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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
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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
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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
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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
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Molecular Hamiltonian -
Born-Oppenheimer Adiabatic approximation
Theoretical Methodology
= 0
Hellmann-Feynman Expansion
- Singular Behavior at degeneracy
Adiabatic representation – Unsuitable for nuclear dynamics
Where,
and
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Adiabatic and Diabatic Representations
H11 H12
H21 H22
Adiabatic Diabatic
cuspSmooth
Lichten, Phys. Rev. 131, 229 (1963); Smith, Phys. Rev. 179, 111 (1969)
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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
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Hamiltonian construction
Symmetry Selection rule
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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
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Electronic structure calculations
Gaussian-03Ground state – MP2/cc-pVDZExcited states – OVGF/cc-pVDZ
RMS error - 0.0029 EH
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Potential energy surfaces and conical intersections
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Vibronic coupling parameters
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Photoelectron spectroscopy
Tn Pn Hn
S. Nagaprasad Reddy and S. Mahapatra, J. Phys. Chem. A., 2013, 117, 8737.
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Nuclear dynamics
Energy differencebetween A minimumand A-B CITn+- 0.08 eVPn+- 0.26 eVHn+- 1.02 eV
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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
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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 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 Pn
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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 ofD2 may cause for Tn+ DIBIt 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
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Pn+ DIBs Pn+ may cause for 8283 and 8321 Å DIBs [3]
From our theoretical results reveals that the second strongest absorption band ofD1 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
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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
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Acknowledgements
Prof. Susanta Mahapatra (supervisor) Present and former labmatesCSIR, DST & UPE for financial support
ISMS-14
C M S D - UoH
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Thank You