Pulse technique

Synchrotron radiation is produced as pulses of extremely short duration lO sec), but the time interval between pulses is so short ( 10 sec) that ions cannot easily be removed from the ionization chamber before succeeding pulses arrive. However, these short times may be useful in the study of autoionization lifetimes and in the measurements of the kinetic energy of photoelectrons by time-of-flight techiques. 

The processes by which photon absorption may produce ions are (1) direct ionization, (2) autoionization, and (3) ion-pair formation. (Processes that depend on collisions or external fields are not included here.) In direct ionization, electron ejection occurs directly upon photon absorption, i.e., no intermediate state is indicated and the process may 

Autoionization involves the existence of an intermediate excited state which can decay by electron ejection as well as by emission of radiation or, in the case of a molecule, by predissociation. Thus, we may have 

Photon absorption may produce an ion pair (H- and —) either by direct dissociation or by predissociation as just mentioned, the respective reaction equations being 

The threshold law for production of a single quantum state of the ion by direct ionization is approximately a step function. That is, the cross section is finite at threshold and usually varies relatively slowly with wavelength above threshold. An example of such behavior is given by Fig. 2, which shows the ionization cross section of He near threshold. One might expect that the formation of several quantum states of an ion by direct ionization would result in an ionization cross-section curve consisting of a series of superimposed step functions. In the absence of complications to be discussed later, such a series of steps may indeed be observed for molecules (but not for atoms) as in the case of the photoionization of shown in Fig. 3, in which the steps correspond to 

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Goez M 1995 Pulse techniques for CIDNP Concepts Magn. Reson. 7 263-79... 

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

It is important to understand the fundamental electrochemistries of analytes before attempting electro analysis. The usual approach is to perform electroanalyses so quickly that kinetic events do not have time to occur before charge-transfer (electrolysis) has provided a response that is unequivocally related to the concentration of the analyte. Pulse techniques figure prominently into this principle. See Reference 10 for a highly useful approach to this problem. 

A frequency response technique was tried first and some results were received. The useful frequency domain was less than one order of magnitude, while in electrical problems five orders of magnitude can be scanned. The single pulse technique was more revealing, but evaluation by moments had the usual accumulation of errors. Fourier transform of the pulse test results was the final method. 

A, Saamanen, l.M. Andersson, R. Niemela, and G. Rosen, Assessment of horizontal displace ment flow with tracer gas pulse technique in reinforced plastic plants, Building and Eninron-ment, 1995,. 30, 135-141. 

The continuous wave technique has a definite advantage over the other techniques a very narrow band of frequencies is needed to transmit the information. The pulse techniques, on the contrary, use a large band of frequencies, and the various noises, pump noises in particular, are more difficult to eliminate. 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

In order to minimize this problem, Ryan (57, 58) combined the pulse techniques of Tal roze (61) with a small continuous repeller field. In this operation, a cluster of ions is formed by a short ionizing pulse and is allowed to react under the influence of a small d.c. field for a certain time. The reaction is then quenched by applying a large (80 volts/cm.)... 

When this is done, the dependence of k(Ee) upon Ee is even greater than predicted by the dipole-alignment model, and the thermal rate constant predicted from this variation, extrapolated to thermal energies, is more than twice the thermal rate constant found experimentally by using the pulsing technique. 

Table I. Comparison between Rate Constants Measured by the Pulsing Technique and the Pressure Method...

In the past few years this pulsing technique has been used by several groups, utilizing both magnetic deflection (16, 31, 37) and time-of-flight (12, 13) instruments, to study ion-molecule reactions at thermal energies. Here we review the results obtained and discuss the applications and limitations of the method, based on our observations and experiences over the past three years. 

Using either of the above approaches we have measured the thermal rate constants for some 40 hydrogen atom and proton transfer reactions. The results are tabulated in Table II where the thermal rate constants are compared with the rate constants obtained at 10.5 volt cm.-1 (3.7 e.v. exit energy) either by the usual method of pressure variation or for concurrent reactions by the ratio-plot technique outlined in previous publications (14, 17, 36). The ion source temperature during these measurements was about 310°K. Table II also includes the thermal rate constants measured by others (12, 13, 33, 39) using similar pulsing techniques. 

Condensation reactions are somewhat more difficult to study by the pulsing technique since the secondary ion usually has a considerably higher mass than the primary ion. However, by restricting the total reaction time to 1 nsec, or less, we have found it possible to study this type of reaction under thermal conditions. Preliminary results are presented in Table IV. Many of the product ions in these systems can be formed in more than one reaction, and the details of reaction identification will be presented elsewhere. 

Pulsed source mass spectrometry 150 Pulsing technique.132, 139... 

Pulse techniques, coupled with the observation of the decay of enhancement (Atkins et al., 1970a, b Glarum and Marshall, 1970 Smaller etal., 1971) constitute the most sensitive procedure for detecting CIDEP. Both net and multiplet polarization have been described. As with CIDNP, the former is believed to arise essentially from the Zeeman interaction and the latter from the hyperfine term. Qualitative rules analogous to Kaptein s rules should be capable of development. 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Brockhinke, A. and Linne, M.A., Short-pulse techniques Picosecond fluorescence, energy transfer and "quench-free" measurements, in Applied Combustion Diagnostics, Kohse-Hoinghaus, K. and Jeffries, J.B. (Eds.), Taylor Francis, New York, 2002, Chapter 5. 

Babu SM, Dhanasekaran R, Ramasamy P (1991) Electrodeposition of CdTe by potentiostatic and periodic pulse techniques. Thin Solid Films 202 67-75... 

The method of potentiostatic pulses is sometimes combined with the DME (called pulse polarography). hi this case the pulse frequency should match the drop frequency, where each pulse is used at a definite time in the drop life, hi Barker s method, large pulse amphrndes are used. Other versions of the potentiostatic pulse technique are square-wave and staircase voltammetry here smaU-amphtude pulses are used. 

For illustration we will present some commonly used techniques for chemisorption measurements. Chemisorption can be measured gravimetrically, volumetrically, or spectroscopically. Also, pulse techniques, and Temperature Programmed Desorption (TPD) can be used. 

Neergat M, Seiler T, Savinova FR, Slimming U. 2006. Improvement of the performance of a direct methanol fuel cell using a pulse technique. J Electrochem Soc 153 A997-A1003. 

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