Probe Laser Pulse Research Articles

Branching ratios for chemical reaction in collisional quenching of excited 5s1 2S1/2 gallium atoms

We report cross sections for fluorescence quenching and branching ratios for chemical reaction in collisional deactivation of Ga(5s1 2S1/2) by added gases at room temperature following resonance 4p1 2P○1/2 →5s1 2S1/2 excitation at 403.30 nm with a pulsed laser. Total quenching cross sections are obtained from Stern–Volmer analysis of fluorescence lifetimes. Chemical contributions to fluorescence quenching are investigated by a pump and probe technique involving saturation of the resonance transition by a pump laser pulse to produce a known fraction of gallium atoms in the excited state, and measurement of the depletion of the gallium atom concentration due to chemical reaction in the excited state. Relative concentration measurements in the presence and absence of the saturating pump pulse are by resonance fluorescence excitation by a probe laser pulse suitably delayed relative to the pump pulse. An analysis of the experiment in terms of rate equations shows how branching ratios for chemical reaction may be obtained from the depletion measurements. Fluorescence quenching cross sections (Å2) are large for N2O (99±10), CO2 (100±20), CH4 (55±6), C2H6 (77±20), C3H8 (100±20), n-C4H10 (130±20), and C2H4 (75±20), moderately large for CO (11±4) and N2 (8±2), and very small for CF4 (<0.3) and H2 (<0.05). Among the efficient quenchers only C2H4 showed no detectable contribution from chemical quenching. Branching ratios for chemical removal of Ga(5 2S1/2) are N2O (0.96+0.04−0.1), CO2 (0.55±0.1), CH4 (0.27±0.07), C2H6 (0.33±0.1), C3H8 (0.26±0.1), and C2H4 (0.0+0.05−0.0). Results for H2 and the alkane hydrocarbons are discussed with reference to simple concepts of orbital interactions in the entrance channel.

Evidence that the variable fluorescence in Chlorella is recombination luminescence

The fluorescence lifetime of oxygen-forming photosynthetic systems as a function of closed traps has been studied by several groups using light and poisons (usually 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)) to fix the closed trap state during the experiment. These measurements have now been carried out using light alone, by means of pump and probe laser pulses and a very efficient fast photomultiplier-digitizing system. It is found that the absolute amplitude of fast fluorescence (mean ψ, approx. 0.3 ns) remains constant until over half the traps are filled. The amplitude of the slow fluorescence ( τ ≈ 1.2 ns) increases with pump energy, and its response is best fit with a lag or finite risetime of approx. 200 ps. This novel result is consistent with the hypothesis that the slow component of the fluorescence is actually recombination luminescence in the trap. Thus, the full trapping time, i.e., the time to form the P +I − state from an excitation in the O 2 photosystem, is relatively slow.

Femtosecond imaging of melting and evaporation at a photoexcited silicon surface

We describe a simple imaging technique that can be used to photograph ultrafast processes with time resolution determined by the duration of pump and probe laser pulses. We demonstrate this technique by photographs having 100-fsec time resolution of a silicon surface undergoing melting and evaporation following intense excitation by an ultrashort laser pulse. These photographs resolve the increase in surface reflectivity caused by surface melting both temporally and spatially. Material evaporation from the melted surface further alters the image of the surface by absorbing and scattering the illuminating laser light. Our analysis of this selectively imaged light suggests that the evaporated material emerges as liquid droplets several hundred angstroms in diameter, which atomize in less than a nanosecond.

Formation of synchronized light pulses for laser plasma diagnostics by a two-transit Pockels cell

Optical probe studies were made of a laser plasma in the ablation regime of heating of thermonuclear targets. Exact synchronization of a short (0.4 nsec) probe laser pulse with a heating radiation pulse (4 nsec) was ensured by a system based on a two-transit Pockels cell. The probe and heating pulses were formed by the same electrical pulse applied to a two-pass Pockels cell in the case of the probe radiation and to a conventional Pockels cell in the case of the heating radiation. The duration of the short probe laser pulse obtained in this way was approximately equal to half the duration of the leading edge of the electrical pulse. Frame interferograms of the laser plasma were recorded.

Theory for selective-vibrational-dephasing experiments with the use of transient stimulated Raman scattering in high laser depletion

A theory for selective-vibrational-dephasing experiments using transient stimulated Raman scattering in high laser depletion is presented. The transient stimulated Raman scattering equations are numerically solved including laser depletion for homogeneously and inhomogeneously broadened vibrational line shapes. A detailed account of the effects of low and high laser depletion on the vibrational amplitudes is given. An interpretation for excite-and-probe vibrational-dephasing experiments undertaken in the low- and high-laser-depletion regimes is developed. For a collinear laser probe pulse, high depletion of the excitation laser pulse leads to enhanced coherent Stokes probe scattering from one vibrational subgroup in the inhomogeneously broadened line shape. This selective scattering is greatly dependent upon the excitation laser pulse width and the off-resonance frequency of the vibrational subgroup. The enhanced selectivity in high laser depletion allows the homogeneous dephasing time ${T}_{2}$ to be measured to a high level of accuracy.

Gain and loss kinetics in a pulsed discharge by scanning in time with a probing laser pulse

Gain and loss kinetics in a pulsed discharge by scanning in time with a probing laser pulse

A Detailed Examination of Time-Resolved Pulsed Raman Temeperature Measurements of Laser Annealed Silicon

ABSTRACTRaman temperature measurements during pulsed laser annealing of Si by Compaan and co-workers are critically examined. It has been shown previously that the Stokes to anti-Stokes ratio depends critically upon the optical properties of silicon as a function of temperature. These dependences, coupled with the large spatial and temporal temperature gradients normally found immediately after the high reflectivity phase, result in large variations in the calculated temperature depending upon the probe laser pulse width and the pulse-to-pulse and spatial variations in the annealing pulse energy density.

THE TRANSIENT BEHAVIOR OF PROBE LASER PULSE IN A TWO-LEVEL SYSTEM STRONGLY DRIVEN BY A NEAR-RESONANT LASER PULSE

The interaction between a weak probe laser pulse and a two-level system strongly driven by a near-resonant laser pulse is studied by dressed model. The probe pulse exhibits absorption or. amplification when its frequency ω2 tuned to the resonance frequency ω2=ω1±21D>. The transient behavior of nonlinear wave-mixing effect, where the resonant enhancement exists, is also obtained.

PULSED RAMAN MEASUREMENTS OF INHIBITED ELECTRON-PHONON COUPLING AT HIGH PLASMA DENSITIES IN SILICON

A 15 nsec duration, frequency doubled Nd : YAG laser at power densities near 1 J/cm2 has been used to create high plasma densities in silicon at room temperature. Phonon Raman scattering generated by a delayed 7 nsec dye laser probe pulse shows lattice temperatures far below the silicon melting temperature of 1412°C. The results suggest the presence of a dense-plasma-induced phase transition with a greatly inhibited electron-lattice coupling.

Nanosecond transient absorption spectrophotometer with dye laser probe

A nanosecond laser spectrophotometer with a tunable dye laser probe pulse continuously delayed up to about 100 ns with respect to the nitrogen laser pump pulse is described. A time scale synchronisation method for reconstruction of the time-resolved transient absorption spectra is employed. Transient spectra of anthracene in acetonitrile from 415 to 630 nm are given as examples.

Interferometric study of laser-produced plasmas

High-quality interferograms of laser-produced plasmas with spatial resolution of 15 μm are obtained using a relatively simple Jamin-type interferometer and a precisely timed 30-psec probing laser pulse from a mode-locked Nd : YAG laser, frequency doubled to 532 nm. The spatial resolution attainable with the present arrangement is limited by the velocity of the expanding plasma and the finite duratiaon of the probing pulse. The plasmas are produced by focusing a Nd : glass Q-switched laser pulse (0.5 GW, 1011–1013 W cm−2) onto slab targets of Al, Mg, and (CH2)n. The electron density distribution, plasma dimension, and expansion velocity for both point and line foci are studied as a function of time delay between the mode-locked and Q-switched laser pulses. The results obtained with different durations of the heating (Q-switched) pulse and with the various target material are compared.

Double absorption photofragment spectroscopy: A new tool for probing unimolecular processes

A new technique, double absorption photofragment spectroscopy (DAPS), has been developed for observing the time evolution of unimolecular processes. Molecules isolated from collisions in a molecular beam are prepared in a well defined intermediate state by an initiation laser pulse. As this state evolves in time, it is monitored by a second probe laser pulse which, after a chosen delay, photodissociates the molecules. Information about the state to which the molecules have evolved by the time of the probe pulse can be extracted from the mass, density, and angular and energy distributions of the recoiling fragments. Simple models are explored for the functional form of the angular distributions which can be expected in DAPS experiments. This new technique is demonstrated on the I2 B state (used as a real rather than virtual intermediate) whose lifetime is found to vary between 770 and 1600 nsec, depending upon the set of B state v′J′ levels prepared by the initiation laser pulse. Future DAPS experiments should be able to observe unimolecular electronic and nuclear change over a time span from 10−12 to 10−3 sec, probing such processes as rearrangement, dissociation, internal vibrational transfer, intersystem crossing, and internal conversion.

Zero-power gain measurements in a CW HF laser using a pulsed-probe laser

Zero-power gain measurements in a supersonic diffusion-type CW HF laser have been made using a pulsed HF probe laser. A peak zero-power gain of 10 percent/cm was measured on a P_{2}(3) vibrational-rotational transition and 7 percent/cm on a P_{1}(6) transition for flow rates corresponding to 1.8 kW of closed-cavity power. The variation of zero-power gain with rotational quantum number J fits a simple rotational equilibrium model indicating increasing rotational temperatures with increasing distance x from the nozzle exit plane. The model also indicates a total inversion for x \leq 0.2 in for the P_{2}(J) transitions and a partial inversion further downstream.