Fast pulse measurements

Furthermore this method can be applied to fast pulse measurements, which are also of great interest in view of memory applications. An example is shown in Figure 17.17. This method can also be applied as small signal measurements on small structures, e.g. C(V) measurements on sub-micron capacitors, and is of great interest to gain further insight in ferroelectric behavior in the nano-scale [12], [13],... 

Ionization chamber The first radon measurements were made with various electrometers, which were primitive ion chambers. Most of the improvements in ion chambers over the years have been in the associated devices or electronics. Since the early applications of the ion chambers, the most important problem to be solved was the discrimination of the radon signal from that of the y-radiation. The earlier system with paired chambers allowed automatic subtraction of background. One chamber, filled with air and sealed, responded to external y-radiation. This response was subtracted from that of the chamber containing the sample. The present generation of ion chambers is based on fast pulse measurement rather than total ionization. In this case, pulse discrimination eliminates any jS- or y-response. 

A precursor of the studies on electron transfer reactions between short-lived radicals and colloidal particles was the development of a fast pulse radiolysis method to measure. the polarograms of radicals in the 10 s range . After considerable information had been acquired about the electron transfer reactions of a few dozen radicals at the mercury electrode, this compact electrode was replaced by metal colloids somewhat later, by semiconductor colloids These studies led to the detection of the electron-storing properties of certain colloids and of reactions of the stored electrons. 

Schaefer and Natusch have shown that for many synthetic high polymers in solution the NOE factors and relaxation times of carbon atoms in or near the main chains eire similcir (.2. In such cases the relative peak areas in the spectra obtained by the noise-decoupled and fast pulsing technique can be used as a good approximation for quantitative microstructure euialysis. However for our investigation of the polymerization of cyclic ethers we are frequently interested in the quantitative measurements of monomers and oligomers as well as the concentrations of the continuously growing polymeric species. Therefore, the assumption of Schaefer and Natusch is not applicable. 

Fast pulse radiolysis studies have shown that geminate recombination occurs on the picosecond time scale [12,13]. Bartczak and Hummel [14] predicted that for -dodecane, 82% of the geminate ions still remain at 5 psec for 1 MeV irradiation. Future accelerators, with pulses of a few picoseconds length, may soon provide experimental measurements of Gtot directly. 

Surface diffusion can be studied with a wide variety of methods using both macroscopic and microscopic techniques of great diversity.98 Basically three methods can be used. One measures the time dependence of the concentration profile of diffusing atoms, one the time correlation of the concentration fluctuations, or the fluctuations of the number of diffusion atoms within a specified area, and one the mean square displacement, or the second moment, of a diffusing atom. When macroscopic techniques are used to study surface diffusion, diffusion parameters are usually derived from the rate of change of the shape of a sharply structured microscopic object, or from the rate of advancement of a sharply defined boundary of an adsorption layer, produced either by using a shadowed deposition method or by fast pulsed-laser thermal desorption of an area covered with an adsorbed species. The derived diffusion parameters really describe the overall effect of many different atomic steps, such as the formation of adatoms from kink sites, ledge sites... 

Other forms of voltammetry are as follows (1) fast-scan cyclic voltammetry useful in neuroelectrochemistry (2) nanosecond voltammetry for a 5-pm disk working microelectrode with RC < 1 gs, scan rates of 2.5 MV/s allow for fast kinetics measurements (3) differential-pulse voltammetry with staircase pulses, potential resolutions of 0.04 V and detection limits of 10 8M can be attained (4) anodic (cathodic) stripping voltammetry traces... 

What constitutes an advance in any field will always be subjective. However, the combination of the inherent ability of MR methods to probe the internal structure and transport processes from the A- to cm-scale phenomena non-invasively, quantitatively and with chemical resolution, and with the ability to acquire these data sufficiently fast so that unsteady state processes can be studied is undoubtedly going to open up new avenues of research and allow us to investigate many phenomena for the first time. This section summarises five recent developments in the field of MR in chemical engineering. The first four sub-sections (Sections III.A-III.D) report developments of fast MR measurement pulse sequences, which have recently been implemented for application in chemical engineering research. The final sub-section (Section III.E) addresses a new and different field of research, that of gas-phase imaging. 

The vibrational energy levels of the B rio electronic state of I2 were studied by absorption spectroscopy in Exp. 39. In the present experiment, selected vibrational-rotational levels of this state will be populated using a pulsed laser. The fluorescence decay of these levels will be measured to determine the lifetime of excited iodine and to see the effect of fluorescence quenching caused by collisions with unexcited I2 molecules and with other molecules. In addition to giving experience with fast lifetime measurements, the experiment will illustrate a Stem-Volmer plot and the determination of quenching cross-sections for iodine. Student results for different quenching molecules will be pooled and the dependence of the cross sections on the molecular properties of the collision parmers will be compared with predictions of two simple models. 

The light decay time constant in Nal is about 0.25 (is. Typical charge sensitive preamplifiers translate this into an output pulse rise time of about 0.5 (is. Fast coincidence measurements cannot achieve the very short resolving times that are possible with plastic, especially at low gamma ray energies. 

D. E. Johnson and C. G. Enke, Bipolar pulse technique for fast conductance measurements. Anal. Chem., 42.329,1970. 

Both types of sources are useful for continuous radiolysis or even time-resolved studies on the several-minute time scale or longer, such studies are limited by the time required to move the sample in and out of the source. Competition kinetics is often used to obtain kinetic information about reactions too fast to measure directly by time-resolved methods (2). Continuous gamma radiolysis is also a convenient method of generating radiolysis products for identification and chemical analysis (instead of using pulsed sources, where the average dose rate is much lower). 

Obendrauf et al. have measured the thermophysical properties of nickel by a fast pulse heating technique using a newly developed highly sensitive pyrometer. The values obtained for A 77°(Ni) are slightly greater than, and the values for C" (Ni, 1) are significantly greater than the calorimetrically determined values on which the quantities in this review are based. 

Fig s 3 to 5 show the comparison between experiment and model for the pulse measurements of cumene at different conversions. Propene diffuses very fast and is weakly adsorbed and therefore its response curve depends on the parameters of cumene. Apart fiem experimental errors of the cumene response curve close to C/Co=10, the ciunene response curve shows only one time constant. With these approximations the analysis of the propene response curve reveals that the cumene reaction takes place in the strong diffusion limitation regime with no adsorption... 

Konig U, Lohrengel MM, Schultze JW (1987) Computer supported pulse measurements of current and capacity during fast oxide formation on iron. Ber Bunsenges Physik Chem 91 426 31... 

Lohrengel MM (1993) Pulse measurements for the investigation of fast electronic and ionic processes at the electrode/electrolyte interface. Ber Bunsenges Physik Chem 112 440-447... 

---