Collision in a gas

Mean free path The average distance travelled by a particle between collisions. In a gas it is inversely proportional to the pressure. 

Higher time moments of Km(0 are negative as well. This is physically accounted for by the anticorrelated nature of successive collisions in a gas [44], A collision from the front is usually followed by a collision from the back (Fig. 1.5). Owing to the opposite direction of collisions the product of the corresponding moments (M(O)M(t)) is negative, and it provides the main contribution to Km for times of order tj. This is a manifestation of the correlated character of the interaction between a molecule and perturbers. 

Fig. 16. Spectroscopic investigation of inelastic collisions in a gas discharge scheme of appropriate levels and corresponding transitions. The level b corresponds in case of Ne to the 3 2 level (Paschen Notation) which is partly depopulated by the laser transitions at X = 3.39 ixm and X = 0.6328 jum...

Rankine has made numerous accurate measurements of -q and calculated the molecular diameters from them. For calculating the number of molecules entering into collision in a gas it is therefore convenient to use Rankine s tables of a and the expression V 2uo2un2. 

The problem of N bound electrons interacting under the Coulomb attraction of a single nucleus is the basis of the extensive field of atomic spectroscopy. For many years experimental information about the bound eigenstates of an atom or ion was obtained mainly from the photons emitted after random excitations by collisions in a gas. Energy-level differences are measured very accurately. We also have experimental data for the transition rates (oscillator strengths) of the photons from many transitions. Photon spectroscopy has the advantage that the photon interacts relatively weakly with the atom so that the emission mechanism is described very accurately by first-order perturbation theory. One disadvantage is that the accessibility of states to observation is restricted by the dipole selection rule. 

Fig. 6.93 Cascade-free lifetime measurements despite the simultaneous excitation of many levels (a) preexcitation by collisions in a gas cell with subsequent laser excitation (b) decay of level k) without cascading curve a), its feeding by cascades curve b), and resulting population Nk t) with cascading and decaying curve c). (c) The fluorescence I x,X) is measured alternately with and without selective laser excitation...

Another mode of stabilization is conceivable. Before the unstable Xg molecule dissociates it could spontaneously undergo a vibrational transition and be stabilized. This is a most unlikely process, even less common than three-body collisions in a gas. It may, however, be important in molecule formation in interstellar space the formation of CH2+ from C+ + H2 appears to follow this pathway. For an introductory review see E. Herbst and W. Klemperer, Phys. Today 29(6), 32 (1976). 

A typical order of magnitude is 5 x 10" cm/sec. It follows that the number of hard sphere collisions in a gas equals... 

The excitation intensity can be increased if the excitation region is placed inside the resonator of a CW dye laser that is tuned to the selected transition. Before they reach the laser beam, the ions can be preexcited into highly excited long-living levels by gas collisions in a gas cell being dif-... 

Several closures have been proposed to calculate the collision density for molecules in gas mixtures [109] (pp. 52-55). To derive these expressions for the collision density one generally perform an analysis of the particle-particle interactions in an imaginary container [80, 109, 135], called the conceptual collision cylinder in kinetic theory, as outlined in Sect. 2.4.2.5. A fundamental assumption in this concept is that the rate of molecular collisions in a gas depends upon the size, number density, and average speed of the molecules. Following Maxwell [95] each type of molecules are considered hard spheres, resembling billiard balls, having diameter dg, mass ntg and number density These hard spheres exert no forces on... 

At 1 mTorr the mean free path for collision in a gas is about 5 centimeters. If the holes in the manifold face outward, the gas density can be non-uniform in regions between the holes. 

Not all collisions are effective in causing a reaction. The average time between collisions in a gas at STP is about 10 ° s and the time that the collision is actually taking place is about 10 s. 

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