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

PAHs in interstellar space?

PAHs in interstellar space? Tielens

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

S0S0 S1S1 Excitation Ionization Excitation energy IE Ionization and detection

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

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

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

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

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

Anharmonic calculations by Spectro 15 Violet anharmonic force field Spectro15

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

Anharmonic calculations by Spectro 15 Violet anharmonic force field Spectro15

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

Anharmonic calculations by Spectro 15 Violet anharmonic force field Spectro15

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

B1 3.3 B2 3.3 A µm enigma

 Charge state  Size  Heteroatom substitutions  Anhamonicty  Edge structure A µm enigma: Interpretation Two-component emission : A 3.3 –B1 3.3 –B /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

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

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?

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

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

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

Infrared Spectroscopy of Matrix Isolated Polycyclic Aromatic Hydrocarbons