We studied the dependence of dissociative ionization in H 2 on carrier-envelope phase (CEP) of few-cycle (6 fs) near-infrared laser pulses. For low-energy channels, we present the first experimental observation of the CEP dependence of combined dissociation yield (with protons emitted in both directions), as well as the highest degree of asymmetry reported to date (40%). The observed modulations in both asymmetry and combined yield could be understood in terms of interference between different n-photon dissociation pathways-n and (n + 1) photon channels for asymmetry, n and (n + 2) photon channels for yield-as suggested by the general theory of CEP effects (Roudnev and Esry 2007 Phys. Rev. Lett. 99 220406). The yield modulation is found to be π-periodic in CEP, with its phase strongly dependent on fragment kinetic energy (and reversing its sign within the studied energy range), indicating that the dissociation yield does not simply follow the CEP dependence of maximum electric field, as a naïve intuition might suggest. We also find that a positively chirped pulse can lead to a higher dissociation probability than a transformlimited pulse.

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.

We investigate the nonlinear optical phenomenon of self-focusing in air with phase-stabilized few-cycle light pulses. This investigation looks at the role of the carrier-envelope phase by observing a filament in air, a nonlinear phenomenon that can be utilized for few-cycle pulse compression [Appl. Phys. B79, 673 (2004)]. We were able to measure the critical power for self-focusing in air to be 18+/-1 GW for a 6.3 fs pulse centered at 800 nm. Using this value and a basic first-order theory, we predicted that the self-focusing distance should deviate by 790 mum as the carrier-envelope phase is shifted from 0 to pi/2 rad. In contrast, the experimental results showed no deviation in the focus distance with a 3sigma upper limit of 180 mum. These counterintuitive results show the need for further study of self-focusing dynamics in the few-cycle regime.

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.

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We present total absolute collision cross sections for neon in the 3 P2 metastable state with ground state, thermal atoms and molecules using a recently developed experimental technique. The technique utilizes a magneto-optical trap (MOT) and involves the measurement of MOT population dynamics to determine the cross section. Collision cross section measurements are presented for metastable neon in the 3 P2 state with He, Ne, Ar, H2, O2, and N2. The average thermal energy of the collision ranges from 11 meV to 27 meV. The measurements made using this technique have small errors, of the order of 10% of the measured cross section and is capable of producing benchmark collision cross sections.

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