Mach disc

Hey That really wasn t a lot of work. Just a lot of talk on Strike s part. All one did was mix an oil with some acid, added water and isolated. One gets some pure propenylbenzene without distillation. Done on a massive scale, this is a cheap method for getting lots of small concentration allylbenzene compounds out of complex oil mixes. And since Strike blew so much dough on this glorified extraction protocol, someone better damn well use it (In an academic lab of course).  [c.51]

As the gas leaves the other side of the sampler orifice, it experiences a vacuum of about 10 torr, and the expanding jet of gas cools very rapidly and reaches supersonic speeds. There is a resulting conical shock wave, inside which is the zone of silence (Figure 14.2). Air and other gases diffuse from outside and into the zone of silence, and the amount eventually reaches a level at which the shock wave heats it along a frontal zone called the Mach disc. Therefore, to sample the fast-moving ions issuing from the sampler, the entrance to the next orifice (the skimmer) needs to be sited within the zone of silence and before the Mach disc, which means it is normally placed about 5 to 6 mm from the sampler. The skimmer orifice needs to be small enough that it prevents too much exterior neutral gas getting through it and into the analyzer region and yet large enough that it exceeds the mean free path of the particles passing through it so as not to interrupt the flow. For these reasons, the skimmer orifice is about 0.8 mm in diameter.  [c.95]

Cia. Quimica Universal de Industrias (Mexico) Degussa (Germany)  [c.19]

Reaction rates often may be improved by using more extreme operating conditions. More extreme conditions may reduce inventory appreciably. However, more extreme conditions bring their own problems, as we shall discuss later. A very small reactor operating at a high temperature and pressure may be inherently safer than one operating at less extreme conditions because it contains a much lower inventory. A large reactor operating close to atmospheric temperature and pressure may be safe for different reasons. Leaks are less likely, and if they do happen, the leak will be small because of the low pressure. Also, little vapor is produced from the leaking liquid because of the low temperature. A compromise solution employing moderate pressure and temperature and medium inventory may combine the worst features of the extremes. The compromise solution may be such that the inventory is large enough for a serious explosion or serious toxic release if a leak occurs, the pressure will ensure that the leak is large, and the high temperature results in the evaporation of a large proportion of the leaking liquid.  [c.263]

The density, distillation curve, viscosity, and behavior at low temperature make up the essential characteristics of diesei fuel necessary for satisfactory operation of the engine.  [c.213]

In these tests it is to be determined if iron inclusions with a mass greater than a minimum value are present and, if yes, at what location within the disk. The large thickness of the disk is a special challenge. While eddy current and ultrasonic test generally fail we operate as follows.  [c.989]

After magnetizing the disk it is placed on a turntable to support and move it during measurements. Fig. 3 shows an example measurement of the remanent magnetization of artifical inclusions with masses of 1 mg, 10 mg, and 15 mg at a depth of 70 mm. The signal to noise ratio is very high. A test which is nearer to the resolution limit is shown in fig. 4. An iron particle with a mass of approximately 10 mg was magnetized and than placed 4 mm below the sensor, i. e. the bottom of the cryostat containing the SQUIDs. The single scan signal to noise ratio is still much larger than 10. Averaging will give further improvments.  [c.989]

Fig. II-6. Capillary rise (capillary much magnified in relation to dish). Fig. II-6. Capillary rise (capillary much magnified in relation to dish).
There is quite a large body of literature on films of biological substances and related model compounds, much of it made possible by the sophisticated microscopic techniques discussed in Section IV-3E. There is considerable interest in biomembranes and how they can be modeled by lipid monolayers [35]. In this section we briefly discuss lipid monolayers, lipolytic enzyme reactions, and model systems for studies of biological recognition. The related subjects of membranes and vesicles are covered in the following section.  [c.544]

In this chapter we discuss the main ideas and results of the equilibrium theory of strongly interacting systems. The partition function of a weakly interacting system, such as an ideal gas, is easily calculated to provide the absolute free energy and other properties (e.g. the entropy). The detemiination of the partition fiinction of a strongly interacting system, however, is much more difficult, if not impossible, except in a few special cases. The special cases include several one-dimensional systems (e.g. hard rods, the one-dimensional (ID) Ising ferromagnet), the two-dimensional (2D) Ising model for a ferromagnet at zero magnetic field and the entropy of ice. Onsager s celebrated solution of the 2D Ising model at zero field profoundly influenced our understanding of strongly interacting systems near the critical point, where the response fiinctions diverge. Away from this region, however, the theories of practical use to most chemists, engineers and physicists are approximations based on a mean-field or average description of the prevailing interactions. Theories of fluids in which, for example, the weaker interactions due to dispersion forces or the polarity of the molecules are treated as perturbations to the harsh repulsive forces responsible for the structure of the fluid, also fall into the mean-field category.  [c.437]

Thus the average velocity decays exponentially to zero on a time scale detennined by the friction coefficient and the mass of the particle. This average behaviour is not very interesting, because it corresponds to tlie average of a quantity that may take values in all directions, due to the noise and friction, and so the decay of the average value tells us little about the details of the motion of the Brownian particle. A more interesting  [c.688]

The characteristic time of the tliree-pulse echo decay as a fimction of the waiting time T is much longer than the phase memory time T- (which governs the decay of a two-pulse echo as a function of x), since tlie phase infomiation is stored along the z-axis where it can only decay via spin-lattice relaxation processes or via spin diffusion.  [c.1576]

SARIS overcomes the limitations of small-area detectors by using a large, time-resolving, position-sensitive microchaimel plate (MCP) detector and TOE methods to capture images of both ions and fast neutrals that are scattered and recoiled from a surface. Due to the large solid angle subtended by the MCP, atoms that are scattered and recoiled in both planar and non-planar directions are detected simultaneously. For example, with a 75 X 95 nun MCP situated at a distance of 16 cm from the sample, it spans a solid angle of -0.3 sr corresponding to an azimuthal range of -26°. Using a beam current of -0.1 uA cm, the four images required to make up a 90° azimuthal range can be collected in -2 min with a total ion dose of-10 ions cm The time gating of the MCP provides resolution of the scattered and recoiled atoms into time frames as short as 10 ns, thereby providing element-specific spatial-distribution images. These SARIS images contain features drat are sharply focused into well defined patterns as a fiinction of both space and time by the crystal structure of the target sample. If the MCP is mounted on a goniometer that provides both horizontal and vertical rotation and translation away from the sample, it is possible to change the solid angle of collection and the flight path lengdi.  [c.1808]

A number of different types of experiment can be designed, in which disc and ring can either be swept to investigate the potential region at which the electron transfer reactions occur, or held at constant potential (under mass-transport control), depending on the infomiation sought.  [c.1937]

So far, we have considered the optimization of wavefiinction and geometry parameters separately. In view of the much shorter timescale and higher energy associated with the fonner, this is reasonable. However, additional savings can be potentially obtained by optimizing the wavefiinction and the geometry simultaneously. This was first proposed for density fiinctional methods [77] and later for traditional quantum chemistry teclmiques [78]. With the large increase of computing speed compared to disk input/output speed, direct techniques [79] were generally adopted. In direct methods, the large disparity between calculating the gradients of the molecular energy with respect to electronic parameters (the Fock matrix in SCF theory) and nuclear coordinates disappeared gradients are now only a few times more expensive than a Fock matrix evaluation, making simultaneous wavefiinction-geometry optimization much more attractive. In spite of this, such methods are not yet widely used, except in the crude fonn of relaxing the SCF convergence criteria if the geometry parameters are far from convergence.  [c.2352]

Fonning an image by scanning the laser spot across the sample, or vice versa, minimizes the light dose received by each molecule and reduces photobleaching. The tradeoff is that it requires some time to gather an image. A fluorescent image can be obtained much more rapidly by irradiating a larger area in the transverse plane and imaging the emission from the entire area at once onto a two-dimensional photodetector. This approach is most useful for highly-photostable molecules at low temperatures [34, 35 and 36]- Photobleaching can be further reduced by employing an automatic positioning system with feedback to locate and centre the excitation on a single molecule as rapidly as possible [32] and also by excluding oxygen [38] and/or working at very low temperatures where most chromophores are more stable, although the latter adds considerable complexity to the experimental configuration [39, 40 and 41]  [c.2489]

T. Matsui and co-workers, Kogakuhu Kenkju Hokoku (Miyazaki Daigaku) 38, 91—100 (1992).  [c.394]

Cia. Quimica Universal de Industrias (Mexico) Degussa-Huls (Geiinany)  [c.109]

The quantitative description of visibility is based on the contrast threshhold dC which is the smallest contrast necessary to recognize an object. The contrast threshold depends on the viewing conditions and the optical abilities of the object. Quantitative correlations are based on experimental investigations which are recommended by the Commission International d Eclairage CIE [6]. The results are presented as dependant on the adaptation luminance and the object dimension as parameter [3]. The object dimension is described by the viewing angle (in angle minutes ) of a circle disc. As usual [5], the presentation time was set on 0.2 s. and the detection probability on 50 %. These quantitative correlations describe the fact, that visibility increases with increasing adaptation luminance if the contrast remains constant. The contrast threshold was determined by test persons under optimal conditions which included viewing and environmental conditions as well as the visual acuity. In any real visual inspection (e.g. detection of small objects) the contrast between the object and the surroundings should be much higher than the contrast threshold. This factor is defined as Visibility Level VL = C/dC. The factor is a quantitative measure for the visibility of objects Recommended values are dependant on the inspection task and are in the range VL = 3 - 60.  [c.670]

In aqueous electrolyte solutions, the oscillations are superimposed on the DLVO potential and additional hydration or hydrophobic forces [69-71]. The repulsive hydration force is generally exponential in form with a decay length of 1-2 nm attributed to the adsorption of hydrated ions to the mica surface [61, 72, 73]. The extensive early literature on this subject has been reviei ed by Detjaguin and Churaev [74] and more recently by Israelachvili [1]. A variety of observations including infrared studies and thermal expansion measurements on clays, quartz, and other systems led numerous researchers to conclude that water at an interface may be structurally perturbed to a depth of tens of nanometers [75-81]. Dielectric constant and nuclear magnetic resonance (NMR) measurements have indicated that water adsorbed to alumina and other solids is structurally perturbed to a considerable depth [82] (see Section XVI-4). The structurally perturbed water at an interface was termed vicinal water by Drost-Hansen and suggestions about its behavior include a lower density than bulk water [79], a liquid crystalline state [80], and polarization layers [83]. Molecular dynamics studies find a perturbation in the dipole moment orientation of vicinal water [84] however, there is some question as to the magnitude of the effect [85]. A statistical mechanical lattice theory, developed by Besseling and Scheut-jens [86] with orientation-dependent interactions, can explain many anomalous  [c.244]

Contact angle is proportional to (ysv - 7sl), therefore addition of a surfactant that adsorbs at the solid-solution interface should decrease ysL d therefore increase the quantity above and make 6 smaller. Yet such addition in flotation systems increases 6. Discuss what is inconect or misleading about the opening statement.  [c.490]

Weak interactions are not invariant to the operation C just as tliey are not invariant to the inversion operation P. One might hope to preserve the exact mirror synnnetry of nature if invariance to the product CP were a fact. Unfortunately, CP synnnetry is not universal [18], although its violation is a small effect that has never been observed outside the neutral Aimeson (kaon) system and the extent of its violation caimot be calculated (unlike the situation with parity violation, which by comparison is a big effect). CP violation pennits unequal treatment of particles and antiparticles and it may be responsible for the domination of matter over antimatter in the universe [19]. Very recent considerations concerning CP violation are summarized in [20] in particular, this reference points out that the study of CP violation in neutral B mesons will probe the physics behind the standard model, which does not predict sufficient CP violation to account, by itself, for the predominance of matter over antimatter in the universe. In the light of the fact that C was introduced as a generalization of the changing-the-sign-of-the-charge operation, it is appropriate that CP violation provides an unambiguous convention-free definition of positive charge it is the charge carried by the lepton preferentially produced in the decay of the long-lived neutral K meson[2 ]. Although CP violation is a fact there is one invariance in nature involving C that is believed to be universal (based on quantum field theory) and that is invariance under the triple operation TCP, which also involves the time reversal operation T.T CP synnnetry implies that every particle has the same mass and lifetime as its antiparticle. However, now, if J C P symmetry is true the observation of CP violation in experiments on neutral K mesons must mean that there is a compensating violation of time reversal synnnetry at the same time. A direct experimental measure of the violation of time reversal synnnetry has not been made, mainly because the degree of violation is very small.  [c.177]

We now make two coimections with topics discussed earlier. First, at the begiiming of this section we defined 1/Jj as the rate constant for population decay and 1/J2 as the rate constant for coherence decay. Equation (A1.6.63) shows that for spontaneous emission MT = y, while 1/J2 = y/2 comparing with equation (A1.6.60) we see that for spontaneous emission, 1/J2 = 0- Second, note that y is the rate constant for population transfer due to spontaneous emission it is identical to the Einstein A coefficient which we defined in equation (Al.6.3).  [c.234]

When a system is not in equilibrium, the mathematical description of fluctuations about some time-dependent ensemble average can become much more complicated than in the equilibrium case. However, starting with the pioneering work of Einstein on Brownian motion in 1905, considerable progress has been made in understanding time-dependent fluctuation phenomena in fluids. Modem treatments of this topic may be found in the texts by Keizer [21] and by van Kampen [22]. Nevertheless, the non-equilibrium theory is not yet at the same level of rigour or development as the equilibrium theory. Here we will discuss the theory of Brownian motion since it illustrates a number of important issues that appear in more general theories.  [c.687]

In this section we consider the generalization of quantum scattering theory to problems with many degrees of freedom, and to problems where the translational motion takes place m three dimensions rather than one. The simplest multidimensional generalization is to consider two degrees of freedom, and we will spend much of our development considering this, as it contains the essence of the complexity that can arise in what is called imiltichanneT scattering theory. Moreover, models containing two degrees of freedom are of use tliroughout the field of chemical physics. For example, this model can be used to describe the collision of an atom with a diatomic molecule with the tliree atoms constrained to be collinear so that only vibrational motion in the diatomic molecule needs to be considered in addition to translational motion of the atom relative to the molecule. This model is connnonly used in studies of vibrational energy transfer [29] where the collision causes changes in the vibrational state of the molecule. In addition, this model can be used to describe reactive collisions wherein an atom is transferred to fomi a new diatomic molecule [23, 23 and 24]. We will discuss both of these processes in the followmg two sections (A3.11.3.1 and A3.11.3.2).  [c.970]

Note that the sums are restricted to the portion of the frill S matrix that describes reaction (or the specific reactive process that is of interest). It is clear from this definition that the CRP is a highly averaged property where there is no infomiation about individual quantum states, so it is of interest to develop methods that detemiine this probability directly from the Scln-ddinger equation rather than indirectly from the scattering matrix. In this section we first show how the CRP is related to the physically measurable rate constant, and then we discuss some rigorous and approximate methods for directly detennining the CRP. Much of this discussion is adapted from Miller and coworkers [44, 45].  [c.990]

If a light beam is used to excite one of the higher singlet states, say S2, a very rapid relaxation occurs to S, the lowest excited singlet state. This non-radiative process just converts the difference in energy into heat in the surroundings. A radiationless transition between states of the same multiplicity is called internal conversion. Relaxation between states of the same multiplicity and not too far apart in energy is usually much faster than radiative decay, so fluorescence is seen only from the state. These radiationless processes in large molecules are the analogue of the perturbations observed in small molecules. They are caused by small temis in the Hamiltonian such as spin-orbital coupling or Bom-Oppenlieimer breakdown, which mix electronic states. The density of vibrational levels of large molecules can be very high and that makes these interactions into irreversible transitions to lower states.  [c.1143]

The main advantage of tlie tln-ee-pulse ESEEM experiment as compared to the two-pulse approach lies m the slow decay of the stimulated echo intensity detemiined by T, which is usually much longer than the phase memory time Ty that limits the observation of the two-pulse ESE.  [c.1579]

If the rate of chemical decay of the RP is desired, the task is complex because the majority of the CIDEP signal decays via relaxation pathways on the 1-10 ps time scale, as opposed to chemical reaction rates which are nominally about an order of magnitude longer than this. There are two ways around this problem. The first is to use a transient digitizer or FT-EPR and signal average many times to improve the signal-to-noise ratio at long delay times where chemical reaction dominates the decay trace. The second is to return to the steady-state method described above and run what is called a kinetic EPR experiment, where the light source is suddenly interrupted and the EPR signal decay is collected over a very long time scale. The begiiming of the trace may contain both relaxation of CIDEP intensity as well as chemical decay however, the tail end of this trace should be dominated by the chemical reaction rates. Much use has been made of kinetic EPR in measuring free radical addition rates in polymerization reactions [65, 66].  [c.1617]

The discussion of electron-specimen interactions shows that, for a given incident electron dose, a certain quantity of resulting scattered electrons and secondary electrons or photons is produced. The majority of energy transfer into the specimen leads to beam damage and, finally, to the destruction of the sample structure. Therefore it is desirable to simultaneously collect as much infonnation from the interactions as possible. This concept could lead to an EM mstmment based on the design of a STEM but including many different detectors for the elastic dark field, phase contrast, inelastically scattered electrons, BSE, SE, and EDX. The complexity of such an instmment would be enonnous. Instead, specific instmments developed in the past can coarsely be categorized as TEM for stnictural studies on thin samples, STEM for analytical work on thin samples and SEM for analytical and surface topography studies.  [c.1630]

In a given aqueous electrolyte there is a certain population of hydrated ions and counterions. In addition, many surfaces, including mica, exhibit a net charge in aqueous solution. Counterions are known to fonu a diflfiise screening layer near a charged molecule, particle or surface with roughly exponentially decreasing concentration into the solution. The characteristic decay length of this double layer is called the Debye length and decreases with increasing ionic strength. This double layer gives rise to an entropically driven.  [c.1739]

The use of million electron volt (MeV) ion beams for materials analysis was instigated by the revolution in integrated circuit technology. Thm planar structures were fonned in silicon by energetic ion implantation of dopants to create electrical active regions and thin metal films were deposited to make intercoimections between the active regions. Ion implantation was a new teclmique in the early 1960s and interactions between metal films and silicon required analysis. For example, the number of ions implanted per square centimetre (ion dose) and thicknesses of metal layers required carefiil control to meet the specifications of integrated circuit teclmology. Rutherford backscattering spectrometry (RBS) and MeV ion beam analysis were developed in response to the needs of the integrated circuit teclmology. In turn integrated circuit teclmology provided the electronic sophistication used in the instrumentation in ion beam analysis. It was a synergistic development of analytical tools and the fabrication of integrated circuits.  [c.1827]

The importance of convection in the system increases as the square of the distance from the electrode surface, and close to the surface it is not a dominant fonn of mass transport. Hence concentration changes will arise due to both diflfiision and convection. In the Nemst diflfiision model, this trend is exaggerated, and for the mass transport behaviour at an RDE, a plot of the concentration of electroactive species, Cj, versus the distance from the electrode surface, v, is divided into two distinct zones (figure Bl.28.6). At the electrode surface, i.e. x= 0, the concentration of the electroactive species will be fand up to a distance 6 away from the electrode, there is a stagnant layer, in which diflfiision is the only fonn of mass transport (the Nemst diffiision layer). Outside this layer, mass transport is dominated by strong convection and the concentration is maintained at the bulk value, The diflhision-layer thickness is detennined by the rotation rate of the disc, the layer becoming thhmer with increasing rotation rate. In this model, the values of r and 8 will depend on the applied potential and the electrode rotation rate, respectively. In a linear-sweep experiment at a given rotation rate, the concentration profiles, (dcj/dx), within the diflfiision layer will vary linearly as the applied potential at the RDE is swept from a value where no electron transfer occurs towards values positive to As the experunent is driven further, the surface concentration of the electroactive species eventually reaches zero, at which point the current response reaches its limiting plateau value. The limiting current density is expressed by [1, 2, 4, 5]  [c.1934]

In order to employ the RRDE for quantitative studies, it is necessary to describe the transport of species from disc to ring. In the absence of homogeneous chemical reactions, the electrogenerated species at the disc reaction is transported to the ring by diffiision across the stagnant layer at the electrode surface, by convection across the gap and diffiision across the stagnant layer at the ring electrode. The collection efficiency, Nq, is defined as the ratio of the mass-transport-controlled current for the electrode reactions at ring and disc, Nq = - ring disc 1 minus sign arises because the reactions at the ring and at the disc occur in tlie opposite direction. The collection efficiency thus represents the fraction of material produced at the disc that is detected at the ring. Analytical solutions of the convective-difflision transport at the ring-disc enables the collection efficiency for specific disc and ring dmiensions to be calculated  [c.1936]

The increased rate of mass transport is one of the most attractive and advantageous properties of microelectrodes over conventional electrodes, as the increased transport of the reactant to the electrode surface allows it to reach steady-state regimes rapidly. The diffiision rates increase with decreasing electrode size beyond that obtained with other steady-state techniques. For example, with a 10 pm diameter disc, the steady-state, mass-transfer coefficient, is comparable to that of a rotating disc revolving at an experimentally impossible 250,000 rpm. The discrimination against charging currents is another very important property. In fact, the magniPide of the charging current depends on the area of the capacitor, and for a microelectrode it decreases with electrode area. Thus, a microelectrode has a very reduced interface capacitance, and the charging current decays much more quickly than with conventional electrodes and faster response times may be achieved. Another property of microelectrodes is the decreased distortion from IR, the potential drop between working and reference electrodes generated by the passage of current tluough a solution and expressed in tenns of the product of the solution resistance and the current flowing in the circuit. With conventional electrodes, it is usual to add supporting electrolyte to minimize the solution resistance, but  [c.1939]

This reaction has been intensively studied because of its accessibility to both experimental and theoretical treatments. This reaction is also important because it is die pumping mechanism for the hydrogen fluoride infrared chemical laser. We present some data from the extensive study in 1985 by Lee and co-workers (Neumark et al [25, 26]) and recent, higher resolution experiments by Faubel et al [27. 28]. Figure B2.3.3 presents a schematic diagram of the apparatus in which Lee and co-workers carried out their experiments [29]. An effiisive beam of fluorine atoms was prepared by thenual dissociation of F2 in a nickel tube at 650 °C. The velocity spread was reduced to 11 % by passage tluough a slotted-disk chopper. The fluorine atom beam was crossed widi a supersonic molecular hydrogen beam, and the incident relative translational energy was varied by changing the temperature of the molecular beam source. The products were detected in a triply differentially pumped mass spectrometer employing electron-impact ionization. Cryogenically cooled surfaces also provided additional pumping to reduce the background signal, primarily from difhise scattering from surfaces. The laboratory velocity distributions of the product at various laboratory scattering angles were measured by a time-of-flight method with a mechanical chopper. Crossed-beam scattering of a nonual  [c.2066]

This tool, which they call pseudospectralmethods, promises to reduce the CPU, memory and disk storage requirements for many electronic structure calculations, thus pemiitting their application to much larger molecular systems. In addition to ongoing developments in the underlying theory and computer  [c.2184]

See pages that mention the term Mach disc : [c.226]    [c.381]    [c.436]    [c.1079]    [c.1190]    [c.1483]    [c.1605]    [c.1716]    [c.1933]    [c.1936]    [c.2067]    [c.2174]    [c.2185]   
Mass Spectrometry Basics (2003) -- [ c.95 ]