more than a dozen ions hitherto identified in the interstellar medium. interstellar chemistry once...
TRANSCRIPT
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) The “Cat’s eye” (Draco) Aurora over Alaska
Interstellar ion chemistry
Ion - neutral reactions ( H2+ + H2 H3
+ + H )
Ion - electron reactions ( H3+ + e- 3 H )
Ion - ion reactions (H- + H+ 2 H ) (quite unexplored)
Important ion reactionsin the ISM
Cosmic RayIonization
Ion-MoleculeReactions
Recombination
CH3+ + H2O CH3OH2
+
CH3OH2+ + e- CH3OH + H
For example: Presumed synthesis of methanol
1. Ion-molecule reaction
2. Dissociative recombination
Feasible pathway of moleculesynthesis in space
h
1rad
kc
1Dis
Redissociation in competition with radiating off of energy
CH3+ + H2O CH3OH2
+ + h
Scheme of a radiative association
Radiative association
AB + h Radiative
recombination(too slow)
AB+(v=m) + e-
Elastic/inelastic/superelastic
(n=m/n<m/n>m)scattering
A+ + B- Resonant ion
pair formation
(high energies) AB+
(v=n) + e-
A + B Dissociative
recombination
Important electron-ion reactions
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.
Dissociative recombination (DR) in interstellar chemistry
Dissociative recombination (DR) in space
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 90’s measurements restricted to afterglow experiments.
Problems to quantifyDR reactions
Bates’s 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
Theoretical prediction of the pathways of DR reactions
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
Flowing afterglow
Glosik et al. 2006
+ 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.
Advantages and disadvantages of flowing afterglow
Schematic view of 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
1
23
45
Storage ring (CRYRING)
Electron cooler
Bendingmagnets
Bendingmagnets
Cooledcathode
Anode
IonBeam Neutral
fragments
ProbabilityT(1-T)
with grid
without grid
Signal with grid(mass spectrum dependent
on branching ratio and T)
Signal without grid(all events lead to full
mass signal)
Surface barrierdetector
e-
GridT=0.3e-
Branching ratio
Particle loss
GRID technique
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.
Advantages and disadvantagesof storage rings
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 Titan’s ionosphere.Saturn’s satellite Titan
N2H+ + e-
The ”Red Rectangle”
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
HCO+ + 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 ?
HCS+ + e-
N2H+ + e- N2 + H H = - 8.47 eV
NH + N H = - 2.40 eV
N2H+ + e- reaction channels
0 1 2 3
0
200
400
600
800
1000
1200
2N+H2N
N+HN
H
Sig
nal
Inte
nsi
ty /
cou
nts
Fragment kinetic energy / MeV
N2H+ fragment energy spectrum
HCO+ + e- H + CO (X 1S+) H = - 7.45 eV
H + CO (a 3P) H = - 1.43 eV
H + CO (a 3S+) H = - 0.75 eV
HC + O H = + 0.17 eV
OH + C H = - 0.75 eV
HCO+ + e- reaction channels
1.0 1.5 2.0 2.5 3.0 3.5
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
Sig
na
l in
ten
sity
/ co
un
ts
Fragment kinetic energy / MeV
HCO+ fragment energy spectrum
C
C+H
O
O+H
C+O
C+O+H
N2H+ + e- / HCO+ + e-
GridT=0.3e-
N2H+ + e- N2 + H H = - 8.47 eV
NH + N H = - 2.40 eV
N2H+ + e- reaction channels2 2
2N H
2N2
N H
N
H
IT T
IT(1 T) 0BR(N H)
I0 T(1 T)BR(NH N)
I0 T(1 T)
IT(1 T) 0
Evaluation matrix
N2H+ + e- / HCO+ + e-
2 22N H
2N2
N H
N
H
IT T
IT(1 T) 0BR(N H)
I0 T(1 T)BR(NH N)
I0 T(1 T)
IT(1 T) 0
Branching ratios
Branching ratioReaction channel
0.36N2 + H
0.64N + NH
Evaluation matrix
Evaluation of the branching ratios
Cross section of N2H+ + e-
k / cm3 mol-1s-1
FALP*Our data
1.7 10-7 (T/300)-0.90 1.0 0.110-7 (T/300)-0.510.02
Reaction rates of N2H+ + e-
•Taken from: Smith, D., & Adams, N. G. 1984, ApJ, 284, L13
Dependence of on relative kineticenergy
k(T)
3/ 21/ 2
kinkin kin kin
B B0
1 2 Ek(T) (E ) E exp dE
k T k T
N2H+ + e-
N2H+ + e- HCO+ + e- HCS+ + e-
Reaction channel
Branching ratio
Reaction channel
Branching ratio
Reaction channel
Branching ratio
N2 + H 0.36 ± 0.05 CO + H 0.92 ± 0.04 CS + H 0.20 ± 0.03
CH + O 0.01 ± 0.02 NH + N 0.64 ± 0.05
C + OH 0.07 ± 0.02
CH + S C + SH
0.80 ± 0.03
Branching ratios
Reaction rates
}
This work Previous FALP Reaction
Rate / cm3 s-1 Rate / cm3 s-1
N2H+ + e- 1.0 0.3 10-7 (T/300)-0.51 0.02 1.7 10-7 (T/300)-0.90 1
HCO+ + e- 3.0 0.9 10-7 (T/300)-0.74 0.02 2.4 10-7 (T/300)-0.69 2
HCS+ + e- 9.7 1.9 10-7 (T/300)-0.57 0.02 -
1 D. Smith, , & N. G Adams, Astrophys. J., 284, L13 (1984) 2 J. B.A. Mitchell, Phys. Rep., 186, 5 (1990)
N2H+ + e- / HCO+ + e-
R / au
HCO+ is depleted in the centre of the core, N2H+ is constant, NH3
slightly enhanced.
Explanation: CO frozen out, N2 isn´t.
Aikawa et al. 2005
N2H+ in prestellar cores
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.
Explanation
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.
Taken from Aikawa et al. 2005
Beamsplitter
MCP
Phosphorus screen
PMP
CCDcamera
Trigger
e-
v
Can we gather information about the product kinetic energy ?
Imaging analysis
HCO+ + e- H + CO (X 1S+) H = - 7.45 eV
H + CO (a 3P) H = - 1.43 eV
H + CO (a 3S+) H = - 0.75 eV
HC + O H = + 0.17 eV
OH + C H = - 0.75 eV
HCO+ + e- reaction channelsReaction channels leading to differentelectronic energy levels of CO
HCO+ + e-
0 10 20 30 40
0
1000
2000
3000
4000
5000
6000
7000
CO - D distance / mm
X1+ + a3
X1+ + a3 + a'3+
exclusively X1+
Cou
nts
Exp. data
X1+ 30%
a3 44%
a'3+ 26%
X1++a3+a'3+
Fit of the different electronic state contributions
Imaging of DCO+
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.
Conclusions N2H+/ HCO+/ HCS+
*Ohishi, M. , Irvine, W. M. Kaifu, N., Astronomy of Cosmic Phenomena, 171
Abundances / N(species/N(H2)
Early time (3.16 105 yr)
Steady state Observed values
(TMC-1) Species
RATE99 This work RATE 99 This work Ohishi et al.*
N2H+ 6.24(-12) 1.23(-12) 1.14(-10) 3.39(-10) 1.77(-10)
NH3 3.59(-9) 2.39(-10) 3.61(-8) 6.73(-8) 2.00(-8)
HCN 2.99(-8) 4.76(-10) 3.42(-9) 8.43(-9) 2.00(-8)
HNC 5.39(-8) 6.85(-10) 2.77(-9) 5.32(-9) 2.00(-8)
OH 1.76(-8) 1.35(-8) 4.07(-8) 1.23(-7) 1.00(-7)
Some notorious troublemakers:
H2O 3.27(-6) 3.60(-9) 3.33(-6) 3.18(-7) 6.00(-10)- 2.00(-8)
HCOOH 1.67(-8) 4.50(-11) 4.00(-8) 6.30(-9) <2.00(-10)
New branching ratios in a model of TMC-1
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.
Conclusions from model calculations
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.
Iupiter´s moon Io
SO2+ + e-
SO2+ + e- reaction channels
SO2+ + e- SO + O H = 6.32 eV
S + O2 H = 6.40 eV
S + 2O H = 1.22 eV
Branching ratios of SO2+ + e-
Reaction rate
k(T) = 4.6 0.110-7 (T/300)-0.520.02 cm3 mol-1s-1
Reaction product
Branching ratio
SO + O 0.61 S + O2 0.00
S + O + O 0.39
SO2+ + e-
Decay of SO2+ in Io’s 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 ?
Consequences
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
Methanol in space
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)
Production of methanol in the ISM
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 !
But...
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 %.......
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
1.2x104
1.4x104
D
2D
3D C+D
C+2D O
O+4D
C+3DO+D
C+4DO+2D
C+5D,O+3D
C+O+2D
C+O+3D
C+O+4D
C+O+5D
In
tens
ity
/ cou
nts
Fragment kinetic energy / MeV
Fragment energy spectrum of CD3OD2
+
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
CD3OD2+ + e- CD4 + OD
CD2 + OD + D2
CD3 + D2 + O
CD3 + D2O
CDO + 2D2
CDO + D2 + 2D
CO + 2D2 + D
CO + D2 + 3D
Some of the channels deliver products with the same mass indistinguishable.
Fragment energy spectrum of CD3OD2
+
Reaction pathway
Branching ratio
CD3OD + D 0.06
CD3 + D2O (CD4 + OD)
0.11
CD3O + D2 0.05
CD3 + OD + D 0.59
CD2 + D2O + D (CD4 + O + D)
0.16
CD + D2O + D2 0.01
CD3O + 2D 0.00
CD2O + D2 + D 0.02
CD2O + 3D 0.00
Reaction pathway
Branching ratio
CH3OH + H 0.03
CH3 + H2O 0.09
CH4 + OH 0.00
CH3O + H2 0.07
CH3 + OH + H 0.51
CH2 + H2O + H 0.21
CH4 + O + H 0.00
CH + H2O + H2 0.00
CH3O + 2H 0.00
CH2O + H2 + H 0.09
CH2O + 3H 0.00
Branching ratios CD3OD2+/CH3OH2
+
Processes
Sum of branching
ratios (CD3OD2
+)
Sum of branching
ratios (CH3OH2
+)
2- body 0.22 0.19
3- body 0.78 0.81
4- body 0.00 0.00
2-,3- and 4-body processes
Cross-section vs. collision energy
10-3 10-2 10-1
10-15
1x10-14
1x10-13
1x10-12
1x10-11
Cro
ss s
ect
ion
/ cm
2
Relative kinetic energy (eV)
= 9.55 10-16 E(eV)-1.2cm2
Thermal reaction rate (CD3OD2
+):
k = 9.11 10-7 (T/300)-
0.63
cm3s-1
For the undeuterated isotopomer (CH3OH2
+):
k = 8.91 10-7 (T/300)-
0.59
cm3s-1
CD3OD2+
2-,3- and 4-body processes
CH3OH + H branching ratio = 1
CH3OH + H branching ratio = 0.06
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.
UMIST (Rate99) model predictions for methanol density in TMC-1
Observed methanol density (TMC-1)
Model Calculations
UMIST (Rate04) model predictions for methanol density in TMC-1
CH3OH + H branching ratio = 1
CH3OH + H branching ratio = 0.06
+ new rate forCH3
+ + H2O
Observed methanol density (TMC-1)
Main gas phase route to CH3OH is now CH3CHO + H3
+ CH3OH + CH3+
k = 1.4 10-9cm3s-1 at 300K
New UMIST model
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)
Conclusions
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
DCO+ CD2OD+ CD2OD2+ CD3OD2
+
Break-up of C-O bond 0.08 0.08 0.43 0.81
C-O bond intact 0.92 0.92 0.57 0.19
Retention versus break-up of CO-bond
Increasing hydrogen saturation favours C-O bond rupture A rule for DR of hydrogen-containing ions ?
DR of other CHxO+ systems
Similar mechanism to methanol postulated for dimethyl ether.
Similar problems ?
CH3+ + CH3OH (CH3)2OH+
(CH3)2OH+ + e- CH3OH + H
DR of (CD3)2OD+
YES !
Production of (CD3)2O only 6 %) !
Grain surface process for formation of dimethyl ether unlikely(Ehrenfreund and co-workers, 2006)
AND:
Anions in space ?
- Negative charge allegedly mostly in form of electrons
- Some anions (OH-, CN-, C- and CH -) found in Halley’s 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 Earth’s ionosphere
Halley 1986
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
Anion chemistry in space
Photodetachment
AB- + h 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
DESIREE storage ring
Double Electrostatic Ion Ring Experiment
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