J. Levesque

Tomographic imaging of molecular orbitals

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.

Electron wavepacket control with elliptically polarized laser light in high harmonic generation from aligned molecules

We study experimentally and theoretically the high harmonic emission from aligned samples of nitrogen and carbon dioxide, in an elliptically polarized laser field. The ellipticity induces a lateral shift of the recombining electron wavepacket in the generation process. We show that this effect, which is well known from high harmonic generation (HHG) in atoms, can be useful to maintain the plane wave approximation in the case of HHG from molecules whose orbitals contain nodal planes. The study of the harmonic signal as a function of molecular alignment also reveals the role of the ellipticity on the recollision angle of the electron wavepacket, which can be used to accurately track the position of resonances in harmonic spectra.

High harmonic generation from aligned molecules–amplitude and polarization

While high harmonic generation from atoms is relatively well understood, the ability to align gas-phase molecules opens an opportunity to more deeply understand the underlying physics. Many assumptions, such as the single active electron approximation, neglect of the Coulomb potential, the strong field approximation, and the assumption of plane waves, are being challenged by new experimental observations. We study high harmonic emission from aligned molecules such as N2, O2 and CO2. We present experimental measurements of the amplitude of the emission as a function of molecular angle, as well as the polarization state.

Transient phase masks in high-harmonic generation

We present a method for controlling the spatial properties of high-harmonic beams with high efficiency. The high nonlinearity of harmonic generation allows weak control beams to induce a phase mask for the extreme UV light as it is formed. We fabricate a phase grating and demonstrate efficient diffraction in the far field. Diffractive elements formed in this way are transient. Since they are induced by the subcycle interaction of the medium with the fundamental and control fields, they can be extended to the attosecond time scale.

Probing the electronic structure of molecules with high harmonics

High harmonics produced in aligned molecules contain structural information of bound-state electronic states. Our recent work has shown that high harmonic generation in aligned molecules is dependent on the symmetry and structure of the highest occupied molecular orbital (HOMO) [ J. Itatani, D. Zeidler, J. Levesque, et al., Phys. Rev. Lett. 94 123902 (2005)]. We show that it is possible to reconstruct the shape of the HOMO by applying a computerized tomography algorithm to the harmonic spectra obtained at different molecular alignments [ J. Itatani, J. Levesque, D. Zeidler, et al., Nature 432 867 (2004)]. In the present paper we review these findings and explain them using a simple but powerful analytical model.

Molecular orbital tomography using high harmonic generation

High harmonics produced in aligned molecules contain the structural information of bound-state electronic states. We have successfully reconstructed tomographic images of the highest occupied molecular orbital (HOMO) of N2 from a set of harmonic spectra

Studying the electronic structure of molecules with high harmonic spectroscopy

High harmonic spectroscopy is a tool to study the valence electronic structure of atoms and molecules. It uses the techniques of high harmonic generation, in which a femtosecond laser ionizes the gas sample and XUV radiation is emitted in the forward direction. The XUV intensity, phase and polarization contain information about the orbital from which an electron was removed by the laser. High harmonic spectroscopy reveals details of electron-electron interactions, motion of electronic wave packets, and can follow a chemical reaction.

Observing the Birth of attosecond pulses

Attosecond pulse trains are produced during the production of high harmonics when a gas sample is irradiated with an intense femtosecond laser pulse. Usually the attosecond pulses are characterized in a second gas source. We show that attosecond pulses can be measured directly as they are produced by using a weak second harmonic of the driving laser, and that such a field can also be used to control the production process. We demonstrate manipulation of the harmonic spectrum as well as the creation of transient diffractive elements in the nonlinear medium itself.

Tomographic Imaging of Molecular Orbital with High Harmonic Generation

High harmonics produced in aligned molecules contain structural information of bound-state electronic states. We have produced high harmonics from N2 molecules aligned in two orthogonal directions. The projeeted images of the highest molecular orbital (HOMO) are successfully reconstrueted using an algorithm of computed tomography using the observed hannonic spectra.

Tomographic imaging of molecular orbitals using high harmonic generation

High harmonics produced in aligned molecules contain the structural information of bound-state electronic states. We have produced high harmonics from N2 molecules aligned to arbitrary directions with 5-degrees steps. From the set of high harnionic spectra, we have successfully reconstructed tomographic images of the highest occupied molecular orbital (HOMO) of N2.