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Register a new userProbing Asymmetric Molecules with High Harmonic Generation. [Original manuscript as prepared on 22/05/2011]
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Asymmetric molecules look different when viewed from one side or the other. This difference influences the electronic structure of the valence electrons, thereby giving stereo sensitivity to chemistry and biology. We show that attosecond and re-collision science provides a detailed and sensitive probe of electronic asymmetry. On each 1/2 cycle of an intense light pulse, laser-induced tunnelling extracts an electron wave packet from the molecule. When the electron wave packet recombines, alternately from one side of the molecule or the other, its amplitude and phase asymmetry determines the even and odd harmonics radiation that it generates. This harmonic spectrum encodes three manifestations of asymmetry; an amplitude and phase asymmetry in electron tunneling; an asymmetry in the phase that the electron wave packet accumulates relative to the ion between the moment of ionization and recombination; and an asymmetry in the amplitude and phase of the transition moment. We report the first measurement of high harmonics from oriented gas samples. We determine the phase asymmetry of the attosecond XUV pulses emitted when an electron recollides from opposite sides of the CO molecule, and the phase asymmetry of the recollision electron just before recombination. We discuss how the various contributions to asymmetry can be isolated in future experiments.
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Using dynamical Hartree-Fock mean-field theory, we study the high-harmonic generation (HHG) in the fullerene molecules C$_{60}$ and C$_{70}$ under strong pump wave driving. We consider a strong-field regime and show that the output harmonic radiation exhibits multiple plateaus, whose borders are defined by the molecular excitonic lines and cutoff energies within each plateau scale linearly with the field strength amplitude. In contrast to atomic cases for the fullerene molecule, with the increase of the pump wave photon energy the cutoff harmonic energy is increased. We also show that with the increase of the electron-electron interaction energy overall the HHG rate is suppressed. We demonstrate that the C$_{70}$ molecule shows richer HHG spectra and a stronger high-harmonic intensity than the C$_{60}$.
We show that the dependence of high-order harmonic generation (HHG) on the molecular orientation can be understood within a theoretical treatment that does not involve the strong field of the laser. The results for H_2 show excellent agreement with time-dependent strong field calculations for model molecules, and this motivates a prediction for the orientation dependence of HHG from the N_2 3s_g valence orbital. For both molecules, we find that the polarization of recombination photons is influenced by the molecular orientation. The variations are particularly pronounced for the N_2 valence orbital, which can be explained by the presence of atomic p-orbitals.
This paper has been withdrawn by the authors because the wave packet propagation used in the ion-dynamics calculation did not allow for electron-nuclei correlation. Hence, the conclusion that the ion-dynamics model is not in agreement with experiment is not substantiated.
We identify that both the dynamic core polarization and dynamic orbital deformation are important in the orientation-dependent high-harmonic generation of CO molecules subjected to intense few cycle laser fields. These polarization dynamics allow for the observation of strong orientation effects and dynamic minimum in the harmonic spectra. The generated attosecond pulses can be greatly affected by these multielectron effects. This work sheds light on future development of dynamic orbital imaging on attosecond time scale.
We develop the strong-field approximation for high-order harmonic generation in hydrogen molecules, including the vibrational motion and the laser-induced coupling of the lowest two Born-Oppenheimer states in the molecular ion that is created by the initial ionization of the molecule. We show that the field dressing becomes important at long laser wavelengths ($approx 2 mu$m), leading to an overall reduction of harmonic generation and modifying the ratio of harmonic signals from different isotopes.
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