1. The 3 μm emission band from space with molecular beam spectroscopy
Elena Maltseva, Cameron J. Mackie, Alessandra Candian, Annemieke Petrignani, Xinchuan Huang, Timothy J. Lee, Alexander G. G. M. Tielens, Jos Oomens and Wybren Jan Buma

2. PAHs in interstellar space?
Tielens 2008

3. 3 µm enigma Irregular shape of 3.29 µm band
Assignment of 3.4 µm feature
Origin 3 µm plateau

4. 3 µm enigma Irregular shape of 3.29 µm band A3.3 B13.3
Assignment of 3.4 µm feature
B23.3
Origin 3 µm plateau

5. Spectroscopy under interstellar conditions
Spectroscopy in molecular beams:
Isolated conditions (high vacuum 10-9 bar)
Cold molecules (< 10 K)
Resonance-enhanced multiphoton ionization (REMPI) and IR ion dip spectroscopy
Conformational selection
Mass spectrometry
Mass-selection S0 S1 IE M+ M
IR hole burn
Probe

6. Experiment vs. Theory: Tetracene
Red (G09-h) - Harmonic force field
G09, D01
B9-71/TZ2P
Green SP15 – SPECTRO 15 :
SP15,16 treats couplings between multiple Fermi resonances
(modes falling within 200 cm-1 of each other)
This experiment

7. Experiment vs. Theory: Tetracene
Red (G09-h) - Harmonic force field
G09, D01
B9-71/TZ2P
Green SP15 – SPECTRO 15 SP15,16 treats couplings between multiple Fermi resonances
(modes falling within 200 cm-1 of each other)
This experiment

8. Experiment vs. Theory: Benz[a]anthracene

9. 3 µm enigma: Interpretation
6 bay Hs
4 bay Hs
2 bay Hs
0 bay H

10. 3 µm enigma: Interpretation
Trios
Quartets
Combination
bands
bay
Solo
Duos

11. 3 µm plateau
L. J Allamandola, A. G. G. M. Tielens & J. R. Barker 1985: A quasi-continuum of overlapping combination bands and overtones of PAH molecules? Yes and No!

12. 3 µm plateau ?
PAHs
Minor features
Plateau

13. Aliphatic C-H Stretch + combination bands
3 µm plateau
PAHs
Aliphatic C-H Stretch + combination bands

14. C-H Stretch vibrations of methylated and hydrogenated anthracene
Much higher density of state due to Methyl-rotors =>
Does not contribute significantly to the 3 µm emission!!!

15. Hydrogenated PAHs

16. Hydrogenated PAHs
PAHs + hydrogenated PAHs
PAHs

17. Experiment vs. observations
2 rings
3 rings
4 rings
5 rings

18. Discussion & conclusions
Naphthalene, anthracene, tetracene, phenanthrene, pyrene, chrysene, triphenylene, 1,2-benzanthracene, perylene, 9,10-dihydroanthracene, 9,10-dihydrophenanthrene, 9,10-dimethylanthracene, 1,2,3,6,7,8-hexahydropyrene, 1,2,3,4-tetrahydronapthalene
Harmonic approximation => Anharmonic calculations with properly treated Fermi resonance must be used in characterization of the 3-μm features in interstellar space
Shape of the 3-μm band is determined by the edge structure of the molecules.
The 3-µm plateau can originate from overlap of combination bands coupled to CH-stretching and aliphatic vibrations
Methylated PAHs do not contribute to the 3 µm emission due to presence of Methyl-rotor

19. and you for your attention !
Acknowledgment
Wybren Jan Buma and Molecular Photonics group (UvA)
Cameron Mackie, Alessandra Candian, Annemieke Petrignani, Xander Tielens (Leiden Observatory)
Jos Oomens (Molecular Structure and Dynamics, FELIX Facility)
Timothy Lee and Xinchuan Huang (NASA Ames Research Center)
and you for your attention !
Dutch Astrochemistry Network
Spinoza prize Prof. Tielens