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

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

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

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

Molecular Hamiltonian -

Born-Oppenheimer Adiabatic approximation

Theoretical Methodology

= 0

Hellmann-Feynman Expansion

- Singular Behavior at degeneracy

Adiabatic representation – Unsuitable for nuclear dynamics

Where,

and

Adiabatic and Diabatic Representations

H11 H12

H21 H22

Adiabatic Diabatic

cuspSmooth

Lichten, Phys. Rev. 131, 229 (1963); Smith, Phys. Rev. 179, 111 (1969)

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

Hamiltonian construction

Symmetry Selection rule

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

Electronic structure calculations

Gaussian-03Ground state – MP2/cc-pVDZExcited states – OVGF/cc-pVDZ

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 differencebetween A minimumand A-B CITn+- 0.08 eVPn+- 0.26 eVHn+- 1.02 eV

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

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

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

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

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

Acknowledgements

Prof. Susanta Mahapatra (supervisor) Present and former labmatesCSIR, DST & UPE for financial support

ISMS-14

C M S D - UoH

Thank You