Interrogation

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

A tremendous amount of work has been done to delineate the detailed reaction mechanisms for many catalytic reactions on well characterized surfaces [1, 45]. Many of tiiese studies involved impinging molecules onto surfaces at relatively low pressures, and then interrogating the surfaces in vacuum with surface science teclmiques. For example, a usefiil technique for catalytic studies is TPD, as the reactants can be adsorbed onto the sample in one step, and the products fonned in a second step when the sample is heated. Note that catalytic surface studies have also been perfonned by reacting samples in a high-pressure cell, and then returning them to vacuum for measurement. 

This interface is critically important in many applications, as well as in biological systems. For example, the movement of pollutants tln-ough the enviromnent involves a series of chemical reactions of aqueous groundwater solutions with mineral surfaces. Although the liquid-solid interface has been studied for many years, it is only recently that the tools have been developed for interrogating this interface at the atomic level. This interface is particularly complex, as the interactions of ions dissolved in solution with a surface are affected not only by the surface structure, but also by the solution chemistry and by the effects of the electrical double layer [31]. It has been found, for example, that some surface reconstructions present in UHV persist under solution, while others do not. 

Studies of the liquid-solid interface can be divided into those that are perfonned ex situ and those perfomied in situ. In an ex situ experiment, a surface is first reacted in solution, and then removed from the solution and transferred into a UFIV spectrometer for measurement. There has recently been, however, much work aimed at interrogating the liquid-solid interface in situ, i.e. while chemistry is occurring rather than after the fact. 

Sedlacek A J, Weston R E Jr and Flynn G W 1991 Interrogating the vibrational relaxation of higly exoited polyatomios with time-resolved diode laser speotrosoopy CgHg, CgDg, and CgFg + COj J. Chem. Rhys. 94 6483-90... 

An introduction to several of the more common methods of surface and interface analysis has been presented in this article. This treatment is certainly not comprehensive. An ever-expanding number of methods for the interrogation of surfaces and interfaces are available to the analyst. The ones chosen for discussion here were meant to be representative of methods that can answer the more general questions posed at the beginning of this article. The reader is encouraged to pursue further reading on other techniques for specific appHcations in the many excellent monographs on the subject of surface and interface analysis. 

In most ultrasonic tests, the significant echo signal often is the one having the maximum ampHtude. This ampHtude is affected by the selection of the beam angle, and the position and direction from which it interrogates the flaw. The depth of flaws is often deterrnined to considerable precision by the transit time of the pulses within the test material. The relative reflecting power of discontinuities is deterrnined by comparison of the test signal with echoes from artificial discontinuities such as flat-bottomed holes, side-drilled holes, and notches in reference test blocks. This technique provides some standardized tests for sound beam attenuation and ultrasonic equipment beam spread. 

In the Lasentec instruments a chord-length distribution is generated, from a rotating infrared beam, and this is converted to a size distribution. Since highly concentrated systems can be interrogated this system can be used for on-hne size analysis. 

Graphic User Interface (GUI)—This consists of screens, which would enable the operator to easily interrogate the system and to visually see where the instruments are installed and their values at any point of time. By carefully designed screens, the operator will be able to view at a glance the relative positions of all values, thus, fully understanding the operation of the machinery. 

On the other hand, if there appears to be a problem, particularly where there may be more issues open than resolved, a second-place vendor may be interrogated. After the two meetings, those involved can make the selection or recommendation, as policy dictates, as to the final candidate. 

Nearly all these techniques involve interrogation of the surface with a particle probe. The function of the probe is to excite surface atoms into states giving rise to emission of one or more of a variety of secondary particles such as electrons, photons, positive and secondary ions, and neutrals. Because the primary particles used in the probing beam can also be electrons or photons, or ions or neutrals, many separate techniques are possible, each based on a different primary-secondary particle combination. Most of these possibilities have now been established, but in fact not all the resulting techniques are of general application, some because of the restricted or specialized nature of the information obtained and others because of difficult experimental requirements. In this publication, therefore, most space is devoted to those surface analytical techniques that are widely applied and readily available commercially, whereas much briefer descriptions are given of the many others the use of which is less common but which - in appropriate circumstances, particularly in basic research - can provide vital information. 

Command modules communicate with other modules through a local area network (LAN). Through this LAN, command modules receive information from the local control modules and store data. These data can be stored from a week to two years, depending on the recording interval and the number of points to be monitored. Unlike host-based systems, which use a central computer to interrogate each command module individually, the computer interface can tap into the network like any other command module. 

Portable sound level meters are also available which can measure percentiles. These either hold the results in a memory which can be separately interrogated or may be connected to a computer for a printout. Larger machines (known as environmental noise analyzers) are available which can record percentiles and Leq readings and produce a printout. These are resistant to weather and can be left on-site for up to a week. 

It would normally be necessary to locate more than one monitoring unit on a structure. Thus each unit will only transmit its information on receiving coded instructions. Surface units can commonly accommodate up to 80 different codes. The ability to interrogate and receive over any distance is dependent upon the acoustic operation frequency. Relatively high frequencies are preferable, as they avoid problems of pulse reverberations and echoes from structural members. Typically data can be transferred over a distance of 2 km. This type of acoustic unit gives accuracies in potential measurement of 10mV. 

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

Science, after all, is a process for interrogating nature, not simply a compendium of facts ... 

It was deemed essential that chemists who might not be computer literate or specially trained should be able to use the interrogation package in a fast, easy and flexible manner to get the precise information needed. The emphasis is on user friendliness, a much over-used term, but here exemplified in the following ways ... 

In the off-line mode of use of the drug file the data-base, stored In a time-shared computer. Is Interrogated through a remote device such as a cathode ray terminal The questions that may be asked Include all the affects of a drug, all the drugs that affect a test, or more specifically does a particular drug affect a particular test procedure by a particular mechanism ... 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

IV. Interrogating Dynamics on Varying Regions of the Excited-State Surface... 

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