Lattice-constant and electron-affinity effects on negative-ion conversion in atom–ionic-crystal-surface grazing scattering

The effects of the lattice constant and electron affinity on the negative-ion conversion of a neutral atom undergoing grazing scattering on an ionic-crystal surface over the complete velocity range were investigated. Here, a comparison of negative-ion conversion of neutral ${\mathrm{O}}^0\text{−}\mathrm{KCl}\left(100\right),{\mathrm{F}}^0\text{−}\mathrm{KCl}\left(100\right)$, and ${\mathrm{O}}^0\text{−}\mathrm{KI}\left(100\right)$ surface systems shows that the pronounced difference in the efficiency of negative-ion formation between ${\mathrm{F}}^0\text{−}\mathrm{KCl}\left(100\right)$ and ${\mathrm{O}}^0\text{−}\mathrm{KCl}\left(100\right)$ is caused by the large difference in their projectile electron affinities, whereas the difference between ${\mathrm{O}}^0\text{−}\mathrm{KI}\left(100\right)$ and ${\mathrm{O}}^0\text{−}\mathrm{KCl}\left(100\right)$ is caused by the difference in their lattice constants.

Figure 1

Sketch of the ${\mathrm{A}}_{\mathrm{gas}}^0$ projectile undergoing grazing scattering from an ionic-crystal surface [here, KCl(100) or KI(100)], where α and $D$ represent the incident angle and diameter of the beam, respectively. The direction of the beam is parallel to the $\langle 100\rangle$ surface axis channel.

Figure 2

Both $\mathrm{\Delta }{E}_{\mathrm{PC}}^{\mathrm{ave}}\left(\mathrm{Z}\right)={\langle \mathrm{\Delta }{E}_{\mathrm{PC}}\left(X,Y,Z\right)\rangle }_S={\langle \mathrm{\Delta }{E}_{\mathrm{PC}}\left(X,Y,Z\right)\rangle }_{S_0}$ and $\mathrm{\Delta }{E}_{\mathrm{PC}}\left(X=Y=0,Z\right)$ as a function of the surface altitude $Z$ are displayed by different types of lines for ${\mathrm{O}}_{\mathrm{gas}}^0$-KCl(100), ${\mathrm{F}}_{\mathrm{gas}}^0$-KCl(100), and ${\mathrm{O}}_{\mathrm{gas}}^0$-KI(100) systems, respectively.

Figure 3

(a–d) ML polarization contributions $P_{\mathrm{ML}}\left(\mathbf{R}\right)$ of ${\mathrm{F}}_{\mathrm{gas}}^0$-KCl(100) and ${\mathrm{O}}_{\mathrm{gas}}^0$-KCl(100) within the surface ${\mathrm{Hal}}_{\text{active}\text{site}}^{-}\left({\mathrm{here}\mathrm{Cl}}_{\text{active}\text{site}}^{-}\right)$ scale $S$ at the projectile-surface altitudes of $Z=3.0,3.5,4.0$, and 4.5 a.u., respectively.

Figure 4

(a) Three-dimensional plot of $U_{\mathrm{image}}$ as a function of both the surface altitude $Z$ and projectile velocity $v$. (b) $U_{\mathrm{image}}$ as a function of the projectile velocity $v$ for fixed surface altitudes of $Z=2.5,3.0,3.5$, and 4.0 a.u. Panels (c,d) are similar to panels (a,b), respectively, except they display the O-KI(100) case.

Figure 5

(a,c,e) Calculated average energy defects $\mathrm{\Delta }{E}_{\mathrm{ave}}\left(Z,v\right)$ on the scale $S$ of the surface ${\mathrm{Hal}}_{\text{active}\text{site}}^{-}\left(\mathrm{C}{\mathrm{l}}^{-}\mathrm{or}{\mathrm{I}}^{-}\right)$ as a function of the surface altitude $Z$ and projectile velocity $v$ for the ${\mathrm{O}}_{\mathrm{gas}}^0$-KCl(100), ${\mathrm{O}}_{\mathrm{gas}}^0$-KI(100), and ${\mathrm{F}}_{\mathrm{gas}}^0$-KCl(100) cases, respectively. (b,d,f) $\mathrm{\Delta }{E}_{\mathrm{ave}}\left(Z,v\right)$ as a function of the projectile velocity $v$ for fixed surface altitudes of $Z=3.0,3.5$, and 4.0 a.u. for the three studied cases, respectively.

Figure 6

(a) Probability of negative-ion conversion for a collision between ${\mathrm{O}}_{\mathrm{gas}}^0$ and a ${\mathrm{Cl}}_{\text{active}\text{site}}^{-}$ at the KCl(100) surface in a single binary collision ${\mathrm{O}}_{\mathrm{gas}}^0+{\mathrm{Cl}}_{\text{active}\text{site}}^{-}\to {\mathrm{O}}_{\mathrm{gas}}^{-}+{\mathrm{Cl}}_{\text{active}\text{site}}^0$. The calculation is performed within the effective region $S$ of ${\mathrm{Cl}}_{\text{active}\text{site}}^{-}$ at the KCl(100) surface. The figure displays the dependence of the probability $P_{\mathrm{S}}^{\mathrm{site}}\left(Z,v\right)$ on the projectile velocity $v$ and the surface altitude $Z$ as well as the projectile velocity dependence of the negative-ion conversion probability $\left[P_{\mathrm{S}}^{\mathrm{site}}\left(Z,v\right)\right]$ in a single binary collision. Four representative values of the surface altitude, $Z=3.0$ (blue dashed line), 3.5 (red solid line), 4.0 (green dashed dotted line), and 4.5 a.u. (black short dashed line), are displayed. Panels (c,e) and (d,f) are similar to panels (a,b), respectively, except they display cases of a single binary collision of ${\mathrm{O}}_{\mathrm{gas}}^0+{\mathrm{I}}_{\text{active}\text{site}}^{-}\to {\mathrm{O}}_{\mathrm{gas}}^{-}+{\mathrm{I}}_{\text{active}\text{site}}^0$ and ${\mathrm{F}}_{\mathrm{gas}}^0+{\mathrm{Cl}}_{\text{active}\text{site}}^{-}\to {\mathrm{F}}_{\mathrm{gas}}^{-}+{\mathrm{Cl}}_{\text{active}\text{site}}^0$. (g) Tunneling dynamics for the potential $V\left(R\right)=1/\phantom{1R}R$. The classically allowed incoming and outgoing trajectories with the energies $E_1$ and $E_2$, respectively, are indicated by the blue dashed lines, and the tunneling is indicated by the green solid line. (h) Tunneling detachment probabilities for ${\mathrm{O}}_{\mathrm{gas}}^{-}+{\mathrm{Cl}}_{\mathrm{site}}^{-}\left({\mathrm{or}\mathrm{I}}_{\mathrm{site}}^{-}\right)\to {\mathrm{O}}_{\mathrm{gas}}^0+e^{-}+{\mathrm{Cl}}_{\mathrm{site}}^{-}\left({\mathrm{or}\mathrm{I}}_{\mathrm{site}}^{-}\right)$ (blue dashed line) and ${\mathrm{F}}_{\mathrm{gas}}^{-}+{\mathrm{Cl}}_{\mathrm{site}}^{-}\to {\mathrm{F}}_{\mathrm{gas}}^0+e^{-}+{\mathrm{Cl}}_{\mathrm{site}}^{-}$ (red solid line).

Figure 7

(a–c) Comparison of the negative-ion yield versus the projectile velocity as measured for ${\mathrm{O}}_{\mathrm{gas}}^0$-KCl(100), ${\mathrm{O}}_{\mathrm{gas}}^0$-KI(100), and $F_{\mathrm{gas}}^0$-KCl(100) [21] for an incidence angle $\alpha \approx 1^0$ as indicted by $\alpha =1^0$. Black solid symbols in (a–c) represent experimental data [21] for three studied systems. The present theoretical results were displayed in (a–c) by different types of lines with the effective crossing number of $N_{{\mathrm{O}}_{\mathrm{gas}}^0\text{−}\mathrm{KCl}}=18-24$, $N_{{\mathrm{O}}_{\mathrm{gas}}^0\text{-KI}}=10-16$, and $N_{F_{\mathrm{gas}}^0\text{-KCl}}=4-9$, respectively, for ${\mathrm{O}}_{\mathrm{gas}}^0$-KCl(100), ${\mathrm{O}}_{\mathrm{gas}}^0$-KI(100), and $F_{\mathrm{gas}}^0$-KCl(100) systems. (d) The calculated results of ${\langle P_S^{\mathrm{site}}\left(Z,v\right)\rangle }_Z$ for $P_S^{\mathrm{site}}\left(Z,v\right)$ average over surface altitude $Z$ values within the $2.0\mathrm{a}.\mathrm{u}.\le Z\le 5.0\mathrm{a}.\mathrm{u}.$ range, where the green dashed line indicates ${\mathrm{O}}_{\mathrm{gas}}^0$-KCl(100); the blue solid line, ${\mathrm{O}}_{\mathrm{gas}}^0$-KI(100); the red short dashed line, ${\mathrm{F}}_{\mathrm{gas}}^0$-KCl(100).