Charge Transfer in He+ - He → He(1s4l, l ≥ 2) - He+ Collisions in Intermediate Energy Range

Abstract

The anticrossing spectra of the helium line λ1s4l D3,F-1s2p P3=447.2 nm emitted after electron capture by He+ ions in He+-He collisions were measured for projectile energies of 10-29 keV. Furthermore, considering the excited states' time evolution, the theoretical intensity functions were calculated. The electric field and density distributions of the target He atoms in the collision volume were taken into account, and by fitting the theoretical intensities to the measured ones, the post-collisional states of the charge-transferred He atoms were determined. The results indicate that for the intermediate projectile energy range, the electronic charge distributions were asymmetric, but the electric dipole moments did not change, as in the case of the target atoms excited directly in the collisions. This result shows that the Paul trap mechanism may play an important role in the charge transfer excitation in this energy range.

Keywords: Stark effect; anticrossing spectroscopy; density matrix; ion–atom collision.

Figure 1

Experimental setup. The diagram of the linear accelerator and collision chamber shows PIS as the peening ion source, PK and PA as the power supplies of the anode and cathode of the PIS, respectively, and PE as the accelerator power supply determining the ion energy. More details are provided in the text.

Figure 2

Scheme of the collision chamber and the detection system.

Figure 3

Scheme of the beams of ${He}^{+}$ ions and He target atoms.

Figure 4

(a) A geometrical cross-section of an ion beam in the form of a cylinder with a radius $r_i=0.6$ mm with a conical beam of atoms with a radius $r_z=0.54$ mm in the plane of the Z axis. (b) The normalised cross-sectional areas. The solid line represents the cross-sections calculated numerically from Figure 4a. The dashed line and the dot-dashed line represent analytical functions given by Equation (6) with n = 2 and 4, respectively.

Figure 5

Electric field intensity distribution in the experimental system along the Z axis. The inset shows an expanded region around $z=0$.

Figure 6

Change in velocity of the helium ions in the experimental system. The solid line is for the ion beam energy of 15 keV, and the dashed line is for the ion beam energy of 30 keV.

Figure 7

Intensities of spectral lines—singlet $\lambda \left(4 {}^{1}D,F-2 {}^{1}P\right)=492.2$ nm and triplet $\lambda \left(4 {}^{3}D,F-2 {}^{3}P\right)=447.2$ nm—along the Z axis (left vertical axis) and the electric field distribution (right vertical axis) for 22 ${\mathrm{k}\mathrm{V}\mathrm{c}\mathrm{m}}^{-1}$ in the midpoint between electrodes. The calculations were made for fast atoms with an energy of 26 keV $(v=1.12·{10}^6 {\displaystyle \frac{m}{s}}$). (a) The area of the atomic beam and the electric field of the AC-B peak ($E_B\approx 18.14 \mathrm{k}\mathrm{V}{\mathrm{c}\mathrm{m}}^{-1}$). (b) The calculation extended to the observation region. The curves of the exponential decays of the intensity of the spectral lines (Equation (25)), with $\tau =37$ ns for the singlet line and $\tau =32$ ns for the triplet line, are presented.

Figure 8

Intensity functions $I_{447}$ calculated for selective excitation of parabolic singlet $\Sigma ,\Pi$ and triplet $\Sigma ,\Pi ,\Delta$ states for 29 keV of ${He}^{+}$ energy.

Figure 9

Intensity functions I₄₄₇ calculated for selective excitation of spherical triplet and singlet states $\left|1\mathrm{s}4\mathrm{l} {}^{3,1}\Lambda ⟩\right.\left(\Lambda =0,1,2\right)$ for 29 keV of ${He}^{+}$ energy as a function of the external electric field.

Figure 10

Recorded fluorescence light intensity $I_{447}\left(F_z\right)$ of the 447 nm line of HeI, measured as a function of an electric field $F_z$ for the charge transfer in collisions ${He}^{+}-He\to {He}^{\mathrm{*}}-{He}^{+}$ at impact energies of 10 keV (a), 15 keV (b), 20 keV (c), 23 keV (d), 26 keV (e), and 29 keV (f).

Figure 11

Intensity function $I_{447}\left(F_z\right)$ measured for ${He}^{+}(29 \mathrm{k}\mathrm{e}\mathrm{V})-He\to {He}^{*}-{He}^{+}$ collisions with a theoretically fitted curve and the components $I\left(\left|x,X⟩\right.\right)$ of the intensity originating from the triplet and singlet state excitations $X=T,S$, respectively (for parabolic and spherical states $x=p,s$ as well, respectively). The solid line at the bottom of the figure illustrates the differences (residues) between the recorded and theoretical signals.