Theoretical Study on Vibronic Interactions and Photophysics of

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

Motivation v PAH (polycyclic aromatic hydrocarbons)-DIB (diffuse interstellar bands) hypothesis and origin of DIBs v Motivated by numerous observations by stellar as well as laboratory spectroscopists v High resolution ZEKE and MIS studies v Tn+ may cause for the 864. 9 nm and 862. 1 nm DIBs v Pn+ may cause for the 828. 3 nm and 832. 1 nm DIBs

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

Introduction Vibronic interactions: v v v 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

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

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

Vibronic effects in Electronic Spectroscopy v v v 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

Hamiltonian construction Symmetry Selection rule

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

Electronic structure calculations Gaussian-03 Ground state – MP 2/cc-p. VDZ Excited states – OVGF/cc-p. VDZ RMS error - 0. 0029 EH

Potential energy surfaces and conical intersections

Vibronic coupling parameters

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

Nuclear dynamics Energy difference between A minimum and A-B CI Tn+- 0. 08 e. V Pn+- 0. 26 e. V Hn+- 1. 02 e. V

Adiabatic population Popualtion decay rates Energy difference between A-B CI and B minimum of B-state Np+ 0. 48 e. V Np+ 240 fs An+ 0. 10 e. V An+ 63 fs Tn+ 0. 027 e. V Tn+ 22 fs Pn+ 0. 016 e. V Pn+ 14 fs Hn+ 0. 6 e. V Hn+ 77 fs

ZEKE spectral comparisons Tn Pn 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 au 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+ 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 Ar 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.

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 Ne Ar 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.

Conclusions 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

Acknowledgements Prof. Susanta Mahapatra (supervisor) Present and former labmates CSIR, DST & UPE for financial support ISMS-14 C M S D - Uo. H

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