We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization

The generation of high-order harmonics in diatomic molecules is investigated within the framework of the strong-field approximation. We show that the conventional saddle-point approximation is not suitable for large internuclear distances. An adapted saddle-point method that takes into account the molecular structure is presented. We analyze the predictions for the harmonic-generation spectra in both the velocity and the length gauge. At large internuclear separations, we compare the resulting cutoffs with the predictions of the simple-man's model. Good agreement is obtained only by using the adapted saddle-point method combined with the velocity gauge.

We calculate the emission times of the radiation in high-order harmonic generation using the Gabor transform of numerical data obtained from solving the time-dependent Schrödinger equation in one, two, and three dimensions. Both atomic and molecular systems, including nuclear motion, are investigated. Lewenstein model calculations are used to gauge the performance of the Gabor method. The resulting emission times are compared against the classical simple man's model as well as against the more accurate quantum orbit model based on complex trajectories. The influence of the range of the binding potential (long or short) on the level of agreement is assessed. Our analysis reveals that the short-trajectory harmonics are emitted slightly earlier than predicted by the quantum orbit model. This partially explains recent experimental observations for atoms and molecules. Furthermore, we observe a distinct signature of two-center interference in the emission times for H 2 and D 2 .

Abstract:We report a new dynamic two-centre interference effect in High-HarmonicGeneration from H 2 , in which the attosecond nuclear motion of H 2 + initiated at ionisation causes interference to be observed at lower harmonic orders than would be the case for static nuclei. To enable this measurement we utilise a recently developed technique for probing the attosecond nuclear dynamics of small molecules. The experimental results are reproduced by a theoretical analysis based upon the strong field approximation which incorporates the temporally dependent two-centre interference term.High-harmonic generation (HHG) has proven to be a rich area of study over the last decade, finding application in a number of fields of laser science, such as coherent X-ray production [1,2] , attosecond pulse generation [3][4][5], and time resolved probing of nuclear dynamics [6,7]. HHG has also led to important advances towards the goal of structural imaging of small molecules [8][9][10][11][12][13][14][15], the harmonic emission depending strongly on the nature of the molecular orbital involved. This is seen most clearly within the Strong Field Approximation (SFA), in which the amplitude for HHG is determined by the Fourier transform of the bound state wavefunction.The wavefunctions relevant to HHG are those describing the propagated continuum electron ( c ψ ), and the bound electronic state from which the electron was ionised ( g ψ ). Recollision of the electron wavepacket with its parent ion results in a high local electron density (described by

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The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The influence of the magnetic-field component of the incident pulse on the emission of photons by multiply charged ions interacting with intense, near-infrared laser pulses is investigated theoretically using a strong-field approximation that treats the coupling of the atom with the incident field beyond the dipole approximation. For peak pulse intensities approaching 10 17 W cm Ϫ2 , the electron drift in the laser propagation direction due to the magnetic-field component of the incident pulse strongly influences the photon emission spectra. In particular, emission is reduced and the plateau structure of the spectra modified, as compared to the predictions in the dipole approximation. Nondipole effects become more pronounced as the ionization potential of the ion increases. Photon emission spectra are interpreted by analysing classical electron trajectories within the semiclassical recollision model. It is shown that a second pulse can be used to compensate the magnetic-field induced drift for selected trajectories so that, in a well-defined spectral region, a single attosecond pulse is emitted by the ion.

The effect of vibrational motion on harmonic generation in molecules is studied within the strong-field approximation. Simple expressions are given for the cut-off energy and for the ratio of spectral intensities from two isotopes. The latter is based on electronic trajectories from the simple-man's model. The influence of vibration is given by a correlation function that is sensitive to the overlap between the initial vibrational wave packet and the wave packet after the evolution in the Born–Oppenheimer potential of the ionized molecule.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The low-energy end of the spectrum of photoelectrons detached from hydrogenic ions exposed to an intense low-frequency few-cycle pulse is calculated within the strong-field approximation ͑SFA͒. The effect on the detached photoelectron of the Coulomb field of the nucleus is taken into account quasiclassically. The results are compared with those of an ab initio solution of the time-dependent Schrödinger equation, for the case of an He + ion irradiated by a 400-nm pulse of 1 ϫ 10 16 W cm −2 peak intensity. Many of the features of the ab initio spectra can be understood within the SFA.