1. High-resolution IR absorption spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs): shining light on the interstellar 3 micron emission bands… Elena Maltseva University of Amsterdam

3. PAHs in interstellar space? www.spitzer.caltech.edu

4. PAHs in interstellar space? Tielens 2008 www.spitzer.caltech.edu

5. Spectroscopy under interstellar conditions Isolated conditions (high vacuum 10 -9 bar) Cold molecules (< 10 K) Vacuum 2 bar

6. S0S0 S1S1 Excitation Ionization Excitation energy IE Ionization and detection

7. ω I I ω1ω1 IR–UV double-resonance spectroscopy S0S0 S1S1 IE ω1ω1 200 ns ω

8. Naphthalene : Harmonic prediction 8 C-H oscillators → 8 normal modes D 2h symmetry → 4 modes IR active G09.D01 B9-71/TZ2P

9. IR–UV double-resonance spectroscopy of naphthalene with harmonic DFT Blue harmonic force field G09, D01 B9-71/TZ2P

10. …and anharmonic calculations Blue harmonic force field Green anharmonic force field G09, D01 B9-71/TZ2P

11. Assignment of naphthalene Experiments on naphthalene-d 8 do not show any structure in 3 μm region → additional bands not solely due to anharmonicity → Fermi resonance Naphthalene

12. Anharmonic calculations by Spectro 15 Violet anharmonic force field Spectro15

13. IR–UV double-resonance spectroscopy of anthracene Blue harmonic force field Green anharmonic force field G09, D01 B9-71/TZ2P

14. Anharmonic calculations by Spectro 15 Violet anharmonic force field Spectro15

15. IR–UV double-resonance spectroscopy of tetracene Blue harmonic force field Green anharmonic force field G09, D01 B9-71/TZ2P

16. Anharmonic calculations by Spectro 15 Violet anharmonic force field Spectro15

17. IR–UV double-resonance spectroscopy of 3- and 4-ring systems phenanthrene pyrene Benz[a]anthracene chrysene Triphenylene

18. B1 3.3 B2 3.3 A 3.3 3 µm enigma

19.  Charge state  Size  Heteroatom substitutions  Anhamonicty  Edge structure A 3.3 3 µm enigma: Interpretation Two-component emission : A 3.3 –B1 3.3 –B2 3.3 3.28/3.30 µm intensity ratio as ≥1, ∼ 0.5 ≤0.2, 1 pair of bay-hydrogens 2 pairs of bay-hydrogens 3 pairs of bay-hydrogens

20.  Charge state  Size  Heteroatom substitutions  Anhamonicty  Edge structure A 3.3 3 µm enigma: Interpretation Two-component emission : A 3.3 –B1 3.3 –B2 3.3 3.28/3.30 µm intensity ratio as ≥1, ∼ 0.5 ≤0.2,

21. 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?

22. Discussion & conclusions 1.High-resolution IR spectra of naphthalene, anthracene, tetracene, phenanthrene, pyrene, chrysene, triphenylene, 1,2-benzanthracene have been obtained 2.Experiments well-suited for studies of the molecular physics of PAHs Isolated and cold molecules Conformation- and mass-selective method 3.Harmonic calculations do not work for 3μm region! 4.Anharmonic calculations with properly treated Fermi resonance must be used in characterization of 3 μm features in interstellar space 5. Two-component emission can be explained by presence of bay-hydrogen sites 6.3 µm plateau can originate from overlapping of combination bands

23. 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) Dutch Astrochemistry Network Spinoza prize Prof. Tielens

25. Gas-phase study of PAHs Done only for Naphthalene, Anthracene, Phenanthrene, Pyrene, Perylene Is it only ‘the right’ absorption? Signal/noise Assignment

26. Infrared Spectroscopy of Matrix Isolated Polycyclic Aromatic Hydrocarbons