The spin-orbit split-off ͑SO͒ exciton is observed in a III-V semiconductor ͑InP͒ using spectrally resolved four-wave mixing ͑SR-FWM͒ with 1.515 eV, 35 fs pulses at 77 K. Interference between the coherent response of the SO exciton and that of electron-hole pairs in continuum states leads to frequency-and time-dependent signatures that can be directly attributed to excitation-induced dephasing ͑EID͒ of the SO exciton. Many-body effects can therefore be isolated from free induction decay contributions, in contrast to experiments that probe the fundamental exciton. Simulations employing the multiband Boltzmann-Bloch equations including EID effects corroborate our experimental results.

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The coherent response of excitons and low-energy continuum excitations has been determined in InP at 5 K using spectrally resolved four-wave mixing employing 40 fs pulses in a self-diffraction geometry. The excitons were studied under simultaneous excitation with free carriers of density 10 15 ϽNϽ10 17 cm Ϫ3 . Excellent agreement between experiment and a simple theory based on excitation induced dephasing is obtained for an exciton dephasing rate which varies as N ␣ with ␣Ϸ0.5 in the density range. The sublinear dependence is suggestive of carrier-exciton screening effects; however, a variation in exciton oscillator strength and/or phasespace filling effects may also be contributing to the density dependence of the four-wave-mixing signal. For the continuum excitations we spectrally resolve the influence of LO phonons on continuum dephasing. Below the threshold for LO-phonon emission by electrons we determine a dephasing rate of ϳ8 ps Ϫ1 at Nϭ5 ϫ10 16 cm Ϫ3 ; for states just above the threshold for LO-phonon emission, the dephasing rate is Ͼ13 ps Ϫ1 , our resolution limit.

Variations of the organometallic cluster known as King's complex, [ (CsH~)Fe(C0)14, have been synthesized to study the excited-state transition responsible for reverse saturable absorption observed at 532 nm in this molecule. Picosecond pumpprobe measurements have been used to measure the excited-state cross sections and lifetimes in King's complex and the analogs methyl-King's complex, [CH3(C5H4)Fe(C0)I4, and triethylaluminum King's complex, [ (CsH5)Fe(COAl(C2H5)3)]4. Within experimental error, the excited-state parameters are unaffected by alterations to either the carbonyl or the cyclopentadienyl ligand of the King's complex. These results are consistent with an excited-state absorption resulting from a second d to d transition within the singlet states of the metal core.

It is shown that Lennard-Jones parameters are not g o d correlating factors; the implications of this fact on the type of interaction potential effective for energy transfer from highly excited molecules are discussed.We have demonstrated reverse saturable absorption of picosecond pulses at 532 nm using the organometallic tetramer cyclopentadienyliron carbonyl [ (C5H5)Fe(CO)], dissolved in methylene chloride. Time-resolved pumpprobe measurements and numerical simulations based on a five-level model indicate that reverse saturable absorption of ultrafast optical pulses is due to excited singlet absorption in this tetramer. We have determined the lifetime of the first excited singlet state to be 120 ps, and have measured a singlet excited-state capture cross-section of 8 X lo-'* cm2, twice that of the ground-state cross-section. In addition, we have placed bounds on the lifetime of the second excited singlet state and the branching ratio between the singlet and triplet systems.