Efficient non-resonant intermolecular vibrational energy transfer

Molecular excited vibrational states are metastable states and we incorporate their finite lifetimes into the theory of vibrational energy transfer between weakly interacting molecules, i.e., at internuclear distances at which they do not have a chemical bond. Expressions for the effective lifetime of an initially vibrationally excited molecule in the presence of a neighbouring molecule are derived in closed form. These expressions allow one to analyse the physics behind the energy transfer. It is shown that due to different finite lifetimes of the isolated excited molecules, a very efficient vibrational energy transfer can take place between them even if their energies are rather off-resonance. Examples are discussed.     

Quenching molecular photodissociation by intermolecular Coulombic decay

In this paper we study the impact of interatomic Coulombic decay (ICD) on molecular photodissociation. The investigation reveals the hitherto unrecognized ability of ICD to quench processes involving nuclear rearrangements. Numerical computations of the nuclear dynamics, initiated by photoexciting the B(1)Σ(+) Rydberg state of CO in CO·Mg complexes, are carried out. The efficiencies of ICD and photoinduced predissociation are compared for the four lowest vibrational levels of the corresponding electronic state. We also show the impact of CO vibrations on the ICD electron spectrum. Finally, we discuss the growing efficiency of ICD to quench the dissociation as the number of neighboring Mg atoms is increased.     

On the unphysical impact of complex absorbing potentials on the Hamiltonian and its remedy

The introduction of complex absorbing potentials as numerical tools to stabilize or increase the efficiency of calculations based on wave-packet propagation or on eigenvalue problems has the drawback of causing a modification of the Hamilton operator of the problem. In this work the consequences of such a modification are analyzed and the corrections required in order to properly describe the original physical process are derived. As an example, the decay of excited molecular states is considered: it is shown that the standard time-independent expression for the decay spectrum loses its validity when a complex absorbing potential is introduced in the nuclear Hamilton operator of the problem. To remedy the situation, a new, very stable formula is derived and tested on relevant model studies. Numerical examples are discussed.     

Coincidence and total photoelectron spectra and their differences induced by internal degrees of freedom

Recent progress in experimental techniques have made it possible to measure photoelectron spectra in coincidence with particles emitted during the decay of the photoionized species. In this work it will be shown that, contrary to intuition, these coincident photoelectron spectra can be qualitatively different from the photoelectron spectra resulting when all photoelectrons are detected. In particular they carry information on the decay mechanism following photoionization as soon as the decay is influenced by internal degrees of freedom of the photoionized system. This is shown explicitly for the case of vibrational degrees of freedom of molecules and demonstrated with a model study.     

Electronic structure of the photoactive yellow protein chromophore: Ab initio study of the low-lying excited singlet states

The excited electronic states of the p-coumaric acid thio-ester chromophore of the Photoactive Yellow Protein (PYP) are characterized in view of identifying the key factors determining the chromophore's isomerisation. These factors include the anionic nature of the chromophore, the presence of sulfur (rather than oxygen or nitrogen) in the ester moiety, and the presence of a hydrogen-bonding environment stabilizing the phenolate moiety. Two twisted stationary S₁ structures are identified, corresponding to a twist around the double bond conjugated with the aromatic ring, and the single bond adjacent to the ring, respectively. The latter structure is accessed directly by relaxation from the Franck–Condon (FC) geometry. These structures are shown to entail a substantial polarization effect (increasing charge separation when moving towards the twisted geometry). Further, an inversion of charge character is observed for the double-bond twisted minimum, which can be accounted for by the vicinity of an S₁–S₀ conical intersection. The S₁–S₀ gap at the minimum geometries depends in a sensitive fashion on the -carbonyl heteroatom. Based upon these observations for the intrinsic properties of the chromophore, we further address the effect of the Arg52 residue, which acts as a counter-ion in the native protein environment.     

Influence of caged noble-gas atom on the superatomic and valence states of C60−

Using high-level ab initio coupled cluster methods, we have conducted a detailed study of two endofullerene anions, He@C^?₆₀ and Ne@C^?₆₀. The main focus was on elucidating the effect of the noble-gas atom on the energy and electronic structure of the stable anion states of the parent C^?₆₀. Our study revealed that the noble-gas atom has no influence on the valence-like states of C^?₆₀, but strongly affects the superatomic-like ²A_g state. The latter was found to undergo pronounced destabilisation in both the He@C^?₆₀ and Ne@C^?₆₀ species, losing more than 70% of its original electron binding energy. The observed destabilisation is due to perturbation of the excess electron density of the ²A_g state inside the C₆₀ cage by the caged noble-gas atom. We show how the results of our analysis can be used to predict the behaviour of the ²A_g state in other closed-shell endohedral complexes. Some possible implications of the observed features of the ²A_g state are discussed.     

Photoinduced isomerization of the photoactive yellow protein (PYP) chromophore: interplay of two torsions, a HOOP mode and hydrogen bonding

We report on a detailed theoretical analysis, based on extensive ab initio calculations at the CC2 level, of the S(1) potential energy surface (PES) of the photoactive yellow protein (PYP) chromophore. The chromophore's photoisomerization pathway is shown to be fairly complex, involving an intimate coupling between single-bond and double-bond torsions. Furthermore, these torsional modes are shown to couple to a third coordinate of hydrogen out-of-plane (HOOP) type whose role in the isomerization is here identified for the first time. In addition, it is demonstrated that hydrogen bonding at the phenolate moiety of the chromophore can hinder the single-bond torsion and thus facilitates double-bond isomerization. These results suggest that the interplay between intramolecular factors and H-bonding determines the isomerization in native PYP.     

Dynamics of interatomic Coulombic decay in quantum dots

In this work we demonstrate that the interatomic Coulombic decay (ICD), an ultrafast electron relaxation process known for atoms and molecules, is possible in general binding potentials. We used the multiconfiguration time-dependent Hartree method for fermions to study ICD in real time in a two-electron model system of two potential wells. Two decay channels were identified and analyzed by using the box stabilization analysis as well as by evaluating the autocorrelation function and measuring the outgoing electron flux during time-propagations. The total and partial ICD widths of an excited state localized in one potential well as a function of the distance between the two potentials was obtained. Finally, we discuss the results with a view to a possible application of ICD in quantum dot technology.