We use sub-10-fs pulses at 400 nm and 15-fs pulses at 800 nm to ionize water molecules and their isotopomers HDO and D2O in a pump–probe scheme. Pulses are generated via spectral broadening of 25-fs pulses of a 1-kHz Ti:sapphire amplifier system by self-phase modulation in a noble-gas-filled hollow waveguide and subsequent compression using chirped mirrors. At this time scale vibronic excitation of the first bending mode of water in the electronic ground state by impulsive Raman scattering is possible (e.g. the fundamental bending mode of H2O: tvib=20 fs). The effect of this pre-excitation on the ionization rate is shown.

Femtosecond pulses tunable between 168 and 181 nm are generated at an energy of 100 nJ by mixing the third-harmonic of a Ti:sapphire laser with pulses from an optical parametric amplifier in an argon-filled capillary.

This work extends the wavelength range of the four-wave mixing (FWM) technique to 170-180 nm using the third-harmonic of a Ti:sapphire laser as the pump and pulses from an optical parametric amplifier pumped by the same Ti:sapphire laser as the idler in the FWM process. By utilizing properly chirped pump and idler pulses, generation of shorter VUV pulses may be possible.

Ultrashort vacuum ultraviolet pulses at 161 nm with energies above 1 muJ were generated by chirped-pulse four-wave difference-frequency mixing in argon-filled aluminum-coated hollow waveguide. The pulses were compressed to 140-fs pulse duration by material dispersion.

Pulse shortening by impulsive high-order stimulated Raman scattering is experimentally demonstrated in the deep ultraviolet for the first time. A single probe pulse at 266 nm is shortened down to 23 fs in N/sub 2/.

We demonstrate compression of ultrashort light pulses in the ultraviolet (UV) by impulsively excited molecular wave-packets in nitrogen filled in a 25 cm long hollow waveguide of 128 microm diameter. After compression with CaF2 prisms the pulse duration was determined by XFROG to be 23 fs with a time-bandwidth product of 0.50. The advantages of our technique are high efficiency and the possibility to use it also for pulses at wavelength shorter than 200 nm. The experimental observations are explained by a theoretical model.

Over the past two decades significant progress was made in generation of high peak power ultrashort pulses utilizing the chirped pulse amplification (CPA) technique. Sub-30-fs pulses with more than 12 mJ energies at 1 kHz repetition rate were generated using Ti:sapphire CPA laser systems. We developed a high-energy Ti:sapphire CPA laser system delivering up to 12 mJ pulses with sub-100-fs duration and spectral bandwidth of 13 nm.

We scaled this technique into the single pulse energy range of 0.1 /spl mu/J, so that time-resolved spectroscopy of small molecules at sufficiently high repetition rates is possible. We use laser pulses of a high-energy Ti:sapphire laser system.

The nonlinear optical response of carbon nanotubes (CNTs) to the interaction with intense ultrashort laser pulses was studied theoretically and experimentally. A full quantum-mechanical theory for harmonics generation from a single-walled CNT has been developed, using the quantum kinetic equations for π-electrons with both intraband and interband transitions taken into account. In the regime of strong driving fields, a non-perturbative approach with the numerical solution of the quantum kinetic equations in the time domain was used to calculate the density of the axial electric current in CNTs. The results of this theory are compared to experiments performed on samples of multi-walled CNTs, using pulses of 160 fs generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The experimental results show indeed an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields of ~ 1010 W/cm2, in very good agreement with the theoretical predictions. The interaction of CNTs with strong laser fields results not only in the generation of harmonics, but also in the generation of a broad spectral background. Generation of a continuous background in the vicinity of the third-harmonic of the laser field was also obtained from the quantum-mechanical calculations, however, with lower intensities than observed experimentally. Possible explanations for this discrepancy will be discussed.

Two-photon excitation with femtosecond laser pulses in the spectral range 240-250 nm was used to prepare vapor phase H(2)O and D(2)O in the C (1)B(1) and D (1)A(1) states. Both states are predissociated via the B (1)A(1) state, forming excited OH/OD(A (2)Sigma(+)) as well as ground state OH/OD(X (2)Pi). We used ultrashort infrared probe pulses (1.65-2.42 microm) to control the ratio between these excited and ground state fragments originating from the dissociation process. Time resolved detection of the OH/OD(A (2)Sigma(+)) --> OH/OD(X (2)Pi) fluorescence allows us to monitor the dynamics of the predissociation. For the heterogeneous predissociation out of the C(1)B(1) state life times of (0.5 +/- 0.1) ps and (1.2 +/- 0.1) ps were found for H(2)O and D(2)O, respectively. The purely homogeneous character of the predissociation out of the D (1)A(1) state was monitored.

Two-photon excitation with femtosecond laser pulses in the spectral range 240–250 nm was used to prepare vapor phase H2O and D2O in the C̃ 1B1 and D̃ 1A1 states. Both states are predissociated via the B̃ 1A1 state, forming excited OH/OD(A 2Σ+) as well as ground state OH/OD(X 2Π). We used ultrashort infrared probe pulses (1.65–2.42 μm) to control the ratio between these excited and ground state fragments originating from the dissociation process. Time resolved detection of the OH/OD(A 2Σ+)→OH/OD(X 2Π) fluorescence allows us to monitor the dynamics of the predissociation. For the heterogeneous predissociation out of the C̃ 1B1 state life times of (0.5±0.1) ps and (1.2±0.1) ps were found for H2O and D2O, respectively. The purely homogeneous character of the predissociation out of the D̃ 1A1 state was monitored.

Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.

Light pulses of 149 μm wavelength and 700 ps duration are generated by non-collinear phase-matched difference frequency mixing of laser pulses at 1053.5 and 1061 nm in a (110) cut GaP crystal. The pump laser pulses are generated in a time-synchronized mode-locked double-frequency Nd:glass laser system consisting of a silicate glass branch and a phosphate glass branch. A photon conversion efficiency of 4 × 10−6 is achieved. The non-linear susceptibility constant of GaP is determined to be d14 = (10 ± 1) pm V−1.

We describe a laser system capable of producing tunable femtosecond VUV pulses around 100 nm by two-photon near-resonant four-wave difference-frequency mixing in argon. Two colour pump-probe ionization experiments allow the characterization of the VUV pulses as well as time-resolved studies of highly excited molecular systems (CS2 , toluene, HDO) in a supersonic jet.

The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump–probe experiments. The vacuum ultraviolet (VUV) pump pulses with a pulse width of 350–450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive à state has been directly observed for the first time: τD⩽20 fs. For molecular oxygen highly excited in the Schumann–Runge band, a decay time of 40±20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times τD=(1.9±0.1) and τD=(90±20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.

Summary form only given. It is well established, that the generation of higher harmonics, plasma sources or higher order wave-mixing schemes provide ultrashort VUV-pulses in a broad spectral range on ultrashort timescales. Our scheme, the two-photon near-resonant four-wave difference-frequency mixing (FWDFM) in a noble gas has the unique property to generate continuously tunable subpicosecond VUV pulses down to the 100 nm wavelength region on a high pulse energy level which is sufficient for nearly every common spectroscopic technique. The aim of the present work is to characterize the obtained sub-picosecond VUV-pulses especially at 155 nm, to discuss the actual limitations of our setup and to demonstrate the intrinsic potential of our source by investigating small molecular systems of interest (toluene, CS/sub 2/, NO, ...).

Nearly bandwidth-limited, time-synchronized, frequency tunable picosecond pulses are generated in an active and passive mode-locked laser consisting of an Nd: silicate glass and an Nd: phosphate glass branch and a common antiresonant ring. An electro-optic Q-switch, an acousto-optic modulator and a saturable absorber in the antiresonant ring are responsible for mode locking and time synchronization. Three-plate birefringent filters in the gain branches are used for spectral narrowing (pulse duration broadening) and wavelength tuning. Pulses of 40 ps duration with a synchronization jitter of 10 ps are generated. A frequency tuning range from 0 to 350 cm-1 is achieved.