The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics

The low frequency profile of the initial distribution of coherent states of a bosonic environment may control and hinder the dissipation of energy and model the dynamics of a strongly coupled quantum oscillator. The energy of the main oscillator is asymptotically stabilized to its initial value with damped oscillations enveloped in inverse power law decays in addition to pure inverse power law relaxations. Two limiting regimes appear. Arbitrarily slowly damped oscillations around the initial value show oscillating exchange of energy with the environment. For special initial conditions the inverse power law relaxations prevail over long times on the oscillations and the dissipation of energy may be hindered by slowing down arbitrarily the decays.

We show that algebraic approximants prove suitable for the summation of the perturbation series for the eigenvalues of periodic problems. Appropriate algebraic approximants constructed from the perturbation series for a given eigenvalue provide information about other eigenvalues connected with the chosen one by branch points in the complex plane. Such approximants also give those branch points with remarkable accuracy. We choose Mathieu's equation as illustrative example.

We report a quantum dynamical treatment of the vibrational excitation of the bending mode of water molecules by collision with low energy positrons in the energy regions close to threshold openings. The exact vibrationally coupled-channel equations derived for the total e+-H2O system are solved in a Body-Fixed-Vibrational-Coupled-Channels (BF-VCC) reference frame, using a single-center expansion of the total wavefunction and of the interaction potential. The vibrationally inelastic cross-sections for transitions from the ground to the lowest excited state of the bending mode clearly show the bending excitation channel to be the dominant inelastic process at low collision energies. Comparisons with our earlier calculations for the other modes and for the excited processes induced by electron impact are also presented and analysed.

The fragmentation statistics of (H2O) n and (NH3) n clusters (n = 100-1000) is investigated by MD simulations at different temperatures. The fragment size distributions are found to be well described by power laws over a wide range of excitation energies. The maximum fragment size depends linearly on the cluster size. A compact analytical model, implying the maximum fragment size and the power law exponent, is shown to fairly fit the average fragment size profiles. Measurements are carried out for the maximum fragment sizes after electron impact ionisation for water and ammonia clusters in the size range of n = 50 to 2100. The measured linear dependence on cluster size is used to estimate the fragment size distributions.

The systematics of the plasmon response in spherical K, Na and Li clusters in a wide size region is studied. Two simplifying approximations whose validity has been established previously are considered: (a) a separable approach to the random-phase-approximation, involving an expansion of the residual interaction into a sum of separable terms, (b) the electron-ion interaction is modeled within the pseudo-Hamiltonian jellium model (PHJM) including nonlocal effects by means of realistic atomic pseudoHamiltonians. In cases where nonlocal effects turn out to be negligible, the Structure Averaged Jellium Model (SAJM) has been used. The leading role of Landau damping in forming the plasmon width in medium and large clusters is demonstrated. Good agreement with available experimental data is achieved for K, Na (using the SAJM) and small Li clusters (invoking the PHJM). The trends for peak position and width are generally well reproduced, even up to details of the Landau fragmentation in several clusters. Less good agreement, however, is found for large Li clusters. The possible reasons of the discrepancy are discussed.

A triple-band perfect metamaterial absorber was achieved in terahertz regime that is made of asymmetric metallic I-shaped resonator and metallic ground layer with dielectric spacer in the middle. The simulated results show that the absorption device has three resonance modes at frequencies 1.655 THz, 1.985 THz and 2.86 THz with corresponding absorption rate closed to 95%. The origin of the triple-band absorber was investigated by electromagnetic field energy distribution. The absorption performance was further analyzed by the structural parameters to verify the underlying mechanisms of these absorption triple-band. Moreover, we also analyze the sensing performances of the absorber for the refractive index and the thickness of the analyte. Two conventional parameters, the sensitivity and figure of merit (FOM), were used to analyze the proposed design for the sensing performance of the device. The refractive index and thickness sensitivities of sensor are 1.2 THz/RIU and 0.0055 THz/µm, and the FOMs are 24.48 and 0.112 which is higher in magnitude compared to the first two resonant peaks and even higher than the resonance peaks of the previously reported works in terahertz regime. The proposed design has a number of applications in sensing, filter and stealth technology.