2 - Instituto de Estructura de la Materia

iem.cfmac.csic.es

2 - Instituto de Estructura de la Materia

Cold Molecular Plasmas.

Highly Reactive Systems

at Low Temperature.

1

Isabel Tanarro

Dept. de Física

Molecular

Inst. de Estructura

de la Materia,

CSIC

itanarro@iem.cfmac.csic.es


Scheme of the talk

2

• Brief introduction to plasmas:

– Historical outline

– Main Plasmas’ Properties

– Kinds of Plasmas

• Characteristics of cold molecular plasmas.

• Plasmas’ reactivity in low pressure discharges.

• Motivation & description of recent experiments in our Lab.

• Kinetic models.


Plasma

4 th State of Matter

3

Predicted by Michael Faraday (1820)

Identified by William Crookes (1880)

in low pressure electric discharges

when studying the cathode rays (e - )

W. Crookes

1832-1919


4

Heinrich Geissler

(1814 - 1879)

Geissler Tube

Discharges

Most Representative Plasmas’ Characteristic:

Their Luminosity ! ⇒ Light Emission


Discharge Cell & Vacuum Pump

(1880)

P = 0,1 Pa = 10 -6 atm !

2,5 · 10 19 molec / m 3 5


6

Plasma

Nombrado así por Langmuir (1920)

Materia gaseosa fuertemente ionizada, con igual

número de cargas libres positivas y negativas”

Diccionario de la Real Academia

de la Lengua Española

Irving Langmuir

1881-1957

Nobel Prize, 1932

Grado de Ionización n = [cargas[

± ] / [neutros] ~ ( 1 – 10 –6 )

Puede darse a presiones de gas muy diferentes


7

COLD PLASMA vs. IONIZED GAS ?

Particle Energy (eV)

10 -2 10 -1 10 0 10 1

GASES

PLASMAS

10 2 10 3 10 4 10 5

Temperature (ºK)


8

Gases

• Independent Particles

• Energy Transfer by Individual Collisions

Plasmas

• Collective Behaviour !!!

• Electro-Magnetic Forces >> Dynamic Forces

UNESCO: 2204- Física de Fluidos –(10) Física de Plasmas

A. Lieberman, A. J. Lichtenberg, Principles of Plasma Discharges

and Material Processing, Wiley, New York 1994.

A. Grill, Cold Plasma in Materials Fabrication, IEEE Press, New York, 1993.

J. A. Bittencourt, , Fundamentals of Plasma Physics, Springer, New York 2004


Fundamental Magnitudes of Plasmas:

N e : Electron Density (= N ion, ≠ N gas )

T e : Electron Energy (≠ T ion , ≠ T gas )

Assuming Maxwell e - energy distribution

& Boltzmann & Saha distributions for neutrals & ions.


T (K)

10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9

12

Solids, Liquids

& Gases

Jupiter core

10 30

Si crystal

Solar core

10 25

Solar

photosphere

Laser focus

10 30 10 10

PLASMAS

10 25

N e

( m - 3 )

10 20

10 15

10 10

Flame

Arc

Glow

Ionospheres

Auroras

Cold

Plasmas

Plasma

Nebula

LLightning

Fusion

plasma edge

Solar

corona

Fusion

reactor core

Interplanetary

space

10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5

E (eV)

10 20

10 15


“COLD” Plasmas

Systems far from thermal equilibrium !

13

T e

>> T N ,T N+ ↔ Low Ionization Degree

Main Interest :

“High-Temperature” Chemistry

via plasma electrons

at “Ambient Gas” Temperatures.

“REACTIVE PLASMAS”

Thin films, sterilization, surface conditioning

of thermo-sensitive materials

Astrophysics, planetary ionospheres, fusion reactors…

F. Gordillo , V. Herrero, I. Tanarro,

Chem. Vapor Deposition (2007, review, in press)


Subject of Interest of This Talk:

Cold Plasmas in Low Pressure Discharges

14

Energy (eV)

1

0.1

0.01

T gas

T e

10 5

10 4

10 3

10 -4 10 -2 10 0 10 2 10 4 10 2

Pressure (Torr)

Temperature (K)

Very Recent Interest:

Microplasmas ( Low T e and N e at ~ Atm. Pressure ! )


DC & Low Frequency Modulated Discharges

( need of electrodes )

15

_

e - +

• Primary Ionization: [N e ] ∼ 10 3 cm -3

• Electron Multiplication

• Secondary electron emission

from the cathode

Glow discharges without electrodes

are used very often at RF & MW frequencies !

Magnetically confined fusion reactors.

Thin film deposition…


Initial Processes

Produced by electron impact

16

• Ionization AB + e – → AB + + 2e –

• Excitation AB + e – → AB * + e –

• Dissociation AB + e – → A + B + e –

But… kinetic energy transfer

from electrons to heavier particles

by elastic collisions is very inefficient!

E

Max

transmited

=

4m

( m +

e

e

M

M

) 2

E

initial

Low gas temperature !!!


In contrast, kinetic energy transfer

is very efficient among electrons,

which gain energy from the electric field !

17

High electron temperature !

e - energy

distributions

σ 1

σ 2

cross

sections

excitacion,

dissociation,

ionization

rate coefficients

k ~ 10 -10 cm 3 s -1


18

Secondary Processes:

• Deexcitation of energy levels ⇒ Light Emission

• Reactions in Gas Phase

• Heterogeneous Reactions

⇒ New Products

M. M. Sanz, I. Tanarro, Recent Res. Devel.

Physics Series (2007, in press)


Reactions in Gas Phase

19

• P Low

⇒ Very low probability of three-body reactions in gas phase !

A+B +C ↛ AB +C k ·[C] ∼ 10 -16 –10 -12 (cm 3 molec -1 s -1 )

• T gas Low

⇒ AB + C → AC + B Very Inefficient for “stable” species ( 10 -9 (cm -3 molec -1 s -1 )


Heterogeneous Processes

• Molecular recombination in the wall

• Physical & Chemical Sputtering

• Deposition processes

• Neutralization of ions

• Secondary electron emission

Large number of species !


Difficulties in plasma diagnostics

21

Stable

Species

Etching Products

• EXTENSIVE : large number of species with

very different energetic and spatial distributions.

• ACCURATE : absolute values [X i ], [X + i], [e - ], T e , T x

• SENSITIVE : many different orders

of magnitude (stable vs. unstable species)

10 16 T (eV)

10 14 Electrons

N (cm -3 )

10 12

Transient

Neutrals

Specific techniques

for each species !

Ions

Cathode

Excited Species

Ions (DC)

10 8

0.01 0.1 1 10 100

10 10


Difficulties in Plasma Modeling

22

– Self-consistent treatment of particles kinetics, gas

dynamics, wave electrodynamics and plasma–wall

interactions.

– Lack of reliable Reaction Cross Sections, specially for

electron impact dissociation and ionization at low

electron energies.

– Lack of data on Wall Reaction Probabilities, largely

dependent on surface conditions.

– Need of simplifying to assign the main mechanisms.


23

Cold Plasmas studied by us during last years :

H 2 , N 2 , CH 4 ,

N x O y (NO , N 2 O, NO 2 ),

Air

Pure or in Mixtures


Experimental Set-up

“Hollow

Cathode

Discharge Reactors

+

Electron gun to ignite the

plasma at lowest pressures

24

Geometry design detemines

Negative Glow Volume

( plasma volume, , E = 0 )

Very effective energy losses

of Ion & Neutral species

in the cathode (T(

gas ~ 300 K)

=

-HV Electron Gun


Diagnostic

Techniques

25

Double Langmuir Probes

Visible Emmision

Spectroscopy

(& Actinometry)

=

Quadrupole Mass

Spectrometry of Neutrals

(with

e- impact ionization)

and Ions (with

ion

energy resolution)

-HV Electron Gun


26

Detail of the

discharge

Laboratory of Cold Plasmas,

Molecular Physics Department, , IEM, CSIC


Experimental set-up (small reactor)

27

Absorption

& Emission

Visible + IR

Langmuir

Probe + QMS


H 2 plasmas : e - , H , H 2 , H + , H 2+ , H 3

+

(in ground and excited levels)

28

Dense Interstellar Clouds:

Sites of Star Generation

T gas = 10 K

N = 10 4 cm -3

H 2 dominant

H 2 formation ?

H + H → H 2

not possible…

except in dust surfaces !


H 3

+

First Lab. . IR spectrum, , 1980 by Oka.

Found in Space & Ionospheres

of Jupiter & Gigant Planets, , 1984.

29

H 3+ formation:

H 2 + Cosmic Rays → H 2+ + e −

H + 2 + H 2 → H + 3 + H

k = 2.6 × 10 -9 cm 3 s -1

T Oka, Phys. Rew. Lett. 45, 531 (1980)

P Drossart et al, Nature 340, 539 (1989)


H

+

3 : Key in interstellar formation of …

30

H 2 O

H + 3 + O → HO + + H 2

H n O + + H 2 → H n+1

O+ + H

H 3 O + + e − → H 2 O + H

HCO +

N 2 H +

H + 3 + CO → HCO + + H 2

H + 3 + N 2 → N 2 H + + H 2

Nitrogen : 5 th Abundance in Universe .

N 2 : non IR Vib-Rot

Transitions.

IR Spectra of N 2 H + , HCO + & CO ⇒ N 2 in dark clouds

S Maret et al, Nature, 442, 425 (2006)


Interstellar Formation of Hydrocarbons

H 3+ + C CH + + H 2

31

CH n+ + H 2 CH n+1+ + H

All these proceses are Non Barrier Reactions !

with Large Rate Constants > 10 9 (cm 3 molec -1 s -1 )

Independent of Temperature !

The whole kinetics is not well understood

Laboratory studies are worthwhile


N e & T e Measurement in H 2 discharges:

Double Langmuir Probe

32

N e / 10 10 (cm -3 )

T e (eV)

4

2

0

10

5

Experiment

Model

N e ∼ ( 1 - 3 ) · 10 10 cm -3

E e

∼ ( 2 - 8 ) eV

⇒ T e = (2 – 9)·10 4 K

0

0,01 0,1

H 2 Pressure (mbar)


T gas & H 2 dissociation: Emission Spectroscopy

From Emission Lines of Fulcher-α H 2

System

& Collisional Radiative Models

T rot (H 2 ) = T trans ∼ 300 K

250

200

H 2

discharge

0.02 mbar, 100 mA

H α

33

Z Qing et al,

J. Appl. Phys. 80, , 1312 (1996)

T vib (H 2 ) ∼ 3000 K

Relative Intensity

150

100

50

H 2

Fulcher-α band

B. P. Lavrov et al,

J. Phys. Rev. E, 59, , 3526 (1999)

0

6000 6100 6200 6300 6400 6500 6600

Wavelength ( A )

From different contributions to

Balmer I Hα

& I Hβ

Intensities of H 2

& H

Far from thermal equilibrium

Ratio H /H 2 ~10 % B.P. Lavrov et al,

Plasma Sources Sci. Tech.12, 576 (2003)


Ion Densities : Mass Spectrometry

34

1,0

H2, 0.008 mbar

Model

Exper.

1,0

H2, 0.2 mbar

Model

Exper. H + 3

0,5

H + H + 2

H + 3

0,5

H + H + 2

0,0

1 2 3

Mass (a.m.u.)

0,0

1 2 3

Mass (a.m.u.)

Dramatic change of relative ion concentrations with pressure!

Méndez, Herrero, Tanarro, Gordillo, J. Phys. Chem. A, 110, 6060 (2006)


H 2 Model

35


Global Results, , H 2 Discharges

36

Neutral, Ion & e - Concentrations (cm -3 )

10 16

10 15

10 14

10 13

H

10 12

e -

10 11

10 10

10 9

10 8

H 2

H + 3

H +

H 2

+

0.01 0.1

H 2 (mbar)

γ = 0.03

Neutrals

e - & Ions

Good agreement Model-Experiment

Neglecting ions doesn’t change predicted neutral’s behaviour


Ratio H / H 2

depends on


H 2 + e 2 H + e

k(T e )

2 H + wall H 2

Model Predictions ⇒

depending on wall

recombination γ SS

Concentración

10 15 MODEL

10 14

P(H 2 )=0.008 mbar

10 13

Te = 8 eV

γ ss γ

Residence time = 1 s ss =0.03 s -1

10 12

10 -4 10 -3 10 -2 10 -1 10 0

T e

= 8 eV

Gamma ( γ )

γ ss = 0.03 s -1 ⇒ Tserepi & Miller, Appl. Phys. 75, 7231 (1994)

Méndez, Herrero, Tanarro, Gordillo,

J. Phys. Chem. A, 110, 6060 (2006)

γ ss = 0.2 s -1 ⇒ Kae-Nune et al. J. Surf. Sci. Letters. L495. (1996)

H2

H


Experimental results

37


Experiment vs. Model for Ions

38

1

Relative Ion Density

γ = 0.03

H 2+

+ H 2

→ H 3+

+ H

H2 + H3 +

H +

0

0.01 0.1

P(H2) (mbar)

With increasing pressure, mean free path decreases

below the negative glow dimensions and H 3+

conversion gains efficiency


Jupiter Ionosphere (model)

H 2 Discharge

39

Agreement with

H 3+

IR detection

(Pioneer)

Concentration (cm -3 )

5.0x10 14

4.5x10 14

1.0x10 14

5.0x10 13

0.0

P(H2) =0.02 mbar

H 2

H

H 3

+

increases

with pressure

Concentration (cm -3 )

H + 3

1x10 10

H + 2

H +

0

10 -9 10 -7 10 -5 10 -3 10 -1

Time (s)

Initiated by Radiation

Pressure ~ μbar

Characteristic dimension (Altitude) ~ 1000 km

Characteristic Evolution Time ~ 100 - 1000 y

Initiated by Electrons

Pressure ~ μbar

Characteristic dimension : 1-10 cm

Characteristic Time : 10 -8 –10 -4 s


H 2 + N 2 Discharges

Ion composition :

Large changes with pressure!

4

1x10 0 H 2

NH 3

N 2

Relative Concentration

1x10 0 mass (a.m.u.)

H 2 +N 2 (7%)

Neutrals

0.008 mbar

Plasma Off

Plasma On

0

0 5 10 15 20 25

H 2

0.02 mbar

Relative Concentration

0

NH 3

N 2

0 5 10 15 20 25

mass (a.m.u.)

5x10 -1 H 2 +N 2 (7%)

0.008 mbar

Ions

N 2

H +

N +

Relative Concentration

NH 3

+H 2+

→ NH 3+

+H 2

k= 5.7·10-9

NH 3

+H 3+

→ NH 4+

+H 2

k= 4.4·10-9

N 2

+ H 3+

→ N 2

H + + H 2

k= 1.9·10-9

N 2

+ H 2+

→ N 2

H + + H k= 2.0·10 -9

N 2 H + , NH 4+ : very stable, do not react with H 2 .

0

NEUTRALS NH + 3

NH 3 : Heterogeneous

formation

mass (a.m.u.)

5x10 -1 NH + 4

Very small change with

0.02 mbar

pressure of chemical

compositionNH + N 2

H +

H + 3

H + 3

2

H + N + N + 2

Relative Concentration

0

H + 2

H + 3

H + NH + 4

N + 2

0 10 20 30

0 10 20 30

mass (a.m.u.)


41

H 2 + CH 4 + N 2 Discharges

Interest in :

• Astrophysics

• Amorphous & Diamond-like Thin C-H Films

• Nanoparticles & Nanotubes

• Plasma-Wall interaction in Fusion Reactors


42

Fusion by

Magnetic Confinement

Tokamak ASDEX-U

(Garching)

Joint European Torus

(J.E.T, U.K.)


PLASMA EDGE: Strong Plasma-Material Interaction

43

Tokamak Poloidal Cross-Section

CORE

∼10 8 K

D + T

Magnetic Flux

Surfaces

Reactor

Surfaces

Interaction of the “Cold” edge plasma

of Hydrogen ( D + T )

with Carbon Surfaces (Graphite)


Deposits of Hydrocarbons rich in

Radioactive Tritium

(T 1/2

=12.4 years)

(T 1/2

Cold

Plasma

G. Federici et al., Nuclear Fusion 41,1967 (2001)


44

Graphite Tile showing evidence for Hydrocarbon Film Formation

( Up to 250 nm thick. Typically 2-32

3 nm s - 1 )

It should be avoided !


45

N 2 recently proposed

to reduce a-C:T deposition

in Fusion Reactor !

Previous attemps to synthesize crystalline β-C 3 N 4

(superior to diamond) and amorphous C:H:N films.

But it was found that nitrogen hampers or even avoids

a-C:H & a-C:H:N film deposition !


H 2 + CH 4 vs. H 2 + CH 4 + N 2 discharges:

Ions

Relative Ion Concentrations

0,4

0,3

0,2

0,1

0,0

CH + 5

CH + 3

NH + 3

NH + 4

C 2

H + 3

N 2

H +

C 2

H + 5

IO N S

H 2 +CH 4 (5%)

H 2 +CH 4 (5%)+N 2 (5%)

CH 3

CNH +

15 20 25 30 35 40

M ass (a.m .u)

• Very stable N x H y+ ions may compete with C x H y+ formation

• Only one mixed C-N ion detected.

46

Tanarro, Herrero, Islyaikin, Méndez, M

Tabarés, , Tafalla,

J. Phys. Chem. . A (send(

to publish)


H 2 + CH 4 vs. H 2 + CH 4 + N 2 discharges:

Neutrals

47

CH 4

C 2 H x

QMS

spectra

CH 4

C 2 H x

Traces of N 2 increase C 2 H x concentration in gas phase !

Tabarés, Tafalla, Tanarro, Herrero, Islyaikin, Maffiotte,

Vacuum 73, 161 (2004)


Decrease in Film Thickness with N 2 / CH 4 ratio

48

Increment of QMS signal

H 2

+CH 4

(5%)+N 2

(5%)

Film Thickness (nm)

Increase of C 2 H x

with N 2 /CH 4 ratio

Decrease of

Film Thickness

• Sputtering ?

• Scavenging ?

N 2

/ CH 4

Mechanisms of Film Inhibition by Nitrogen are not clear !

Tabarés, Tafalla, Tanarro,Herrero, Islyaikin

Plasma Phys. & Controlled Fusion 44, L37 (2002)


Intensity (counts / s)

10 3

49

Other plasmas containing N 2 +CH 4 :

Titan Ionosphere

Titan: Only moon

1.5 Atm, 94 K

90-95% N 2 , 0.5-4% CH

in Solar System

4

3-12% Ar , 0.1% H 2

with dense atmosphere Dust

Organic reactions?

Titan Ionosphere

CH 3 CNH +

H 2

+N 2

H

C 2

H + 2

+N 2

+ CH 4

4

N

H 2

+CH 4

CH 3 CNH +

Cassini-Huygens

Mission to Saturn (2004/5)

10 4 +

C 3

H 5

+

C 3

H 7

C 3

H 3

+

Altitude (km)

35 40 45

Ions (a.m .u)

Ionic Fraction


50

N x O y ( NO, N 2 O, NO 2 )

& Air ( N 2 + O 2 )

Plasmas

Useful in :

• Plasma assisted Catalytic Formation of N x O y

• Plasma Sterilization at Low Temperature

• Microelectronics...


Earth Atmosphere


Earth Troposphere

& Stratosphere

52 52

N x O y

Atmospheric Cycle

Troposphere & Stratosphere

Steinfeld J I et al, Chemical Kinetics & Dynamics, Prentice Hall (1999)


Earth Ionosphere

53

Ion Density

distribution

Plasma initiated by

incident photons

and cosmic rays

Altitude (km)

F region

O 2

+

NO +

E region

N 2

+ e -

O 2

+

NO +

Ion density (cm -3 )

Wayne R P 2000 Chemistry of Atmospheres ,Oxford University Press


FTIR absorption: N 2 O discharge 54

FTIR Absorbance Spectra of a N 2

O DC discharge

N 2 O dissociation ⇒ NO + NO 2

Detection of N 2

O decrease and NO and NO 2

form ation

1 mbar, 36 sccm, 40 mA

Spectral Absolute resolution:0.25 Concentrations cm -1

IR spectroscopy

plasma off

D i s c h a r g e O

f f

ν 3

N 2

O

W a te r tra c e s

ν 1

+ ν 2

N 2

O

ν

N O

3

N O 2

D i s c h a r g e O n

plasma on

1550 1600 16 50 1700 1750 1800 1850 19 00 1950

2 ν 2

N 2

O

ν 1

N 2

O

Red:Discharge ON

Black:Discharge Off

Spectral resolution:

2 cm -1

ν 1

+2ν 2

N 2

O

2 ν 1

N 2

O

1000 1500 2000 2500

W avenumber (cm -1 )


P (mbar)

0.6

0.3

0.0

0.6

0.3

0.0

0.6

0.3

0.0

0.01

0.00

NO 2

O 2

N 2

NO

N2 O

0 3 6 9

time (s)

NO 2 Modulated Discharge

Time resolved QMS

Dissociation of NO 2

&

O 2 , N 2 , NO, N 2 O formation

Good Agreement

Experiment - Model

55

M. Castillo, V. J. Herrero,

I. Méndez, I. Tanarro

Plasma Sources Sci. Tech. 13, 39 (2004)


AIR discharge

56

Air, 0.007 mbar , 100 mA

Molar Fraction

0.9

0.6

0.3

N 2

Exper.

Model

Neutrals

NO O 2

N 2

+

NO + Ions

O 2

+

3x10 -5

2x10 -5

1x10 -5

[ NO ] ∼ [ O 2 ]

Heterogeneous

NO formation

[ NO + ] > [ N 2+ ]

0.0

28 30 32

Mass (a.m.u)

28 30 32

Mass (a.m.u)

0

NO + Major Ion,

like in the E region

of Earth Ionosphere

M. Castillo, I. Méndez, A. Islyaikin,

V. J. Herrero, I. Tanarro,

J. Phys. Chem. A, 109, 6255 (2005)


Transients and Steady States of

N x O y & Air Modulated Discharges

+

Comparison with an Unified Model


k Dissociation of N x O y at low T e (not known)

&

Wall Reaction Probabilities for N x O y formation

&

Assignment of the main processes

57

I. Tanarro & Collaborators

J. Phys. Chem. A 104, 8183 (2000)

J. Phys. Chem. A 104, 3974 (2000)

Vacuum 64 (3-4), 457 (2002)

Plasma Sources Sci. Tech. 13, 39 (2004)

Plasma Sources Sci. Tech. 13, 343 (2004)


Kinetic

Model

N x O y

&

N 2 + O 2


Low pressure Air Plasmas

59

N 2

10 -1

NO O 2

O 2

NO

Molar Fraction

10 0 *

CHARGES

10 -2

10 -3

10 -4

10 -5

10 -6

*

e −

N

+

2 NO + +

O 2

*

*

Sheath Collisions

0.003 0.01

0.05

Pressure (mbar)

*

NEUTRALS

Experiment

& Model

M. Castillo, I. Méndez, A. Islyaikin, V. J. Herrero,

I. Tanarro, J. Phys. Chem. A, 109, 6255 (2005)


SUMMARY & CONCLUSIONS

• Brief introduction to plasma properties.

• Brief description of different kinds of plasmas.

• Characteristics of cold (very reactive) molecular plasmas.

• Outline of some applications and fields of interest.

• Description of recent experiments in Lab. of Cold Plasmas.

• Descripition of the kinetic models developed, showing:

• Relevance of barrierless homogeneous reactions

• Relevance of surface reactions, increasing at low pressure.

• Neutral chemistry not affected by ions in present cases.

• The models fit very well the experimental results.

60


THE TEAM

61

RESEARCHERS

Concepción Domingo

Víctor José Herrero

Francisco Gordillo*

Francisco Tabarés**

David Tafalla**

Isabel Tanarro

* Inst. de Óptica, CSIC

** Asociación EURATOM - CIEMAT

PhD. STUDENTS

Maria del Mar Sanz

Teresa de los Arcos

Marco Castillo

Isabel Méndez

TECHNICIANS

José Manuel Castillo

Miguel Ángel Moreno

Javier Rodríguez


Thanks a lot

for your attention !

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