From Stars to Life, April 3-6 2013, Gainesville FL Simulating chemistry on interstellar dust grains in the laboratory Nicolas Polfer University of Florida Department of Chemistry http://www.cosmotography.com/images/cosmic_nurseries.html
Simulating chemistry on interstellar dust grains in the laboratory starting with known molecules (e.g. polycyclic aromatic hydrocarbons) as simple analogues to dust grains
Background Polycyclic aromatic hydrocarbons (PAHs) in the ISM emission spectrum from the Orion ionization ridge C-H in-plane C-C stretching C-H out-of-plane Orion ionization ridge mix of lab absorption spectra of PAHs. Ap J 1999, 511, L115. 3
Project 1: Formation of H 2 in the Interstellar Medium (ISM) Polycyclic Aromatic Hydrocarbons (PAHs) Hypothesis Photolysis reactions contribute to enhanced H 2 column densities in ISM Aims Experimentally: establish reaction rates and branching ratios for photolysis Theoretically: understand reaction pathway(s) UV/vis -H 2 Rates? Yields? Pathways? 4
Project 2: Depletion of metal atoms (Ni, Fe) in the ISM Depletion Factors for Selected Interstellar Elements compared to Solar Gas-Phase Elements Many metals are depleted Hypothesis Gas-phase elements depleted in ISM due to binding to PAHs (dust grains)? Aims Experimentally: measure IR spectra and compare to Theoretically: calculated IR spectra establish binding energies Phys. Scripta 165, T47, 1993 Binding pattern and binding energy?
Experimental techniques 12.95 Temperature Controller 12 K Helium Cycle Cryostat Pump Rotary Feed- Through cryogenic set-up (12 K) simulate ices on dust grains preparation of complexes: - UV lamp irradiation (photolysis) - laser ablation of metal structural interrogation of complexes by infrared spectroscopy (FTIR) Gold (Fe doped)- Constantant TC Copper Sample Mount Resistive Heater CsI 12K Window Ni Rod Ar gas FT PAHs Oven IR Nd-YAG Laser RGA
Project 1: Formation of H 2 Classes of PAHs anthracene coronene PAHs with aliphatic (sp 3 ) carbons 1,2-dihydronaphthalene 9,10-dihydroanthracene 5,12-dihydrotetracene 7
H 2 Loss of 9,10-dihydroanthracene (9,10-DHA) 9 10 1 2 -H 2 One UV photon 9,10-DHA Anthracene
Compute dissociation pathway for H atoms removal from C9 and C10 How to confirm experimentally? = 546 nm Density functional theory B3LYP/6-311++G(d,p), Zero Point Energy(ZPE) scaling factor 0.98 9
UV-vis absorbance spectrum Hg lamp emission Low-pass filter
Identify photolysis tetracene product based on reference IR spectrum Photolysis products Precursor CO 2 bands Intensity after 34 hr UV-photolysis after 12 hr UV-photolysis DHT without photo Wavenumber (cm -1 )
Photolysis: confirmation by IR spectroscopy Quantify depletion of precursor and appearance of products * anthracene -H 2 UV (254 nm) dihydroanthracene Ap. J., 2012, 744, 61 * Methanol band Unidentified fragments bands are overlaped by strong precursor bands 12
DHA: Branching ratios and rates 9 10 -H 2 1 2 90% yield One UV photon dihydroanthracene Rate Experiment 40% depletion for 21 hrs @ 4.4x10 14 photons cm -2 s -1 10%? ISM anthracene In dark clouds @ 10 3 photons cm -2 s -1 ~770 million years At ionization ridges @ 10 7 photons cm -2 s -1 ~77,000 years
dihydroanthracene dihydrotetracene 2.595 ev = 478 nm 12 5 1 2 +H 2 0 ev 1.008 ev 1.007 ev B3LYP/6-311++G(d,p)
Dihydrotetracene: Branching ratios and rates (cf. 90% for dihydroanthracene) 32% yield -H 2 One UV photon dihydrotetracene Rate Experiment cf. 40% cf. 21 hrs 20% depletion for 34 hrs @ 4.4x10 14 photons cm -2 s -1 68%? ISM tetracene Add one aromatic ring 9 times less efficient dehydrogenation In dark clouds @ 10 3 photons cm -2 s -1 ~6 billion years At ionization ridges @ 10 7 photons cm -2 s -1 ~600,000 years
Cationic PAHs + coronene VERY different conditions: Gas-phase ions @ room temperature
Binding pattern and binding energy? Project 2: Metal binding Gas-phase elements depleted in ISM due to binding to PAHs
Binding energies of Metal-PAH s (Metal = Ni, Fe, or Fe + ) MPW1PW91/6-31+G(d,p)) (Fe-PAH) + Ni-PAH D 0 / ev Dissociation Energy, D o = E ZPE (Ligand) + E(Ni) -E ZPE (Ni(Ligand)) Fe-PAH* Naphthalene Fluorene Pyrene Coronene *Chem. Phys. 2007, 342, 107 Metal-PAH s
Example: Ni on pyrene 0.0 ev 0.145 ev B C A Pyrene 0.116 ev 0.261 ev 0.0 ev
Exp. Ni- Pyrene 0.145 ev 0.261 ev 0.0 ev A Calc C Calc B Calc Exp.
Project 1: Formation of H 2 Summary -Confirm H 2 formation for photolysis from neutral PAHs - BUT rates/yields decrease with size - Cationic PAHs more promising Project 2: Binding of metal atoms - Open questions about accuracy of computations - IR spectroscopy for benchmarking of theory Nathan Roehr Acknowledgements Jan Szczepanski Yi Fu Poster #74 $$$ Opportunity Seed Fund (UF) $$$
Proof of H 2 : detection of mass 2 1.2x10-6 H 2 /Ar (0.1%) matrix H 2 Partial Pressure [Torr] 240 200 160 120 80 40 0 30 20 10 0 8.0x10-7 4.0x10-7 5 10 15 20 25 30 35 Matrix Temperature [K] 0.0 H 2 trapped in matrix, detected at m/z 2 5 10 15 20 25 30 35 Photolyzed dihydroanthracene/ar matrix Photolyzed Ar only matrix H 2 trapped in matrix, detected at m/z 2