1. Theoretical Study on Vibronic Interactions and Photophysics of Low-lying Excited Electronic States of Polycyclic Aromatic Hydrocarbons S. Nagaprasad Reddy School of Chemistry University of Hyderabad India

2. Motivation  PAH (polycyclic aromatic hydrocarbons)-DIB (diffuse interstellar bands) hypothesis and origin of DIBs  Motivated by numerous observations by stellar as well as laboratory spectroscopists  High resolution ZEKE and MIS studies  Tn + may cause for the 864.9 nm and 862.1 nm DIBs  Pn + may cause for the 828.3 nm and 832.1 nm DIBs

3. Outline of work  Electronic structure calculations and PESs  Hamiltonian construction  Nuclear dynamics and photoelectron spectra  Relation with DIBs  Relative comparison with previous work on Np and An

4. Introduction Vibronic interactions:  Interaction of electronic and nuclear motion  separation of motions: B-O approximation  Breakdown at degeneracy and near-degeneracy  Conical Intersections  Photoinduced dynamics – Spectroscopy and Reaction Dynamics

5. Molecular Hamiltonian - Born-Oppenheimer Adiabatic approximation Theoretical Methodology = 0 Hellmann-Feynman Expansion - Singular Behavior at degeneracy  Adiabatic representation – Unsuitable for nuclear dynamics Where, and

6. Adiabatic and Diabatic Representations H 11 H 12 H 21 H 22 AdiabaticDiabatic cusp Smooth Lichten, Phys. Rev. 131, 229 (1963); Smith, Phys. Rev. 179, 111 (1969)

7. Vibronic effects in Electronic Spectroscopy  Broad and Diffuse vibronic bands  appearance of forbidden bands  excitation of non-totally symmetric modes  low quantum yield or lack of fluorescence  Fast decay of electronic states  Loss of mirror symmetry of absorption and emission

8. Hamiltonian construction Symmetry Selection rule

9. Nuclear dynamics Simulation of eigenvalue spectrum:  Time-independent approach  Time-dependent approach - Fermi’s Golden Rule - Diagonalization by Lanczos algorithm - Eigen Values (Energetic positions) - Convolution with Lorentzian Function - Wavepacket Propagation - Fourier Transform of Autocorrelation Function - Electronic populations and life times - Heidelberg MCTDH Method

10. Electronic structure calculations Gaussian-03 Ground state – MP2/cc-pVDZ Excited states – OVGF/cc-pVDZ RMS error - 0.0029 E H

11. Potential energy surfaces and conical intersections

12. Vibronic coupling parameters

13. Photoelectron spectroscopy Tn PnHn S. Nagaprasad Reddy and S. Mahapatra, J. Phys. Chem. A., 2013, 117, 8737.

14. Nuclear dynamics Energy difference between A minimum and A-B CI Tn + - 0.08 eV Pn + - 0.26 eV Hn + - 1.02 eV

15. Adiabatic population Popualtion decay rates of B-state Np + 240 fs An + 63 fs Tn + 22 fs Pn + 14 fs Hn + 77 fs Energy difference between A-B CI and B minimum Np + 0.48 eV An + 0.10 eV Tn + 0.027 eV Pn + 0.016 eV Hn + 0.6 eV

16. ZEKE spectral comparisons In our case, the peaks at 475 and 766 cm -1 are assigned to the X-A coupling vibrational modes, which are in contrast, assigned to combination bands of a u modes, in the study of Zhang et al. J. Zhang et al., J. Chem. Phys, 128, 104301, 2008. A.Amirav et al., J. Chem. Phys, 71, 2319, 1979. J. Zhang et al., APJ, 715, 485, 2010. Tn Pn

17. Tn + DIBs Tn + may cause for 8649 Å DIB [3] Tn + may cause for 8621 Å DIB [2] Our theoretical results reveals that the first strongest absorption band of D 2 may cause for Tn + DIB It is coming at ~8680 Å 1) J. Szczepanski et al. Chem. Phys. Let, 245, 539, 1995. 2) F. Salama et al. APJ, 526, 265, 1999. 3) F. Salama. Origins of Life and Evolution of the Biosphere, 28, 349, 1998. Ar

18. Pn + DIBs Pn+ may cause for 8283 and 8321 Å DIBs [3] From our theoretical results reveals that the second strongest absorption band of D 1 may cause for Pn + DIB It is placing at ~8271 Å 1) J. Szczepanski et al. Chem. Phys. Let, 232, 221, 1995. 2) F. Salama. Origins of Life and Evolution of the Biosphere, 28, 349, 1998. 3) F. Salama et al. APJ, 526, 265, 1999. Ar Ne

19. 1)All theoretical results are in good agreement with experimental photoelectron spectral results, ZEKE and MIS studies 2)In case of Tn + 0-0 peak may cause for 8649 Å DIB, is in good agreement with our theoretical value 8680 Å 3)In case of Pn + second strongest absorption peak may cause for 8283 Å DIB, is in good agreement with our theoretical value 8271 Å Conclusions

20. Acknowledgements Prof. Susanta Mahapatra (supervisor) Present and former labmates CSIR, DST & UPE for financial support C M S D - UoH