees +/- 1 degree and 21 degrees +/- 1 degree for the values of the average angles between the CO bond and the normal to the heme planes in the alpha and beta subunits, respectively. Our present results and the work of previous investigators [Nagai, M., Yoneyama, Y., & Kitagawa, T. (1991) Biochemistry 30, 6495-6503] suggest that a change in the polar interactions of the bound CO with the heme pocket environment upon substitution of tyrosine for the distal histidine and a less bent structure for the Fe-C-O unit in the alpha subunits are responsible for the difference in the bound CO absorption frequencies in the alpha and beta subunits. A spectrum of the depletion of the bound 13CO peaks following photolysis indicates that both subunits photodissociate CO with the same quantum yield and neither subunit exhibits significant recombination within 1 ns.