Potential Effects in the Interaction of Highly Charged Ions with Solid Surfaces∗

In the field of nuclear fusion technology, Er2O3 thin film is a candidate for the coating material of the blanket of thermonuclear fusion reactor. It is known that the luminescence of Er2O3 provides information on its crystallinity. We observed luminescence of various samples including Er2O3 during the irradiation with highly charged ions produced by an electron beam ion source (EBIS) as a function of charge state and kinetic energy of incident Ar ions. We found that the luminescence intensity non-linearly rises as the charge state increases and is independent of kinetic energy. This demonstrates that the luminescence arises from the potential energy of highly charged ions rather than the kinetic energy. Emission spectra from various samples indicate that emission from sputtered atoms, mostly atomic hydrogen, is remarkable, while emission lines from the surface layers due to transitions among Stark splitting were observed for Er2O3 samples. [DOI: 10.1380/ejssnt.2016.1]


I. INTRODUCTION
The highly charged ion (HCI) has a large potential energy which induces emission of photons ranging from visible to X-ray, hundreds of secondary electrons and various kinds of particles, and leaves traces of HCI impact in nanometer scale when a single HCI hits the surface [1][2][3].We have measured mass spectra of secondary ions released from the surface and observed microscopic surface structure (crater or hillock in nanometer size) created by the impact of single HCI using STM and AFM [4,5].The potential effect in the secondary ion emission and surface modification has characteristics that the potential energy of HCI is dissipated as multiple electron transfer processes and subsequent Auger de-excitation processes over few atomic layers from the topmost surface, and the nanometer region becomes positively charged then collapses.
It is expected that erbium oxide (Er 2 O 3 ) thin film fabricated by metal organic chemical vapor deposition (MOCVD) method is useful for blanket of thermonuclear reactor [6].For bulk Er 2 O 3 sample visible light emission during the irradiation with Ar + ions has been measured [7].
In the present study, we have observed visible light emitted from the surface region irradiated with HCIs.

II. EXPERIMENTAL
We used a HCI source (electron beam ion source, EBIS) installed at Kobe University, Japan [8,9].An EBIS consists of an electron gun, drift tubes, an electron collector and a super-conducting magnet.In the EBIS HCIs are created successive ionization through the electron impact of high current density (∼ 1000A/cm 2 ) electron beam formed by the strong magnetic field.The electron gun and collector of Kobe EBIS are floated on negative high voltage in order to lower the electric potential of drift tube region (i.e.lower the kinetic energy of HCI extracted from the EBIS), which is essential to emphasize the potential energy effect of HCIs in the interaction with material surfaces.The potential of the drift tubes was 3 kV in the present experiment.The electron gun and drift tubes are installed in a vacuum tube which is inserted in a superconducting magnet (3T) with the bore size of 180 mmϕ.The vacuum system is evacuated by a turbo-molecular pump, a titanium getter pump and a sputter ion pump.The ultimate pressure of the Kobe EBIS with the electron beam (100 mA) is 2-3×10 −8 Pa after the bake-out procedure for 100 h at 500 K.
In the present experiment, the EBIS is operated under epulse modef operation where the electron beam was intermittently extracted by modulated anode voltage [10].The modulation is periodic repetition of on/off of electron beam; the period is in the order of 0.1 s and the width of beam off time is around 1ms.Although the electron beam is almost turned on and for only ∼1/100 of the period the beam is turned off, the charge state distribution for the higher charge state region is improved very much.For the normal operation the intensity of Ar 16+ is negligible (< 1 pA), on the other hand, in the range of 100 pA is obtained under the pulse mode operation.
The HCIs extracted from the EBIS were charge separated using a bending magnet, and introduced to a vacuum chamber for the irradiation of a sample which is mounted on the manipulator for 3-axes translation, rotation and vertical long travel [5].The pressure of the irradiation chamber was in the range of 10 −6 Pa.The HCI beam is focused on the sample by an electrostatic lens system with the spot size of ∼1 mm in FWHM.The incident beam current can be monitored applying electric potential to sample up to 2 kV.The chamber is also equipped with detectors (a Faraday cup and a MCP detector) in order to monitor the intensity and profile of primary HCI beam.The light emitted from the sample during the irradiation is transferred through a sapphire window to a liquid N 2 cooled CCD detector (VersArray 1300, Princeton Instruments) with a camera lens.Spectral distribution of the emission was measured using a homemade visible spectrometer designed for the simultaneous detection of spectral range from 400 to 700 nm with a flat-field concave grating (52066BK01-002C, Richardson Gratings).The optical axis of emitted light measurement is perpendicular to that of incident ion beam.
The various samples (Cu, HOPG, Si and Er 2 O 3 ) were irradiated with Ar q+ (q = 6-16) ions and optical images and spectra of emitted light from the region around the sample were taken with the CCD camera.We used two types of Er 2 O 3 samples, one is a sintered bulk polycrystalline Er 2 O 3 , the other is a thin film deposited on stainless steel substrate by metal organic chemical vapor deposition (MOCVD) method.All the samples were rinsed in solvent, however, no cleaning procedure was used after they are introduced to the irradiation chamber.It is to be noted that the light is emitted from both the surface layers of sample and particles (sputtered or back scattered atoms and ions) in space departed from the surface.In order to discriminate the former (surface emission) from the latter (space emission), the optical image was recorded for two kinds of configurations; the angle of sample surface normal against the ion beam is 0 • and 45 • (Fig. 1).If the angle is 0 • , only the space emission is observed whereas surface emission is superposed on space emission at the angle of 45 • .

III. RESULTS AND DISCUSSION
Figure 2 shows the charge state dependence of the intensity of observed visible light.The intensity nonlinearly increases with the charge state.If the intensity is fitted in a power of charge state, the intensity is proportional to 3.5∼ 4th power of the charge state.The intensity for the incident angle of 45 • slightly exceeds that for 0 • .Relative difference of intensities between 0 • and 45 • becomes negligible at higher charge states.This implies that the light emission for higher charge states mainly consists of space emission.The light intensity at specific charge states (Ar 7+ and Ar 14+ ) was measured applying various retarding potentials in the range of 0-1.5 kV, i.e. the kinetic energy of HCI incident on the sample is suppressed by a half of extracted one at most.Since the light intensity showed negligible retarding potential (i.e.kinetic energy) dependence, the charge state dependence in Fig. 2 is the consequence of potential effect in the interaction of HCI with surface [11].When a HCI hits on a surface, an irradiation trace is created in the nanometer scale.For a graphite sample, hillock-like nanostructure is produced.Such structural modification is usually ascribed to potential effect like Coulomb explosion, however, similar structure appears when the surface is irradiated with a swift heavy ion [12] and it is difficult to extract potential effect from the experimental data on structural modification of surface layers.The present result on the charge state dependence of light intensity is unique in a sense that only the potential energy of HCI governs the phenomena.
Spectroscopic measurement of emitted light during irradiation was also performed.Figure 3  Fig. 3 and its vertical direction corresponds to the dispersion of wavelength.The emission spectrum of Er 2 O 3 film is shown in Fig. 4 which is constructed as the intensities of pixels with same vertical position within the region indicated with a red rectangle in Fig. 3 are summed.The broad structure between 500 to 600 nm comes from the inter-band transitions among Stark splitting in crystalline Er 2 O 3 which was also observed from bulk Er 2 O 3 irradiated with Ar + ions [7].The sharp and strong peak at 656 nm corresponds to Balmer alpha (Hα) line from hydrogen atom released from the surface of sample, which was not observed in the spectrum for Ar + irradiation.Balmer beta (Hβ) line is also observable at 486 nm.The relative intensity of Hβ increases with charge state.This Balmer series is observed for all the samples used in the present experiment.
Since the pressure of the irradiation chamber is in the order of 10 −6 Pa, and hydrogen and water molecules exist in ambient gas, excited hydrogen atom seems to be originated from hydrogen or water molecules adsorbed on the sample.As a consequence of energy transfer between HCI and surface as described before, secondary particles are released from the impact site of single HCI.Compared to the case for the irradiation with singly charged ion, yield of secondary particle is considerably high, and proton yield increases as the 5th power of charge state for the surface of hydrogen terminated Si [13].Although sputtering yield and penetration depth would depend on the kinetic energy, emission intensity itself did not depend on the kinetic energy [11].Thus the emission of Balmer series is the consequence of the potential effect in the interaction of HCI with surface.

IV. CONCLUSION
In conlcusion, we observed light emission from various samples including Er 2 O 3 irradiated with highly charged ions (HCIs).The observed charge state dependence is proved to be the potential effect in the interaction of HCI with surface.For Er 2 O 3 , emission lines from the surface layers due to transitions among Stark splitting, however, emission from atoms sputtered from the surface such as Balmer series from atomic hydrogen is also remarkably observed for all the samples.
FIG. 1. Directional relations between incident ion beam and optical axis of observation of light emission.(a) Incident angle of HCI beam is 45 • against the sample surface, (b) the direction of HCI beam is perpendicular to the sample.

FIG. 2 .
FIG. 2. Charge state dependence of the intensity of light emission from Er2O3 irradiated with Ar highly charged ions.
FIG. 4. Emission spectrum of the MOCVD film of Er2O3 irradiated with Ar 11+ ions.Intensity of each wavelength is calculated as a summation of counts at the same vertical position within the region indicated with two red lines in Fig. 3.