A current induces forces on atoms inside the conductor that carries it. It is now possible to compute these forces from scratch, and to perform dynamical simulations of the atomic motion under current. One reason for this interest is that current can be a destructive force--it can cause atoms to migrate, resulting in damage and in the eventual failure of the conductor. But one can also ask, can current be made to do useful work on atoms? In particular, can an atomic-scale motor be driven by electrical current, as it can be by other mechanisms? For this to be possible, the current-induced forces on a suitable rotor must be non-conservative, so that net work can be done per revolution. Here we show that current-induced forces in atomic wires are not conservative and that they can be used, in principle, to drive an atomic-scale waterwheel.

We derive and employ a semiclassical Langevin equation obtained from path integrals to describe the ionic dynamics of a molecular junction in the presence of electrical current. The electronic environment serves as an effective nonequilibrium bath. The bath results in random forces describing Joule heating, current-induced forces including the nonconservative wind force, dissipative frictional forces, and an effective Lorentz-type force due to the Berry phase of the nonequilibrium electrons. Using a generic two-level molecular model, we highlight the importance of both current-induced forces and Joule heating for the stability of the system. We compare the impact of the different forces, and the wide-band approximation for the electronic structure on our result. We examine the current-induced instabilities (excitation of runaway "waterwheel" modes) and investigate the signature of these in the Raman signals.

We present calculations of intense-field multiphoton ionization processes in helium at XUV wavelengths. The calculations are obtained from a full-dimensional integration of the two-electron time-dependent Schrödinger equation. A momentum-space analysis of the ionizing two-electron wavepacket reveals the existence of double-electron above threshold ionization (DATI). In momentum-space two distinct forms of DATI are resolved, namely non-sequential and sequential. In non-sequential DATI correlated electrons resonantly absorb and share energy in integer units of ℏωlaser.

A method for correlated quantum electron–ion dynamics is combined with a method for electronic open boundaries to simulate in real time the heating, and eventual equilibration at an elevated vibrational energy, of a quantum ion under current flow in an atomic wire, together with the response of the current to the ionic heating. The method can also be used to extract inelastic current–voltage corrections under steady-state conditions. However, in its present form the open-boundary method contains an approximation that limits the resolution of current–voltage features. The results of the simulations are tested against analytical results from scattering theory. Directions for the improvement of the method are summarized at the end.

We present calculations of single-and double-ionization rates of helium at 390 nm, accurate to within 10%, obtained from a full-dimensional integration of the time-dependent Schrödinger equation. The theoretical results are compared with experimental data at the same wavelength. Excellent agreement is obtained, allowing for likely uncertainties in the experimental determination of laser intensity.

A recent result for the curl of forces on ions under steady-state current in atomic wires with noninteracting electrons is extended to generalized forces on classical degrees of freedom in the presence of mean-field electron-electron screening. Current is described within a generic multiterminal picture, forces within the Ehrenfest approximation, and screening within an adiabatic, but not necessarily spatially local, mean-field picture.

We report a new method which allows sequential and non-sequential double-ionization rates in laser-driven helium to be distinguished and calculated separately. The method is applied to calculate such rates for two laser pulses, one of 0.236 au frequency and 8.0 × 1015 W cm-2 peak intensity, the other of 1.0 au frequency and also of 8.0 × 1015 W cm-2 peak intensity.

Abstract. Intense-field ionization of the hydrogen molecular ion by linearly-polarized light is modelled by direct solution of the fixed-nuclei time-dependent Schrödinger equation and compared with recent experiments. Parallel transitions are calculated using algorithms which exploit massively parallel computers. We identify and calculate dynamic tunnelling ionization resonances that depend on laser wavelength and intensity, and molecular bond length. Results for λ ∼ 1064 nm are consistent with static tunnelling ionization. At shorter wavelengths λ ∼ 790 nm large dynamic corrections are observed. The results agree very well with recent experimental measurements of the ion spectra. Our results reproduce the single peak resonance and provide accurate ionization rate estimates at high intensities. At lower intensities our results confirm a double peak in the ionization rate as the bond length varies.The mechanism of high-intensity ionization by infrared and optical wavelength light is often considered a static tunnelling process. The simplicity of this model is hugely appealing because of the ease of calculation. The ionization rates are effectively independent of wavelength, and to some extent the internal structure of the molecule can be ignored [1]. A rough criterion for validity of this model is given by the Keldysh parameter, γ k ≡ |E i |/2U p , where the internal binding energy is (|E i |) and the external laser-driven kinetic energy is (U p ). When the conditions are such that γ k ≪ 1, the ionization process is dominated by static tunnelling in which the shape of the potential strongly (exponentially) affects the ionization rate. At certain critical distances between the nuclei, discovered by Codling and co-workers [2], the ionization rate can rise sharply producing a sequence of fast fragments ions at sharply-defined energies. Predictions for ion yields and energies based on classical arguments [1] agree very well with experiments even for large diatomic molecules such as I 2 . The presence of critical distances would be evident in polyatomic molecules and is also seen in small rare-gas clusters [3]. The tunnelling process is generally relatively fast compared to the vibrational motion of the molecule, so the fixed-nuclei approximation is reasonable. However the tunnelling time may be longer than the optical period of the laser. Under these conditions the process is more accurately termed a dynamic tunnelling process. In this paper we provide evidence of just such effects for Ti:Sapphire light λ ∼ 790 nm at intensities I ∼ 10 14 W cm −2 . Our theoretical results in this wavelength region do not agree with cycleaveraged static field models, however the results do agree well with the features observed in experimental studies.In a molecule with few electrons the ionization process can be studied quantally with few approximations. For one-electron models, static-field ionization resonances in the potential