Potential Sputtering from Rare Gas Solid Surface
by Multiply-Charged Ion impact

SAWA Hiroyoshi and HIRAYAMA Takato
Department of Physics, Rikkyo University
Tokyo JAPANhirayama@rikkyo.ac.jp
http://s.rikkyo.ac.jp/HIRAHIRAYAMA, 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 = 21.6 eV for solid Ne )• Very small cohesive Energy (0.02 eV/atom for solid Ne)• Excited states (Exciton) can migrate through the solidgWhy RGS for sputtering/desorption study?
✓ Atoms in the solid have almost the same electronic statewith isolated atoms because of inactivity of rare gas atoms.
➡ Gas phase information can be used to explain theprimary 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
Potential Energy : PE(A ) =q+q−1n+IP(A)∑n=0q+Ar Potential Energyq1234567…

18PE (eV)

15.8

43.4

84.1

144219310434…

14k

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

Desorption of Ions

Potential Sputtering (PS)
Kinetic Sputtering (KS)HIRAYAMA, Takato, Rikkyo University

MoD-PMI 2019 @NIFS (18 - 20 June 2019)3

Experimental Apparatus
QMS

S
QM

BAG

RGS/ Cu
RGS/CuFC

Ion Beam

Mechanical Cryostat
with Heat Shield(Rotatable)

MCP

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

HIRAYAMA, Takato, Rikkyo University

to Gas Handling
System100mmTMP

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

MoD-PMI 2019 @NIFS (18 - 20 June 2019)4

Total Desorption Yield

Surprisingly Large
Desorption Yield

Absolute Desorption Yield (atoms/ion)

Total Desorption Yield of Solid Ne by Arq+ Ion Impact

6000

Ar

q+

Ne

No Dependence onCharge Stateq = 1
q = 14= 65000

4000

ir ng

3000

2000

CIET
N
KI

1000

tetu
Sp00500

1000

1500

2000

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 Arq+ 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：Neq+ (q = 1~4)
：Arq+ (q = 1 ~6)

Solid Ne Target：Krq+ (q = 2~6)Target Solid：Ar, Ne

Incident Energy：100 - 2000 eV(Thicker color : Higher energy)
Solid Ar TargetYion ∝ No. of Created Ions ∝ σion
Ion Creation: Charge Transfer(Negligible Contribution of Direct Ionization)?

Charge Transfer Cross Section: σCT
σCT(Ar) > σCT(Ne)

Ban et al., Low Temp. Phys. in press.
HIRAYAMA, Takato, Rikkyo University

Ion Desorption Yield : Yion
Yion(Ar) < Yion(Ne)MoD-PMI 2019 @NIFS (18 - 20 June 2019)7

Ion Desorption Model fro PSYion∝ No. of Created Ions ∝ σion

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

Ion Desorption with Very Large
Number of Neutral Atoms

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

Yion ∝ σ CT ×Ytotal
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

Yion ∝ σ CT ×Ytotal
Total Desorption Yield : Ytotal

Charge Transfer Cross Section : σCT
No Available Data for Solid Target
KIMURA Scaling (NICE group in NIFS)
2σ CT = 4π q / E IP

σ CT Å2( )q : Charge State of Incident Ion (q ≧ 5)
EIP : Ionization Potential of Target AtomBan et al., Low Temp. Phys. in press.
Fujita et al., J. Phys. Conf. Ser. 163, (2009) 012083.

Yion ∝HIRAYAMA, Takato, Rikkyo Universityq×Ytotal ≡ Γ
2E IP

24π q / E IP

Kimura et al.,J. Phys. B28, L643 (1995)
MoD-PMI 2019 @NIFS (18 - 20 June 2019)10

Desorption Mechanism
New Model

Absolute Ion Desorption Yield (ions/ion)

3.5

Scaling Parameter(3.0)

2Γ ≡ q E IP×Ytotal

2.5
2.0
1.5
Solid Ne target
q=5
q=6

1.0

Incident Ion: Neq+, Arq+, Krq+ (q ≧ 5)

Target Solid: Ar, Ne

Incident Energy : 100 - 2000 eV

Solid Ar target
q=5
q=60.50.0020406080100

Γ (arb. units)

HIRAYAMA, Takato, Rikkyo University

MoD-PMI 2019 @NIFS (18 - 20 June 2019)11

Observation of Desorbed Ionsin coincidence with Scattered Ions
MCP for Desorbed Ion

Charge Separator

B+Aq+A(q-n)+θ = 5!
RGS/CuSlit

MCP + PSD
for Scattered Ion

Aq+ + 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 Ar6+ → Solid Ne
TOF of Desorbed Ne ionsin coincidence with Ar ions

Reflected Arq+ Ion

yrani
im

1548

Intensity (arb. units)

300

ChargeDist.
200
1000Ang. Dist

00100200ler
P

300

102030

Coincidence with Ar

Intensity (arb. units)

Coincidence with Ar+

4050-6Flight Time (x10 s)

60

01020304050

Flight Time (x10 s)• Most of the scattered ions are Ar , with weaker signal of Ar , Ar , Ar
• Almost no signal of q > 3• Correlation of desorbed ion mass spectra with the charge state of1+60-602+3+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 yieldfrom 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