[Legenda]
[Contents of the thesis]
[Comments on this module]
[Legenda]
[Contents of the thesis]
[Comments on this module]

Quantitative interpretation: potential and deflection function =A08-m5bi

We determine the potential parameters that govern the chemi-ionization reaction Na + I Na⁺ + I according to the atom-atom model for ion-pair formation in molecular collisions , . From the potential parameters, the theoretical cross section of the chemi-ionization reaction can be calculated via the deflection function. We determine the potential parameters, and simultaneously the deflection curve, by fitting the calculated cross section (that is based on assumed potential parameters) with the experimental cross sections.

We now restrict ourselves as much as possible to the determination of some potential parameters and to the comparison of measurements and calculations used for that purpose. The calculated differential cross section and its discrepancies with the measurements will be treated in module Quantitative interpretation A08-m5bii .

Figure A08-m5bi-F1: Polar differential cross section for chemi-ionization (CM system) at Ei=13.1 eV. (b) Differential cross section calculated with the stationary-phase approximation and uniform rainbow approximation showing separated the long-wavelength interference structures due to a + c ($\tau=0\to65$, full curve), b + c ($\tau=65\to250$, full curve) and d + e ($\tau=0\to 2300$, dashed curve) interferences. (c) Full bars indicate the measured maxima of the interference structure on the differential cross section due to net-attractive scattering. Dashed bars indicate the maxima due to net-repulsive scattering.

Fig. A08-m5bi-F1b shows the differential cross section with simplified interference structure. An additional simplification in Fig. A08-m5bi-F1b is the separate reproduction of the attractive and repulsive scattering contribution as though they could be distinguished. Because the origin of the oscillatory features of the calculated cross sections can be seen easily from Fig. A08-m5bi-F1b and the latter figure is more easily related to the deflection function and the potential curves, Fig. A08-m5bi-F1b is more suitable to fit the interference structure of the calculated and measured differential cross section by adjustment of the potential parameters. Moreover, convolution of the differential cross section of the complete calculated cross section, which is given in Fig. A08-m5bi-F1a , and Fig. A08-m5bi-F1b even with the smallest angular resolution of the detector of 0.3º fwhm will lead to equal results.

By comparison of measurements and calculations, the missing parameters of the covalent potential curve and the value of H₁₂ can be estimated. For that purpose are very important particularly the wavelengths of the rainbow and Stueckelberg oscillations , which result from the semi-classical interference of different contributions to the scattering angle .

For a fitting procedure of the potential curves to the measurements, it is very helpful that the interference wavelengths can be estimated directly from the deflection curves. The oscillations are generated by the cosine of the Eqs. (E1)  

$$I(\MT<0)=A^2+C^2+2AC\cos(\alpha-\gamma){\rm }$$ (E1a)

or  

$$I(\MT<0)=B^2+C^2+2BC\cos(\beta-\gamma){\rm }$$ (E1b)

and  

$$I(\MT\gt)=D^2+E^2+2DE\cos(\delta-\vare).$$ (E1c)

Figure A08-m5bi-F2: Deflection curves for chemi-ionization scattering (CM system) at Ei=13.1 eV.

The difference in the cosine argument for two neighbouring scattering angles $\theta_1$ and $\theta_2$ can be shown easily, with the help of Fig. A08-m5bi-F2 and Eqs. (E2) for the contributions to the scattering angle in lowest-order stationary-phase approximation .
      

For interferences as indicated in Eqs. (E1) this difference is exactly the part of the deflection curve enclosed by $\theta_1$ and $\theta_2$ and multiplied by a factor of $k/2\pi$. Then the local oscillation frequency is linearly proportional to the distance along the b-scale of the relevant branches of the deflection curve. For net-attractive and net-repulsive scattering Fig. A08-m5bi-F2 shows two slices with equal areas. It is clearly shown that in the chosen angular ranges the repulsive-scattering wavelength is much larger compared to the attractive one.

Figure A08-m5bi-F4: Na_I adiabatic potential curves. The pseudo-crossing potentials are all of the same species $^1{\mit\Sigma}^+$. [Copied to the Treated results (link type: 'output to'; target: A08-m4bii1)]

 Repulsive scattering oscillation A_cov and $\rho_\cov$

By fitting the repulsive scattering oscillations of the experimental and the theoretical cross section curves, we found A_cov=3150 eV and the repulsive steepness to be $\rho_\cov$=0.435 Å. Then the calculated repulsive oscillation is in perfect agreement with the experimental one.   

Figure A08-m5bi-F3: Wavelength of the oscillatory differential cross section (CM system) due to net-repulsive scattering, versus the colliding energy. The measurement give the wavelength averaged over ten oscillations just beyond the rainbow angle. The error bars only indicate the error in the relative position of first and tenth oscillation. The curve gives the corresponding calculated wavelengths

Inelastic energy $\Delta E$

The general shape of the polar differential cross section is the cross section averaged over the quantal oscillations. The angular positions of special features at different collision energies agree very well: namely the maximum of the peak at $\tau\approx35$ due to covalent scattering, the minimum at $\tau\approx65$ due to scattering with maximal impact parameter and the maximum of the primary rainbow. These agreements determine that $\Delta E$, the potential-energy difference of the ionic and covalent state at infinite internuclear separation, is =2.075eV.

Potential well-depth $\vare$

The agreement of the angular positions of the special features of the general shape also establishes the well-depth of the ionic potential curve: =-3.1 0.2 eV.

Coupling H₁₂

The relative intensities of several parts of the general shape of the cross section lead to an estimation of H₁₂ of 0.05 eV for energies in the range of 30-55 eV. However, for the 13.1 eV curve the estimation is H₁₂=0.065. Now we have determined the potential parameters of the ionic and the covalent system Na - I. The parameters are summarized in Table A08-m5bi-T1. The ionic ground state is well known. It is described by the Rittner potential :

$$-\frac{e^2}{R}-\frac{e^2(\alpha_{\Na^+}+\alpha_{\I^-})}{2R^4}$$ U_ion(R) =

$$-\frac{2e^2\alpha_{\Na^+}\alpha_{\I^-}}{R^7}-\frac{C_\ion}{R^6}$$

$$+A_\ion\, \er^{-R/\rho_\ion}+\Delta E.$$ (E3)

We describe the covalent potential only by two terms:

$$U_\cov(R)=-\frac{C_\cov}{R^6}+A_\cov\,\er^{-R/\rho_\cov}$$

(E4)

We have determined for the covalent Na-I system most of the potential parameters; the missing parameters have been chosen to construct the potential curves in Fig. A08-m5bi-F4 and the deflection functions in Fig. A08-m5bi-F2.

The ionic potential curve is overdefined by the given parameters. That is why the values for A and r_e have not been used but the other parameters give rise to the values given in parentheses.

For small values of the internuclear distance R the ionic and covalent potential curves bend over to negative values, leading to $\lim_{R\to 0} U(R)=-\infty$. This is due only to the mathematical form of the potential-energy expressions [Eqs. (E3) and (E4)]. Using the parameters of Table A08-m5bi-T1, the maxima of the potential curves are U_ion=29.7 eV and U_cov=21.4 eV for R=1.16 Å and R=1.70 Å, respectively. However, this effect does not handicap the calculations. Even for the smallest impact parameter considered, the distances of closest approach are R₀=2.02 Å and R₀= 2.84 Å corresponding to the potential energies U_cov=-0.3 eV and U_cov=2.7 eV, respectively.