Potential Sputtering from Rare Gas Solid Surface by Multiply-Charged Ion impact SAWA Hiroyoshi and HIRAYAMA Takato Department of Physics, Rikkyo University Tokyo JAPAN hirayama@rikkyo.ac.jp http://s.rikkyo.ac.jp/HIRA HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 1
INTRODUCTION Rare Gas Solid (RGS) • van der Waals Solid • Insulator with a large band gap energy ( E g = 21.6 eV for solid Ne ) • Very small cohesive Energy (0.02 eV/atom for solid Ne) • Excited states (Exciton) can migrate through the solid Why RGS for sputtering/desorption study? ✓ Atoms in the solid have almost the same electronic state with isolated atoms because of inactivity of rare gas atoms. ➡ Gas phase information can be used to explain the primary excitation/ionization processes ✓ Rare gas solid is very fragile ➡ Excitation/ionization effectively leads to the desorption HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 2
Multiply-Charged Ion (MCI) LARGE Potential Energy q − 1 ∑ PE(A q + ) = IP(A n + ) Potential Energy : n = 0 Ar q + Potential Energy 1 2 3 4 5 6 7 … 18 q PE (eV) 15.8 43.4 84.1 144 219 310 434 … 14k MCI can ionize (take electrons from) the surface/bulk atoms Desorption of Ions using its potential energy. Potential Sputtering (PS) Kinetic Sputtering (KS) HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 3
Experimental Apparatus QMS BAG BAG QMS RGS / Cu RGS/Cu FC Ion Beam Mechanical Cryostat with Heat Shield (Rotatable) MCP to Gas Handling System Electron Cyclotron 100mm Resonance (ECR) Type Ion Source: P : <10 -8 Pa TMP NANOGAN T : < 5K 10 GHz, 100W θ : 500 - 1000 ML HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 4
Total Desorption Yield Total Desorption Yield of Solid Ne by Ar q+ Ion Impact q + Ne q + Ne 6000 6000 Ar Ar Absolute Desorption Yield (atoms/ion) Absolute Desorption Yield (atoms/ion) No Dependence on q = 1 5000 5000 Charge State q = 1 = 4 = 6 4000 4000 Surprisingly Large KINETIC Sputtering Desorption Yield 3000 3000 2000 2000 1000 1000 0 0 0 0 500 500 1000 1000 1500 1500 2000 2000 Incident Ion Energy (eV) Incident Ion Energy (eV) Fujita et al., J. Phys. Conf. Ser. 163 , (2009) 012083. HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 5
Absolute Ion Desorption Yield Absolute Ion Desorption Yield of Solid Ne by Ar q+ Ion Impact Clear Correlation between Potential Energy and Desorption Yield! Ban et al., Low Temp. Phys. in press. HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 6
Absolute Ion Desorption Yield Absolute Ion Desorption Yield of Solid Ne and Ar by MCI Impact Incident Ion ： Ne q + ( q = 1~4) ： Ar q + ( q = 1 ~6) Solid Ne Target ： Kr q + ( q = 2~6) Target Solid ： Ar, Ne Incident Energy ： 100 - 2000 eV (Thicker color : Higher energy) Y ion ∝ No. of Created Ions ∝ σ ion Solid Ar Target Ion Creation: Charge Transfer (Negligible Contribution of Direct Ionization) ? Charge Transfer Cross Section: σ CT σ CT (Ar) > σ CT (Ne) Ion Desorption Yield : Y ion Y ion (Ar) < Y ion (Ne) Ban et al., Low Temp. Phys. in press. HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 7
Ion Desorption Model fro PS Y ion ∝ No. of Created Ions ∝ σ ion Charge Transfer Cross Section: σ CT σ CT (Ar) > σ CT (Ne) Ion Desorption Yield : Y ion Y ion (Ar) < Y ion (Ne) • Coulomb Explosion Model (Bitensky and Parilis, 1989) • Defect-Mediated Sputtering (Neidhart et al., 1995) • Kinetically Assisted Potential Sputtering (Hayderer et al., 2001) • Desorption by Charge Accumulation, Defect Accumulation • Exciton Induced Desorption • etc... HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 8
Desorption Mechanism New Model MCI Ion Desorption with Very Large Ion Creation by Incident Ion Number of Neutral Atoms Ion Desorption Yield ∝ Number of Created Ions × Total Desorption Yield Y ion ∝ σ CT × Y total HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 9
Desorption Mechanism Ion Desorption Yield ∝ Number of Created Ions × Total Desorption Yield Y ion ∝ σ CT × Y total Charge Transfer Cross Section : σ CT Total Desorption Yield : Y total No Available Data for Solid Target KIMURA Scaling (NICE group in NIFS) σ CT = 4 π q / E IP 2 q : Charge State of Incident Ion ( q ≧ 5) E IP :Ionization Potential of Target Atom ) σ CT Å 2 ( Ban et al., Low Temp. Phys. in press. Fujita et al., J. Phys. Conf. Ser. 163 , (2009) 012083. 4 π q / E IP 2 Y ion ∝ q 2 × Y total ≡ Γ Kimura et al., E IP J. Phys. B 28 , L643 (1995) HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 10
Desorption Mechanism New Model 3.5 Scaling Parameter Absolute Ion Desorption Yield (ions/ion) ( ) × Y total 3.0 Γ ≡ q E IP 2 2.5 2.0 Incident Ion : Ne q + , Ar q + , Kr q + ( q ≧ 5) 1.5 Target Solid : Ar, Ne Solid Ne target q = 5 1.0 Incident Energy : 100 - 2000 eV q = 6 Solid Ar target 0.5 q = 5 q = 6 0.0 0 20 40 60 80 100 Γ (arb. units) HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 11
Observation of Desorbed Ions in coincidence with Scattered Ions MCP for Desorbed Ion Charge Separator B + A (q-n)+ A q+ θ = 5 ! Slit MCP + PSD RGS/Cu for Scattered Ion A q+ + B (solid) → A (q-n)+ + nB + (solid) B + (desorption) Sawa et al., in press HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 12
Preliminary Results 3 keV Ar 6+ → Solid Ne TOF of Desorbed Ne ions Reflected Ar q + Ion in coincidence with Ar ions Preliminary Coincidence with Ar + Coincidence with Ar 1548 Intensity (arb. units) 300 Intensity (arb. units) Charge Dist. 200 100 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Ang. Dist 0 -6 s) -6 s) Flight Time (x10 Flight Time (x10 0 100 200 300 • Most of the scattered ions are Ar 1+ , with weaker signal of Ar 0 , Ar 2+ , Ar 3+ • Almost no signal of q > 3 • Correlation of desorbed ion mass spectra with the charge state of scattered ions HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 13
SUMMARY • We have succeeded in measuring the ABSOLUTE ion desorption yield from solid Ne and Ar by multiply-charged ion impacts. • New ion desorption model has been proposed : Contribution of both Kinetic and Potential energy of incident ions • Coincidence measurements of desorbed ions with the scattered ion are now in progress. HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 14
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