Guennaddi K. Paramonov

Bio: Guennaddi K. Paramonov is an academic researcher from University of Potsdam. The author has contributed to research in topics: Laser & High harmonic generation. The author has an hindex of 5, co-authored 7 publications receiving 53 citations.

Controlled surface photochemistry: Bond- and isotope-selective photodesorption of neutrals by adsorbate vibrational preparation with infrared laser pulses

Abstract: The possibility of controlling surface photochemistry by the selective vibrational preparation of adsorbates with infrared (ir) laser pulses is investigated theoretically. In particular, the selective ir plus ultraviolet (uv) light-induced desorption of different isotopomeric neutral adsorbates from metal surfaces is studied with the help of nuclear density matrix theory. As a concrete example the system NH3/ND3/Cu(111) is chosen. In a first step of the “vibrationally mediated chemistry” advocated here, based on computed two-mode dipole functions and model potentials, optimal infrared laser pulses are designed to selectively excite the umbrella mode ν2 of either adsorbed NH3 or ND3. In a second step, an uv/visible photon enforces an electronic transition, leading, after ultrafast quenching, to desorption induced by electronic transitions (DIET). It is argued that despite strong dissipation, the proper vibrational preparation not only increases desorption yields substantially, but also allows for an almost...

Quantum dynamics, isotope effects, and power spectra of H 2 + and HD + excited to the continuum by strong one-cycle laser pulses: Three-dimensional non-Born-Oppenheimer simulations

Abstract: Non-Born-Oppenheimer quantum dynamics of ${\mathrm{H}}_{2}{}^{+}$ and ${\mathrm{HD}}^{+}$ excited by single one-cycle laser pulses linearly polarized along the molecular ($z$) axis have been studied within a three-dimensional model, including the internuclear distance $R$ and electron coordinates $z$ and $\ensuremath{\rho}$, by means of the numerical solution of the time-dependent Schr\"odinger equation on the timescale of about 200 fs. Laser carrier frequencies corresponding to the wavelengths of ${\ensuremath{\lambda}}_{l}=400$ and 50 nm have been used and the amplitudes of the pulses have been chosen such that the energies of ${\mathrm{H}}_{2}{}^{+}$ and ${\mathrm{HD}}^{+}$ are above the dissociation threshold after the ends of the laser pulses. It is shown that excitation of ${\mathrm{H}}_{2}{}^{+}$ and ${\mathrm{HD}}^{+}$ above the dissociation threshold is accompanied by formation of vibrationally ``hot'' and ``cold'' ensembles of molecules. Dissociation of vibrationally ``hot'' molecules does not prevent the appearance of post-laser-pulse electronic oscillations, parallel $z$ oscillations, and transversal $\ensuremath{\rho}$ oscillations. Moreover, dissociation of ``hot'' molecules does not influence characteristic frequencies of electronic $z$ and $\ensuremath{\rho}$ oscillations. The main difference between the laser-induced quantum dynamics of homonuclear ${\mathrm{H}}_{2}{}^{+}$ and its heteronuclear isotope ${\mathrm{HD}}^{+}$ is that fast post-laser-pulse electronic $z$ oscillations in ${\mathrm{H}}_{2}{}^{+}$ are regularly shaped with the period of ${\ensuremath{\tau}}_{\mathrm{shp}}\ensuremath{\approx}30$ fs corresponding to nuclear oscillations in ${\mathrm{H}}_{2}{}^{+}$, while electronic $z$ oscillations in ${\mathrm{HD}}^{+}$ arise as ``echo pulses'' of its initial excitation and appear with the period of ${\ensuremath{\tau}}_{\mathrm{echo}}\ensuremath{\approx}80$ fs corresponding to nuclear motion in ${\mathrm{HD}}^{+}$. Accordingly, corresponding power spectra of nuclear motion contain strong low-frequency harmonics at ${\ensuremath{\omega}}_{\mathrm{shp}}=2\ensuremath{\pi}/{\ensuremath{\tau}}_{\mathrm{shp}}$ in ${\mathrm{H}}_{2}{}^{+}$ and ${\ensuremath{\omega}}_{\mathrm{echo}}=2\ensuremath{\pi}/{\ensuremath{\tau}}_{\mathrm{echo}}$ in ${\mathrm{HD}}^{+}$. Power spectra related to both electronic and nuclear motion have been calculated in the acceleration form. Both higher- and lower-order harmonics are generated at the laser wavelength ${\ensuremath{\lambda}}_{l}=400$ nm, while only lower-order harmonics are well pronounced at ${\ensuremath{\lambda}}_{l}=50$ nm. It is also shown that a rationalized harmonic order, defined in terms of the frequency of the laser-induced electronic $z$ oscillations, agrees with the concept of inversion symmetry for electronic motion in diatomic molecules.

Excitation of muonic molecules ddμ and dtμ by super-intense attosecond soft X-ray laser pulses: Shaped post-laser-pulse muonic oscillations and enhancement of nuclear fusion

Abstract: The quantum dynamics of muonic molecular ions ddμ and dtμ excited by linearly polarized along the molecular (z)-axis super-intense laser pulses is studied beyond the Born–Oppenheimer approximation by the numerical solution of the time-dependent Schrodinger equation within a three-dimensional model, including the internuclear distance R and muon coordinates z and ρ. The peak-intensity of the super-intense laser pulses used in our simulations is I0 = 3.51 × 1022 W/cm2 and the wavelength is λl = 5 nm. In both ddμ and dtμ, expectation values 〈z〉 and 〈 ρ 〉 of muon demonstrate "post-laser-pulse" oscillations after the ends of the laser pulses. In ddμ post-laser-pulse z-oscillations appear as shaped nonoverlapping "echo-pulses". In dtμ post-laser-pulse muonic z-oscillations appear as comparatively slow large-amplitude oscillations modulated with small-amplitude pulsations. The post-laser-pulse ρ-oscillations in both ddμ and dtμ appear, for the most part, as overlapping "echo-pulses". The post-laser-pulse oscillations do not occur if the Born–Oppenheimer approximation is employed. Power spectra generated due to muonic motion along both optically active z and optically passive ρ degrees of freedom are calculated. The fusion probability in dtμ can be increased by more than 11 times by making use of three sequential super-intense laser pulses. The energy released from the dt fusion in dtμ can by more than 20 GeV exceed the energy required to produce a usable muon and the energy of the laser pulses used to enhance the fusion. The possibility of power production from the laser-enhanced muon-catalyzed fusion is discussed.

Isotopic effects in vibrational relaxation dynamics of H on a Si(100) surface.

Abstract: In a recent paper [U. Lorenz and P. Saalfrank, Chem. Phys. 482, 69 (2017)], we proposed a robust scheme to set up a system-bath model Hamiltonian, describing the coupling of adsorbate vibrations (system) to surface phonons (bath), from first principles. The method is based on an embedded cluster approach, using orthogonal coordinates for system and bath modes, and an anharmonic phononic expansion of the system-bath interaction up to second order. In this contribution, we use this model Hamiltonian to calculate vibrational relaxation rates of H–Si and D–Si bending modes, coupled to a fully H(D)-covered Si(100)-( 2 × 1 ) surface, at zero temperature. The D–Si bending mode has an anharmonic frequency lying inside the bath frequency spectrum, whereas the H–Si bending mode frequency is outside the bath Debye band. Therefore, in the present calculations, we only take into account one-phonon system-bath couplings for the D–Si system and both one- and two-phonon interaction terms in the case of H–Si. The computat...

Excitation of H + 2 with one-cycle laser pulses: shaped post-laser-field electronic oscillations, generation of higher- and lower-order harmonics

Abstract: Non-Born–Oppenheimer quantum dynamics of H+2 excited by shaped one-cycle laser pulses linearly polarised along the molecular axis have been studied by the numerical solution of the time-dependent Schrodinger equation within a three-dimensional model, including the internuclear separation, R, and the electron coordinates z and ρ. Laser carrier frequencies corresponding to the wavelengths λl = 25 nm through λl = 400 nm were used and the amplitudes of the pulses were chosen such that the energy of H+2 was close to its dissociation threshold at the end of any laser pulse applied. It is shown that there exists a characteristic oscillation frequency ωosc ≃ 0.2265 au (corresponding to the period of τosc ≃ 0.671 fs and the wavelength of λosc ≃ 201 nm) that manifests itself as a ‘carrier’ frequency of temporally shaped oscillations of the time-dependent expectation values ⟨z ⟩ and ⟨∂V/∂z ⟩ that emerge at the ends of the laser pulses and exist on a timescale of at least 50 fs. Time-dependent expectation val...

Local coherent-state approximation to system-bath quantumdynamics

Abstract: A novel quantum method to deal with typical system-bath dynamical problems is introduced. Subsystem discrete variable representation and bath coherent-state sets are used to write down a multiconfigurational expansion of the wave function of the whole system. With the help of the Dirac-Frenkel variational principle, simple equations of motion--a kind of Schrodinger-Langevin equation for the subsystem coupled to (pseudo) classical equations for the bath--are derived. True dissipative dynamics at all times is obtained by coupling the bath to a secondary, classical Ohmic bath, which is modeled by adding a friction coefficient in the derived pseudoclassical bath equations. The resulting equations are then solved for a number of model problems, ranging from tunneling to vibrational relaxation dynamics. Comparison of the results with those of exact, multiconfiguration time-dependent Hartree calculations in systems with up to 80 bath oscillators shows that the proposed method can be very accurate and might be of help in studying realistic problems with very large baths. To this end, its linear scaling behavior with respect to the number of bath degrees of freedom is shown in practice with model calculations using tens of thousands of bath oscillators.

Optimal control in a dissipative system: Vibrational excitation of CO/Cu(100) by IR pulses

Abstract: The question as to whether state-selective population of molecular vibrational levels by shaped infrared laser pulses is possible in a condensed phase environment is of central importance for such diverse fields as time-resolved spectroscopy, quantum computing, or “vibrationally mediated chemistry.” This question is addressed here for a model system, representing carbon monoxide adsorbed on a Cu(100) surface. Three of the six vibrational modes are considered explicitly, namely, the CO stretch vibration, the CO-surface vibration, and a frustrated translation. Optimized infrared pulses for state-selective excitation of “bright” and “dark” vibrational levels are designed by optimal control theory in the framework of a Markovian open-system density matrix approach, with energy flow to substrate electrons and phonons, phase relaxation, and finite temperature accounted for. The pulses are analyzed by their Husimi “quasiprobability” distribution in time-energy space.

Theoretical study on quantum control of photodissociation and photodesorption dynamics by femtosecond chirped laser pulses

Abstract: We have theoretically studied the effect of chirping one-photon incident laser pulses on (I) the branching ratio of the HOD molecule in the photochemical reaction D+OH←HOD→H+OD and (II) the UV photodesorption dynamics of NH3 and ND3 from Cu(111). As was predicted in our previous 1D model, wave packet calculations have demonstrated that it is possible, in practice, to control the branching ratio of reaction (I) and to greatly enhance the desorption probability of the photodesorption reaction (II) by negatively chirped laser pulses. It was found that two characteristics of (negatively) chirped laser pulses contribute to this remarkable effect; the mechanism of adiabatic rapid passage for the population transfer between the ground and excited states, and the intrapulse pump-dump process for determining the branching ratio and photodesorption yield.