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The fluorescence lifetimes of 9-(N-carbazolyl)-anthracene (C9A) in selected vibronic states |1˜ l> are calculated by means of a simple model. Specifically, the states |1˜ l> are excited to the bright electronic state S[sub 1] which is coupled to a dark state X, as well as to the l-th vibrational excitation of the torsional mode of C9A. The model takes into account the torsional moments of inertia of C9A, the empirical diabatic torsional potentials, the potential couplings, the dipole couplings, and the vibrational couplings between the torsion and the other vibrations of C9A. The corresponding model parameters are fit to the experimental spectra and fluorescence lifetimes of Monte et al. [J. Chem. Phys. 98, 2580 (1993)]. Three competing processes are described by the model, i.e.: (i) fluorescence from doublets of states |1˜ l,+> and |1˜ l,-> with + and - parity; (ii) intramolecular vibrational redistribution IVR from the torsion to the other vibrational modes; and (iii) the coupling between states S[sub 1] and X. The resulting fluorescence lifetimes decrease systematically from ca. 20 ns for |1˜ 0> to ca. 6 ns for |1˜ 50>, with an exceptional decrease to ca. 7 ns for intermediate states such as |1˜ 24>, with energies close to the crossings of the diabatic potentials of the bright and dark states S[sub 1] and X. These systematic and exceptional trends agree well with the experimental results of Monte et al., and they are explained and interpreted as consequences of the three processes (i)-(iii) within our model. Accordingly, the fluorescence lifetimes contain some information about the intramolecular dynamics, e.g., IVR from the torsion to the other vibrations should be faster in the dark state X than in the bright state S[sub 1], where it proceeds on the extraordinarily long time scale of several ns. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed. A pump-probe experimental approach for separating these different NSIs is then proposed.

A systematic approach is presented to describe nonresonant multiphoton transitions, i.e., transitions between two electronic states without the presence of additional intermediate states resonant with the single-photon energy. The method is well suited to describe femtosecond spectroscopic experiments and, in particular, attempts to achieve laser pulse control of molecular dynamics. The obtained effective time-dependent Schrodinger equation includes effective couplings to the radiation field which combine powers of the field strength and effective transition dipole operators between the initial and final states. To arrive at time-local equations our derivation combines the well-known rotating wave approximation with the approximation of slowly varying amplitudes. Under these terms, the optimal control formalism can be readily extended to also account for nonresonant multiphoton events. Exemplary, nonresonant two- and three-photon processes, similar to those occurring in the recent femtosecond pulse-shaping experiments on CpMn(CO)(3), are treated using related ab initio potential energy surfaces.