Femtosecond laser pulses have proven to provide valuable insight into the dynamics of microscopic systems by using pump-probe techniques. Applied to atomic clusters even a single pulse of varying pulse duration can reveal how and when energy from the laser pulse is transferred effectively to the cluster. We review the main experimental observables for energy transfer to a cluster and the major theoretical approaches which have been devised. Most importantly, we compare the cluster response to standard 780 nm light pulses with the response to 100 nm pulses, already obtained at a VUV free electron laser (FEL) source, and with 3 nm light which will be available from x-ray FEL sources.
We introduce soft recollisions in laser-matter interaction. They are characterized by the electron missing the ion upon recollision in contrast to the well-known head-on collisions responsible for high-harmonic generation or above-threshold ionization. We demonstrate analytically that soft recollisions can cause a bunching of photo-electron energies through which a series of low-energy peaks emerges in the electron yield along the laser polarization axis. This peak sequence is universal, it does not depend on the binding potential, and is found below an excess energy of one fifth of the ponderomotive energy.PACS numbers: 34.80. Qb,32.80.Rm, 32.80.Wr,32.80.Fb Recollision of an electron with its parent ion under a linearly polarized strong laser field has been shown to be the basis of a plethora of phenomena in atoms [1, 2], molecules [3], clusters [4] and solids [5]. In principle the recollision process is very simple and a single degree of freedom along the laser polarization axis is sufficient to describe it (often referred to as the three-step model [1]): Firstly, the bound electron is released from an atom due to the strong electric field of a laser. Secondly, it is accelerated and driven back to the ion. In the third step it either recombines in the atomic potential or is scattered from it. In the former case, high-order harmonics are generated (HHG) due to recombination of the electron [6]. In the latter case, the elastic head-on collision induces the high-energy phenomenon of above-threshold ionization (ATI) with fast electrons emitted [7,8]. The enormous impact of HHG up to recent proposals for imaging of molecular orbitals [3] and the generation of attosecond pulses [9] is not the least due to the simple yet accurate description with the three-step model. Recently, a surprising strong peak -the "low-energy structure" (LES) -was observed at few eV in the photoelectron spectrum of atoms in strong infra-red (a few µm wavelength) laser pulses [10,11] and confirmed numerically with classical calculations [12,13]. Although the LES peak contains about half of the photo electrons it was not seen in any of the numerous experiments done with 800 nm laser pulses.Here, we will give an analytical explanation of the LES by introducing a low-energy soft-recollision mechanism. It gives rise to a universal series of low-energy peaks in the momentum spectrum of the photo electron with well defined relative positions of 3/5, 5/7, 7/9 . . . on an absolute energy scale of about one fifth of the ponderomotive energy F 2 /(4ω 2 ), where F is the amplitude and ω the frequency of the laser field. These peaks do not require a special binding potential, e. g., long range, nor do they need more than one degree of freedom to appear, and they can be derived classically since they rely essentially on the well known strong-field trajectories as will become clear later.We will begin by working out the classical structures which are responsible for the LES [10], i.e., we consider a Hamiltonian H = H 0 + V with (throughout the paper we...
We determine the ionization time in tunneling ionization by an elliptically polarized light pulse relative to its maximum. This is achieved by a full quantum propagation of the electron wave function forward in time, followed by a classical backpropagation to identify tunneling parameters, in particular, the fraction of electrons that has tunneled out. We find that the ionization time is close to zero for single active electrons in helium and in hydrogen if the fraction of tunneled electrons is large. We expect our analysis to be essential to quantify ionization times for correlated electron motion.
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