Fullerenes: their role in
interstellar extinction and
diffuse microwave emission
in our Galaxy.
Susana Iglesias Groth
Instituto de Astrofísica de Canarias.
1
Fullerenes: the third alothropic form of carbon
(graphite+diamond+fullerenes)
1985 The discovery of C60 Experiments
by Kroto and Smalley aimed to reproduce the
chemistry of the atmospheres of red giant stars
(H.W.Kroto et al.,Nature 318,1985)
C60
• General characteristics C60
- 60 carbon atoms
Hollow
- atoms follow simmetry of a truncated icosahedral
molecule
distributed in pentagons (12) and hexagons (20)
- Approximated radius of the sphere 3.55 Å
π
•Electronic structure:
-Each of the 60 atomic orbitals 2pz (π molecular states)
• Orbitals 2s, 2px y 2py distributed in a plane tangential to the molecular surface --> σ orbitals
Their combination produces 3 hybrid orbitals sp2
2
Fullerene families
-Fullerenes with a number of atoms 20 (m2 + n2 + nm) Æ icosahedrum
simmetry groups I ó Ih. Higher stability m=n (60 n2)
Æ (C60 ,C240 ,C540 ,C960 , C1500 , C2160 , C2940 , C3840 , C4860 ...)
-Buckyonions, multishell fullerenes
Higher stability than individual fullerenes
Many shells with separations 3.4-3.5 Å
Discovery: Ugarte (Nature, 359, 1992)Æelectronic
bombardment on carbon soot .
-annealing of carbon soot and nanodiamonds
( T approx. 2000 K ) ( Kuznetsov et al., 1994;
C60@C240@C540@C960@C1500
Tomita et al., 1999 and 2001)
-carbon ions in metallic substrates
(Cabioc´h 1995; 1997)
Real picture
Electronic microscopy
3
Fullerenes in Meteorites
and Earth
Mass spectrum measured in the Allende
Meteorite (carbonaceous chondrite)
0.1 ppm
Met. Allende.
Becker et al
1994,1999
S. Vries et al. 1993, Geochim. Cosmochim. Acta 57, 933 (1993); L. Becker
et al., Nature 372, 507 (1994); L. Becker and T.E.Bunch, Meteorities &
Planetary Science 32, (1997) Æ 5-10 ppb, + C60HX
Met. Munchinson L. Becker et al., Proc.Natl.Acad Sci. USA 97,
2979 507 (2000)
Met. Lake Tagisch (Canadá) S. Pizzarello et al., Science 293
(2000).
Detected in sedimentary layers of the
cretaceous-Tertiary (KTB) boundary
¿Formation mechanism ?
Red giant stars and outflows of carbon stars
4
Goal of this study:
Calculate the photoabsorption spectrum
of fullerenes and buckyonions and
compare with the main features of
interstellar extinction
5
Photoabsorption spectrum of icosahedric fullerenes: a semiempirical model
-Theoretical approach based:
Hückel, Pariser-Parr-Pople (PPP)
Berkowitz
Spectra obtained in
gaseous phase
-Strong electronic correlation -> screening effects are
relevant (Random Phase Approximation)
Kamke
2 Prominent spectral features of C60 bands at 6 and 23 eV
Molecular monoelectronic Hamiltonian:an effective monoelectronic model
-Born-Openheimer approximation
-Use of valence electrons
-Separability−π/σ ÆThe effective electronic hamiltonian is built as sum of monoelectronic contributions
-The spatial part of the spin-orbital is built as a lineal combination of atomic orbitals
-The molecular wave function for the fundamental state of the He0 is built as a Slater determinant from
monoelectronic spin-orbitals.
-The excited state functions are monoelectronic transitions between the fundamental and excited state.
6
Monoelectronic hamiltonians in the formalism of tight-binding as a function of its N orbitals
and in the notation of second quantization
•Hamiltonian associated to electrons π
Adopted as matrix elements
effective values that reproduce the energy levels
and wave functions selfconsistently calculated following the PPP
Repulsion integrals γss=γ ,γst=χ/rst
Hopping between orbitals 2pz Æ β
The eigenvalue equation is solved in the formalism HF-SCF
•Hamiltonian associated to electrons σ
7
Unscreened polarizabilities
Icosahedric fullerenes: isotrophic
polarizability tensor
Cartesian components of the
dipolar moment operator
Screened polarizability
RPA
Interaction effects
(canje y correlación)
ζ=1/R3e
•Photoabsorption
cross sections
8
Results for single fullerenes
9
Photoabsorption cross section of C60 and
experimental spectra Berkowitz and Kamke
Photoabsorption cross sections calculated
for five icosahedric fullerenes.
Width : Γω=0.001 eV y Γω =0.06 ω
0.06ω
0.001
Γω=0.3eV (ω<8eV)
Γω=0.3+0.2(ω−8) (eV) (ω>8eV)
S. Iglesias-Groth, Ruiz, Bretón, Gómez Llorente, Journal of Chemical Physics 116, 1648 (2002)
Thesis, S. Iglesias-Groth, Univ. La Laguna (2003)
10
Results obtained for buckyonions
(multishell fullerenes)
11
Polarizabilities and photoabsorption spectra of buckyonions
-Assumption: each shell contributes independently to the global monoelectronic structure
-The effects of inter and intrashell electronic interactions (screening) Æ RPA Æ use of classical
electrostatic model
Theoretical model
Distribution of dipolar charge (l=1) on each sphere
+ boundary conditions
Induced dipolar moment
Coupled polarizability
It is only required knowledge of the effective monoelectronic
structure for each fullerene shell
12 , 2003
S. Iglesias-Groth, A. Ruiz, J. Bretón and J.M.Gómez Llorente, J.Chem. Phys., 118, 7103
Photoabsorption spectra
of buckyonions
C60@C240@C540
C240@C540
C60@C540
C60@C240@C540@C960@C1500
C60@C240@C540@C960
PhD Thesis, S. Iglesias-Groth (2003)
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Fullerenes and buckyonions
in the
interstellar medium
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Fullerenes and buckyonions in the interstellar medium.
1.-UV Bump
-The extinction of radiation λi can be approximated
abs
abs
The reddening factor in the
diffuse interstellar medium is
RV =3.1
(N surface density of particles)
Extinction curves from
Fitzpatrick 1999
-Suggested explanations:
Graphite : peak 4.68-4.8 µ -1;
Band 2175Å
5.7eV
4.6µ−1
PAHs : in general produce bands
at lower energies than graphite;
Multishell spherical layers:
Range of peak position 2193 y 2157 Å
electrostatic approach (promising)
range of widths : 0.96-1.55 eV
2.-The diffuse interstellar bands
-More than 400 bands known between 4000 y
10000 Å see e.g. Herbig (An. Rev. Astron.
Astroph. 1995)
9577 and 9632 Å bands associated to C60+
(Foing and Ehrenfreund Nature, 369, 1994)
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Fullerenes and buckyonions:carriers of interstellar absorption?
Our theoretical spectra and the 2175 Å band
Comparison between cross sections of buckyonions and extinction curves
C60@C240@C540
Curve
Fitzpatrick
Rv=3.1
C180@C720
16
2175Å
Best fit for a grain
size distribution
consisting
of a power law
dn/dR ∝ R-3.5 ±1.0
Buckyonions spectra
Æ3840
monoshells+ buckyonions
60 Æ3840
Estimation of porcentage of carbon locked in fullerenes and buckyonions in the ISM
0.2-0.08 fullerenes per million Hydrogen atoms (ppm)
-Interstellar Carbon in fullerenes and buckyonions
n(C ) /n(H) = 90-110 x 10-6 (25%)
Consistency with carbon budget for ISM.
Iglesias-Groth, 608:L37, ApJ Letter 2004
17
Theoretical spectra and DIBs
Monoshell fullerenes
Buckyonions with a
complete number of shells
18
Comparison of predicted transitions with the wavelengths of
the 30 stronger DIBs
Æ
+ Single fullerenes
Buckyonions complete shells
--- stronger observed DIBs
19
Another prediction:
Fullerene based molecules should
rotate at high speed in the
interstellar medium....
If they had a dipole moment,
then
we may expect electric dipole
radiation
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Εlectric dipole emission of fullerene based molecules
Processing affects to the rotation molecules in ISM
HC60
ω
Η
(Drain & Lazarian, 1998)
Rotational damping
mechanisms:
Rotation excitation
mechanisms:
Collisional drag
Excitación by the plasma
Plasma drag
Infrared emission
Infrared emision
Photoelectric emission
Electric dipole damping
CNM
25% Hydrogenation
25% Hidrogenation
(1 - 7 Debyes)
Dipole moment C-H =0.3 Debye
Dipole moment of fullerane with N H atoms
µ=0.3(N)0.5
υ (GHz)
rotation
angular
velocity
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The emissivity as a function of frequency
30% Hydrogenation
for
Individual molecules
Εmissivity (Jy sr-1/H)
υ(GHz)
Asuming the law R-3.5
Peak at 15-30 GHz
for
the mixture
propoused
to explain
UV bump
The values obtained are close
to the measurements of
anomalous
diffuse galactic emission
obtained by the COBE and
Tenerife CMB experiments
22
Conclusions:
•
The cross sections obtained for single fullerenes and buckyonions reproduce the
behaviour of the interstellar medium UV extinction curve. The prominent band
obtained between 5 and 6 eV in the theoretical cross sections can explain the
position and widths observed for the 2175 Å bump. They also reproduce the rise in
the extinction curve at higher energies.
•
We infer ISM densities of 0.2 and 0.08 ppm for single fullerenos and buckyonions
respectively (very similar to the densities of fullerenes observed in meteorites 0.1
ppm). We show that these results are consistent with estimates for the carbon budget
in the ISM. Fullerenes and buckyonions are possibly the most abundant form of
carbon in the ISM.
Our computations also show that fullerenes and buckyonions present weaker
transitions in the optical and near infrared with their number decreasing towards
longer wavelengths. These transitions may be responsible for some of the known but
unexplained diffuse interstellar bands.
•
•
Finally, hydrogenated forms of fullerenes may produce electric dipole radiation and
contribute to the so far unexplained anomalous microwave emission detected by
Cosmic Microwave Background experiments like COBE and Tenerife.
23