Diffusion cross section

Until much improved flux-to-background ratios are available it does not seem likely that much progress will be made with this type of experiment either by observation of total diffuse cross-sections, magnetic-switching cross-sections, or polarization analysis measurement of paramagnetic cross-sections, and conventional form factor measurement of paramagnetic as well as of magnetically ordered systems will be much more fruitful. 

F = Faraday s constant Do2 = diffusion coefficient Q = effective diffusion cross section L = effective diffusion length. 

Recently Schearer measured the cross section of the excitation transfer of 2 Po to 2 Pi and 2 Pj levels as a result of collisions with the ground-state helium atoms. For the 2 5 state, Colegrove et al. measured the excitation transfer cross section and compared with the calculation by Buckingham and Dalgarno. Fitzsimmons et al. and Pakhomov and Fugol discussed the transfer cross section obtained by the empirical potential from diffusion cross sections. 

Fig. 5. Excitation diffusion cross section for He(P5) + He (2 / ) as a function of incident energy. The meaning of C = 23.75 and C = 0 is the same as before. The isotope effect of He + He is negligibly small in this case.

INCIDENT ENERGY IN ELECTRON VOLTS 8. Excitation diffusion cross section for He(l 5) +... 

The momentum-transfer or diffusion cross section is and the viscosity cross section is... 

Figure 11.35 Influence of unhydrated cation size on dissolution rate, showing linear correlation between the reciprocal diffusion cross section l/(Trr ) of the unhydrated...

A different approach to probe the ion-neutral potential involves the analysis of collision-broadened ion cyclotron resonance line shapes. For ions undergoing elastic collisions, the power absorption curve is Lorentzian in shape with a half-width at half-maximum equal to the collision frequency (J, which is then simply related to the reduced collision frequency, the diffusion cross section, and the ion mobility. For a series of nonreactive alkyl cations in methane, excellent agreement is found between experimental values for these quantities and theoretical values predicted from the Langevin model.H Unexpectedly, this finding may be interpreted to suggest that other mechanisms for collision broadening, namely inelastic collisions and, specifically, collisions involving complex formation, appear to be unimportant. ... 

Typical quenching cross-sections by cold ( 1 K) helium yield a rotational quench on the order of every 10 to 100 elastic collisions whereas it can take more than 10 elastic collisions before a vibrational quench [12]. DeLucia and coworkers measured rotationally inelastic cross-sections with He of H2S, NO, and H2CO. Typical values were on the order of 1 to 10 x 10 cm at 1K [13,15,56], which is about 10 to 100 times smaller than a typical diffusive cross-section at 1 K. We therefore expect buffer-gas cooling to effectively thermalize the rotational temperature of the target molecules while leaving the vibrational temperature out of thermal equilibrium. Because the rotational and translational energy transfer cross-sections are similar, thermalization of both happens rapidly and in tandem in butfer-gas cooling. We do not detect nonequilibrium rotational populations. 

For this particular cell, using an assumed cold diffusion cross-section Oc 3 x 10 cm , the crossover between effusive and boosted flow (the condition d = X) occurs for nne 10 cm . We characterize the beam source for both species within a range of densities around the anticipated optimal condition for Na, namely nne 0.2 - 5 X lOi cm-3. 

For Na, in-cell laser absorption spectroscopy is used to determine the number of thermalized Na atoms, WNa. and the cold collision diffusion cross-section, Oc,Na-We find A/Na 10 " /pulse for both the metallic Na and NaCl ablation targets. We measure diffusion lifetimes of x (msec) 4 x 10 x nne (cm ). From this we inferac,Na 3 x 10 cm. Previous workwithPbOhas measured an ablation yield of 10 /pulse [38], and our in-cell LIF measurements indicate a comparable yield. 

A = diffusion cross-sectional area (cm ) p = partial pressure of sample at T° (atm)... 

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