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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

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

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

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

INTRODUCTION

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Rare Gas Solid (RGS)

  • van der Waals Solid
  • Insulator with a large band gap energy ( Eg = 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

✓ 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

Why RGS for sputtering/desorption study?

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

Multiply-Charged Ion (MCI)

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LARGE Potential Energy

PE(Aq+) = IP(An+)

n=0 q−1

∑

Potential Energy : q 1 2 3 4 5 6 7 … 18 PE (eV) 15.8 43.4 84.1 144 219 310 434 … 14k

Arq+ Potential Energy

MCI can ionize (take electrons from) the surface/bulk atoms using its potential energy. Desorption of Ions

Kinetic Sputtering (KS) Potential Sputtering (PS)

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

Experimental Apparatus

P : <10-8 Pa T : < 5K θ : 500 - 1000 ML

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100mm

RGS/Cu to Gas Handling System

TMP Ion Beam BAG

FC Mechanical Cryostat with Heat Shield

(Rotatable)

MCP

QMS QMS BAG

RGS / Cu

Electron Cyclotron Resonance (ECR) Type Ion Source: NANOGAN 10 GHz, 100W

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 5 6000 5000 4000 3000 2000 1000

Absolute Desorption Yield (atoms/ion)

2000 1500 1000 500

Incident Ion Energy (eV) q = 1

Ar

q+ Ne

6000 5000 4000 3000 2000 1000

Absolute Desorption Yield (atoms/ion)

2000 1500 1000 500

Incident Ion Energy (eV) q = 1 = 4 = 6

Ar

q+ Ne

Total Desorption Yield of Solid Ne by Arq+ Ion Impact

Fujita et al., J. Phys. Conf. Ser. 163, (2009) 012083.

No Dependence on Charge State Surprisingly Large Desorption Yield KINETIC Sputtering

Total Desorption Yield

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 6

Absolute Ion Desorption Yield of Solid Ne by Arq+ Ion Impact

Absolute Ion Desorption Yield

Clear Correlation between Potential Energy and Desorption Yield!

Ban et al., Low Temp. Phys. in press.

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 7

Incident Ion ：Neq+ (q = 1~4) ：Arq+ (q = 1 ~6) ：Krq+ (q = 2~6) Target Solid ：Ar, Ne Incident Energy：100 - 2000 eV (Thicker color : Higher energy) Solid Ar Target Absolute Ion Desorption Yield of Solid Ne and Ar by MCI Impact

Absolute Ion Desorption Yield

Solid Ne Target

Ban et al., Low Temp. Phys. in press.

Ion Desorption Yield : Yion Yion(Ar) < Yion(Ne) Charge Transfer Cross Section: σCT σCT(Ar) > σCT(Ne)

?

Yion ∝ No. of Created Ions ∝ σion Ion Creation: Charge Transfer
 (Negligible Contribution of Direct Ionization)

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

Ion Desorption Model fro PS

  • 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...

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Ion Desorption Yield : Yion Yion(Ar) < Yion(Ne) Charge Transfer Cross Section: σCT σCT(Ar) > σCT(Ne) Yion ∝ No. of Created Ions ∝ σion

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 9

New Model MCI Ion Creation by Incident Ion Ion Desorption with Very Large Number of Neutral Atoms

Ion Desorption Yield ∝ Number of Created Ions × Total Desorption Yield

Desorption Mechanism

Yion ∝ σ CT ×Ytotal

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 10

Ion Desorption Yield ∝ Number of Created Ions × Total Desorption Yield

Yion ∝ σ CT ×Ytotal

Total Desorption Yield : Ytotal

Ban et al., Low Temp. Phys. in press. Fujita et al., J. Phys. Conf. Ser. 163, (2009) 012083.

Charge Transfer Cross Section : σCT

KIMURA Scaling (NICE group in NIFS)

σ CT = 4πq /EIP

2

q : Charge State of Incident Ion (q ≧ 5) EIP :Ionization Potential of Target Atom

4πq /EIP

2

σ CT Å2

( )

Yion ∝ q EIP

2 ×Ytotal ≡ Γ

No Available Data for Solid Target

Desorption Mechanism

Kimura et al.,

  • J. Phys. B28, L643 (1995)
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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019) 11

Incident Ion : Neq+, Arq+, Krq+ (q ≧ 5) Target Solid : Ar, Ne Incident Energy : 100 - 2000 eV

Desorption Mechanism

New Model

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Absolute Ion Desorption Yield (ions/ion)

100 80 60 40 20

Γ (arb. units)

Solid Ne target q = 5 q = 6 Solid Ar target q = 5 q = 6

Γ ≡ q EIP

2

( ) ×Ytotal

Scaling Parameter

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

Observation of Desorbed Ions in coincidence with Scattered Ions

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Aq+ B+

RGS/Cu MCP for Desorbed Ion

A(q-n)+

Charge Separator MCP + PSD for Scattered Ion Aq+ + B (solid) → A(q-n)+ + nB+ (solid) B+ (desorption) Slit

θ = 5!

Sawa et al., in press

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

Preliminary Results

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3 keV Ar6+ → Solid Ne

Reflected Arq+ Ion TOF of Desorbed Ne ions in coincidence with Ar ions

300 200 100 300 200 100 1548

Charge Dist.

  • Ang. Dist

Intensity (arb. units)

60 50 40 30 20 10

Flight Time (x10

  • 6 s)

Coincidence with Ar+ Coincidence with Ar

Intensity (arb. units)

60 50 40 30 20 10

Flight Time (x10

  • 6 s)
  • Most of the scattered ions are Ar1+, with weaker signal of Ar0, Ar2+, Ar3+
  • Almost no signal of q > 3
  • Correlation of desorbed ion mass spectra with the charge state of

scattered ions

Preliminary

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HIRAYAMA, Takato, Rikkyo University MoD-PMI 2019 @NIFS (18 - 20 June 2019)

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.

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