Pump-probe experiments using a delay line are one important approach in the investigation of molecular dynamics on the femtosecond to picosecond time scale. As the pulse energies for femtosecond pulses are usually small, the measured signal has to be obtained from the overlap region of two focused laser beams. Due to the low density in the gas phase high sensitivity experiments are essential, particularly if one or both laser pulses are in the infrared (IR) or near-IR range with much smaller absorption cross-section as compared to the visible or ultraviolet.
To increase the interaction volume between pump- and probe-pulse, the two laser beams can be focused into a hollow waveguide with an inner diameter dID = 100 to 500 &mgr;m. We have calculated the focusing condition for a near-IR pump- and an UV probe-beam to excite nearly exclusively the lowest HE11-mode within the waveguide. For molecular samples with a low absorption coefficient (alpha < 0.1 m-1 for the probe beam in the ultraviolet) an enhancement of the measured probe signal of a factor of 9-10, relative to a confocal arrangement in a cell, is calculated from the intensity distribution within a hollow waveguide with an inner diameter dID = 250 &mgr;m and length Lwg = 500 mm. The theoretical calculations were confirmed in pump-probe experiments of intramolecular vibrational energy redistribution (IVR) in CH3I vapour. In the experiments the first overtone of the CH-stretching vibration is excited with a near-IR pump-pulse and the redistribution of the vibrational energy to other vibrational degrees of freedom, especially to the CI-stretching vibration, is detected through a change of the UV-spectrum by a probe pulse around 310 nm.
KEYWORDS: Ultraviolet radiation, Molecules, Picosecond phenomena, Liquids, Absorption, Energy transfer, Near infrared, Laser beam diagnostics, Femtosecond phenomena, Molecular energy transfer
The rapid flow of vibrational energy within a molecule is central for the control and as well for the theory of
unimolecular reactions. It defines the lifetime of vibrationally excited states and thus the time during which
a specific vibrational excitation can control the outcome of chemical reactions. Times for intramolecular vibrational energy redistribution (IVR) can be deduced either indirectly from time-independent high resolution infrared(IR) spectra or measured directly in kinetic pump-probe experiments. We have applied delayed ultraviolet(UV) absorption spectroscopy with a time resolution of 150 fs to measure intramolecular vibrational energy
redistribution after near-IR excitation of the CH-stretching vibration around 5900 cm-1 in CF3CHFI, CH3I, C2H5I, and C7H8. Intramolecular relaxation times T(IVR) between 3 and 7 ps have been found in the gas phase. For CH3I an additional short time of 250 fs has been measured. In the liquid phase IVR is followed by a
fast collisional energy transfer of the excitation energy to the solvent molecules. Assuming a two step kinetic
mechanism intermolecular relaxation times T(transfer) between 10 and 30 ps have been determined.
We report experimental and theoretical investigation of supercontinuum generation in broad spectral range from 500 nm to 2500 nm induced in water and bulk fused silica by 1mJ 150-fs pulses at 780 nm excitation wavelength. We find that experiments may be modeled theoretically by considering Raman- and Kerr-type nonlinearities such as transient stimulated Raman scattering, parametric four-photon mixing, self-phase modulation and cross-phase modulation.
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