Embed Size (px)
Citation preview
Laboratory formation of fullerenes from PAHs:Top-down interstellar chemistry.
Junfeng Zhen, Pablo Castellanos, Daniel Paardekooper,Harold Linnartz & Alexander Tielens
Leiden Observatory
Photodissociation in AstrochemistryFebruary 3, 2015
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 1 / 15
Outline
1 Introduction
2 Top-down Chemistry
3 Set-up
4 Samples Properties and Goals
5 Results and Future Work
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 2 / 15
Why C60?
C60 (and C70) are the largest molecules detected in the ISM.
Highly stable molecule.
Its formation is related to other carbon bearing compounds.
Ubiquitous in the ISM.
Possible culprit of some DIBs.
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 3 / 15
Observations
First confirmed detection (along with C70) in circumstellarenvironment of PN Tc 1 (Cami et al. 2010).Later detections in wide variety of environments and physicalconditions: PNe, post-AGB stars, YSOs, Herbig Ae/Be and PDRsassociated with RNe and Hii regions.
Cami et al. (2010).
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 4 / 15
C60 Formation
Formation in the envelope ofAGBs has limited efficiency(Bernard-Salas et al. 2012).
Berne & Tielens (2012) proposedthat PAH dehydrogenation,followed by C2-losses, can formC60.
Micelotta et al. (2012) propose asimilar mechanism, but startingfrom HAC.
Sellgren et al. (2010).
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 5 / 15
Top-down Chemistry
Berne & Tielens (2012).
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 6 / 15
i-PoP
Ion GateElectron Gun Ion Trap TOF
Nd:YAG laserOven
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 7 / 15
“Magic Numbers”
C44+ C48
+
Peaks enhanced in PAH dissociation.
Formation of fullerenes shows peaks withenhanced intensity.
“Magic numbers”: C44, C50 and C56. Kroto et al. (1987)
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 8 / 15
Electronic Absorption Spectra
C60 has an absorptionminimum at ∼500–600 nm.
C70 and large PAHs absorbefficiently.
At shorter wavelengths theabsorption becomescomparable for the three.
Tatsuhisa et al. (1991), Malloci et al. (2007)
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 9 / 15
Samples
Fullerenes follow “cage route”only.
Irradiation with 266, 355 and532 nm.
C +60 does not dissociate at
532 nm.
Goals:
Dehydrogenation and C2 loss.
Compare dissociation patterns.
C2 loss necessary forisomerization?
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 10 / 15
Dehydrogenation
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 11 / 15
266 and 355 nm Irradiation
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 12 / 15
532 nm Irradiation
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 13 / 15
C78H26 Fragmentation
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 14 / 15
Conclusions
C60 can be formed from large PAHs efficiently.
Large PAHs pass through a first step of fast dehydrogenation.
C2 losses are a necessary step for isomerization of closed-cage.
Smaller (and larger) cages can also be formed.
Future Work:
Confirmation with IR spectroscopy.
Derive energies involved using synchrotron radiation.
Pablo Castellanos (Leiden Obs.) Fullerenes and PAHs PD in Astrochem. 15 / 15
