Topics CoveredTopics Covered

(1)

Ultra

Ultra--cold Reactionscold Reactions..

Richard D. Thomas

Topics Covered Topics Covered

Dissociative Recombination

What is it and What do we want to learn ? What are the most instructive Techniques ? Investigation

Results

From experiments, what do we Obtain and Learn?

What can we now Ask?

Introduction

Molecules. Why Molecules ? What do these molecules do ? Why is that important?

Conclusions

The Future

Merged

Merged--beams Tools for the Study of Ultrabeams Tools for the Study of Ultra--cold Reactionscold Reactions..

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Molecules are present in many environments, from …

Knowledge on how molecules interact and react is vital to understanding these environments.

Introduction:

Introduction: Molecules. Why Molecules

to

large small

to

hot cold

to

life on earth life beyond Molecules and philosophy …

• How do molecules determine life?

• Are amino acids in space?

• Molecules arriving on comets and meteorites?

How are they involved in …

• ice formation?

• aurorae?

• pollution?

How can molecules …

•Raise combustion efficiency?

•Make combustion cleaner?

•Influence nuclear fusion?

How are molecules …

• created?

• destroyed?

How do they influence …

• Protein folding and structure?

• Radiation induced cell damage?

• DNA fragmentation?

• Drug efficiency?

How can molecules …

• form clouds ?

• probe temperature?

• effect isotopic abundance?

Planetary Atmospheres Molecular Clouds Combustion & Flames

Barrierless reactions; ion-neutral & e^--ion Low temperature:

High Level of Ionisation: Stellar Radiation, Cosmic Rays & Heat Low density: Binary reactions only

What about these environments – a few examples

Introduction Introduction

Merged

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GAS PHASE GAS PHASE

ION REACTIONSION REACTIONS

Types Of Reaction

Resonant Ion-Pair Formation Excitation

Ionization

Associative Attachment Charge/ Ion Exchange Neutralisation

Charge Transfer Dissociative Charge Transfer

Ion-Atom Exchange Association

Non Dissociative Attachment Dissociative Electron Attachment

Ion-Neutral Ion-Ion Electron-Neutral

Electron-Ion

Laboratory Techniques

Introduction:

Introduction: ReactionsReactions

Afterglows Flow Tubes

X-/Merged Beams Traps

Ion Mass SpectrometerImportant process of ionisation balance

and production of reactive radicals in these regions:

Reactions of molecular ions with electrons

Dissociative Recombination Dissociative Recombination

The electron feels attraction of the ion A⁺ A⁺

e^-

[A]**

and is captured!

Special Case if A is a molecule.

FRAGMENTATION

A^* + B + Energy

UNSTABLE!

TOO MUCH ENERGY Lose electron

Autoionisation A⁺ + e^-

Emit Photon Radiative Decay A + γ

A^*+ B + C + Energy AB + C + Energy A + BC + Energy AC + B + Energy

Dissociative Recombination:

What is it - I?

[AB]** [ABC]**

τ~ 10^-11 s

τ ~ 10^-11s τ ~ 10^-7s

Merged

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What is it - II?

Potential Energy

Internuclear separation

AB⁺

AB^* AB^**

AB⁺

AB^* AB^**

e^-

Capture

Dissociation

Direct Mechanism Indirect Mechanism

e^-

Capture Autoionisation

Dissociation Autoionisation

What is it - III?

AB⁺ + e^- ^⎯→ ^…

A + B^* + e^- + KE₂ A^* + B + e^- + KE₃

AB⁺ AB AB^**

A^* + B

A + B A + B^*

∞

KE₂

KE₃ KE₁

A + B + e^- + KE₃

Ground and Excited State Fragments

Ground State Fragments Final state depends on

many factors, and exit channels compete.

Merged

(5)

… in diffuse interstellar clouds

Dissociative Recombination …

What do we want to Learn?

H₃⁺

DR rate of H₃⁺ is

fundamental

DR important for observed

neutrals

H₂ H₂⁺ H₃⁺ H

Cosmic

ray 10⁹ years

H₂⁺ H₂

2 months Observe

ν₂

0 ortho para

McCall, Geballe, Hinkle and Oka, Science279, 1910 (1998)

Density of H₃⁺

Length of cloud Column density

DR Electron density Cosmic ionization

rate

n N

L k

n

e

n e

( ) ( ) ( )

H H H ( )

3

+ 3

+

= = ς

2

H₃⁺column density

DR

k

_e

=

Length cloudof

Cosmic ionization

rate

L ς ^N( ^H

₃⁺

⁾

Electron column density

N( H

₂⁺

) N e ( )

H₂⁺column density

Merged

(6)

… in planetary atmospheres

In the ionosphere:

DR of O₂^+:

O₂⁺+ e^-→O(¹D) + O ? O(¹S) + O ?

In the atmosphere:

DR of N₂⁺

14N¹⁵N⁺ + e^-→

15N + ¹⁴N + KE m(¹⁵N) > m(¹⁴N)

∴ v(¹⁴N) > v(¹⁵N)

Isotope Fractination

O₂⁺(ν,n,l) ? State effects

In the ionosphere

green skyglow

red skyglow

14N:¹⁵N = 6x10^-3 1.62 x Earth!

Accurate Models

DR rate & product info. is VITAL

AXY⁺ + e^- →AX + Y + KER₁

→AY + X + KER₂

→A + XY + KER₃

→A + Y + X + KER₄

What do we want to Learn - Polyatomics?

Early Theory AXY⁺ + e^- →AX + Y + KER₁

Storage Ring Results

60-80% -

Complete fragmentation

AXY⁺ + e^- →

→A + Y + X + KER₄

WHY and HOW?

Kinetic Energy Release (eV)

0 1 2 3 4 5 6 7

3-body breakup (%)

0.0 0.2 0.4 0.6 0.8 1.0

^XH₂⁺^:

Heavy X atom, light H atoms Merged

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What are the most instructive Techniques?Investigation

Experimental Requirements High Beam

Energy Good Vacuum Low Background

High Energy

Low c.m. energy Multi-pass Good Data Rate

CRYRING

Ion Storage Ring

Neutral products Source gas

Ion beam Electrons reactionDR

Movie

Electron cooler

Neutral fragments Cathode &

superconducting magnet

BeamIon

Dipole bending magnets Toroidal bending magnets

Anode

DR Reaction Merged

(8)

What are the most instructive Techniques - I?Investigation

spectrum dependent on branching

ratio and T

Branching ratio

all events lead to full mass signal

Info loss-

XYZ⁺beam e^-beam

Neutrals XYZ⁺

Dipole Magnet Electron Cooler

Reaction region XYZ, XY, XZ …

0^oDetection Arm

Detectors XYZ⁺

with grid

XYZ⁺beam e^-beam

XYZ⁺

Electron Cooler

Reaction region

XYZ⁺ Neutrals

XYZ, XY, XZ …

Surface barrier detector XYZ⁺beam

e^-beam

XYZ⁺

Electron Cooler

Reaction region

XYZ⁺

Solve using Solve using Grid Technique Grid Technique

Neutrals

XYZ, XY, XZ …

Probability T(1-T)

Detectors

T=0.3Grid

Grid

Mass/Energy sensitive detection

Surface Barrier Detector

60 mm

XYZ⁺beam e^-beam

Neutrals XYZ⁺

0^oDetection Arm

All events @ full mass signal

Rel. Energy (eV)

+1 0 -1

Cathode Voltage

5 10

Electron cooler 15

Electron Cooler Reaction region

Low collision energies < 10 meV !

Collision energy-dependence

Merged

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XYZ⁺beam e^-beam

Neutrals XYZ⁺

0^oDetection Arm

XYZ⁺beam e^-beam

XYZ⁺

Electron Cooler

Reaction region

XYZ⁺

BE =1/2 mv² Neutrals

XYZ, XY, XZ …

Flight distance, D

Detectors

Separation, RMax Extra KE

For standard conditions:

KE/BE ~ 10^-5, D ~ 6 m

R ~ 10 mm BE

D KE α

R More channels?

Position sensitive detection

BE =1/2 mv²

Flight distance, D Extra KE Extra

KE

Sep., Max R

Sep.,Max Different channels have R

different KEs.

∴ Different Rs R₁ ↔ KE₁ R₂ ↔ _KE₂

II ^CCD

PMT 3-Stacked Multi

Channel Plates

Phosphor Screen

H atom

O atom 250 keV/A

25 keV/A Stopped

Slowed

NOVEL:

A thin foil was placed in-front of the MCP.

60 mm

Movie

Merged

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H Atom 1 H Atom 2

X Atom

COM-H1 COM-H2

Centre of Mass (c.m.) COM-X

XH₂⁺: Heavy X atom, light h atoms

H: Take most energy, large r &

sensitive to internal excitation of the heavy atom.

0 10

20 30

40 50

60 0 10

20 30

40 50

6

X Pixels in CCD Fram Y P e

ixels in CCD Frame 0

10 20

30 40

50 60 0

10 20

30 40

50 6

X Pixels in CCD Fram Y P e

ixels in CCD Frame

BH₂⁺,CH₂⁺ OH₂⁺,NH₂⁺

SD₂⁺,PD₂⁺

∠ H-c.m.-HKER E(H)

From experiments, what do we Obtain andResults Learn?

DR

k

_e

=

Length cloudof

Cosmic ionization

rate

L ς N( H

₃⁺

)

N( H

₂⁺

) N e ( )

Cross sectionRate coefficient 23 K

= 2.6×10^-7cm³s^-1

k

_e

Merged

(11)

From experiments, what do we Obtain andResults Learn?

DR

k

_e

=

Length cloudof

Cosmic ionization

rate

L ς ^N( ^H

₃⁺

⁾

N( H

₂⁺

) N e ( )

H₂⁺column density H₃⁺column

density

DR

k

_e

=

Length cloudof

Cosmic ionization

rate

L ς N( H

₃⁺

)

N( H

₂⁺

) N e ( )

8,000 cm s^-1

L ς ₌

L = 2.1 pc

∴ζ = 1.2 x 10^-15 s

Cosmic ray ionization rate …

40x higher than previously assumed

… in planetary atmospheres

From experiments, what do weResults Obtain and Learn?

In the ionosphere:

O₂⁺ + e^-→ O(¹D) + O ? O(¹S) + O ?

O₂⁺ + e^- → O(¹D) + O 9

O(¹S) + O 9

DR is only source of O(¹S) in the nighttime

F- region

Cross section

DR rate is O₂⁺(v,n,l) dependent

O(¹S) yield is T-dependent

∴Altitude dependent

Merged

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… in planetary atmospheres In the atmosphere:

14N¹⁵N⁺+ e^-→

15N + ¹⁴N + KE m(¹⁵N) > m(¹⁴N)

∴ v(¹⁵N) > v(¹⁴N)

14N¹⁵N⁺ + e^- →

N(⁴S) + N(²D) + 3.44 eV 9

3.44 eV

1.78 1.66

14N ¹⁵N

SPEED LIMIT

5200

14N

escape trapped

15N

DR leads to isotope fractionation

From experiments, what do weResults Obtain and Learn?

0 3 6 9 1 2 1 5 1 8 2 1 2 4 2 7

0 0 . 5 1 1 . 5 2 2 . 5

P(c.m.-H) (Arb. Units)

c .m .-H D is t a n c e (P ix e ls )

X = C (a rb o n ) X = N (itro g e n ) X = O (x y g e n )

Fragment energy distribution OH₂⁺ + e^- → O(³P)/O(¹D)

NH₂⁺ + e^- → N(⁴S)/N(²D)/N(²P) CH₂⁺ + e^- → C(³P)/C(¹D)

From experiments, what do we Obtain and Learn?

Results

0 5 10 15 20 25 30 35

0 1000 2000 3000 4000

T D (Pix els)

Counts (Arb. Units) X = C(arbon)

X = N (itrogen) X = O (xygen)

Ground/Excited state competition

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0

Counts (Arb. units)

H - c . m . - H A n g l e ( D e g r e e s )

X = C ( a r b o n ) X = N ( itr o g e n ) X = O ( x y g e n )

Molecular Geometry on break-up

Merged

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State populations

Ion CH₂

NH₂ OH₂

N(⁴S) 3.94 O(³P)

3.04 Ground

C(eV³P) 2.45

First C(eV¹D) 1.24 N(²D)

1.56 O(¹D)

1.07

Ratio 1.0:1.0

1.1:1.0 3.5:1.0

Angular distributions

0 5 0 10 0 15 0 2 0 0

0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 10 0 0 0 12 0 0 0

H -c .m .H A n g le (D e g r e e s ) Counts (Arb. Units) Iso tro pic

CH₂⁺ N H₂⁺

Reaction energy distribution

0 3 6 9 12 15 18 2 1 2 4 2 7 3 0

0 2 4 6 8 10

P(c.m.-H) (Arb. units)

D is t a n c e (P ix e ls )

ρ = c ρ = 1.0 ρ = 0 .1 N H₂⁺ O H₂⁺

From experiments, what do we ObtainResults and Learn?

Insights into molecular motion and energy distribution during the reaction, sequential or

concerted reaction mechanisms.

Conclusions

Molecules are present in many environments Dissociative Recombination is a critical mechanism The presence of ions and electrons plays a huge role

CRYRING is a perfect tool for studying DR DR of H₃⁺

ζ = 1.2 x 10^-15 s 40x higher than previously assumed

DR of N₂⁺,O₂⁺…

Isotope fractionation Excited, radiating

products

Combustion &

Flames

Excited, reactive products Improved air- breating engines

0 10

20 30

40 50

60 0 10

20 30

40 50

60

X Pixels in CCD Frame Y Pixe

ls in CC D Frame 0

10 20

30 40

50 60 0

10 20

30 40

50 60

X Pixels in CCD Frame Y Pixe

ls in CC D Frame

Polyatomics

Large system differences

Extremely violent

DR, so “simple”, is extremely complicated and needs to be understood.

Merged

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… in the primordial universe

Formation of H₂

Cooling of Gas in Primordial Galaxies

k₂<< k₁

Destruction of H^- ?

H

H^- hν e Photodissociaiton

H

H^- H⁺ H Mutual Neutralisation

Savin: Recent review: For low T factor 3-4 uncertainty in MN

Most Fundamental Reaction desires

Accurate Measurement

H e H^- hν

k₁

Radiative association H H^- H₂

k₂

e Associative detachment

Double Double ElectroElectroSStatic Itatic Ion on RRing ing EExperimxperimEEnt: nt: DESIREEDESIREE

How to solve that

Desire Desire

^?^?

Merged

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Ion Storage Rings:

Ion Storage Rings: Magnetic vs ElectrostaticMagnetic vs Electrostatic

But for electrostatic storage But for electrostatic storage For magnetic storage

For magnetic storage

”No”No”” mass limit for the ions mass limit for the ions that can be stored

that can be stored Can study small, astrophysically

Can study small, astrophysically and atmospherically important and atmospherically important

systems ...

HH₃₃⁺⁺,H,H₂₂⁺⁺ HH₂₂OO⁺⁺

CHCH⁺⁺ OO₂₂⁺⁺, NO, NO⁺⁺

......

Can now study large biologically Can now study large biologically

important systems ...

amino acids amino acids

peptides peptides proteins proteins chromophores chromophores DNA fragmentsDNA fragments

RNA fragments RNA fragments

......

q=1q=1

B•B•ρρ=1.44 Tm=1.44 Tm mm^{For 0-}^{For 0}_max_max-eV collisions~ 100 amu~ 100 amueV collisions

Two 8.7-m-circumference electrostatic rings

Ion source/

pre trap 100 kV platform

Heavy ions

Up to 100 kDa at 2 kV acc before magnet

Typical resolving power:

1000

Positive ions

Lighter ions

25 kV platformNegative ions

Ion source/pre trap

Outer heat shield (60 K) Inner heat shield (4-10 K) Cryogenerators

Titanium sublimation pumps Vacuum < 10^-13 mbar

Pre-cooling of ions in pre-traps

Double Double EElectrolectroSStatic tatic IIon on RRing ing EExperimxperimEnt:Ent:

DESIREE DESIREE

Merged

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160° cylindrical bends

10°

bends

correcting bend Q-poles

Q-poles

10°

bends

Maximum mass ratio for storage of two beams: 20:1

Heavy ions, m_H

Positive ions

Merging section (0.8 m) q_L, m_L, v_L

q_H, m_H, v_H

Lighter ions, m_L

Negative ions

Double Double ElectroElectroSStatic Itatic Ion on RRing ing EExperimxperimEEnt: nt: DESIREEDESIREE

T_H=m^Hv_H² 2 T_L= m^Lv_L²

2

The Merging section –

How to control collision energies

q_L, m_L, v_L q_H, m_H, v_H

m_Hm_L m_H + m_L The velocity of the centre-of-mass, v = m_H

m_H + m_Lv_H m_L m_H + m_Lv_L

+

Collision energy available in

the center-of-mass, T_CM = T_H

m_H T_L m_L

[

-

]

²

Merged

(17)

For m_H= m_L; q_H= +1; q_L = -1

we get: T_CM = 1

2

[

T_H-U_tune- ^T^L^+U^tune

]

²

( In this special case T_CM = 0 for U_tune = ( T_H- T_L )/2 )

The Merging section – How to control collision energies

Example: T_H =20 keV; T_L=19 keV

1.0x10^-6 500

0.256 400

1.60x10^-2 475

2.308 200

6.41x10^-2 450

6.411 0

T_CM(eV) U_tune(V)

T_CM(eV) U_tune (V)

Fine-tune the collision energy with a voltage, U_tune, on the merging section:

U_tune

m_Lv_L² T_L=

2 - q_LU_tune T_H=m^Hv_H²

2 - q_HU_tune q_L, m_L, v_L

q_H, m_H, v_H

U_tune (V)

350 400 450 500 550 600 650

T cm (meV)

0 50 100 150 200 250 300

The Merging section – How to control collision energies

Fine-tune the collision energy with a voltage, U_tune, on the merging section:

U_tune

m_Lv_L² T_L=

2 - q_LU_tune T_H=m^Hv_H²

2 - q_HU_tune q_L, m_L, v_L

q_H, m_H, v_H

For m_H≠ m_L; q_H= +1; q_L = -1 we get:

m_LT_H m_H + m_L

T_CM = (T_L +U_tune) m_H (T_H-U_tune) m_L

[

^1-

]

²

Example: m_H = 2m_L, T_H =38 keV; T_L=20 keV

2.1x10^-6 -667

0.17 -760

3.39x10^-3 -680

0.34 -800

5.42x10^-2 -720

1.04 -900

U_tune(V)

U_tune (V) T_CM (eV) T_CM (eV)

U_tune (V)

-800 -750 -700 -650 -600 -550

T' cm

(meV)

0 100 200 300 400 Merged

(18)

I_HI_L e²

Rate = v_rel v_H v_L

Lσ A_max

I_H and I_Lare the stored currents The largest beam cross section area in the merging section is A_max

The rate of, e.g., mutual neutralisation in the merging section of length L is given by:

The Merging section – Reaction Rates

L= 0.8 m

U_tune

q_L, m_L, v_L q_H, m_H, v_H

Assuming again m_H = m_L, q_H= +1, q_L = -1 we get:

(T_H-U_tune) (T_L+U_tune) I_HI_L

e²

Lσ ( ) A_max

Rate = m_L T_CM T_CM σ = σ ( )

T_CM

r_min = b

1 + [ T_CM(eV) b/13.6 ]² 1 +

[T_CM(eV) b/13.6]

..dependens on the relation between the distance of closest approach, r_min, and the impact parameter b

The energy dependence for the electron capture cross section σ = σ (T_CM)

b r_min

q_H = +1

q_L = -1

The Merging section – Cross sections

Merged

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The Merging section – Cross sections

1 + r_min = b

1 + [ 8T_CM bπε₀/e²]² [8T_CM bπε₀/e²]

For small T_CM

(in SI units): ≈ ^4TCM b²πε₀

e²

For small T_CM: e²

4ε₀

R_crit T_CM σ = σ (T_CM) = ^πb²≈

For large T_CM : r_min ≈ b σ = σ (T_CM) = ^πb²≈ π R_crit² R_crit = r_min : Over the barrier models

Collision Energy (eV)

0.01 0.1 1

e-transfer cross-section (10-14 cm2 )

0 200 400 600 800 1000

σ ∼ e² R_crit 4ε_o T^'_cm

πR_crit² σ ∼

The Merging section – Count rates

I_HI_L e²

Rate = v_rel v_H v_L

Lσ A_max I_H = I_L= 100 nA,

m_H = H⁺, m_L= H^-,

T_H = 21 keV, T_L = 20 keV, A_max = 1 cm², L = 80 cm, R_crit = 13.5 a_o

U_tune (V)

440 460 480 500 520 540 560

0 200 400 600 0 20 40 60 80 100

TCM(eV)Count Rate (s-1) Merged

Topics CoveredTopics Covered

fundamental

n N

L k

n

n e

( ) ( ) ( )

H H H ( )

= = ς

k

=

L ς N( H

)

N( H

) N e ( )

WHY and HOW?



Solve using Solve using Grid Technique Grid Technique

k

=

L ς N( H

)

N( H

) N e ( )

k

k

=

L ς N( H

)

N( H

) N e ( )

k

=

L ς N( H

)

N( H

) N e ( )

L ς =

Desire Desire

[

]

[

]

[

]

L ς ^N( ^H

⁾

L ς ^N( ^H

⁾

L ς ₌