[Legenda]  
[Contents of the thesis] 

[Comments on this module] 



[Legenda]  
[Contents of the thesis] 

[Comments on this module] 
Chemiionization in collisions between Na and I can be explained via crossing of the potential energy surfaces of the collisions . The LandauZener formula gives the transition probability between translationally coupled states. However, the LandauZener transition probability does not give an adequate quantitative explanation of the experimentally determined differential cross sections of the chemiionization process. It is likely that rotationally induced transitions could explain the discrepancy of measured and calculated differential cross section.
The discrepancy that the rotational coupling could explain is where the calculated cross section is too small for collisions with large impact parameters . Fig. A08m5bii2F1 clearly shows for increasing kinetic energy an increasing deviation of the relative differential cross section at eV degree without rotational coupling. Even the relative cross sections more separated from the minimum would lead for a fitting procedure to an unphysical energy dependence of the LandauZener coupling parameter H_{12}.
We have calculated
the general shape of the differential cross sections of the chemiionization process again, now taking into account the rotational coupling
.
Fig. A08m5bii2F2 gives again the measured general shapes of the differential cross sections at kinetic colliding energies of 13.1, 20.7, 29.7, 38.7 and 55.0 eV. A comparison has been made with calculated cross sections taking into account some rotational coupling.
Indeed, the minimum in the differential cross section has been increased to a degree dependent on the kinetic energy.
Moreover, it is very obvious that now it is possible to find one set of coupling constants giving a good fit of measured and calculated cross sections for all energies. This set consists of the values H_{12}=0.065 eV and H_{rot}=3 x 10^{17} eV corresponding to the values of 0.0024 a.u. and 0.04 a.u., respectively.
A comparison of the corresponding cross section curves of Fig. A08m5bii2F1 and Fig. A08m5bii2F2 shows that rotational coupling at low kinetic energies increases the differential cross section only at the very minimum but for higher kinetic energies there is a rise over a larger range. This feature makes it possible to use only one value of H_{12}.
On the range eV degree Fig. 0.2f gives the dependence of the cross section on some values of _{rot} at E_{i} = 55 eV. For increasing coupling constant, the maximum contribution of rotational coupling to chemiionization moves to collisions with smaller impact parameters.
It must be noted that for impact parameters Eq. (E1)
(E1) 
Reliability
The estimated value H_{12}=0.065 eV differs rather much from the value of 0.05 eV, estimated from total crosssection
measurements
on the collision energy range 220 eV. The effect of
these two values on the absolute differential cross section can be
observed from Fig. 0.1 (H_{12}=0.05 eV) and Fig. 0.2
(H_{12}=0.065 eV). At E_{i} = 55
eV the cross sections of
Fig. F2 have
hardly different values, while at E_{i} = 13.1
eV the cross sections
differ by a factor of two. Of course, relative measurements on the
differential cross section versus the kinetic energy should give a
hint to the correct value of H_{12}. However, at the present
measurements it is impossible to distinguish in this way these values
of H_{12}. It is expected that the surfaceionization detector as well
as the scatteredion detector have large and unknown energydependent
efficiencies.