Femtosecond chemical dynamics in solution. Wavepacket evolution and caging of I2

The relative fluorescence intensities of all the strong electronic transitions from five of the first cluster of ion-pair (IP) states of I2 and from three states in the second cluster have been measured. The fluorescence, in most cases from ν′ = 0, was dispersed and comprised both discrete and continuous (bound → free) spectra. These were fitted and each simulated electronic spectrum integrated separately to give relative values of the Einstein A-coefficients (summed over final vibrational states) for each transition. Using previously reported lifetimes, transition dipoles for each of the parallel transitions from nine IP states are deduced, together with some of the stronger perpendicular transition dipole moments. At R e of the ion-pair states, μ‖ (five transitions observed) is almost constant at 0·2–0·3 D, but μ‖ ranges between 0·7 and 3·7 D. There is a marked difference in the parallel transition moments associated with some g/u pairs in the same cluster of IP states which can be accounted for by the ...

Einstein A-coefficients and transition dipole moments for some ion-pair to valence transitions in I2

Ion-pair states are a distinctive sub-set of the valence states of molecules. In a few cases they constitute the ground electronic state, but more usually their term values lie close to the lowest Rydberg states. Their unusual spectroscopic properties and the techniques used to access them are discussed with particular reference to the diatomic halogens. Their electronic structure is best approached from two complementary points of view, as a single molecular orbital configuration or the product of separated ion configurations, according to the ionic separation. The consequences of this dual character for absorption and fluorescence spectroscopy are outlined. Finally, the state-specific coupling of Rydberg and ion-pair states is surveyed.

Spectroscopy and electronic structure of ion-pair states

The ion-pair states of molecular iodine provide a unique system for studying the efficiency, selectivity, and mechanisms of collision-induced non-adiabatic transitions. Non-adiabatic transitions between the first-tier ion-pair states in collisions with molecular partners and rare gases are analyzed and discussed. The qualitative features of the rate constants and product state distributions under single collision conditions are summarized and interpreted in terms of appropriate theoretical approaches. Two mechanisms for the non-adiabatic transitions are clearly identified. The first, operative for collisions involving molecular partners possessing permanent or transition electrostatic moments, is highly selective. It connects the initially prepared level in the E 0+g electronic state with the near-resonant vibronic level of the D 0+u state with a minimum change of the total angular momentum. In an extreme quasi-resonant case when the gap between initial and final rovibronic level is less than 1 cm−1, this mechanism has a giant cross section, 40 times that of a gas kinetic collision. An electrostatic model, which includes the coupling of the giant E–D transition dipole moment with a moment of the colliding partner and the semiclassical Born approximation, provides a plausible interpretation of this mechanism. A second mechanism is shown to govern collisions with rare gas atoms. It results in population of several ion-pair states and broad distributions over rovibronic levels. This mechanism is successfully interpreted by quantum scattering calculations based on the diatomics-in-molecule diabatic potential energy surfaces and coupling matrix elements. The calculations provide good agreement with experimental measurements and reveal different mechanisms for the population of different electronic states. Unexplained features of the non-adiabatic dynamics and directions of future work are outlined.

/pdf/collision-induced-non-adiabatic-transitions-between-the-ion-2rhigql4f8.pdf

Collision-induced non-adiabatic transitions between the ion-pair states of molecular iodine: A challenge for experiment and theory

Double resonance ionisation nozzle cooled spectroscopy (DRINCS) of the E(3P2), f(3P0) and f′(1D2) O+g ion-pair states of I2

Determinations of electronic transition moments rely generally on the r-centroid approximation. This method is no longer valid when, in the stationary phase approximation, two transition points occur. A purely quantal treatment is applied here to the determination of electronic transition moments M e(r) from both line intensities and lifetime measurements for the 1 g (3 P 2) → A1 u and 0+ g (3 P 2) → B0+ u transitions of the I2 molecule.

Electronic transitions moments from both fluorescence intensities and lifetime measurements for transitions involving the I2 ionic states

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Electronic transitions moments from both fluorescence intensities and lifetime measurements for transitions involving the I2 ionic states