Simulating chemistry on interstellar dust grains in the laboratory

Transcription

1 From Stars to Life, April , Gainesville FL Simulating chemistry on interstellar dust grains in the laboratory Nicolas Polfer University of Florida Department of Chemistry

2 Simulating chemistry on interstellar dust grains in the laboratory starting with known molecules (e.g. polycyclic aromatic hydrocarbons) as simple analogues to dust grains

3 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

4 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

5 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?

6 Experimental techniques 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

7 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

8 H 2 Loss of 9,10-dihydroanthracene (9,10-DHA) H 2 One UV photon 9,10-DHA Anthracene

9 Compute dissociation pathway for H atoms removal from C9 and C10 How to confirm experimentally? = 546 nm Density functional theory B3LYP/ G(d,p), Zero Point Energy(ZPE) scaling factor

10 UV-vis absorbance spectrum Hg lamp emission Low-pass filter

11 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 )

12 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

13 DHA: Branching ratios and rates H % yield One UV photon dihydroanthracene Rate Experiment 40% depletion for x10 14 photons cm -2 s -1 10%? ISM anthracene In dark 10 3 photons cm -2 s -1 ~770 million years At ionization 10 7 photons cm -2 s -1 ~77,000 years

14 dihydroanthracene dihydrotetracene ev = 478 nm H 2 0 ev ev ev B3LYP/ G(d,p)

15 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 x10 14 photons cm -2 s -1 68%? ISM tetracene Add one aromatic ring 9 times less efficient dehydrogenation In dark 10 3 photons cm -2 s -1 ~6 billion years At ionization 10 7 photons cm -2 s -1 ~600,000 years

16 Cationic PAHs + coronene VERY different conditions: Gas-phase room temperature

17 Binding pattern and binding energy? Project 2: Metal binding Gas-phase elements depleted in ISM due to binding to PAHs

18 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

19 Example: Ni on pyrene 0.0 ev ev B C A Pyrene ev ev 0.0 ev

20 Exp. Ni- Pyrene ev ev 0.0 ev A Calc C Calc B Calc Exp.

21 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 $$$ Opportunity Seed Fund (UF) $$$

22 Proof of H 2 : detection of mass 2 1.2x10-6 H 2 /Ar (0.1%) matrix H 2 Partial Pressure [Torr] x x Matrix Temperature [K] 0.0 H 2 trapped in matrix, detected at m/z Photolyzed dihydroanthracene/ar matrix Photolyzed Ar only matrix H 2 trapped in matrix, detected at m/z 2