Line collisional

This alone, however, yielded a value for the product that was 3-4 orders of magnitude too large to account for the observed curvature. There was clearly at least one other contributory factor to the curvature and that was found to be due at least in part to collisional broadening effects. The spectral line collisional broadening effect in a two component mixture can be described by... 

High-resolution spectroscopy used to observe hyperfme structure in the spectra of atoms or rotational stnicture in electronic spectra of gaseous molecules connnonly must contend with the widths of the spectral lines and how that compares with the separations between lines. Tln-ee contributions to the linewidth will be mentioned here tlie natural line width due to tlie finite lifetime of the excited state, collisional broadening of lines, and the Doppler effect. 

Spectral lines are fiirther broadened by collisions. To a first approximation, collisions can be drought of as just reducing the lifetime of the excited state. For example, collisions of molecules will connnonly change the rotational state. That will reduce the lifetime of a given state. Even if die state is not changed, the collision will cause a phase shift in the light wave being absorbed or emitted and that will have a similar effect. The line shapes of collisionally broadened lines are similar to the natural line shape of equation (B1.1.20) with a lifetime related to the mean time between collisions. The details will depend on the nature of the intemrolecular forces. We will not pursue the subject fiirther here. 

The 3 Pi/2, 3 P2/2 excited states involved in the sodium D lines are the lowest energy excited states of the atom. Consequently, in a discharge in the vapour at a pressure that is sufficiently high for collisional deactivation of excited states to occur readily, a majority of atoms find themselves in these states before emission of radiation has taken place. Therefore... 

Figure 7. Experimental data (symbols) for TNB s viscosity [78] superimposed on the results of the fitting procedure (line) from Lubchenko and Wolynes [47] are shown. Ta is diown by a tickmark. (TNB = trinaphthyl benzene). The temperature Ter signifies a crossover from activated to collisional viscosity, dominant at the lower and higher temperatures, respectively (see text). The temperature is varied between the boiling point and the glass transition. The right-hand side panel depicts the temperature dependence of the length scales of cooperative motions in the liquid. The thick solid and dashed lines are the critical radius and the cooperativity length respectively. Taken from Ref. [47] with permission.

Jongerius MJ, Van Bergen ARD, Hollander T, Alkemade CTH (1981) An experimental study of the collisional broadening of the Na-D lines by Ar, N2 and H2 perturbers in flames and vapor cells—I. The line core. J Quant Spectrosc Radiat Transfer 25 1-18... 

Reactants AB+ + CD are considered to associate to form a weakly bonded intermediate complex, AB+ CD, the ground vibrational state of which has a barrier to the formation of the more strongly bound form, ABCD+. The reactants, of course, have access to both of these isomeric forms, although the presence of the barrier will affect the rate of unimolecular isomerization between them. Note that the minimum energy barrier may not be accessed in a particular interaction of AB+ with CD since the dynamics, i.e. initial trajectories and the detailed nature of the potential surface, control the reaction coordinate followed. Even in the absence (left hand dashed line in Figure 1) of a formal barrier (i.e. of a local potential maximum), the intermediate will resonate between the conformations having AB+ CD or ABCD+ character. These complexes only have the possibilities of unimolecular decomposition back to AB+ + CD or collisional stabilization. In the stabilization process,... 

This is a similar value to the temperature of the Sun derived optically or from A.max in the black body spectrum. In a colder star the Balmer series is weaker still, but in a hotter star the Balmer series lines are stronger. In very hot stars the Balmer series may become weaker again due to collisional ionisation of H atoms, removing the electrons from the atoms completely. 

The excess energy (liv — 13.6 eV) of the ionizing photons supplies heat to the ionized gas, which is cooled chiefly by the emission of collisionally excited lines... 

Intensities of collisionally excited lines relative to hydrogen lines depend on the ionic abundance and on the balance between excitation by electron collisions and de-excitation by both electron collisions and radiation. The emission rates per unit volume are given respectively by ... 

The intra-cluster gas in clusters of galaxies is generally hotter and in collisional ionization equilibrium, and the continuum is dominated by bremsstrahlung, making the interpretation of at least the hydrogen-like and helium-like K-shell emission lines relatively straightforward, but they are comparatively weak and an accurate determination of the temperature(s) is critical. 

Fluorescence and collisional excitation, arising primarily from the metastability of the 23S level (see Fig. 4.9), in which consequently a high population accumulates which can cause additional emission from lines such as X 4471, X 5876 by either collisional excitation or radiative transfer effects following absorption of higher lines in the 23S — n3P series. The singlet line X 6678 can also be enhanced by collisional excitation from 23S. The collisional effects can be calculated from the known electron temperature and density, and are quite small at... 

The major requirement of the light source for atomic absorption is that it should emit the characteristic radiation (the spectrum) of the element to be determined at a half-width less than that of the absorption line. The natural absorption line width is about 10 4 (A), but due to broadening factors such as Doppler and collisional broadening, the real or total width for most elements at temperatures between 2000 ° and 3000 °K is typically 0.02 — 0.1 A. Hence, a high resolution monochromator is not required. 

Ideally, the emission line used should have a half-width less than that of the corresponding absorption line otherwise equation (8.4) will be invalidated. The most suitable and widely used source which fulfils this requirement is the hollow-cathode lamp, although interest has also been shown in microwave-excited electrodeless discharge tubes. Both sources produce emission lines whose halfwidths are considerably less than absorption lines observed in flames because Doppler broadening in the former is less and there is negligible collisional broadening. 

Collisional line broadening can be written in a form similar to Eq. (5.6)... 

One of the first applications of this chopped-beam irradiation technitriplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

Figure 2. Simplified picture of atom-atom collisional ionization with crossing distance r. Heavy solid lines represent trajectories of neutral systems. At the first crossing (r= rj some fraction (1 - PJ of trajectories make adiabatic transitions and are represented by dashed lines (ion pairs). Those making diabatic transitions remain neutral and continue their flight relatively unaffected. Each of these trajectories then encounters r = r<- again, and again each trajectory can make an adiabatic or diabatic transition, resulting in ion pairs or neutrals depending on the trajectory. The ultimate production of ions requires one transition to be diabatic and one to be adiabatic, in either order. The inner circle represents the repulsive core.

In passing it is interesting to note that Fig. 5 qualitatively explains the reason for the difference in the effect of motion on spectral lines in radiofrequency and optical spectroscopy. In radiofrequency spectroscopy one refers to motional narrowing, while collisional broadening is used to... 

---