• Radicals have large electron affinities, typically EA(X^•) > 1.5 eV (144 kJ mol^-1)

— but the largest IS radicals yet found (C₃N, C₅N, C₄H, C₅H, C₆H) aren't highly polyatomic

... so are IS fullerenes (if they exist) likely to form negative ions?

Smith, Spanel & Märk 1993:

• Attachment to C₆₀ slow below 300 K

— attributed to 26 kJ mol ^-1 barrier

Canosa, Parent, Pasquerault, Gomet, Laubé & Rowe 1994:

• Attachment to anthracene (C₁₄H₁₀) is also barrier-inhibited

Spanel & Smith 1994:

• Attachment to C₇₀ lacks an apparent barrier

Data suggest that a case-by-case study is needed ...

If attachment doesn't work, do other options exist?

(Petrie, 1996)

• e.g. from PAH^— to small radicals (CN, C₂H, C₂H ...)

• RA alone gives n(CN^—) ~ 2 ¥ 10^-15 n(H₂)

— much too faint to detect!

• If assume n(PAH^—) = 0.03 n(e), then charge transfer gives n(CN^—) ~ 2 ¥ 10^-13 n(H₂)

— still too faint, but getting close to the detection limit


RA and CT are probably the only two viable pathways to CN^— in a dense cloud

3. Dissociative attachment

• Consider the following reaction

• endothermic for all dense-cloud XCN yet detected [D₀(X—CN) > EA(CN)]

• but exothermic for MgNC and MgCN (found in IRC +10216)

• assume reaction proceeds at 1/4 efficiency (i.e. on singlet, not triplet, surface)

— get n(CN^—) ~ 2 ¥ 10^-15 n(H₂) in IRC +10216

— this is potentially detectable

An analogous reaction

competes with RA to CCCN as a possible pathway in TMC-1, but RA probably wins out (Petrie & Herbst 1998)

4. Electron-catalyzed isomerization

• The radicals CCCH and c-C₃H both exist in TMC-1

• Radicals usually have largish EA values

• Will electrons attach to these species?

• Not permanently, but ...

• this process should convert CCCH into c-C₃H

— unfortunately, this depletes CCCH too much to agree with observed CCCH:c-C₃H ratio

Are doubly-charged ions possible interstellar cloud species?

Leach 1986:

• sequential ionization by the ISRF

— dication formation is feasible in the diffuse interstellar medium (if PAHs exist there)

• will work for any species having IE(X⁺) < IE(H)

— e.g. all PAHs, all fullerenes, some metal atoms (Ca and others)

• WON'T work in dense clouds

Omont 1986:

• suggested the following reaction

• This is exothermic, but why should it occur?

— many accessible monocationic product channels (e.g. dissociation) exist also


e.g. He ⁺ + C₁₀H₈ (Petrie, Javahery, Fox & Bohme 1993)

He ⁺ / Naphthalene:

• main product (~25%) occurs at m/z 64, i.e. C₅H₄⁺

— but this doesn't seem credible, in terms of required fragmentation

• why should the most complex fragmentation process be preferred?

• m/z 64 signal is more credibly explained as C₁₀H₈²⁺ than as C₅H₄⁺

Why should dication formation occur?

• similar mechanism accounts for He ⁺ + C₆₀ Æ C₆₀²⁺ + He

• this is a viable pathway to dications in dense interstellar clouds

Won't dications just react with everything in sight ?

• Note that many of the non-reactions have exothermic channels (e.g. charge transfer)

• C₆₀²⁺ may not dissociate on recombination, either (but less certain)

— so ionization isn't a destruction mechanism for fullerenes?

• fullerenes are also likely to be impervious to photodissociation in the ISM

Do you need an ionization source to get ions?

Not all dissociative recombination channels are exothermic:

• the opposite of "dissociative recombination" is "associative ionization" (AI)

— this is assumed to be one of the dominant early sources of CO in dense (& diffuse) clouds

Other exothermic associative ionization reactions can be identified:

• AI with atomic O is exothermic for most early transition metals, & most lanthanide atoms

• Dense clouds, especially at start, should contain free atoms of all stable elements

— but abundances may be very low due to accretion onto dust

When the AI product ion is diatomic (e.g. the metal oxide ions):

• no exothermic dissociative recombination reactions exist

— so these ions must recombine by radiative stabilization

• such metal oxide ions, & their neutrals (e.g. TiO⁺, TiO) are good candidates for detection

— no such species identified to date

Information provided by: http://rsc.anu.edu.au