Mise au point. Theorie des effets de couplage vibronique multimodes. Probleme a 2 etats. Couplage vibronique mettant en jeu des modes et des etats degeneres. Effets du couplage vibronique multimodes en spectroscopie. Comportement statistique des niveaux d'energie vibroniques. Intersections coniques et evolution temporelle de la fluorescence

Multimode molecular dynamics beyond the Born-Oppenheimer approximation

Multimode Molecular Dynamics Beyond the Born‐Oppenheimer Approximation

Fluorescence and phosphorescence from higher excited states of organic molecules.

The fluorescence excitation spectrum of NO2 was measured in the region 5708–6708 A using a tuneable dye laser as an excitation source. The NO2 was cooled to a rotational temperature of ∼3 K by expansion with argon as a carrier gas through a supersonic nozzle. This cooling drastically reduced the rotational structure and thereby permitted a clear separation and analysis of 140 vibronic bands found in this 1000 A region of the spectrum. The results indicate that most of the fluorescence in this region is due to a 2B2 electronic state, and that this state is so heavily perturbed by high lying levels of the ground electronic state that a substantial fraction of all ground state levels of appropriate vibronic symmetry in this region have appreciable fluorescence intensity.

The fluorescence excitation spectrum of rotationally cooled NO2

As the scope of quantum chemistry broadened from the consideration of stable molecules near equilibrium to encompass potential curves and surfaces, transition states, radicals, ions, and excited states, the shortcomings of the Hartree— Fock (HF) approximation for the description of the electronic structure of molecular systems became increasingly evident. The energy error of the restricted HF wave function, i.e., the difference between the HF limit energy (which is the limit approached by restricted self-consistent field calculations as the basis set approaches completeness) and the exact solution of the nonrelativistic Schrodinger equation, has been termed the correlation energy.(1) It reflects the fact that the HF Hamiltonian contains the average, rather than instantaneous, interelectron potential, and thus neglects the correlation between the motions of the electrons.

The Method of Configuration Interaction

Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self‐consistent‐field (MC–SCF) wavefunctions have been computed for the low‐lying X 2A1, A 2B2, B 2B1, C 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the A 2B2, B 2B1, and C 2A2 states have all been found to be within 2 eV of the minimum of the X 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double‐zeta‐plus‐polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained: The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 A for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 A and 1.42 A and the inversion barrier is approximately 800 cm−1.

The electronic structure of nitrogen dioxide. I. Multiconfiguration self‐consistent‐field calculation of the low‐lying electronic states

The results of an accurate ab initio study of the electronic structure of NO2 have been applied to an analysis of the two important visible and near infrared absorption systems of this molecule. The long wavelength absorption (λ≳6000 A) arises from an ? 2B2←? 2A1 transition. A theoretical absorption spectrum that is generated from the C2V ab initio potential surfaces of these two states qualitatively reproduces most of the features of an experimental low resolution absorption spectrum between 9000 and 6000 A. The (0–0) band of the transition is predicted to be several times less intense than nearby hot bands even at temperatures as low as 300 K. The computed ? 2B2 spectroscopic parameters are Te=1.18 eV, Re=1.26 A, ϑe=102°, ω1=1461 cm−1, ω2=739 cm−1, and μ=0.46 D. There is a marked difference between experimentally determined ? 2B2 rotational constants and those deduced from the ab initio equilibrium geometry; this datum adds to the rapidly increasing evidence for strong vibronic coupling of the ? 2B2 sta...

The electronic structure of NO2. II. The ? 2B2← ? 2A1 and ? 2B1←? 2A1 absorption systems

The S2→S1 fluorescence of azulene in a naphthalene mixed crystal has been measured from 740 to 1000 nm at 4.2 K. Of the 25 in‐plane fundamentals (exclusive of C–H stretches) of the S1 state, 20 are readily identifiable in emission. A number of other bands are present in the spectrum and these are tentatively identified as S1 out‐of‐plane vibrations induced by vibronic coupling to σπ* and/or πσ* excited electronic states. An emission previously assigned as T1→S0 phosphorescence is shown to be a combination band of S2→S1 fluorescence. The branching ratio of S2 →S0 fluorescence to S2→S1 fluorescence is approximately 7500 for xenon arc lamp excitation; this is an order of magnitude larger than the value reported for short pulse laser excitation. A summary of photophysical parameters for the low‐lying electronic states of azulene, based on the most recent and reliable experimental data, is also presented.

Gregory D. Gillispie

Papers

The electronic structure of nitrogen dioxide. I. Multiconfiguration self‐consistent‐field calculation of the low‐lying electronic states

The electronic structure of NO2. II. The ? 2B2← ? 2A1 and ? 2B1←? 2A1 absorption systems

Vibrational analysis of the S2→S1 fluorescence of azulene in a naphthalene mixed crystal at 4.2 K

T2→T1 fluorescence in substitute anthracenes

S2→S1 fluorescence of azulene in a Schpol’skii matrix