Phase changing collisions

Often the collisional broadening of the Lamb dip and of the Doppler profile can be measured simultaneously. A comparison of both broadenings allows the separate determination of the different contributions to line broadening. For phase-changing collisions there is no difference between the broadenings of the two different line profiles. However, velocity-changing collisions do affect the Lamb-dip profile (see above), but barely affect the Doppler profile because they mainly cause a redistribution of the velocities but do not change the temperature. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

Phase- and Energy-Changing Collisions in Iodine Gas Studies by Optical Multiple-Pulse Spectroscopy, E. T. Sleva and A. H. Zewail, Chem. Phys. Lett. 110, 582 (1984). 

In order to understand the meaning of the different transport coefficients arising in the liquid vapour phase change we consider a liquid surface at temperature T with an adjacent vapour phase at temperature T (Fig.l). The vapour pressure is assumed to be so low that gas collisions can be neglected (Knudsen gas). The entropy flux can be expressed in terms of the flux of internal energy and the mass flux J by the following equation... 

Phase interrupting collisions and J changing collisions affect all hyperfine components alike and thus can account for the basic broadening seen for all components- The coupling between the nuclear spin and the... 

The elastic collisions do not change the amplitude, but the phase of the damped oscillator is changed due to the frequency shift As(jo R) during the collisions. They are often termed phase-perturbing collisions (Fig. 3.11). 

Some techniques of laser spectroscopy, such as the method of separated fields optical Ramsey fringes. Sect. 14.4), coherent transient spectroscopy (Sect. 12.4), or polarization spectroscopy (Sect. 7.4) allow one to distinguish between phase-changing, velocity-changing, or orientation-changing collisions. 

When a material undergoes a phase change from liquid to gas, there is a further loosening of the molecular bonds. The gas molecules, free to move in any direction and restrained only by random collisions, have a significantly lower probability that contact areas will exist. As a result, the thermal conductivity of gases is quite low. For example, the thermal conductivity of air is 0.0024 W/m-K. This is many orders of magnitude lower than the thermal conductivity of water [9]. 

The simplest vibrational spectra are obtained for molecules isolated in a matrix of a condensed inert gas, for which very sharp peaks are usually observed (Figure 8.7(d)). In all other phases the peaks are broadened and/ or split, but for different reasons. In the gas phase, changes in rotational quantum numbers accompany the vibrational transitions for gases (Section 8.6.2). At high resolution, these result in fine structure at lower resolution, various band envelopes, some of them characteristic, can be observed (Figure 8.7(a)). We find these only in the spectra of gases because rotation is not quantized in condensed phases, because the rate of intermolecular collision is greater than the rotation rate. 

PrPJ f) where p is the atomic density. Note that in 15.4 the speed of crystal growth becomes a constant for nuclei much larger than the critical one. The expressions 15.3 and 15.4, refers to a diffusion-limited crystallization, which seems to apply to phase change materials as they show a maximum in the crystallization speed for temperatures in between and T. In a collision limited crystallization (which does not apply to phase change materials) D jX has to be replaced by the velocity of sound which is weakly dependent on temperature. The formation energy and the radius of the critical nucleus are given in turn hy AGc =... 

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