Hydrogenation of PAHs and its effect on the UIR band spectrum Rachel Gover Supervisors Prof. Peter Sarre & Prof. Andrei Khlobystov

About me and my project Originally from Congleton, Cheshire Graduated from University of Nottingham with MSci in Chemistry Now in second year of PhD with Professor Peter Sarre and Professor Andrei Khlobystov Project focussed on carbon-based species in the ISM: Transformations of carbon and resulting changes in vibrational spectra Mix of theoretical and some lab-based work Manchester Congleton Nottingham

NGC 7023 Photo-dissociation region in which variation in UIR band characteristics can be tracked with distance from star Blind Signal Separation (BSS) carried out on spectra of this region by Berné et al. (2007) and Rosenberg et al. (2011) O. Berné, C. Joblin, Y. Deville, J. D. Smith, M. Rapacioli, J. P. Bernard, J. Thomas, W. Reach, and A. Abergel, A&A, 2007, 469, 575 M. J. F. Rosenberg, O. Berné, C. Boersma, L. J. Allamandola and A. G. G. M. Tielens, A&A, 2011, 532, A128

Blind Signal Separation 3 separate source signals, attributed to neutral PAH molecules, PAH cations, and Very Small Grains (VSGs). 3 signals vary significantly – most notably the 11.2 µm solo CH oop bend peak. Most contributing signal to overall spectrum varies as distance from exciting star increases. M. J. F. Rosenberg, O. Berné, C. Boersma, L. J. Allamandola and A. G. G. M. Tielens, A&A, 2011, 532, A128

Blind Signal Separation Most contributing signal to overall spectrum varies as distance from exciting star increases. M. J. F. Rosenberg, O. Berné, C. Boersma, L. J. Allamandola and A. G. G. M. Tielens, A&A, 2011, 532, A128

Blind Signal Separation Most contributing signal to overall spectrum varies as distance from exciting star increases. M. J. F. Rosenberg, O. Berné, C. Boersma, L. J. Allamandola and A. G. G. M. Tielens, A&A, 2011, 532, A128

Hydrogenated PAHs PAHs with excess peripheral H atoms (Hn-PAHs) Aliphatic carbons with 2 H atoms have tetrahedral geometry – break planarity of PAH Less stable than aromatic species so are believed to exist in areas of low UV irradiation Spectral features include aliphatic C-H stretch occurring at 3.4 µm which has been detected Lab studies suggest that PAHs trapped in H20-rich ices can be transformed to H-PAHs; these species could be found on perimeter of dense molecular cloud irradiated by nearby star S. A. Sandford, M. P. Bernstein, and C. K. Materese, ApJSS, 2013, 205, 8

Outline of work 1. DFT calculations run on PAHs/hydrogenated PAHs - beginning with acenes. 3. Spectra of some molecules coadded and compared with VSG signal. 2. Vibrational spectra obtained and trends with increasing hydrogenation observed.

Results Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.983, 0.980 and 0.974 for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

Results Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.983, 0.980 and 0.974 for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

10 – 15 µm region: Anthracene Upon hydrogenation: Shift in solo CH oop bend peak to higher wavelengths (red-shifted). Decrease in intensity of quartet oop bend peak. Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.983, 0.980 and 0.974 for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

6 – 9 µm region: Anthracene Upon hydrogenation: Addition of features at 6.6 and 6.8 µm. Complex from 7.2 – 7.7 µm arises and increases in intensity. Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.983, 0.980 and 0.974 for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

3.4 µm region: Anthracene Upon hydrogenation: 3.3 µm aromatic CH str peak decreases in intensity. 3.4 µm aliphatic CH str peaks increase dramatically. Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.983, 0.980 and 0.974 for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

Tetracene and pentacene Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.985 for tetracene/hydrogenated tetracenes and 0.983 for pentacene/hydrogenated pentacenes.

Ovalene Calculations run on Qchem using DFT B3LYP/6-31G*. Results scaled by 0.975

Spectral features of VSG signal Spectra of particular hydrogenated molecules were co-added and compared with VSG signal.

Spectral features of VSG signal Spectra of particular hydrogenated molecules were co-added and compared with VSG signal. Red-shifted 11.2 µm peak

Spectral features of VSG signal Spectra of particular hydrogenated molecules were co-added and compared with VSG signal. Plateau between ~ 12 – 13 µm

Spectral features of VSG signal Spectra of particular hydrogenated molecules were co-added and compared with VSG signal. Possible features at ~ 12, 12.5 and 12.8 µm.

Spectral features of VSG signal Spectra of particular hydrogenated molecules were co-added and compared with VSG signal. Generally flattened spectrum from 12.0 µm onwards – specifically absence of 13.5 µm quartet peak.

Co-added spectra Ovalene plus hydrogenated forms. Slight features at ~ 12.0 and 12.8 µm. No quartet peak. Removing ovalene removes 12.0 µm feature. Feature at 12.8 µm still present, feature at 12.5 µm can be seen more clearly.

Comparison with VSG signal Red-shifted 11.2 µm peak. Plateau between 12 and 13 µm. Feature at 12.5 µm.

Conclusions H-PAHs are feasible candidates for ‘Very Small Grains’ based on: Trends in peak position with increasing hydrogenation. Comparisons of coadded spectra with VSG signal Other spectral regions, specifically 3.4 µm feature Their greatest contribution being seen in the region of low UV intensity Their position at the edge of molecular cloud – possibility that they are formed as a result of photo processing of H20 rich, mixed molecular ices

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