Hiromichi Niikura

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.

Sub-laser-cycle electron pulses for probing molecular dynamics

Experience shows that the ability to make measurements in any new time regime opens new areas of science. Currently, experimental probes for the attosecond time regime (10-18–10-15 s) are being established. The leading approach is the generation of attosecond optical pulses by ionizing atoms with intense laser pulses. This nonlinear process leads to the production of high harmonics during collisions between electrons and the ionized atoms. The underlying mechanism implies control of energetic electrons with attosecond precision. We propose that the electrons themselves can be exploited for ultrafast measurements. We use a ‘molecular clock’, based on a vibrational wave packet in H2+ to show that distinct bunches of electrons appear during electron–ion collisions with high current densities, and durations of about 1 femtosecond (10-15 s). Furthermore, we use the molecular clock to study the dynamics of non-sequential double ionization.

Probing molecular dynamics with attosecond resolution using correlated wave packet pairs

Spectroscopic measurements with increasingly higher time resolution are generally thought to require increasingly shorter laser pulses, as illustrated by the recent monitoring of the decay of core-excited krypton using attosecond photon pulses. However, an alternative approach to probing ultrafast dynamic processes might be provided by entanglement, which has improved the precision of quantum optical measurements. Here we use this approach to observe the motion of a D2+ vibrational wave packet formed during the multiphoton ionization of D2 over several femtoseconds with a precision of about 200 attoseconds and 0.05 ångströms, by exploiting the correlation between the electronic and nuclear wave packets formed during the ionization event. An intense infrared laser field drives the electron wave packet, and electron recollision probes the nuclear motion. Our results show that laser pulse duration need not limit the time resolution of a spectroscopic measurement, provided the process studied involves the formation of correlated wave packets, one of which can be controlled; spatial resolution is likewise not limited to the focal spot size or laser wavelength.

Frequency-resolved high-harmonic wavefront characterization

We introduce and demonstrate a novel concept of frequency-resolved wavefront characterization. Our approach is particularly suitable for high-harmonic, extreme-UV (XUV) and soft X-ray radiation. The concept is based on an analysis of radiation diffracted from a slit scanned in front of a flat-field XUV spectrometer. With the spectrally resolved signal spread across one axis and the spatially resolved diffraction pattern in the other dimension, we reconstruct the wavefront. While demonstrated for high harmonics, the method is not restricted in wavelength.

Attosecond and Angstrom Science

Abstract When a strong laser field ionizes atoms (or molecules), the electron wave packet that tunnels from the molecule moves under the influence of the strong field and can re-collide with its parent ion. The maximum re-collision electron kinetic energy depends on the laser wavelength. Timed by the laser field oscillations, the re-colliding electron interferes with the bound state wave function from which it tunneled. The oscillating dipole caused by the quantum interference produces attosecond optical pulses. Interference can characterize both interfering beams—their wavelength, phase and spatial structure. Thus, written on the attosecond pulse is an image of the bound state orbital and the wave function at the re-collision electron. In addition to interfering, the re-collision electron can elastically or inelastically scatter from its parent ion, diffracting from the ion, and exciting or even exploding it. We review attosecond technology while emphasizing the underlying electron–ion re-collision physics.

Coherent imaging of an attosecond electron wave packet

Attosecond pulses image the quantum mechanical nodal structure as an electron is expelled from a neon atom. A detailed look at an electrons exit When a burst of light ejects an electron from an atom, the later detection of two charged particles masks a great deal of intermittent quantum mechanical complexity. Villeneuve et al. provide a striking look at the wavelike properties of the electron just as it emerges from neon, expelled by two photons from an attosecond pulse train in a strong infrared field. The phase distribution displays the characteristic three-node structure of an f-wave, which the Stark shift from the strong field appears to select with a single magnetic quantum number of 0. Science, this issue p. 1150 Electrons detached from atoms or molecules by photoionization carry information about the quantum state from which they originate, as well as the continuum states into which they are released. Generally, the photoelectron momentum distribution is composed of a coherent sum of angular momentum components, each with an amplitude and phase. Here we show, by using photoionization of neon, that a train of attosecond pulses synchronized with an infrared laser field can be used to disentangle these angular momentum components. Two-color, two-photon ionization via a Stark-shifted intermediate state creates an almost pure f-wave with a magnetic quantum number of zero. Interference of the f-wave with a spherically symmetric s-wave provides a holographic reference that enables phase-resolved imaging of the f-wave.

Order-dependent structure of high harmonic wavefronts

The physics of high harmonics has led to the generation of attosecond pulses and to trains of attosecond pulses. Measurements that confirm the pulse duration are all performed in the far field. All pulse duration measurements tacitly assume that both the beams wavefront and intensity profile are independent of frequency. However, if one or both are frequency dependent, then the retrieved pulse duration depends on the location where the measurement is made. We measure that each harmonic is very close to a Gaussian, but we also find that both the intensity profile and the beam wavefront depend significantly on the harmonic order. Thus, our findings mean that the pulse duration will depend on where the pulse is observed. Measurement of spectrally resolved wavefronts along with temporal characterization at one single point in the beam would enable complete space-time reconstruction of attosecond pulses. Future attosecond science experiments need not be restricted to spatially averaged observables. Our approach paves the way to recovery of the single molecule response to the strong field.

Attosecond dynamics using sub-laser-cycle electron pulses

We demonstrate that an electron wave packet produced during intense field ionization can be used for probing molecular dynamics with an attosecond time resolution. When a H2 molecule is ionized by an intense infrared laser field, a vibrational and an electronic wave packet are simultaneously produced. Using the vibrational wave packet in H as a molecular clock, we deduce the time structure and magnitude of the re-collision electron wave packet. Delaying the re-collision time by changing the laser wavelength, we measure the D vibrational motion with 200 as and 0.05 Å resolution. We also show that attosecond measurements can be achieved without attosecond optical pulse because of entanglement between vibrational and electron wave packets. Attosecond science will be developed using both optical and electron pulses.

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.

Extracting Electron-Ion Differential Scattering Cross Sections for Partially Aligned Molecules by Laser-induced Rescattering Photoelectron Spectroscopy

We extract large-angle elastic differential cross sections (DCSs) for electrons scattering from partially aligned O2+ and CO2+ molecules using rescattering photoelectrons generated by infrared laser pulses. The extracted DCSs are in good agreement with those calculated theoretically, demonstrating that accurate DCSs for electron-ion scattering can be extracted from the laser-induced rescattering spectra, thus paving the way for dynamic imaging of chemical reactions by rescattering photoelectron spectroscopy.