This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 14531–14541 14531 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 14531–14541 Theoretical study of the electronically excited radical cations of naphthalene and anthracene as archetypal models for astrophysical observations. Part II. Dynamics consequenceswz S. Ghanta, V. Sivaranjana Reddy and S. Mahapatra* Received 7th April 2011, Accepted 3rd June 2011 DOI: 10.1039/c1cp21084j Nuclear dynamics is investigated theoretically from first principles by employing the ab initio vibronic models of the prototypical naphthalene and anthracene radical cations developed in Part I. This Part is primarily aimed at corroborating a large amount of available experimental data with a specific final goal to establish an unambiguous link with the current observations in astrophysics and astronomy. The detailed analyses presented here perhaps establish that these two prototypical polycyclic aromatic hydrocarbon radical cations are indeed potential carriers of the observed diffuse interstellar bands. I. Introduction Vibronic energy level structure and dynamics of the six energetically lowest electronic states of the radical cation of naphthalene (Np + ) and anthracene (An + ) are examined in this paper. It should be recognized from the results of Part Iz that coupling between the electronic and nuclear motion is crucial and is expected to play a pivotal role in the spectro- scopy and dynamics of these electronic states. Such coupling triggers nonadiabatic transitions of nuclei among the electronic states and a theoretical description of the dynamics necessarily goes beyond the adiabatic Born–Oppenheimer 1 approach. In particular, conical intersections (CIs) 2–7 of electronic states are well established as mechanistic bottlenecks for ultrafast transition of nuclei among various electronic states. The identity of an independent vibrational level structure is lost in such a situation and the electronic spectrum often appears as a complex composite of lines of different vibronic symmetries. The motivation behind the present work stems from a large number of experimental studies that appeared in the current literature to elucidate the spectroscopy and dynamics of the low-lying electronic states of Np + and An + . 8–16 These laboratory experimental results have been utilized recently to assign the new interstellar bands recorded from different observatories by the astronomers. 17–19 The peak positions and widths measured in the laboratory experiments by mimicking the exotic conditions of the interstellar medium (ISM) 20 are directly compared with the observed diffuse interstellar bands (DIBs) 21–24 by the stellar spectroscopists. As mentioned in section I of Part I,z these vast amounts of experimental data await complementary support from rigorous theoretical studies to date. The present effort is aimed towards this goal. We in the following therefore, attempt to unravel the details of the vibronic energy level structure and time-dependent properties of the six low-lying electronic states of Np + (X ˜ –A ˜ –B ˜ –C ˜ –D ˜ –E ˜ ) and An + (X ˜ –A ˜ –B ˜ –C ˜ –D ˜ –E ˜ ) by undertaking the nuclear dynamics study from first principles. The vibronic Hamiltonians of Np + and An + developed in Part Iz (eqn (3)–(6)) are employed here for the dynamics studies. The results of Part I attest to the existence of a whole sequence of CIs in the considered electronic states of both radical cations. A critical analysis also revealed the importance of a large number of vibrational degrees of freedom (DOF) (29 for Np + and 31 for An + ) in the nuclear dynamics. Fortuitously, the number of relevant nuclear DOF does not grow in parallel to the increase in their total number from Np + to An + . The symmetry rule discards (in first-order) many of them to enter into the dynamics. The photoelectron spectra of Np and An have been recorded by several experimental groups. 25–33 The gas-phase photoelectron spectrum of Np recorded by da Silva et al. 26 using 60 eV photon revealed well resolved vibronic structures of the X ˜ and B ˜ electronic states and a broad band for the A ˜ state of Np + . In the 11–12 eV ionization energy range a signature of highly overlapping C ˜ ,D ˜ and E ˜ electronic states is observed in their experiment. Sa´nchez-Carrera et al. 28 recorded the gas phase photoelectron spectrum of An. The spectrum revealed well resolved vibronic structures of the X ˜ and A ˜ electronic states and a broad and diffuse structure of the B ˜ electronic state of An + . Analogous to Np + , highly overlapping band structures are also observed in this case in the 10–11 eV ionization energy range. It is shown below that these spectral broadenings School of Chemistry, University of Hyderabad, Hyderabad 500 046, India. E-mail: smsc@uohyd.ernet.in; Fax: +91-40-23011357, 23012460 w Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cp21084j z For Part I, please see DOI: 10.1039/c1cp21083a. PCCP Dynamic Article Links www.rsc.org/pccp PAPER Published on 12 July 2011. Downloaded by Indian Institute of Technology Kharagpur on 15/12/2014 06:39:48. View Article Online / Journal Homepage / Table of Contents for this issue