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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

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.

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

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

11. 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, and for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

12. 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, and for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

13. 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, and for anthracene, tetrahydroanthracene and octahydroanthracene, respectively.

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

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

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

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

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

19. 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.

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

21. 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.

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

23. 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

24. Thank you for listening