M. Spanner

Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy

Coincident vibrational and electronic rearrangements in a photoexcited molecule are tracked in fine detail. Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection. At longer delays, we observe the onset of statistical dissociation dynamics. The present results demonstrate that high-harmonic spectroscopy could become a powerful tool to highlight electronic dynamics occurring along nonadiabatic chemical reaction pathways.

Demonstration of attosecond ionization dynamics inside transparent solids

Attosecond science has arisen from intense light pulses interacting with low density gases. We show that the initiating process—sub-cycle ionization—also survives in large band gap condensed media. Using fused SiO2 as an example, we measure the differential nonlinear absorption between the major and minor axis of elliptically polarized light. Through simulations that include ionization and light propagation, we confirm that changes in the ellipticity between the incident beam and the transmitted beam encode sub-cycle absorption dynamics. As the pulse duration is increased, we observe that sub-cycle ionization is masked by collisional processes. We propose a general class of methods for measuring attosecond dynamics in condensed media.

Coulomb asymmetry and sub-cycle electron dynamics in multiphoton multiple ionization of H 2

We present a systematic study of the molecular-frame photo-electron angular distributions produced by multiphoton double ionization of H2 using circularly polarized 800 nm, femtosecond laser pulses. We compare experimental results to numerical results obtained from a reduced-dimensionality time-dependent Schr¨ odinger equation (TDSE) model. In addition, we implement a TDSE-like version of the strong-field approximation to isolate the effect of the parent ion’s Coulomb potential on the continuum electron in our simulations. Thereby we identify the contributions of the parent ion potential, and light induced sub-optical cycle electron dynamics on the observable energy and angular distributions.

Phase sensitivity of high harmonic transient grating spectroscopy

We study the spatial profile of high order harmonics generated by a transient grating of rotational excitation. We show that the phase modulation of the harmonic emission as a function of molecular alignment is encoded in the diffraction pattern. In molecular nitrogen, the phase difference between aligned and isotropic molecules decreases from 1.6 rad for harmonic 19 to less than 0.3 rad for harmonic 27. In CO2 we observe a strong phase jump for the highest harmonics. The position of this phase jump in the harmonic spectrum depends on the laser intensity, reflecting the contribution from multiple molecular orbitals to the harmonic emission.

Alignment Dependent Enhancement of the Photoelectron Cutoff for Multiphoton Ionization of Molecules

The multiphoton ionization rate of molecules depends on the alignment of the molecular axis with respect to the ionizing laser polarization. By studying molecular frame photoelectron angular distributions from N(2), O(2), and benzene, we illustrate how the angle-dependent ionization rate affects the photoelectron cutoff energy. We find alignment can enhance the high energy cutoff of the photoelectron spectrum when probing along a nodal plane or when ionization is otherwise suppressed. This is supported by calculations using a tunneling model with a single ion state.

Transition between Mechanisms of Laser-Induced Field-Free Molecular Orientation

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 × 10(13) W/cm(2), the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrating absolute rates of nonperturbative strong-field molecular ionization.

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

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.

Streak Camera for Strong-Field Ionization

Ionization of an atom or molecule by a strong laser field produces suboptical cycle wave packets whose control has given rise to attosecond science. The final states of the wave packets depend on ionization and deflection by the laser field, which are convoluted in conventional experiments. Here, we demonstrate a technique enabling efficient electron deflection, separate from the field driving strong-field ionization. Using a midinfrared deflection field permits one to distinguish electron wave packets generated at different field maxima of an intense few-cycle visible laser pulse. We utilize this capability to trace the scattering of low-energy electrons driven by the midinfrared field. Our approach represents a general technique for studying and controlling strong-field ionization dynamics on the attosecond time scale.