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...

The intermediate state dependence of photoelectron circular dichroism (PECD) in resonance-enhanced multi-photon ionization of fenchone in the gas phase is experimentally studied. By scanning the excitation wavelength from 359 to 431 nm, we simultaneously excite up to three electronically distinct resonances. In the PECD experiment performed with a broadband femtosecond laser, their respective contributions to the photoelectron spectrum can be resolved. High-resolution spectroscopy allows us to identify two of the resonances as belonging to the B- and C-bands, which involve excitation to states with 3s and 3p Rydberg character, respectively. We observe a sign change in the PECD signal, depending on which electronic state is used as an intermediate, and are able to identify two differently behaving contributions within the C-band. Scanning the laser wavelength reveals a decrease of PECD magnitude with increasing photoelectron energy for the 3s state. Combining the results of high-resolution spectroscopy and femtosecond experiment, the adiabatic ionization potential of fenchone is determined to be IP=(8.49±0.06) eV.

Photoelectron circular dichroism (PECD) is experimentally investigated with chiral specimens with varying amounts of enantiomeric excess (ee). As a prototype, we measure and analyze the photoelectron angular distribution from randomly oriented fenchone molecules in the gas phase that result from ionization with circularly polarized femtosecond laser pulses. The quantification of these measurements shows a linear dependence with respect to the ee values. In addition, differences in the ee values (denoted as detection limit) of below one percent can be distinguished for nearly enantiopure samples, as well as for almost racemates. In combination with the use of a reference, the assignment of absolute ee values is possible. The present measurement time is a few minutes, but this could be reduced. This table-top laser-based approach should facilitate widespread implementation in chiral analysis.

Using a model methane-like chiral system, we theoretically demonstrate a possibility to access photoelectron circular dichroism (PECD) by a single experiment with two overlapping laser pulses of carrier frequencies ω and 2ω, which are linearly polarized in two mutually-orthogonal directions. Depending on the relative phase, the resulting electric field can be tailored to have two different rotational directions in the upper and lower hemispheres along the polarization of the ω-pulse. We predict a strong forward/backward asymmetry in the emission of photoelectrons from randomly oriented samples, which has an opposite sign in the upper and lower hemispheres. The predicted PECD effect is phase-and enantiomer-sensitive, providing new insight in this fascinating fundamental phenomenon. The effect can be optimized by varying relative intensities of the pulses. 33.20.Xx, 33.55.+b, 81.05.Xj In 1976, it was predicted theoretically that photoionization of chiral molecules is sensitive to the helicity of ionizing light [1]. The effect emerges already in the electric-dipole approximation and manifests itself as a forward/backward asymmetry in the emission of photoelectrons from randomly oriented chiral molecules in the gas phase. It took about 25 years to verify these theoretical predictions experimentally [2,3]. Later on, this enantiomer-and helicity-selective effect was termed as photoelectron circular dichroism (PECD) [4]. During the last decade, one-photon ionization of chiral molecules by circularly polarized radiation, and the emerging PECD effects, were studied in the numerous experimental and theoretical works (see, e.g., review articles [5][6][7]).Recently [8,9], a similar PECD effect has been observed in the multiphoton ionization of chiral molecules by circularly polarized laser pulses. Since then, extensive experimental studies provided many important details on the multiphoton PECD, like, e.g., on its dependence on the pulse intensity and ellipticity [10-12], on the enantiomeric excess of the target [13,14], or on the intermediate electronic states involved in different multiphoton ionization schemes [15,16]. In addition, an impact of the nuclear and electron dynamics during multiphoton ionization of chiral molecules have been demonstrated by pump-probe experiments [17][18][19][20]. Theoretical approaches to describe the multiphoton PECD range from time-independent perturbative ab-initio calculation of the two-photon absorption followed by one-photon ionization in hydrogenic continuum spectrum [21] to nonperturbative time-dependent methods [22,23].Employing overlapping bichromatic laser pulses is a particularly important example of coherent control schemes [24,25]. Here, interference between the nphoton route of one of the fields and the m-photon route of the other field can be controlled through the relative phase. According to the selection rules for multiphoton transitions in atoms, there are two control scenarios [24].If n and m are both odd or even numbers, the integral and differential cross section ca...

Photoelectron circular dichroism (PECD) is a highly sensitive enantiospecific spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization or multiphoton ionization. In the short pulse limit investigated with femtosecond lasers, resonance-enhanced multiphoton ionization (REMPI) is rather instantaneous and typically occurs simultaneously via more than one vibrational or electronic intermediate state due to limited frequency resolution. In contrast, vibrational resolution in the REMPI spectrum can be achieved using nanosecond lasers. In this work, we follow the high-resolution approach using a tunable narrow-band nanosecond laser to measure REMPI-PECD through distinct vibrational levels in the intermediate 3s and 3p Rydberg states of fenchone. We observe the PECD to be essentially independent of the vibrational level. This behaviour of the chiral sensitivity may pave the way for enantiomer specific molecular identification in multi-component mixtures: one can specifically excite a sharp, vibrationally resolved transition of a distinct molecule to distinguish different chiral species in mixtures.