Gas-phase C60 photoionization and photofragmentation experiments were performed using a sub-50 fs Ti Sapphire laser system and reflectron time-of-flight (RETOF) mass spectrometer. The dependence of the C60+ and C602+ signals on the laser intensity for the fundamental (795 nm) and second harmonic (ca. 400 nm) has been determined. For low laser intensities, before the onset of fragmentation, single ionization is a direct multiphoton process. Double ionization is a sequential process in which C602+ originates from already singly ionized fullerenes. At laser intensities beyond the onset of C602+ there is considerable metastable fragmentation indicating a strong coupling of electronic excitation energy into vibrational degrees of freedom that appears to be in competition with multiple ionization.

We report on the observation of a Raman frequency conversion mechanism different from the usual Stokes (anti-Stokes) stimulated process. A continuous down-shift of laser pulse frequency was measured for sufficiently short ͑#40 fs͒ pulses propagating in gaseous SF 6 . We attribute this behavior to the extremely nonstationary nuclear response of SF 6 molecules to the light field with a duration shorter than the period of the Raman-active vibration. [S0031-9007(98)06773-8] The purpose of this Letter is to present some new experimental and theoretical characteristics of stimulated Raman scattering of light in the region of fs-laser pulse durations. Stimulated Raman scattering (SRS) was extensively studied during the past three decades, and nonlinear properties of this phenomenon are well established in the literature: The effect occurs for laser field intensities exceeding some threshold, and the frequency spectrum of generated coherent radiation generally contains discrete Stokes (anti-Stokes) frequency components whose number increases with an increase of the pump laser intensity and the interaction length [1]. In the ps-pulse regime the physical picture is more complicated, and such factors as nonstationary molecule response, group velocity dispersion, self-phase modulation due to electronic Kerr effect, etc., come into play [2]. Considerable progress in the understanding of short-pulse mechanisms of SRS has been made in nonlinear optics of fibers where SRS is involved in the processes of pulse nonlinear self-action and soliton formation [3].The transient properties of SRS are usually discussed in the connection with the vibrational dephasing time T 2 . For pulses shorter than T 2 the amplitude of resonantly driven molecular vibrations has no time to settle within the interaction time t p which gives rise to a number of Raman-type coherent propagation effects observed in gases and liquids. Delayed Stokes generation, shortening of the Stokes pulse, an oscillating structure of the pump, and Stokes field were reported and discussed by many authors [2,4,5]. Despite the different interaction regimes and different parameters of the Raman media used, the main features of the SRS process established in the first experiments are normally observed down to fs-pulse durations.We draw attention to the fact that a very specific behavior of the SRS process could be expected for laser pulse durations comparable or shorter than the molecular vibrational period: t p , T y 2pV 21 y .(1) The nature of nonstationarity here originates from the transient character of the buildup of the vibrational response itself. If one remembers that the vibrational frequencies of many Raman-active materials lie in the region of several hundreds of cm 21 , it can be seen that the discussed extreme regime of SRS is rather a regular trend for nonlinear optical interactions with pulses of sub-100-fs duration. Within the short time (1), a quantum oscillator has no time to exhibit its vibrational properties, and therefore it cannot be resonantly driven by...

We study the reaction dynamics of OClO in cyclohexane, acetonitrile, and water by femtosecond pump–probe spectroscopy. In all solvents we observe a quantum beat in a 403 nm one-color pump–probe experiment with 55 fs temporal resolution, that decays with a 1.3–1.5 ps time constant. From this we conclude that, in contrast to previous reports, not all OClO molecules dissociate after excitation with 403 nm light. In both cyclohexane and water we observe in the 403 nm experiment an increase in stimulated emission between 0.5 and 2 ps that appears to be connected to the quantum beat decay. We explain these results as the consequence of vibrational relaxation of the bending mode of OClO. Relaxation from (ν1,1,0) to (ν1,0,0) leads to population of a state with a two times higher transition dipole moment, which accounts for the increased stimulated emission. Further proof that not all OClO molecules dissociate immediately after excitation is found in the identification of a stimulated emission contribution in femtosecond 400 nm pump/800 nm probe experiments, which also decays with about a 1.5 ps time constant. Femtosecond 400 nm pump/267 nm probe measurements indicate that a fraction of the OClO molecules dissociate very rapidly, with dissociation times of ⩽60, 80, and 140 fs, in acetonitrile, water, and cyclohexane, respectively. An anisotropy decay is resolved at 267 nm of the formed ClO in water and cyclohexane, with anisotropy decay times of 0.17 and 0.27 ps, respectively. In all solvents a fraction of the ClO+O fragments recombine, with time constants of 1.2 and 4.1 ps in water, 6.0 ps in acetonitrile, and 8.9 ps in cyclohexane. In acetonitrile a secondary dissociation pathway is identified with a 2.1 ps time constant. This pathway might also be responsible for the biexponentiality of the recombination process in water. In particular, in acetonitrile and cyclohexane the data indicate cage escape of a significant amount of fragments.

Frequency-doubled pulses from a sub-40-fs, 1-kHz Ti:sapphire amplifier system are spectrally broadened in an argon-filled hollow waveguide. Compression of the self-phase-modulated pulses is implemented with chirped mirrors and a prism pair, yielding 8-fs, 15-muJ pulses in the violet spectral range.

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