We have measured the beam-normal single-spin asymmetry An in the elastic scattering of 1-3 GeV transversely polarized electrons from 1 H and for the first time from 4 He, 12 C, and 208 Pb. For 1 H, 4 He and 12 C, the measurements are in agreement with calculations that relate An to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the 208 Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new An measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.

Accurate knowledge of the intensity of focused ultra-short laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching 10 15 W/cm 2 with an accuracy of 1%. This accuracy is achieved by comparing experimental photoelectron yields from atomic hydrogen with predictions from exact numerical solutions of the three-dimensional time-dependent Schrödinger equation. Our method can be extended to relativistic intensities and to the use of other atomic species.

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We study transverse electron momentum distribution in strong field atomic ionization driven by laser pulses with varying ellipticity. We show, both experimentally and theoretically, that the transverse electron momentum distribution in the tunneling and over the barrier ionization regimes evolves in a qualitatively different way when the ellipticity parameter describing polarization state of the driving laser pulse increases.

We present the first experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an ab initio simulation over a wide range of laser intensities and electron energies.The interaction of intense few-cycle infrared laser pulses with matter induces tunneling ionization and subsequent quantum dynamics of freed electrons. Intense few-cycle pulses are difficult to generate and use because of the stringent requirements on dispersion control over a broad bandwidth. However, they offer unparallelled opportunities to reveal and control the electronic dynamics of atoms [1,2] and molecules [3,4] and to generate isolated attosecond pulses in the extreme ultraviolet [5]. The few-cycle regime is particularly challenging for simulations, as intensities approaching 10 15 W/cm 2 can be reached before the ionisation response saturates. At these intensities, a photoelectron driven by intense longwavelength radiation can travel a distance hundreds of times larger than the size of the parent atom and can have energies of many tens of eV, imposing stringent requirements on the simulation grid. Ab initio simulations in this regime can be carried out only for atomic H due to its simple electronic structure.Here we describe an experiment on the interaction of intense few-cycle laser pulses with atomic hydrogen (H), the simplest of all atomic systems and the traditional test case for atomic physics. No data on H has previously been available in this regime of laser interaction. Previous strong-field experiments with atomic H [6,7] used relatively short-wavelength pulses that were many optical cycles in duration with maximum intensities of 10 14 W/cm 2 . Our data show excellent quantitative agreement, at the 10% level, with ab initio simulation over a wide range of electron energies and laser intensities.The experimental apparatus is composed of an atomic H beam interacting with a few-cycle strong-field laser (Fig. 1). The laser used is a commercial Femtolaser 'Femtopower Compact Pro'. Each pulse has energy of 150 µJ and the pulse repetition rate is 1 kHz. The spectral width of the laser is 150 nm at full width half maximum (FWHM) centered at 750 nm. The pulse duration at FWHM of the intensity envelope is 6.3 ± 0.2 fs at the interaction region, or alternatively ∼ 2.5 optical cycles. An off-axis parabolic mirror of 750 mm focal length is used to focus the beam to a spot size of 47 µm 1/e 2 radius. The laser carrier-envelope phase was not stabilized in these experiments.The atomic H beam is created via collisional dissociation in a radio frequency (RF) discharge powered by a helical resonator [8]. An RF signal at a frequency of 75 MHz and power of 8 W is applied to the resonator and the dissociation efficiency is determined via emission spectroscopy of the discharge. The atomic beam emerging from the discharge is 80 ± 15% H atoms by number with the remainder being undissociated H 2 . The atomic H beam passes through two apertures, producing a...

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

Recent experiments in ultrafast physics have established the importance of above-threshold ionization (ATI) experiments in measuring and controlling the carrier-envelope phase (CEP) of few-cycle laser pulses. We have performed an investigation of atomic hydrogen subjected to intense CEPstable few-cycle laser pulses. The experimental ATI spectra have been compared to predictions from an ab initio numerical solution of the time-dependent Schrödinger equation in three dimensions. Good agreement between experiment 7

We present a new interferometer technique whereby multiple extreme ultraviolet light pulses are generated at different positions within a single laser focus (i.e., from successive sources) with a highly controllable time delay. The interferometer technique is tested with two generating media to create two extreme ultraviolet light pulses with a time delay between them. The delay is found to be a consequence of the Gouy phase shift. Ultimately the apparatus is capable of accessing unprecedented time scales by allowing stable and repeatable delays as small as 100 zs.