Extreme-ultraviolet high-order-harmonic pulses with 1.6·10(7) photons/pulse at 32.5 eV have been separated from multiple harmonic orders by a time-preserving monochromator using a single grating in the off-plane mount. This grating geometry gives minimum temporal broadening and high efficiency. The pulse duration of the monochromatized harmonic pulses has been measured to be in the range 20 to 30 fs when the harmonic process is driven by an intense 30 fs near-infrared pulse. The harmonic photon energy is tunable between 12 and 120 eV. The instrument is used in the monochromatized branch of the Artemis beamline at the Central Laser Facility (UK) for applications in ultrafast electron spectroscopy.

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms 1 . This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected 2 . Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-

The Coulomb explosion of carbon dioxide in a 55 fs laser pulse of intensity 1.5-3 × 10 16 W cm −2 has been studied using a variety of techniques based on time-of-flight mass spectroscopy. Covariance mapping has been used to identify Coulomb explosion channels and to measure the associated kinetic energy release. By comparing time-of-flight spectra taken with linearly and circularly polarized light, a clear signature of laser-induced reorientation is found, which is strongest for the lowest Coulombic channel. Ion-momentum imaging coupled with Monte Carlo simulation shows that the zero-point bend-angle distribution is better preserved than for longer laser pulses. However, some residue of the sequential processes dominant in much longer pulses is found.

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The experimental study of molecular dissociation of H2+ by intense laser pulses is complicated by the fact that the ions are initially produced in a wide range of vibrational states, each of which responds differently to the laser field. An electrostatic storage device has been used to radiatively cool HD+ ions enabling the observation of above threshold dissociation from the ground vibrational state by 40 fs laser pulses at 800 nm. At the highest intensities used, dissociation through the absorption of at least four photons is found to be the dominant process.

a b s t r a c tRecent advances in the study of quantum vibrations and rotations in the fundamental hydrogen molecules are reported. Using the deuterium molecules (D + 2 and D 2 ) as exemplars, the application of ultrafast femtosecond pump-probe experiments to study the creation and time-resolved imaging of coherent nuclear wavepackets is discussed. The ability to study the motion of these fundamental molecules in the time-domain is a notable milestone, made possible through the advent of ultrashort intense laser pulses with durations on sub-vibrational (and sub-rotational) timescales. Quantum wavepacket revivals are characterised for both vibrational and rotational degrees of freedom and quantum models are used to provide a detailed discussion of the underlying ultrafast physical dynamics for the specialist and non-specialist alike.

A fast beam of H2+ ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused laser intensity and angular distributions of neutral dissociation products have been measured. Significant differences are observed between the present results and those obtained from experiments involving neutral H2 molecules. These differences are indicative of the precursor H2 molecule playing an important and hitherto neglected formative role in the laser-induced fragmentation processes.

The effects of electronic structure and symmetry are observed in laser driven high-order harmonic generation for laser aligned conjugated polyatomic molecular systems. The dependence of the harmonic yield on the angle between the molecular axis and the polarization of the driving laser field is seen to contain the fingerprint of the highest occupied molecular orbitals in acetylene and allene, a good quantitative agreement with calculations employing the strong field approximation was found. These measurements support the extension of the recently proposed molecular orbital imaging techniques beyond simple diatomic molecules to larger molecular systems. DOI: 10.1103/PhysRevLett.98.203007 PACS numbers: 33.80.Rv, 42.65.Ky, 42.65.Re High-order harmonic generation (HHG) in atomic gases by high intensity laser fields has proven a fruitful method for producing coherent extreme ultraviolet beams and attosecond pulses [1][2][3]. In molecules, HHG presents a rich set of new physical phenomena, as both the ionization and electron recombination steps of HHG are dependent on the particular symmetry of the highest occupied molecular orbital (HOMO) and its orientation with respect to the laser field [4 -9]. Because of this orientation dependence, HHG in molecules can provide a unique time resolved probe of the electronic orbital structure. Recent work has shown that the HHG spectrum from aligned molecular samples can be used to determine the molecular structure for small molecules such as N 2 and CO 2 [10 -15]. So far these studies have been confined to simple diatomic and triatomic linear molecules. The present work extends this approach to significantly more complex polyatomic molecules, and we find that the angular dependence of the HHG signal can be used to map more complex electronic orbitals.In order to perform a full tomographic reconstruction of the molecular orbital, a high degree of spatial alignment of the molecules is required; furthermore, a large enough number of harmonic orders must be produced in order to provide a complete characterization of the orbital in momentum space. The structure retrieval is then possible by either the examination of distinct interference signatures in the spectrum [10 -12] or tomographic reconstruction [13,14] or the iterative optimization of the wave functions to best match the measured spectrum [15]. The mathematical steps in structural retrieval from HHG are based upon the strong field approximation (SFA). The SFA is a single active electron model and, additionally, the influence of the laser field upon the bound electronic states, and the influence of the Coulomb potential upon the continuum electron states are both neglected. HHG in the strong field limit can be seen as a three step process [16]: (i) ionization; (ii) acceleration in the laser field; (iii) recombination back into the bound electronic state. It is known that multielectron processes become significant with increasing molecular size [17] as, for example, the polarization of the ionic core by the laser field modi...