Electron capture in collisions between ${\mathrm{O}}^{6+}$ ions and ${\mathrm{H}}_2\mathrm{O}$ molecules

By means of photon emission spectroscopy, state selective electron capture cross section for low energy $(0.1–7.5\mathrm{keV}∕\mathrm{amu})$ collisions of ${\mathrm{O}}^{6+}$ on ${\mathrm{H}}_2\mathrm{O}$ molecules have been measured. Over the range of interaction energies the state selective cross sections change strongly, i.e., by factors up to 5, while the total one-electron capture cross section turns out to increase by approximately 40%. Possibilities of using $\mathrm{O}\mathrm{VI}$ line emission ratios as a remote diagnostics of astrophysical plasma are indicated.

Figure 1

Velocity dependence of charge exchange spectra for collisions between ${\mathrm{O}}^{6+}$ and ${\mathrm{H}}_2\mathrm{O}$. Shown are the first order $(10–20\mathrm{nm})$ and the second order $(22–32\mathrm{nm})$ spectra for collision energies of $7.5\mathrm{keV}∕\mathrm{amu}$ (top), $0.38\mathrm{keV}∕\mathrm{amu}$ (middle), and $0.11\mathrm{keV}∕\mathrm{amu}$ (bottom). The spectra are corrected for the accumulated charge and target pressure, but not for the spectrometer’s wavelength dependent response. Second order spectra are blown up for the ease of presentation.

Figure 2

Grotrian diagram of $\mathrm{O}\mathrm{VI}$ $\left(1s^{2}nl\right)$. Observed transitions are indicated with solid lines, other transitions by dashed lines. Wavelengths are given in nanometers, the numbers between brackets are the branching ratios.

Figure 3

Velocity dependence of state selective, single electron capture cross sections for ${\mathrm{O}}^{6+}+{\mathrm{H}}_2\mathrm{O}$. Upper panel: Capture into $n=4$. ◻: $\sigma \left(4s\right)$; ●: $\sigma \left(4p\right)$; ▵: $\sigma \left(4d\right)$; ▾: $\sigma (3d+4f)$. Lower panel: Capture into $n=3$. ●: $\sigma \left(3s\right)$; ◻: $\sigma \left(3p\right)$. Lines are drawn to guide the eye. Only relative errors are given. The systematic uncertainty is approximately 25%.

Figure 4

Over-the-barrier predictions for one electron capture in ${\mathrm{O}}^{6+}+{\mathrm{H}}_2\mathrm{O}$ collisions. Reaction windows for collision energies of 0.25 and $4\mathrm{keV}∕\mathrm{amu}$. $\mathrm{O}\mathrm{VI}$ states are indicated.

Figure 5

Comparison of measured (gray) and theoretical distribution (black) over the $4\ell$ states. Top panel: Measured distribution at $0.11\mathrm{keV}∕\mathrm{amu}$, compared with a radial coupling determined low energy distribution [35, 36]. Bottom panel: Measured distribution at $7.5\mathrm{keV}∕\mathrm{amu}$, compared with a statistical $\ell$ distribution typical for higher collision velocities.

Figure 6

Velocity dependence of total one electron capture cross sections for collisions between ${\mathrm{O}}^{6+}$ and ${\mathrm{H}}_2\mathrm{O}$, ●, compared with charge exchange cross sections for ${\mathrm{H}}_2$ (▵: [16]; ▿: [20]) and H (◻: [16]). 엯: Total charge changing cross section from [24]. ▴: Total charge changing cross section for ${\mathrm{C}}^{6+}+{\mathrm{H}}_2\mathrm{O}$ [38]. Lines are drawn to guide the eye. Only relative errors are given. The systematic uncertainty is approximately 25%.