Vitali Averbukh

On the interatomic Coulombic decay in the Ne dimer

The interatomic Coulombic decay (ICD) in the Ne dimer is discussed in view of the recent experimental results. The ICD electron spectrum and the kinetic energy release of the Ne+ fragments resulting after Coulomb explosion of Ne2 (2+) are computed and compared to the measured ones. A very good agreement is found, confirming the dynamics predicted for this decay mechanism. The effect of the temperature on the electron spectrum is briefly investigated.

High harmonic generation spectra of aligned benzene in circular polarized laser field

We present model calculations of high-order harmonic generation in benzene, aligned in the polarization plane of circular polarized laser field. The resonance states of the system are obtained using complex scaling Floquet approach (i.e., within non-Hermitian quantum mechanics) combined with (t,t′) time propagation method. Our results show that the photo-induced dynamics of the model benzene molecule at the laser wavelength of 800 nm is dominated by a single long-lived resonance state up to the intensity of about 90u2009TWu200acm−2. The high-order harmonics emitted by the system obey the selection rules derived in [Phys. Rev. Lett. 80, 3743 (1998)] on the basis of the dynamical symmetry of the system, namely the emitted harmonics possess the frequencies (6±1)ω,(12±1)ω,…, where ω is the incident laser frequency. These symmetry-allowed harmonics are found to be the dominant ones in the spectrum also when the laser polarization deviates from the “ideal” circular one by about 5%. The nonlinear response of the model b...

High activity of helium droplets following ionization of systems inside those droplets

Relaxation processes following inner-valence ionization of a system can be modified dramatically by embedding this system in a suitable environment. Surprisingly, such an environment can even be composed of helium atoms, the most inert species available. As demonstrated by the examples of Ne and Ca atoms embedded in He droplets, a fast relaxation process [interatomic Coulombic decay (ICD)] takes place merely due to the presence of the He surroundings. We have computed ICD widths for both {sup 4}He{sub N} and {sup 3}He{sub N} droplets doped with Ne and Ca and discuss the findings in some detail. In the case of Ne, ICD is by far the dominating relaxation pathway. In Ca, atomic Auger decay is also possible but ICD becomes a competitive relaxation pathway in the droplets.

Decay rates of inner-valence excitations in noble gas atoms.

A Fano - algebraic diagrammatic construction - Stieltjes method has been recently developed for ab initio calculations of nonradiative decay rates [V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005)] of singly ionized states. In the present work this method is generalized for the case of electronic decay of excited states. The decay widths of autoionizing inner-valence-excited states of Ne, Ar, and Kr are calculated. Apart from the lowest excitation of Kr, they are found to be in good to excellent agreement with the experimental values. Comparison with the other theoretical studies shows that in many cases the new method performs better than the previously available techniques.

High harmonic generation of X-rays by carbon nanotubes

We have shown that in order to achieve a selective generation of high harmonics, one has to devise a system possessing a very high order spatio-temporal symmetry. Using the dynamical symmetry approach to the formulation of the selection rules for the high harmonic generation spectra, we have derived that a planar molecular target, the N-th order symmetry axis of which is aligned with the wave vector of the circularly polarized laser beam, emits only kN/spl plusmn/1-th (k=1,2,...) harmonics of the incident laser frequency. We extend the method for the formulation of selection rules for high harmonic generation spectra beyond the dipole approximation and apply it to single wall carbon nanotubes interacting with a circularly polarized laser field. Our results show that the carbon nanotubes, especially the chiral ones, can be excellent systems for a selective generation of high harmonics, up to the X-ray regime. The exact symmetry analysis of carbon nanotubes has revealed that the interaction of chiral nanotubes and circularly polarized incident radiation is characterized by a very high order symmetry and can lead to the generation of harmonics of the order higher than 100 in a selective fashion. Thus, already the first high harmonic emitted by a nanotube can fall within the desired (soft X-ray or X-ray) range. For example, for the chiral (8,3) single wall carbon nanotube the first emitted harmonics are the 193-rd and 195-th. In such a case, the wavelength of the first harmonic, generated by a circularly polarized 200 nm incident radiation, is about 1 nm. The chiral nanotubes represent the first example of a realistic physical system giving rise to a high harmonic generation spectrum in which all harmonics but very high ones are forbidden by symmetry.

Interatomic (Intermolecular) Decay Processes in Clusters: Current Status and Outlook

Since their theoretical prediction a decade ago, interatomic (intermolecular) Coulombic decay (ICD) and related processes have been in the focus of intensive theoretical and experimental research. The spectacular progress in this direction has been stimulated both by the fundamental importance of the new electronic decay phenomena and by the exciting possibility of their practical application, for example in spectroscopy. We review the current status of the research of interatomic (intermolecular) decay phenomena in clusters and discuss some perspectives of this new field.

Atoms, Molecules, Crystals and Nanotubes in Laser Fields: From Dynamical Symmetry to Selective High-Order Harmonic Generation of Soft X-Rays

Abstract The interaction of atoms, molecules, crystals and nanotubes as well as of artificial nanostructures with time-periodic laser fields can lead to high-order space–time symmetries. These space–time symmetries (also referred to as dynamical symmetries (DSs)) are to the Floquet Hamiltonian what spatial symmetries are to the field-free Hamiltonian. Consequently, DS properties of Floquet states can be studied by group theory. Here we use DSs to characterize the modifications of electronic energy levels induced by circularly polarized laser fields in quantum rings, thin crystals and carbon nanotubes (CNTs). Furthermore, we show that DSs underlie the formulation of selection rules for high-order harmonic generation processes in strong laser fields. This enables one to predict outputs of harmonic generation experiments and even suggests new experiments which would lead to ‘engineered’ spectra. By doing so, we place DS analysis of harmonic generation processes in high-intensity fields on a firm foundation, analogously to that possessed by symmetry analysis in ‘conventional’ spectroscopy. This formalism is applied to single-walled CNTs interacting with circularly polarized laser fields. We find that CNTs can be excellent candidates for a selective and efficient generation of high-order harmonics, up to the soft X-ray regime.

Molecular Photoionization Cross‐Sections by Application of Stieltjes Imaging to Lanczos Pseudospectra

Stieltjes imaging technique is widely used for ab initio computation of photoionization cross‐sections and decay widths. The main problem hampering the application of the standard Stieltjes imaging algorithms in conjunction with high‐level ab initio methods to polyatomic molecules is the requirement of full diagonalization of excessively large Hamiltonian matrices. Here we show that the full diagonalization bottleneck can be overcome by applying the Stieltjes imaging procedure to Lanczos pseudospectrum of the molecular Hamiltonian. The new technique is applied to calculation of the total photoionization cross‐section of benzene within an ab initio approach explicitly taking into account single and double electronic excitations. A good agreement with the experimental cross‐section is obtained.