For a pencil-shaped extended medium with Fresnel number equal to 1, we have quantum mechanically derived a description of the initiation of superfluorescence in terms of Maxwell-Bloch equations with a fluctuating source due to the zero-point fluctuations of the vacuum field. By the introduction of classical behavior, these equations are extended to include nonlinear behavior due to decreasing atomic inversion. The principal assumption in the derivation is that the main features of superfluorescence are governed by the interaction of atoms with field modes inside two small solid angles around the pencil axis. The delay 7~, defined as the time at which the mean-squared tipping angle of the collective Bloch vector attains the value i, turns out to be given by rn = (rs/4)[ln(2n X)"'],where rs is the radiation time for collective decay and N is the number of atoms. The corresponding effective initial tipping angle roughly equals 2/(N)" . A Fokker-Planck equation is derived to describe the statistics of the initial development of the tipping angle.The variance 4v"of the delay of the superfluorescence pulse satisfies approximately br~= 2.3/lnN. A brief comparison with previous treatments of superfluorescence is given.

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Sum-frequency (SF) spectra of a monolayer of thiophenol on silver are reported in the mid and far-IR (infrared). The free-electron laser FELIX was used to reach wavelengths up to 54 μm. Molecular vibrations of thiophenol are observed at wavelengths near 10 μm (three modes), 14 μm (1 mode), and 24 μm (1 mode). The appearance of the different vibrational modes in the spectra varies dramatically due to interference between the resonant sum-frequency signal and the nonresonant sum-frequency signal from silver. The standard model used to describe line shapes in SF spectra is shown to be insufficient to explain the different line shapes for the various vibrational modes of thiophenol on silver.

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