More than a dozen ions hitherto identified in the interstellar medium.

1
Interstellar ion chemistry

• More than a dozen ions hitherto identified in
  the interstellar medium.
• Interstellar chemistry once thought to be
  dominated by ion chemistry.
• Ions found in interstellar clouds, shock waves,
  ionospheres, etc.

The Horsehead nebula (Ori)
Aurora over Alaska
The Cats eye (Draco)
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Important ion reactions in the ISM

• Ion - neutral reactions ( H2 H2 ?
  H3 H )
• Ion - electron reactions ( H3 e- ? 3 H
  )
• Ion - ion reactions (H- H ? 2 H )
• (quite unexplored)

3
Feasible pathway of molecule synthesis in space
Cosmic Ray Ionization
Ion-Molecule Reactions
Recombination
For example Presumed synthesis of methanol 1.
Ion-molecule reaction 2. Dissociative
recombination
CH3 H2O ? CH3OH2
CH3OH2 e- ? CH3OH H
4
Radiative association
Scheme of a radiative association
Redissociation in competition with radiating off
of energy
CH3 H2O ? CH3OH2 hn
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Important electron-ion reactions
A B- Resonant ion pair formation (high
energies)
AB(vm) e- Elastic/inelastic/ superelastic
(nm/nltm/ngtm) scattering
AB(vn) e-
AB hn Radiative recombination (too slow)
A B Dissociative recombination
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Dissociative recombination (DR) in space
Dissociative recombination (DR) in interstellar
chemistry

• Negative charge in the interstellar medium
  (ISM) thought to
• present mostly in the form of electrons.
• DR often the final step of synthesis of
  neutral molecules in the ISM.
• (HX e- X H
  )
• Dissociative recombination(DR) often the
  only way to destroy cations.
• Ample data on of DR rates, little on
  branching ratios.
• Branching ratios often hard to explain by
  common sense.
• DR can lead to excited states that emit
  characteristic lines.

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Problems to quantify DR reactions

• General rule Conditions must match
  interstellar ones
• Ions have to be rotationally and vibrationally
  cool.
• Three-body processes must be excluded.
• Low relative translational energies of
  reactants.
• Additionally Clear identification of the ion
  (isomeres) and
• products.
• Up to the 90s measurements restricted to
  afterglow experiments.

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Theoretical prediction of the pathways of DR
reactions

• Batess theory 1986 Dissociative
  recombinatons favour the pathway(s) which
  involve(s) least orbital rearrangement, e. g.
  
• N2H e- N2 H
• N2OH e- N2O H
• Difficult to obtain reliable potential
  surfaces due to
• involvement of highly excited states
• very few high-level ab initio studies
  on DR reactions
• available

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Flowing afterglow

• 4 steps
• 1. Production of He by discharge in He
• He e- ? He 2
  e-
• 2. Reaction of He with H2
• He H2 ? H2 He
• H2 H2 ? H3 H
• 3. Reaction of H3 with other substances, e.g.
  CO
• H3 CO ? HCO H2
• 4. Recombination of the ion
• HCO e- ? H
  CO

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Glosik et al. 2006
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Advantages and disadvantages of flowing afterglow

• Low operational costs.
• Thermal equilibrium
• of reactants.
• Detection of products by
• mass spectromtry.
• Detection of electron
• degradation by Langmuir probe.

• - Impure reactants - except
• very simple systems like H3.
• - Mearurements only at high
• (room) temperatures.

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Storage ring (CRYRING)
Steps during the experiment

• 1. Formation of the
• ions in the source
• 2. Mass selection by
• bending magnet
• 3. Injection via RFQ
• and acceleration
• 4. Merging with
• electron beam
• 5. Detection of the
• neutral products

3
2
1
4
5
Schematic view of CRYRING
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Cooled cathode
Anode
Ion Beam
Neutral fragments
Bending magnets
Bending magnets
Electron cooler
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GRID technique
without grid
Particle loss
Surface barrier detector
Signal without grid (all events lead to full mass
signal)
with grid
Grid T0.3
e-
Branching ratio
Signal with grid (mass spectrum dependent on
branching ratio and T)
Probability T(1-T)
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Advantages and disadvantages of storage rings

• Low (interstellar) relative
• kinetic energies
• of the reactants.
• Mass selection of the ion
• produced.
• All products can be identified.
• Low background.

• Only radiative cooling
• possible.
• No straightforward
• identification of
• product internal states.
• High set-up and operation
• costs.

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N2H e-

• One of the most prominent ions in dark
  interstellar clouds.
• N2 lost through protonation might be fully
  recovered by DR of N2H
• N2 H3 N2H H2
• N2H e- N2 H
• Most of interstellar nitrogen
• thought to be stored as N2.
• Tracer for the unobservable N2.
• Present in Titans ionosphere.

Saturns satellite Titan
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HCO e-

• HCO formed easily in the interstellar medium
  from CO through
• protonation (e. g. by H3).
• One of the most important carbon- containing
  interstellar ions.
• Cameron bands in Red Rectangle maybe due to
  excited CO from
• DR of HCO.

Cameron bands in the Red Rectangle
The Red Rectangle
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HCS e-

• HCS is the most important sulfur-containing
  interstellar
• molecular ion.
• In dark clouds, a high HCS/CS ratio is found.
• CS presumably formed by DR of HCS.
• Very low rate of DR used in astrophysical
  models.
• How does the rate and branching ratio of the DR
  affect the
• HCS/CS ratio ?

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N2H e- / HCO e-
HCO e- reaction channels
N2H e- reaction channels
HCO e- H CO (X
1S) DH - 7.45 eV H CO (a 3P) DH -
1.43 eV H CO (a 3S) DH - 0.75
eV HC O DH 0.17 eV OH
C DH - 0.75 eV
N2H e- N2 H DH -
8.47 eV NH N DH - 2.40
eV
N2H fragment energy spectrum
HCO fragment energy spectrum
CO
O
COH
CH
C
OH
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N2H e- / HCO e-
Evaluation matrix
N2H e- reaction channels
N2H e- N2 H DH -
8.47 eV NH N DH - 2.40
eV
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Evaluation of the branching ratios
Evaluation matrix
       
Branching ratios    
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N2H e-
Cross section of N2H e-
Dependence of s on relative kinetic energy
Reaction rates of N2H e-
k(T)

• Taken from Smith, D., Adams, N. G. 1984, ApJ,
  284, L13

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N2H e- / HCO e-
Branching ratios

       
Reaction rates
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N2H in prestellar cores
Aikawa et al. 2005
R / au

• HCO is depleted in the centre of the core,
  N2H is constant, NH3
• slightly enhanced.
• Explanation CO frozen out, N2 isnt.

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BUT

• Temperature desorption behaviour of N2 and CO
  differs only slightly.
• (Schlemmer and co-workers 2006)
• No explanation for enhancement of ammonia near
  the core centre.

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Explanation
Taken from Aikawa et al. 2005

• Two destruction mechanisms for N2H, only DR
  for HCO
• N2H CO ?
  HCO N2
• N2H
  e- ? Products
• At low temperatures DR becomes the only
  degradation process
• (CO frozen out, but N2 also)
• Formation of NH leads to enhancement of NH3.

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Imaging analysis
Can we gather information about the product
kinetic energy ?
PMP
Trigger
MCP
e-
Beam splitter
Phosphorus screen
v
CCD camera
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HCO e-
Reaction channels leading to differentelectronic
energy levels of CO
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Imaging of DCO
Fit of the different electronic state
contributions
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Conclusions N2H/ HCO/ HCS

• In the DR of N2H, the break-up of the N-N
  bond dominates.
• In the DR of HCO, the CO H channel is
  preeminent.
• Recombination of HCO partly leads to CO in
  the 3Pu state,
• which can explain the Cameron bands in the
  Red Rectangle.
• In the DR of HCS, the break-up of the C-S
  bond is favoured.
• Reaction rate in the N2H and HCO DR
  reactions in agreement
• with previous FALP measurements.

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New branching ratios in a model of TMC-1

Ohishi, M. , Irvine, W. M. Kaifu, N., Astronomy
of Cosmic Phenomena, 171
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Conclusions from model calculations

• Abundances of N-containing compounds predicted
  better
• assuming an older age of TMC-1.
• Some improvements for molecule densities that
  proved difficult
• to model (H2O, HCOOH).
• No big influence on models of circumstellar
  envelopes, planetary
• nebulae and diffuse clouds.

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SO2 e-

• Influence on interstellar sulfur
  chemistry.
• SO2 is found in atmospheres of planets
  (Venus) and satellites (Io).
• Important role of SO2 in the ionosphere of
  Io.
• Three-body break-up energetically allowed.

Iupiters moon Io
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SO2 e-
SO2 e- reaction channels
SO2 e- SO O DH 6.32
eV S O2 DH 6.40 eV S 2O
DH 1.22 eV

Branching ratios of SO2 e-
Reaction rate
k(T) 4.6 ? 0.1?10-7 (T/300)-0.52?0.02 cm3
mol-1s-1
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Consequences

• Decay of SO2 in Ios ionosphere during
  eclipse probably
• caused by DR.
• Strong observed UV lines of O(I) and S(I) could
  be due to
• increased S- and O-atom production by
  three-body break-
• up in DR.
• Possible role in the ionosphere of Venus ?

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Methanol in space

• Responsible for maser emission in
  star-forming regions.
• Evolution indicator in star-forming regions
• Used for determination of
• kinetic temperature and H2
• density simultaneously.
• From CH3OH2/CH3OH ratio
• electron temperature in
• cometary coma derived.

The Bear Claw Nebula, where a strong methanol
maser was detected
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Production of methanol in the ISM
CH3 H2O ? CH3OH2
CH3OH2 e- ? CH3OH H
With a high rate of DR, the radiative
association rate should be about 1.2 ? 10-10
cm3s-1 at 50 K. (Herbst et al. 1985)
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But...
Ion trap experiments yielded a an upper
limit of 2 ? 10-12 cm3s-1 at dark cloud
temperatures (Luca et al. 2002). a factor of 60
too low !
However...

• CH3 not detected so far, densities only
  estimates
• from models.
• Uncertainties in water densities.
• If the DR of CH3OH2 leads to methanol with a
• branching ratio of close to 100 .......

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Fragment energy spectrum of CD3OD2
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Fragment energy spectrum of CD3OD2
CD3OD2 e- CD4 OD CD2 OD D2
CD3 D2 O CD3 D2O CDO 2D2 CDO
D2 2D CO 2D2 D CO D2 3D

CD3OD2 e- CD3OD D CD3 OD D CD2
D2O D CD D2O D2 CD3O
2D CD3O D2 CD2O D2 D CD2O 3D
CD4 O D
Some of the channels deliver products with the
same mass ? indistinguishable.
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Branching ratios CD3OD2/CH3OH2
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2-,3- and 4-body processes
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2-,3- and 4-body processes
Thermal reaction rate (CD3OD2) k 9.11 ?
10-7 (T/300)-0.63 cm3s-1
s 9.55 ? 10-16 E(eV)-1.2cm2
For the undeuterated isotopomer (CH3OH2) k
8.91 ? 10-7 (T/300)-0.59 cm3s-1
Cross-section vs. collision energy
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Model Calculations
Observed methanol density (TMC-1)
CH3OH H branching ratio 1
CH3OH H branching ratio 0.06
UMIST (Rate99) model predictions for methanol
density in TMC-1
Including new rates for the radiative association
of CH3 and H2O, (Luca et al. 2002) the peak
methanol relative abundance sinks to 7 ? 10-13.
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New UMIST model
CH3OH H branching ratio 1
CH3OH H branching ratio 0.06
new rate for CH3 H2O
Observed methanol density (TMC-1)
UMIST (Rate04) model predictions for methanol
density in TMC-1
Main gas phase route to CH3OH is now CH3CHO
H3 ? CH3OH CH3
k 1.4 ? 10-9cm3s-1 at 300K
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Conclusions

• Three-body break-ups dominate.
• Production of CH3OH only 3 (CD3OD only 6 ).
• No big isotope effects
• Gas-phase mechanism for interstellar methanol
  very unlikely.
• In line with the following facts
• Formation of methanol on CO ice surfaces
  possible at 10 K.
• (Watanabe et al. 2004)

• Models including grain surface desorption
  reproduce methanol
• densities
• (Herbst 2006)

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Can we close the books ?

• Anticorrelation of CO and CH3OH in dense
  clouds.
• (Buckle, 2006)
• No experimental evidence for surface
  desorption of freshly
• formed methanol

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DR of other CHxO systems
Retention versus break-up of CO-bond
?? Increasing hydrogen saturation favours C-O
bond rupture A rule for DR of
hydrogen-containing ions ?
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DR of (CD3)2OD

• Similar mechanism to methanol postulated for
  dimethyl ether.
• Similar problems ?

CH3 CH3OH ? (CH3)2OH
(CH3)2OH e- ? CH3OH H
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YES !
Production of (CD3)2O only 6 ) !
Grain surface process for formation of dimethyl
ether unlikely (Ehrenfreund and co-workers, 2006)
AND
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Anions in space ?
- Negative charge allegedly mostly in form of
electrons - Some anions (OH-, CN-, C- and CH -)
found in Halleys coma (Chaizy et al. 1991) -
CNO- and possibly HCOO- in interstellar ices
(Pontopiddan et al. 2002, Schutte et al.
2001) - anions and cluster anions present in
Earths ionosphere
Halley 1986
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Possible importance of anions in space ?
- Role of atomic anions in early universe
H H- ? H H - Diffuse interstellar
bands possibly PAH anions and carbon-chain
anions - CNO- and possibly HCOO- in
interstellar ices (Pontopiddan et al. 2002,
Schutte et al. 2001) - High electron sticking
coefficient of lage PAHs - Anion abundance
constrains electron density
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Anion chemistry in space

• Photodetachment
• AB- hn ? AB e-
• Mutual neutralisation
• AB- C ? AB C
• ? other neutral products
• very little experimental data

The negative charges may reside more in the form
of anions than electrons and mutual
neutralization may replace dissociative
recombination as the main mechanism for removing
positive ions. Alex Dalgarno, 1999
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DESIREE storage ring
Double Electrostatic Ion Ring Experiment
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Features of DESIREE

• Cryostat cooling down to at least 10K
• No restriction on ion mass
• Electrospray ion source for large ions (PAHs)
• Windows for laser spectroscopy

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