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.
Transient SRS in mixture of two Raman active media (doubled gas mixtures) with pumping by ultrashort (2 ps) pulses was theoretically analysed. The numerical simulations show that there is strong energy exchange between direct Stokes waves and combination Stokes line during ultrashort pulse propagation in gas mixture. It is necessary to achieve the same phase and temporal dependencies of direct Stokes pulses for effective energy conversion in combination Stokes line. One-peak pulse generation in combination Stokes line with temporal compression is possible at pump intensity from threshold value up to doubled level. The decrease of duration of pulse by 12 times at conversion efficiency of 6% was shown. Increase of input pump energy causes destruction of Stokes pulses form and formation of multipeak Stokes pulses. The results show that the optimum ratio of gas pressure for transient SRS generation of sum and difference combination Stokes lines is proportional to the relation of the g/T2 of mixture components or to the relation of their Raman cross-sections. Our calculation is in good agreement with experimental results1.
The concept for a new approach to generating and receiving optical system for sodium guide star excitation based on solid-state laser technology and the stimulated Raman scattering/amplification in compressed gases is presented. Requirements to laser system for laser guide star generation are analyzed. The most promising SRS media are chosen.
We present the review of femtosecond SRS generation and amplification in compressed gases. the aim of our work is to study SRS spectral and temporal structures in compressed gases with femtosecond light pulses and to optimize conditions of excitation in order to obtain pulses with the desired spectral, temporal, and energy properties. In what follows, we present the result of our studies of SRS amplification in compressed hydrogen pumped by femtosecond pluses of the second harmonic of radiation of a titanium- doped sapphire laser. Our aim was to estimate the feasibility of increasing efficiency of SRS conversion and the potentialities of using transient SRS for spectral-time selection and amplification of weak signals.
The equations describing the propagation dynamics of optical pulses consisting of several electrical field cycles and pulses with spectrum bandwidth comparable with the central frequency are presented. The numerical simulation of nonlinear optical phenomena observed in isotropic media with pumping by the pulses with the spectrum in area of the normal group dispersion is carried out. The spectrum ultrabroadening of a short pulse with the temporal broadening, the self-induced changes of the pulse polarization, the simultaneous generation of several Stokes, anti-Stokes components, the third harmonic and sum and difference frequencies generation with self-phase and cross- phase modulation are considered.
We report an experimental investigation of recording of slanted fringe holograms in DuPont HRF-800X071-20 photopolymer film under pulse exposure. An initially weak polymerization ability was improved significantly by using incoherent preillumination. The line shape and position of spectral absorbance curves are discussed for a wide range of slant angles.
We present results of investigations of modern cw-laser diodes, emitting in the red spectral range from 635 nm to 670 nm with power from 5 mW to 100 mW for recording and reconstructing reflection and transmission holograms. High- quality reflection holograms were recorded in silver halide materials and in DuPont photopolymers. We demonstrated a brilliant reconstruction of transmission holograms with a depth of image up to 3 m.
We demonstrate the recording of high-efficiency reflection holograms in DuPont photopolymer films HRF-800X071-20 and HRF-700X001-20 by using, for the first time to our knowledge, nanosecond laser pulses. The weak reflectance efficiency (6.5%) of the mirror holograms could be significantly improved up to 75% by using pre-illumination. With incoherent pre-illumination, significantly increased diffraction efficiency and sensitivity of the holograms are obtained, while pulsed pre-illumination leads to an enhancement of the reflectance only. The results are compared with cw laser exposure under identical processing conditions, including pre-illumination.
We present the results on stimulated Raman scattering in hydrogen excited by 200 fs-duration 1 kHz repetition rate and 0.6 mW average power Ti:sapphire laser (780 nm) and 0.2 W average power second harmonic (390 nm) pulses with linear and circuit polarization. We report on the spectral and temporal measurements of the various generated vibrational and rotational components at Stokes- and anti-Stokes frequencies in the wavelength range 289 - 1154 nm. At optimal conditions at 390 nm pump wavelength the SRS signal of 30 mW average power was obtained at the first (465 nm) and the second (577 nm) vibrational Stokes. The shortest measured duration of the vibrational Stokes components was by 3 times less than the duration of the pump pulses.
In LilO3 and BBO crystals noncollinear parametric generation with tuning range 0.45 - 2.9 micrometers is investigated by pumping with the second harmonic of 150 fs Ti:sapphire laser pulses. In LilO3 propagation angles simultaneous phase- and group-velocity-matching are determined. Noncollinear parametric amplification with the wavelength tunability from 0.6 to 1.2 micrometers is demonstrated.
We present results on stimulated Raman scattering in H2, D2, HD and CH4 excited by 200 fs-duration 1 kHz repetition rate and 0.6 mW average power Ti:sapphire laser (780 nm) and 0.2 W average power second harmonic (390 nm) pulses with linear and circular polarization. Spectral and temporal measurements of the vibrational and rotational components at Stokes- and anti-Stokes frequencies in the wavelength range 289 - 1154 nm are reported. We observed 2.5 times shortening of the vibrational Stokes component pulses in H2 and D2 exited by 350 fs-duration second harmonic pulses.
We report efficient amplification of weak femtosecond signals by a stimulated Raman scattering process on vibrational and rotational components of pressurized H2 gas excited with 200-fs-duration frequency-doubled pulses from a regenerative-amplified Ti:sapphire laser. The amplification factor of 108 is obtained at the wavelength of 465 nm for weak seed pulses produced by white light continuum generation in glass.
We studied both experimentally and theoretically, the conversion efficiency of 150-fs-duration Ti:sapphire laser pulses to the second, third and fourth harmonic. The laser pulses at the wavelength of 780 nm have a spectral width of 10 nm and were amplified to 0.8 mJ energy in a regenerative amplifier. With a repetition rate of 1 kHz intensity of up to 150 GW/cm2 could be reached by focusing the beam with a lens. KDP, BBO and LilO3 crystals of different length were used for the conversion process. The experimentally observed conversion efficiency to the second, third and fourth harmonic does not exceed 50%, 15% and 8%, correspondingly. It is shown theoretically that one possible process limiting the conversion efficiency of the femtosecond pulses at low as well as high intensity is the modulation of the phase of the fundamental wave.
The sensitivity of a fine-grained silver halide VRP photoplates exposed by laser radiation over a wide range of durations (102 - 10-3 sec) is studied. The drastic deviation of sensitivity in the time range from 10-9 sec to 10-13 sec is found and discussed.
The results of the investigation of a pulsed hololaser or recording of monochrome (sized 600x400 mm ) and two-color (size 300x400 mm ) holograms and topograms (included living beings) are presented. The holosystem is based on Nd phosphate glass laser with frequency doubling (A nm) and Raman conversion in compressed hydrogen (A nm). 1.
The results of work of the holographic studio of the State Optical Institute which is equipped with multicolor pulsed lasers and continuous wave lasers as well as appropriate photomaterials are presented. In this studio any technique of recording can be realized in pratice for various objects including alive. Maximum size of monochrome holograms is 1. 2 x 0. 8 m and color holograms is 60x40 cm 1. INTRODtXT ION At present the display ho lography is the most mass appl icat ion of ho lographic methods popul ar among the populat ion. Due to the permanent interest of holographers to production of display holograms the methods of their recording are now brought to perfection and quality of holograms is of such a level that an observer in a number of cases prefers the holographic image to the original. Such progress in display holography allows transition from technical problems of image quality improvement to problems of artistic representation of objects and forms of the world around. Organization of holographic studios is one of the ways for realization of such a transition. These holographic studios provide the possibility of systematic recording of a wide range of objects and compositions including peopl. nd animals. The first steps in this direction were made in the USSR and USA in 19671970 now the holographic studios exist also in other countries. Broad prospects in realization of artistic
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