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 ( H H 2  H H ) * Ion - electron reactions ( H e -  3 H ) * Ion - ion reactions (H - + H +  2 H ) (quite unexplored) Important ion reactions in the ISM

Cosmic Ray Ionization Ion-Molecule Reactions Recombination CH H 2 O  CH 3 OH 2 + CH 3 OH e -  CH 3 OH + H For example: Presumed synthesis of methanol 1. Ion-molecule reaction 2. Dissociative recombination Feasible pathway of molecule synthesis in space

Redissociation in competition with radiating off of energy CH H 2 O  CH 3 OH 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) 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 quantify DR reactions

Bates’s theory 1986: Dissociative recombinatons favour the pathway(s) which involve(s) least orbital rearrangement, e. g.: N 2 H + + e - N 2 + H N 2 OH + + e - N 2 O + 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 e - 2. Reaction of He + with H 2 : He + + H 2  H He H H 2  H H 3. Reaction of H 3 + with other substances, e.g. CO: H CO  HCO + + H 2 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 H Mearurements only at high (room) temperatures. Advantages and disadvantages of flowing afterglow

Schematic view of CRYRING Steps during the experiment Storage ring (CRYRING)

Electron cooler Bending magnets Bending magnets Cooled cathode Anode Ion Beam Neutral fragments

Probability T(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 barrier detector e-e- Grid T=0.3 e-e- 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. 5 No straightforward identification of product internal states. 5 High set-up and operation costs. Advantages and disadvantages of storage rings

One of the most prominent ions in dark interstellar clouds. N 2 lost through protonation might be fully recovered by DR of N 2 H + : N 2 + H 3 N 2 H + + H 2 N 2 H + + e - N 2 + H Most of interstellar nitrogen thought to be stored as N 2. Tracer for the unobservable N 2. Present in Titan’s ionosphere. Saturn’s satellite Titan N 2 H + + e -

The ”Red Rectangle” HCO + formed easily in the interstellar medium from CO through protonation (e. g. by H 3 + ). 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 -

N 2 H + + e - N 2 + H  H = eV NH + N  H = eV N 2 H + + e - reaction channels N 2 H + fragment energy spectrum HCO + + e - H + CO (X 1 S + )  H = eV H + CO (a 3 P)  H = eV H + CO (a 3 S + )  H = eV HC + O  H = eV OH + C  H = eV HCO + + e - reaction channels HCO + fragment energy spectrum C C+H O O+H C+O C+O+H N 2 H + + e - / HCO + + e -

Grid T=0.3 e-e- N 2 H + + e - N 2 + H  H = eV NH + N  H = eV N 2 H + + e - reaction channels Evaluation matrix N 2 H + + e - / HCO + + e -

Branching ratios Branching ratioReaction channel 0.36N 2 + H 0.64N + NH Evaluation matrix Evaluation of the branching ratios

Cross section of N 2 H + + e - k / cm 3 mol -1 s -1 FALP*Our data 1.7  (T/300)  0.1  (T/300)  0.02 Reaction rates of N 2 H + + e - Taken from: Smith, D., & Adams, N. G. 1984, ApJ, 284, L13 Dependence of  on relative kinetic energy k(T) N 2 H + + e -

Branching ratios Reaction rates } N 2 H + + e - / HCO + + e -

R / au ( HCO + is depleted in the centre of the core, N 2 H + is constant, NH 3 slightly enhanced. ( Explanation: CO frozen out, N 2 isn´t. Aikawa et al N 2 H + in prestellar cores

BUT ( Temperature desorption behaviour of N 2 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 N 2 H +, only DR for HCO + : N 2 H + + CO  HCO + + N 2 N 2 H + + e-  Products ( At low temperatures DR becomes the only degradation process (CO frozen out, but N 2 also) ( Formation of NH leads to enhancement of NH 3. Taken from Aikawa et al. 2005

Beam splitter MCP Phosphorus screen PMP CCD camera Trigger e-e- v Can we gather information about the product kinetic energy ? Imaging analysis

HCO + + e - H + CO (X 1 S + )  H = eV H + CO (a 3 P)  H = eV H + CO (a 3 S + )  H = eV HC + O  H = eV OH + C  H = eV HCO + + e - reaction channels Reaction channels leading to differentelectronic energy levels of CO HCO + + e -

Fit of the different electronic state contributions Imaging of DCO +

In the DR of N 2 H +, 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 3 P u 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 N 2 H + and HCO + DR reactions in agreement with previous FALP measurements. Conclusions N 2 H + / HCO + / HCS +

* Ohishi, M., Irvine, W. M. Kaifu, N., Astronomy of Cosmic Phenomena, 171 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 (H 2 O, HCOOH). No big influence on models of circumstellar envelopes, planetary nebulae and diffuse clouds. Conclusions from model calculations

Influence on interstellar sulfur chemistry. SO 2 is found in atmospheres of planets (Venus) and satellites (Io). Important role of SO 2 + in the ionosphere of Io. Three-body break-up energetically allowed. Iupiter´s moon Io SO e -

SO e - reaction channels SO e - SO + O  H = 6.32 eV S + O 2  H = 6.40 eV S + 2O  H = 1.22 eV Branching ratios of SO e - Reaction rate k(T) = 4.6  0.1  (T/300)  0.02 cm 3 mol -1 s -1 SO e -

Decay of SO 2 + 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 H 2 density simultaneously. From CH 3 OH 2 + /CH 3 OH ratio electron temperature in cometary coma derived. The Bear Claw Nebula, where a strong methanol maser was detected Methanol in space

CH H 2 O  CH 3 OH 2 + CH 3 OH e -  CH 3 OH + H With a high rate of DR, the radiative association rate should be about 1.2  cm 3 s -1 at 50 K. (Herbst et al. 1985) Production of methanol in the ISM

Ion trap experiments yielded a an upper limit of 2  cm 3 s -1 at dark cloud temperatures (Luca et al. 2002). a factor of 60 too low ! But... However... CH 3 + not detected so far, densities only estimates from models. Uncertainties in water densities. If the DR of CH 3 OH 2 + leads to methanol with a branching ratio of close to 100 %

Fragment energy spectrum of CD 3 OD 2 +

CD 3 OD e - CD 3 OD + D CD 3 + OD + D CD 2 + D 2 O + D CD + D 2 O + D 2 CD 3 O + 2D CD 3 O + D 2 CD 2 O + D 2 + D CD 2 O +3D CD 4 + O + D CD 3 OD e - CD 4 + OD CD 2 + OD + D 2 CD 3 + D 2 + O CD 3 + D 2 O CDO + 2D 2 CDO + D 2 + 2D CO + 2D 2 + D CO + D 2 + 3D Some of the channels deliver products with the same mass  indistinguishable. Fragment energy spectrum of CD 3 OD 2 +

Branching ratios CD 3 OD 2 + /CH 3 OH 2 +

2-,3- and 4-body processes

Cross-section vs. collision energy  = 9.55  E(eV )-1.2 cm 2 Thermal reaction rate (CD 3 OD 2 + ): k = 9.11  (T/300) cm 3 s -1 For the undeuterated isotopomer (CH 3 OH 2 + ): k = 8.91  (T/300) cm 3 s -1 2-,3- and 4-body processes

CH 3 OH + H branching ratio = 1 CH 3 OH + H branching ratio = 0.06 Including new rates for the radiative association of CH 3 + and H 2 O, (Luca et al. 2002) the peak methanol relative abundance sinks to 7  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 CH 3 OH + H branching ratio = 1 CH 3 OH + H branching ratio = new rate for CH H 2 O Observed methanol density (TMC-1) Main gas phase route to CH 3 OH is now CH 3 CHO + H 3 +  CH 3 OH + CH 3 + k = 1.4  cm 3 s -1 at 300K New UMIST model

Three-body break-ups dominate. Production of CH 3 OH only 3 % (CD 3 OD 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 CH 3 OH in dense clouds. (Buckle, 2006) * No experimental evidence for surface desorption of freshly formed methanol

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 CH x O + systems

* Similar mechanism to methanol postulated for dimethyl ether. * Similar problems ? CH CH 3 OH  (CH 3 ) 2 OH + (CH 3 ) 2 OH + + e -  CH 3 OH + H DR of (CD 3 ) 2 OD +

YES ! Production of (CD 3 ) 2 O 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