Generalized molecular orbital theory (original) (raw)
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Generalized molecular orbital tomography
Nature Physics, 2011
The emission of coherent XUV radiation from atomic or molecular gases exposed to intense infrared laser pulses, known as high harmonic generation, is of paramount interest in atomic and molecular physics as well as in attosecond science. The emitted radiation contains a wealth of information about the structure of its generating medium, which inspired vigorous efforts to tomographically image the valence orbital of atoms and molecules. The orbital retrieval is nevertheless seriously hindered by the complexity of the harmonic emission process, as recently demonstrated by several theoretical and experimental works. Here we present a novel approach for molecular orbital tomography that contributes to overcome those difficulties, opening intriguing perspectives on coherent XUV imaging of complex species by high-order harmonic generation.
Probing Orbital Structure of Polyatomic Molecules by High-Order Harmonic Generation
Physical Review Letters, 2007
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.
High-order harmonic spectroscopy for molecular imaging of polyatomic molecules
Faraday discussions, 2014
High-order harmonic generation is a powerful and sensitive tool for probing atomic and molecular structures, combining in the same measurement an unprecedented attosecond temporal resolution with a high spatial resolution of the order of an angstrom. Imaging of the outermost molecular orbital by high-order harmonic generation has been limited for a long time to very simple molecules, like nitrogen. Recently we demonstrated a technique that overcame several of the issues that have prevented the extension of molecular orbital tomography to more complex species, showing that molecular imaging can be applied to a triatomic molecule like carbon dioxide. Here we report on the application of such a technique to nitrous oxide (N(2)O) and acetylene (C(2)H(2)). This result represents a first step towards the imaging of fragile compounds, a category which includes most of the fundamental biological molecules.
Physical Review A, 2010
High-order harmonic generation (HHG) from molecules produces spectra that are modulated by interferences that encode both the static structure and the electron dynamics initiated by interaction with the laser field. Using a midinfrared (mid-IR) laser at 1300 nm, we are able to study the region of the harmonic spectrum containing such interferences in CO 2 over a wide range of intensities. This allows for isolation and characterization of interference minima arising due to subcycle electronic dynamics triggered by the laser field, which had previously been identified but not systematically separated. Our experimental and theoretical results demonstrate important steps toward combining attosecond temporal and angstrom-scale spatial resolution in molecular HHG imaging.
Axis-dependence of molecular high harmonic emission in three dimensions
Nature Communications, 2014
An important goal in molecular physics and chemistry today is to obtain structure-dependent information about molecular function to obtain a deeper understanding into chemical reactions 1-4. However, until now, asymmetric tops, which comprise the widest and most general class of molecules, remain principally unexplored. This gap is particularly evident in high harmonic generation (HHG). HHG has successfully obtained structural information about electron hole pairs 5 or orbitals 6 for simple linear molecules. Unfortunately, for more complicated molecules, the emission from different molecular directions interfere, concealing individual angular signatures. Here we introduce a method to extract orientation-dependent information from asymmetric tops and apply it to the sulfur dioxide (SO 2) molecule. We use the rotational revival structure to decompose the angular contributions of HHG emission. This method also extends HHG-based tomographic imaging into three dimensions and makes it applicable to a much wider class of systems than previously envisioned. Our results suggest that HHG is a powerful tool to probe electron orbital structure and dynamics of complex molecules. HHG has shown promise as a technique for understanding electron structure and dynamics. Since HHG is a phase-matched process, the signal it produces is a result of a large aggregation of molecules rotating at different
Physical Review A, 2012
We study numerically pump-probe schemes for monitoring electron-nuclear motion in a dissociating molecule using a midinfrared, intense few-femtosecond probe laser pulse which generates molecular high-order harmonics (MHOHG) from a coherent superposition of electron-nuclear wave packets prepared by a weak femtosecond UV pump pulse from an initial bound state. We show that by varying the time delay between the intense probe pulse and the UV pump pulse by a few hundred attoseconds one alters the MHOHG signal intensity by many orders of magnitude. The periodicity of the MHOHG intensity variations as function of the time delay is equal to the period of the electron oscillation in the coherent superposition which varies with internuclear distance. We use the strong field approximation (SFA) and three-step model to explain this high sensitivity of the harmonic intensity to pulse delay time and to the overlap of nuclear wave packets. We also solve numerically the three-dimensional (3D) time-dependent Schrödinger equation describing harmonic generation for a hydrogen atom prepared in a superposition of its two lowest atomic states, in order to investigate the dependence of the same effect (in a simpler system but in 3D) on the probe-pulse carrier-envelope phase (CEP) and on the probe duration. We also relate these strong effects in the intensity of harmonics to the correlation between the velocity of the recolliding electron wave packet and the electron velocity in the coherent superposition of the electron bound states.
Theoretical analysis of dynamic chemical imaging with lasers using high-order harmonic generation
Physical Review A, 2007
We report theoretical investigations of the tomographic procedure suggested by Itatani et al. ͓Nature ͑Lon-don͒ 432, 867 ͑2004͔͒ for reconstructing highest occupied molecular orbitals ͑HOMOs͒ using high-order harmonic generation ͑HHG͒. Due to the limited range of harmonics from the plateau region, we found that even under the most favorable assumptions, it is still very difficult to obtain accurate HOMO wave functions using the tomographic procedure, but the symmetry of the HOMOs and the internuclear separation between the atoms can be accurately extracted, especially when lasers of longer wavelengths are used to generate the HHG. Since the tomographic procedure relies on approximating the continuum wave functions in the recombination process by plane waves, the method can no longer be applied upon the improvement of the theory. For future chemical imaging with lasers, we suggest that one may want to focus on how to extract the positions of atoms in molecules instead, by developing an iterative method such that the theoretically calculated macroscopic HHG spectra can best fit the experimental HHG data.
Physical Review Letters, 2008
By analyzing accurate theoretical results from solving the time-dependent Schrödinger equation of atoms in few-cycle laser pulses, we established the general conclusion that laser-generated high-energy electron momentum spectra and high-order harmonic spectra can be used to extract accurate differential elastic scattering and photo-recombination cross sections of the target ion with free electrons, respectively. Since both electron scattering and photoionization (the inverse of photo-recombination) are the conventional means for interrogating the structure of atoms and molecules, this result implies that existing fewcycle infrared lasers can be implemented for ultrafast imaging of transient molecules with temporal resolution of a few femtoseconds.