Helium collision dynamics

Following the same methodology than the one used to understand the case of proton and He collision, the planar-integrated density was computed along the direction of impact. It is represented in Fig. 7. As for the proton-Helium collision, such charge density representation allows to visualize the dynamic evolution of the electronic clouds. The two clouds, initially weU separated, merge, and then separate, while a small component is present outside of the two major regions. 

To take one example, let us consider the effects of rotational relaxation in BrF. The excited 53FI(0+) state in BrF is crossed by another 0+ state which leads to predissociation of the B state in vibrational levels 7 and 6. The initial study of the dynamics of the B state was carried out in a discharge flow system where the minimum operating pressure was 50 m Torr. The gas-kinetic collision rate coefficient at 298 K for He + BrF(B) collisions is 4.4 x 10-10 cm3 molecule-1 s-1. Thus, at the minimum pressure of 50 m Torr, the average time between collisions of excited BrF molecules and helium buffer gas is 1.5/us. This time is short compared with the radiative lifetime of BrF (42—56/ns [43]) and therefore significant redistribution in the excited state can occur before it radiates. 

Barocchi F, Zoppi M (1981) Experimental determination of two-body collision-induced light scattering spectrum of helium. In Van Kranendonk J (ed) Intermolecular spectroscopy and dynamical properties of dense systems, Proceedings of the International School Enrico Fermi , Course LXXV, Amsterdam, North-Holland, pp263-274... 

The Raman spectra of solids have a more or less prominent collision-induced component. Rare-gas solids held together by van der Waals interactions have well-studied CILS spectra [656, 657]. The face-centered, cubic lattice can be grown as single crystals. Werthamer and associates [661-663] have computed the light scattering properties of rare-gas crystals on the basis of the DID model. Helium as a quantum solid has received special attention [654-658] but other rare-gas solids have also been investigated [640]. Molecular dynamics computations have been reported for rare-gas solids [625, 630, 634]. 

F. Barocchi and M. Zoppi. Experimental Determination of Two-Body Collision Induced Light Scattering of Helium." In J. van Kranendonk (ed.), Intermolecular Spectroscopy and Dynamical Properties of Dense Systems—Proceedings of the International School of Physics "Enrico Fermi," Course LXXV, North-Holland, Amsterdam, 1980, pp. ICi-llA. 

Recently, the sapt approach has been applied (32) to compute the interaction-induced polarizability for the helium diatom. The computed polarizability invariants have been analytically fitted, and used in quantum-dynamical calculations of the binary collision-induced Raman spectra. The results of the dynamical calculations are summarized in Fig. 1. 

Baranowski, R., Wagner, B., Thachuk, M., Molecular dynamics study of the collision-induced rotational abgnment of N2" drifting in helium. J. Chem. Phys. 2001,114,6662. 

In the next section some important experimental aspects needed for understanding the new experiments are surveyed. A decisive result coming out of this new experimental direction is the beneficial influence of the helium environment on the microscopic dynamics. In many of the experiments the vibrations, especially of large molecules which are initially highly excited are very rapidly quenched. As a result the dynamics and the laser interrogation are greatly simplified. The downside is that translations of molecules moving very rapidly inside the droplets are appreciably slowed by collisions with the individual atoms of the droplet. To provide perspective... 

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