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Situation=A08-m2a

In the research programme from which this article issues [More about the situation in a mesoscopic module (link: 'elaborated in/project/wider range'; target: MESO-m2a)], total and differential cross sections have been measured for collisions between electropositive and electronegative atoms and molecules, using molecular beam techniques [Particulars on those techniques in the mesoscopic Experimental methods module (link type: 'is detailed in/focused on in/project/wider range'; target: MESO-m3a]. These cross sections have been interpreted tentatively in terms of a simple classical atom-atom model for ion-pair formation in molecular collisions [Particulars on that model in a mesoscopic Theoretical methods module (link type: 'is detailed in/focused on in/project/wider range'; target: MESO-m3c-mod], according to which the transition to the ionic state takes place via crossing of the neutral and ionic ground states, with a probability given by the Landau-Zener approximation.

We have studied total cross sections for collisions between alkali atoms and O2[link type: 'project/elaborated in'; target: A02-m1], alkali atoms and Br2, I2, Cl2, NO2, N2 and CO2[link type: 'project/elaborated in'; target: A02-m1]. The total cross section of alkali atoms and I2 and Br2 has also been measured as a function of the secondary beam temperature [link type: 'project/elaborated in'; target: A04-m1]and taking into account the fragmentation of the negative ion [link type: 'project/elaborated in'; target: A07-m1]

We have also published measurements on the differential cross section for chemi-ionization in some alkali atom-halogen molecule collisions in an earlier paper [link type: 'project/elaborated in'; target: A05-m1]. The measurements have been compared with classical calculations, using the two-state approximation and the Landau-Zener theory to describe diabatic transitions at the crossing of the ionic and covalent potential surfaces [particulars on the model used for these calculations in a mesoscopic Theoretical methods module (link type: 'is detailed in/focused in/project/wider range'; target: MESO-m3c-mod)], via the determination of the deflection function using the numerical method described in [link type: 'project/elaborated in'; target: A06-m*].

As a stringent simplification we have considered the diatomic halogen molecule as a single particle, forming with the alkali atom an isotropic potential field with an isotropic crossing of ionic and neutral potential surfaces. The consequence of the simplification is the neglect of rotation and vibration of the halogen molecule and an application of the Landau-Zener transition formula independent of the angular position of the halogen molecule with respect to the radius vector between the two collision partners. A comparison of measurements and calculations has shown that the quantitative agreement is very poor [link type: 'project/depends on'; target: Findings A05-m6a]. Moreover, a semiclassical approximation with the same simplification does not improve the agreement [Arguments for this are given in a subsequent article (link type: 'elucidated in/To cause/external '; target: R2-m*)].

Because at that time the lack of agreement was not well understood, we started measurements on atom-atom scattering of the same type

to test the semiclassical calculation method and the suitability of the Landau-Zener theory for this type of collision processes.

We have studied the $\Na+\I\to\Na^++\I^-$ collision process.

Of course this process does not need the simplifications described above. Moreover, this process is convenient for calculating the differential cross section because at least the shape of one of the potential curves, the ionic Na+ + I- curve, is well known while the measurements allow estimations of the covalent NaI curve.

We use atomic beam techniques allowing for the experimental resolution of interferences of two types, viz. the rainbow structure and the Stueckelberg oscillations [Bernstein elucidates rainbow and Stueckelberg oscillations in a macroscopic module (link type: 'elucidated in/elaborated in/wider range/external'; target: MACRO-m3c-Bern)] . Stueckelberg oscillations have been observed before at elastic and inelastic scattering from noble-gas ion on noble-gas atom collisions, by Lorents and Smith et al. [(link type: 'external/elaborate'; target R3-m*)]and by Barat et al. [linktype: 'external/elaborate'; target: R4-m*].

We also consider rotational coupling in our calculations. Rotational coupling couples a covalent state with the ionic state and is generally expected to be most effective for collisions with large impact parameters. A theoretical description of this phenomenon has been given, for example, by Russek [[link type: 'external/elaborate'; target: R5-m*], but rotational coupling has not yet been observed in combination with Landau-Zener transitions.