D. Zeidler

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.

Observation of Coulomb focusing in tunnelling ionization of noble gases

We ionized He, Ne and Ar in the tunnelling regime with a linearly polarized laser pulse and measured the ion-recoil momentum distribution on an axis perpendicular to laser polarization for two different wavelengths (λ = 800 nm and λ = 1800 nm). We observed a significantly narrower distribution for 800 nm than for 1800 nm. Classical simulations of the electron wave packets evolution under the combined influence of the ions Coulomb potential and the laser field show that the narrowing is caused by Coulomb focusing of the electron wave packet during recollisions. The narrowing is sensitive on the longitudinal momentum of the wave packet after tunnelling, which suggests a way of measuring it. Measurements at 800 nm in circular polarization–for which no recollision occurs–do not exhibit this narrowing of the perpendicular momentum distribution. Simulations for circular polarization show another aspect of Coulomb focusing: the Coulomb potential also affects the wave packets transverse momentum as it leaves the saddle point immediately after tunnelling.

Generation of 11 fs pulses by using hollow-core gas-filled fibers at a 100 kHz repetition rate

Using self-phase modulation in a hollow-core fiber filled with xenon, we were able to produce 2.3 microJ laser pulses with a duration of 10.9 fs at a repetition rate of up to 100 kHz. We started with 45 fs, 4.4 microJ, 800 nm pulses generated by a Coherent RegA Ti:sapphire regenerative amplifier system, then spectrally broadened the 30 nm bandwidth to more than 100 nm. Dispersion compensation was achieved with two pairs of chirped mirrors. This is believed to be the first time this type of compression was achieved at a repetition rate as high as 100 kHz. This brings the advantages of few-cycle laser pulses to experiments that require high-repetition-rate, low-energy laser systems, for example, coincidence experiments.

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

Controlling High-Harmonic Generation via Molecular Alignment

The yield of high harmonic generation with N2 and O2 molecules is enhanced when the molecules are aligned parallel to the laser field that produces the harmonics and suppressed for the perpendicular case, compared to randomly oriented molecules. The alignment is produced via creation of a rotational molecular wave packet with a non-ionizing 800-nm, 60-fs Ti:Sa pulse and probed at its revival via high harmonic generation with a delayed intense 800-nm, 30-fs pulse.

Laser-induced orbital projection and diffraction of O-2 with velocity map imaging

Abstract In a velocity map imaging spectrometer, we measured the electron momentum distributions from the ionization of O molecules with 800 nm wavelength, 40 fs laser pulses at a peak intensity of W cm. The molecules were aligned at 0, 45 and 90 relative to the laser polarization prior to ionization. We show that for all alignments the low momentum region – populated by direct electrons which do not recollide with the parent ion – is consistent with the ionized orbital being filtered and projected onto the continuum electron wave packet. In the high momentum region – populated by rescattered electrons – we observe that the pattern created by diffraction of the recolliding wave packet by the ion core disappears as the alignment gets closer to the laser field axis. We find that a two-slit diffraction model agrees well with the results for molecules aligned at 90, but only partially predicts the decrease in the diffraction signature for smaller alignment angles.

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.

Tomographic imaging of electron orbital using high-harmonic generation

High harmonics produced in aligned molecules contain the structural information of the outermost electron orbital that preferentially ionizes in intense laser fields. We show a method to reconstruct a 3-dimensional (3D) structure of the molecular orbital. The method is based on the technologies to align molecules and to produce attosecond XUV pulses, both of which utilize intense ultrashort laser pulses. We measured a set of high harmonic spectra produced in differently aligned N2 molecules, and successfully reconstructed the image of the highest occupied molecular orbital (HOMO) with sub-angstrom resolution.