Lasers pulse width

The time resolution of these methods is detennined by the time it takes to mitiate the reaction, for example the mixing time in flow tubes or the laser pulse width in flash photolysis, and by the time resolution of the detection. Relatively... 

A series of measurements in which the pump wavelength is varied reveal that at some energies the oscillations predominate for times beyond lOps, whilst at others the decay of population by curve-crossing wins out within 400 fs or so. The time resolution of the experiment is in this example is determined by the convolution of the two laser pulse widths, here about 125fs. 

For near-field imaging based on nonlinear or ultrafast spectroscopy, light pulses from a femtosecond Ti sapphire laser (pulse width ca. 100 fs, repetition rate ca. 

The characterization of the laser pulse widths can be done with commercial autocorrelators or by a variety of other methods that can be found in the ultrafast laser literature. " For example, we have found it convenient to find time zero delay between the pump and probe laser beams in picosecond TR experiments by using fluorescence depletion of trans-stilbene. In this method, the time zero was ascertained by varying the optical delay between the pump and probe beams to a position where the depletion of the stilbene fluorescence was halfway to the maximum fluorescence depletion by the probe laser. The accuracy of the time zero measurement was estimated to be +0.5ps for 1.5ps laser pulses. A typical cross correlation time between the pump and probe pulses can also be measured by the fluorescence depletion method. 

OL behavior is assessed simply by monitoring the transmission of a (usually solution) sample as a function of the incoming laser fluence measured in joules per square centimeter (rather than intensity in watts per square centimeter).22,23 Limiting thresholds Fth, defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, are then commonly quoted. Since excited-state absorption processes generally determine the OL properties of molecules, the excited-state structure and dynamics are often studied in detail. The laser pulse width is an important consideration in the study of OL effects. Compounds (1-5)58-62 are representative non-metal-containing compounds with especially large NLO and/or OL... 

The drift time as calculated by means of Eq. 4.7 is not fully identical to the total time-of-flight. Obviously, the time needed for acceleration of the ions has to be added. Furthermore, a short period of time Iq may be attributed to the laser pulse width and the process of desorption/ionization, which is typically in the order of a few nanoseconds. Thus, the total time-of-flight itotai is given by... 

The Raman spectra of Ni(PP) in coordinating solvents (Figures 2 and 3) clearly show that photoinduced ligation changes occur within the 10 nsec laser pulse width. The appearance of the modes lo and V2 at 1658 cm , 1520 cm", ... 

The PLP-SEC method, like the rotating sector method, involves a non-steady-state photopolymerization [Beuermann, 2002 Beuermann and Buback, 2002 Komherr et al., 2003 Nikitin et al., 2002], Under pulsed laser irradiation, primary radicals are formed in very short times ( 10 ns pulse width) compared to the cycle time ( 1 s). The laser pulse width is also very short compared to both the lifetimes of propagating radicals and the times for conversion of primary radicals to propagating radicals. The PLP-SEC method for measuring kp requires that reaction conditions be chosen so that no significant chain transfer is present. The first laser pulse generates an almost instantaneous burst of primary radicals at high... 

Fig. 3.18 To show more vividly how ions are counted one by one, we show a small time window of an oscillograph containing atom-probe signals of W3+ and HeW3+ and He2W3+ ions, taken with the Penn State pulsed-laser ToF atom-probe in its early stage of development when the flight path length was 200 cm and the laser pulse width was still 5 ns.

The data for the ablation of PTFE by 248-nm and 300-fs pulses obtained by Kuper and Stuke" q-h, modeled. By reducing the laser pulse width from... 

The fitting function used was constructed from functions derived for use in the analysis of pump-probe dynamics data [7]. The function accounts for the cross correlation of the pump and probe laser pulses and deconvolutes the laser pulse width. A pulse width of 120 fs was used for the analysis of all of the data as this width was found to best represent the cross correlation of the second and third harmonic laser pulses of the laser system. 

A major advance in the investigation of the intramolecular dynamics of spin equilibria was the development of the Raman laser temperature-jump technique (43). This uses the power of a laser to heat a solution within the time of the laser pulse width. If the relaxation time of the spin equilibrium is longer than this pulse width the dynamics of the equilibrium can be observed spectroscopically. At the time of its development only two lasers had sufficient power to cause an adequate temperature rise, the ruby laser at 694 nm and the neodymium laser at 1060 nm. Neither of these wavelengths is absorbed by solvents. Various methods were used in attempts to absorb the laser power, with partial success for microsecond relaxation times. 

To date, similar experiments on dissociative ionization of molecules by adaptively shaped ultrashort laser pulses have been reported, where specific ion ratios were targeted during iterative control using GA and SA [14,15]. However, less detailed comparisons between adaptive control and open-loop control experiments regarding the laser pulse width, pulse train, and peak intensity exist. Moore et al. claimed that not only accelerated specific bondbreaking, but even new bond formation, can specified by adaptive pulse shape control [14]. Because ethanol molecules have a relatively simple main struc-... 

If delayed extraction increases the mass resolution without degradation of sensitivity compared with continuous extraction, it also has limitations. Indeed, delayed extraction complicates the mass calibration procedure. It can only be optimized for part of the mass range at a time and is less effective at high mass. Delayed extraction partially decouples ion production from the flight time analysis, thus improving the pulsed beam definition. However, calibration, resolution and mass accuracy are still affected by conditions in the source. For instance, in the usual axial MALDI-TOF experiments, optimum focusing conditions depend on laser pulse width and fluence, the type of sample matrix, the sample preparation method, and even the location of the laser spot on the sample. 

With intense laser pulses, new nonlinear optical phenomena are possible. The prime example is two-photon excitation (TPE). The peak power in a laser pulse from a Ti sapphrre laser (pulse width 100fs) can readily reach 10 W or higher, with a focused intensity of lO W/cm. Under these conditions, excitation can occur with two photons that have half of the energy (twice the wavelength) of the corresponding one-photon transition (see Fig. 2a). The rate of TPE is given by ... 

Both TCSPC and frequency-domain fluorimetry are limited in time resolution by the response of available detectors, typically >25 ps. For cases in which higher time resolution is needed, fluorescence up-conversion can be used (22). This technique uses short laser pulses (usually sub-picosecond) both to excite the sample and to resolve the fluorescence decay. Fluorescence collected from the sample is directed through a material with nonlinear optical properties. A portion of the laser pulse is used to gate the fluorescence by sum frequency generation. The fluorescence is up-converted to the sum frequency only when the gate pulse is present in the nonlinear material. The up-converted signal is detected. The resolution of the experiment therefore depends only on the laser pulse widths and not on the response time of the detectors. As a result, fluorescence can be resolved on the 100-fs time scale. For a recent application of fluorescence up-conversion to proteins, see Reference 23. 

In the investigation of non-thermal damage to dielectrics, the Fokker-Planck equation is applied to describe the transient behaviors of electron densities, and to predict the damage threshold fluences for various laser pulse widths ranging from 10 femtoseconds to 10 picoseconds [13]. This model includes the effects of electron avalanche and multiphoton ionization on the generation of electrons. 

Laser photothermal melting and fragmentation of gold nanorods Energy and laser pulse-width dependence. J. Phys. Chem. A 1999, 103, 1165-1170. 

Since both electronic and dynamic components can be measured, the choice of the laser pulse width is very important to determine which components will be measured. In organic materials, the peak power is usually in the 10 MW/cm- — 1 GW/cm range. The laser pulses have to be well resolved temporally and spatially and pulse fluctuations should be minimized. With long laser... 

The transient absorption method utilized in the experiments reported here is the transient holographic grating technique(7,10). In the transient grating experiment, a pair of polarized excitation pulses is used to create the anisotropic distribution of excited state transition dipoles. The motions of the polymer backbone are monitored by a probe pulse which enters the sample at some chosen time interval after the excitation pulses and probes the orientational distribution of the transition dipoles at that time. By changing the time delay between the excitation and probe pulses, the orientation autocorrelation function of a transition dipole rigidly associated with a backbone bond can be determined. In the present context, the major advantage of the transient grating measurement in relation to typical fluorescence measurements is the fast time resolution (- 50 psec in these experiments). In transient absorption techniques the time resolution is limited by laser pulse widths and not by the speed of electronic detectors. Fast time resolution is necessary for the experiments reported here because of the sub-nanosecond time scales for local motions in very flexible polymers such as polyisoprene. 

The photophysical dynamics of o-HBP was investigated by the Temp.G method with a picosecond pulsed laser (pulse width 30ps) [164]... 

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