Molecular flux/intensity

Analysis of thermal decomposition of molecules on hot surfaces of solids is of considerable interest not only for investigation of mechanisms of heterogeneous decomposition of molecules into fragments which interact actively with solid surfaces. It is of importance also for clarifying the role of the chemical nature of a solid in this process. Furthermore, pyrolysis of molecules on hot filaments made of noble metals, tungsten, tantalum, etc., is a convenient experimental method for producing active particles. Note that it allows continuous adjustment of the intensity of the molecular flux by varying the temperature of the filament [8]. 

In a number of cases, the temperature of the filament and thermodynamic parameters allow one to calculate [9] the flux intensity of free atoms produced in dissociation of molecules. Specifically, in the case of dissociation of hydrogen, oxygen, and nitrogen molecules on hot metal filaments under pressures of molecular gases higher than lO" Torr, the flux intensity of atoms A originating from A2 molecules is given by... 

Note that this method enables one to observe variation of electric conductivity of a sample due to adsorption of hydrogen atoms appearing as a result of the spillover effect, no more. In a S3rstem based on this effect it is rather difficult to estimate the flux intensity of active particles between the two phases (an activator and a carrier). The intensity value obtained from such an experiment is always somewhat lower due to the interference of two opposite processes in such a sample, namely, birth of active particles on an activator and their recombination. When using such a complicated system as a semiconductor sensor of molecular hydrogen (in the case under consideration), one should properly choose both the carrier and the activator, and take care of optimal coverage of the carrier surface with metal globules and effect of their size [36]. 

If we abandon the very unlikely case of anisotropic diffusion with principal tensor axes which are not perpendicular to the crystal faces normal, molecular fluxes may quite generally be assumed to be directed perpendicular to the crystal surfaces. Hence, molecular uptake and release may be considered to proceed via one-dimensional diffusion quite generally, as long as the fluxes stemming from different crystal faces do not superimpose upon each other. This includes in particular the initial phases of uptake and release. We shall see that due to this reason, by measuring surface permeabilities, interference microscopy is in general able to quantify the intensity of surface resistances. 

If the laser intensity is sufficiently large the molecular transitions are saturated. In the case of complete saturation (5 I, Sect. 7.1) every molecule in the absorbing level /) passing through the laser beam will absorb a photon. The intensity of the absorption line is then independent of the transition probability B but only depends on the molecular flux N vi, Ji) = m(u/, Ji) X n(vi, Ji). We then obtain instead of (9.16a)... 

Square brackets around a molecular species indicate atmospheric concentration. The rate constants k times the reactant concentration product refers to the rates of the chemical reactions of the indicated number. The photolytic flux term /l4 refers to the photodissociation rate of N02 in Reaction R14, its value is proportional to solar intensity.]. RO2 stands for an organic peroxyl radical (R is an organic group) that is capable of oxidizing NO to NO2. Hydrocarbons oxidize to form a very large number of different RO2 species the simplest of the family is methylperoxyl radical involved in R5, R6 and R8. 

Birch and coworkers studied the time-intensity interrelationships for the sweetness of sucrose and thaumatin, and proposed three thematically different processes (see Fig. 47). In mechanism (1), the sweet stimuli approach the ion-channel, triggering site on the taste-cell membrane, where they bind, open the ion-channel (ionophore), and cause a flow of sodium and potassium ions into, or out of, the cell. Such a mechanism would correspond to a single molecular event, and would thus account for both time and intensity of response, the intensity of response being dependent on the ion flux achieved while the stimulus molecule binds to the ionophore. 

Underneath all of the ideas of atomic and molecular detection, counting the number of molecules in a particular line of sight, requires the intensity of the transition to be calculated via the transition moment to the Einstein B coefficient. If the total photon flux through a sample is known and the transition moment is also known, then the absolute number of atoms or molecules present can be determined. 

Imagine that system B is the water column of a lake and system A is the pore space of the lake sediments. In B, mixing is by turbulence and fairly intensive while in system A transport is by molecular diffusion. The above case corresponds to a situation in which at time t the concentration of a compound in the water suddenly rises to the value Cg. Then Eqs. 19-25 and 19-26 describe the cumulative and incremental mass flux of the compound into the infinitely deep sediment column. 

Although we shall not directly use these four postulates of irreversible thermodynamics as a foundation to our study of molecular transport in separations, a number of important principles are illuminated here. For instance, postulate 2 permits us to use—and this is in no way obvious— equilibrium parameters such as entropy and temperature in descriptions of systems where no equilibrium exists. The importance of this is evident when we ask ourselves how we would describe a system if these parameters were not available. Postulate 3 demonstrates that in the range of our typical experiences, the fluxes of matter or of heat are proportional to the gradients or forces that drive them. However, there are exceptions nonlinear terms enter if the forces become intense enough. 

Simultaneously the intensity of the atomic Balmer lines (in this case D7) increases by nearly a factor of two. Thus, in the presence of molecules it obviously turns out that corrections to the estimated hydrogen flux may be required in such a form that the atomic S/XB (which is about 15 for densities 1018-1019 m 3 and temperatures above 15 eV) is replaced by an effective one, which is determined by the molecular deuterium flux rd-> ... 

The probability of photodissociation depends on the spectral and intensity distribution of the light flux, the absorption cross section and the photolysis branching ratio(s). The key molecular property is the transition dipole that governs the intensity or oscillator strength of the transition. Due to violation of space and spin... 

There are several means of lowering the detection limit in vibrational spectroscopy. The intensity of a Raman band is for a given spectrometer and sample arrangement proportional to the flux of the exciting radiation the concentration c (in molecules per cm ), the effective molecular Raman scattering cross section and the effective thickness of the scattering sample d ... 

Another method was used by Nieman and Naaman to estimate the concentration of (802)2 in n mixture with their monomers. The amount of SO2 monomers in a molecular beam of neat SO2 was determined using its fluorescence intensity. It was established before that while SO2 monomers produce very intense excitation sjjectra, no significant contribution could be obtained from the dimers in the beam. In this case the total emission from the laser-excited species in the beam was monitored as a function of the SO2 flux and stagnation pressure. 

---