Total cross section helium

A common result of all the experiments is that most molecules quench the alkali resonance radiation very effectively with total cross sections ranging from 10 A2 to over 200 A2. However, if the molecule BC is replaced by a rare-gas atom, the quenching cross sections become very small at thermal energies. They are probably below 10 2 A2 for quenching by helium, neon, argon, krypton, and xenon.55 The latter result is easily understood in terms of Massey s adiabatic criterion.67 If Ar is a characteristic interaction range, v the impact velocity, and AE the energy difference between initial and final electronic states E(3p) and E(3s), respectively, then we must have a Massey parameter... 

As an example of the energy dependence of the total cross section for positron-atom scattering, a schematic of the data for helium atoms is shown in Figure 2.1, together with the corresponding data for electrons. It is seen that [Pg.41]

These equations provide useful checks on the consistency of the experimentally measured values of aT with the calculated values of the real part of the forward scattering amplitude, or the scattering length they have been used as such by Bransden and Hutt (1975). For helium, these authors took the measured total cross sections of Coleman et al. (1976b) in the energy range 2-800 eV below 2 eV they used an extrapolation based on a least-squares fit of the functional form... 

A similar comparison of measured total cross sections and calculated values of the elastic scattering parameters was also made by Bransden and Hutt (1975) for positron-neon collisions, using the theoretical polarized-orbital phase shifts of Montgomery and LaBahn (1970). These are almost certainly less accurate than the corresponding results for helium, and there is poorer agreement between the values of the two sides of equation (2.6). 

Fig. 3.13. Total (tot, upper solid line) and elastic (el, lower solid line) cross sections for positron-noble gas scattering near the positronium formation threshold from the R-matrix analysis of Moxom et al. (1994). Graphs (a)-(e) correspond to helium through to xenon. The data points shown are total cross section measurements from the literature (see Chapter 2 and Moxom et al., 1994, for details) except for the solid diamonds for helium, which are the

Bransden, B.H., Hutt, P.K. and Winters, K.H. (1974). Total cross sections for the scattering of positrons by helium. J. Phys. B At. Mol. Phys. 7 L129-L131. 

Mizogawa, T., Nakayama, Y., Kawaratani, T. and Tosaki, M. (1985). Precise measurements of positron-helium total cross sections from 0.6 to 22 eV. Phys. Rev. A 31 2171-2179. 

Van Reeth, P. and Humberston, J.W. (1999a). A significant feature in the total cross section for positron-helium scattering at the positronium formation threshold. J. Phys. B At. Mol. Opt. Phys. 32 L103-L106. 

Table 8.8. Total cross sections for electron-helium scattering. CCO, coupled-channels-optical (equivalent local) method (McCarthy et al.,1991) experiment. Nickel et al. (1985). Units are KT cmi ...

The collinear model (Eq. (15)) has been successfully used in the semiclassical description of many bound and resonant states in the quantum mechanical spectrum of real helium [49-52] and plays an important role for the study of states of real helium in which both electrons are close to the continuum threshold [53, 54]. The quantum mechanical version of the spherical or s-wave model (Eq. (16)) describes the Isns bound states of real helium quite well [55]. The energy dependence of experimental total cross sections for electron impact ionization is reproduced qualitatively in the classical version of the s-wave model [56] and surprisingly well quantitatively in a quantum mechanical calculation [57]. The s-wave model is less realistic close to the break-up threshold = 0, where motion along the Wannier ridge, = T2, is important. 

Fig. 13 shows charge exchange cross sections based on available experimental data for and h (upper left) [211,212], He " (lower left) [199], He" (lower right) [213,214], and He (upper right) [214,215]. There is a probability of two-electron transfer for helium atom, such as (T20 and (7o2- Total cross sections for electron capture crio for and electron loss analytical functions developed by Miller and Green [202]. Cross sections for He ions were least-square fitted by a simple polynomial function similar to Eq. (14). Smooth extrapolation was carried out where the experimental... 

If the reverse of Reaction 1 is slow compared to 2 ( the colli sional stabilization step) then overall cluster growth will not depend strongly upon the total helium pressure. This is found to be the case using RRK estimates for k n and hard sphere collision cross sections for ksn for all clusters larger than the tetramer. The absence of a dependence on the total pressure implies that the product of [M] and residence time should govern cluster growth. Therefore, a lower pressure can be compensated for by increasing the residence time (slower flow velocities). 

Figure 13 Plots of (

Fig. 2.1. Schematic illustration of the behaviour of the positron-helium and electron-helium total scattering cross sections. Notable are the large differences in magnitude of the cross sections at low energies, their merging at approximately 200 eV and the onset of inelastic processes at the positronium formation threshold EPS in the positron curve.

Fig. 2.8. Low energy positron-helium total scattering cross sections. Experimental data, main diagram x, Costello et al. (1972) , Canter et al. (1972, 1973) , Wilson (1978), after correction by Sinapius, Raith and Wilson (1980) , Stein et al. (1978) A, Coleman et al. (1979) A, Brenton et al. (1977) o, Griffith et al. (1979a). The experimental data in the inset are from Mizogawa et al. (1985) and they are compared there with the theoretical work of Campeanu and Humberston (CH, see text). Theoretical curves, main diagram — —, CH ...

Fig. 2.9. Intermediate energy positron-helium total scattering cross sections. Experiment A, Coleman et al. (1979) o, Griffith et al. (1979a) V, Brenton...

The total elastic scattering cross sections for the three helium models, calculated using the accurate variationally determined phase shifts for l < 2 and equation (3.67) for l > 2, are shown in Figure 2.8, together with several sets of experimental measurements. Excellent agreement is obtained between the results for the two helium models H5 and H14 and the experimental measurements of Canter et al. (1973) and, more recently, those of Mizogawa et al. (1985). Also, the accurate theoretical results... 

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