Laser-Pulsed Measurements

A particularly interesting result from the laser-pulsed measurements deals with the determination of the potential where the laser-induced response is negligible. At this potential, 

In the above equation, it has been taken into account that the constant in Eq. (95) does not vary with the temperature, and also, that the temperature dependence of the ionic contribution, g ion), can be considered as zero, since d and s are molecular properties that should be only weakly dependent on the temperature.  

The first term in the right-hand side of Eq. (96) corresponds to the thermal coefficient of the work fimction of the electrode, and it is usually small (for a Pt(lll) electrode (dO/dr)/e -0.15 mV/K). The second term is usually considered to be negli- 

The following Sections will summarize some recent results on the application of the laser-induced temperature jump method to characterize the net orientation of interfacial water on platinum single-crystal electrodes, under electrochemical conditions. 

The behavior of interfacial water on the three platinum basal planes, under electrochemical conditions, has been studied, for the first time, by means of the laser-induced temperature jump method. Moreover, the comparison of these results with charge density data has contributed to the understanding of the electrostatic and chemical interactions governing the reorientation of the interfacial water network on platinum electrodes. Below we provide a brief summary of the main results of this study. 

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Figure 14. Fluorescence decay time of hydroxypyrene trisulfonate bound to apomyoglobin (A) 60pAf apomyoglobin, 50pAf hydroxypyrene trisulfonate in lOmAf Mes buffer pH 5.0. The emission was measured (in arbitrary units) at a streak speed of 15 mm/nsec at the wavelengths 400—450 nm using BG-3 (3 mm) and GG 400 Schott glass filters. (B) The excitation laser pulse measured under identical conditions as seen by reflection from the front and back surfaces of an empty 0.5-cm cuvette.

There are also some drawbacks to DLTS as it has been applied to a-Si H. First of all, the instrumentation and analysis is rather complex compared with, for example, capacitance methods. In addition, DLTS is restricted for the most part to the study of doped samples. Although the picture of g E) obtained for reasonably -type samples by using voltage and laser pulse measurements together extends over a wide energy range, it is restricted approximately to the range to E — AE. This is due to the... 

It requires additional experimental data, such as CO displacement experiments at different temperatures and laser-pulsed measurements. 

Noteworthy, the double-layer response is, in general, virtually instantaneous within the time scale of the laser-pulsed measurements. Accordingly, imder conditions where the contribution from charge-transfer processes is negligible, the laser-induced potential transients, AE, would be given by ... 

The laser-pulsed measurements on Pt(lll), Pt(lOO) and Pt(llO) were performed in (0.1 - x) MKCIO4 + xM HCIO4 solutions, where x equals 10, 10, 10 and 0.1. These solution compositions were selected in order to minimize the extent of anion... 

K.W. DeLong, D.N. Fittinghoff, and R. Trebino, Practical Issues in Ultrashort-Laser-Pulse Measurement Using Frequency-Resolved Optical Gating , IEEE Quant. Electr. 32, 1253 (1996). 

David E. Hayes and Donald R. Emmons, Laser Pulse Measurements for Safe and Proper Trimming System Alignment, Hybrid Circuit Tech., pp. 35 10, November 1987. 

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

SFIG or SFG from a medium that has a strong response in a separate detection anu. By this means, one may fiilly compensate for variations not only in pulse energy, but also in the temporal and spatial substructure of the laser pulses. Some experiments may require measurement of the phase of the nonlinear signal [57]. 

Kane D J, Taylor A J, Trebino R and DeLong K W 1994 Single-shot measurement of the intensity and phase of a femtosecond UV laser pulse with frequency-resolved optical gating Opt. Lett. 19 1061-3... 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

An important consequence of shortening a laser pulse is that the line width is increased as a result of the uncertainty principle as stated in Equation (1.16). When the width of the pulse is very small there is difficulty in measuring the energy precisely because of the rather small number of wavelengths in the pulse. For example, for a pulse width of 40 ps there is a frequency spread of the laser, given approximately by (2 iAt), of about 4.0 GFIz (0.13 cm ). 

A promising technique is cavity ringdown laser absorption spectroscopy (307), in which the rate of decay of laser pulses injected into an optical cavity containing the sample is measured. Absorption sensitivities of 5 x 10 have been measured on a ]ls time scale. AppHcations from the uv to the ir... 

In Laser Ionization Mass Spectrometry (LIMS, also LAMMA, LAMMS, and LIMA), a vacuum-compatible solid sample is irradiated with short pulses ("10 ns) of ultraviolet laser light. The laser pulse vaporizes a microvolume of material, and a fraction of the vaporized species are ionized and accelerated into a time-of-flight mass spectrometer which measures the signal intensity of the mass-separated ions. The instrument acquires a complete mass spectrum, typically covering the range 0— 250 atomic mass units (amu), with each laser pulse. A survey analysis of the material is performed in this way. The relative intensities of the signals can be converted to concentrations with the use of appropriate standards, and quantitative or semi-quantitative analyses are possible with the use of such standards. 

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

A Q-switched, frequency-quadrupled Nd—YAG laser (X, = 266 nm) and its accompanying optical components produce and focus the laser pulse onto the sample surface. The typical laser spot size in this instrument is approximately 2 pm. A He-Ne pilot laser, coaxial with the UV laser, enables the desired area to be located. A calibrated photodiode for the measurement of laser energy levels is also present... 

Typical mass resolution values measured on the LIMA 2A range from 250 to 750 at a mass-to-charge ratio M/ Z= 100. The parameter that appears to have the most influence on the measured mass resolving power is the duration of the ionization event, which may be longer than the duration of the laser pulse (5—10 ns), along with probable time broadening effects associated with the l6-ns time resolution of the transient recorder. ... 

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

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