arXiv subjects

Philip H. Bucksbaum

Publications and source records attributed to Philip H. Bucksbaum.

At least 37 records · Page 2Linked to original sources

arXiv2020

Nonlinear resonant X-ray Raman scattering

We report the observation of a novel nonlinear effect in the hard x-ray range. Upon illuminating Fe and Cu metal foils with intense x-ray pulses tuned near their respective K edges, photons at nearly twice the incoming photon energy are emitted. The signal rises quadratically with the incoming intensity, consistent with two-photon excitation. The spectrum of emitted high-energy photons comprises multiple Raman lines that disperse with the incident photon energy. Upon reaching the double K-shell ionization threshold, the signal strength undergoes a marked rise. Above this threshold, the lines cease dispersing, turning into orescence lines with energies much greater than obtainable by single electron transitions, and additional Raman lines appear. We attribute these processes to electron-correlation mediated multielectron transitions involving double-core hole excitation and various two-electron de-excitation processes to a final state involving one or more L and M core-holes.

physics.atom-ph↗

arXiv2020

Strictly Non-Adiabatic Quantum Control of the Acetylene Dication Using an Infrared Field

We demonstrate the existence of a strictly non-adiabatic control pathway in deprotonation of the acetylene dication. This pathway is identified experimentally by measuring a kinetic energy shift in an ion coincidence experiment. We use a TDSE simulation to identify which properties most strongly affect our control. We find that resonant control around conical intersections is limited by the speed of non-adiabatic dynamics.

physics.chem-ph↗

arXiv2019

Characterizing multiphoton excitation using time-resolved X-ray scattering

Molecular iodine was photoexcited by a strong 800 nm laser, driving several channels of multiphoton excitation. The motion following photoexcitation was probed using time-resolved X-ray scattering, which produces a scattering map $S(Q,τ)$. Temporal Fourier transform methods were employed to obtain a frequency-resolved X-ray scattering signal $\tilde{S}(Q,ω)$. Taken together, $S(Q,τ)$ and $\tilde{S}(Q,ω)$ separate different modes of motion, so that mode-specific nuclear oscillatory positions, oscillation amplitudes, directions of motions, and times may be measured accurately. Molecular dissociations likewise have a distinct signature, which may be used to identify both velocities and dissociation time shifts, and also can reveal laser-induced couplings among the molecular potentials.

physics.chem-ph↗

arXiv2019

Attosecond Transient Absorption Spooktroscopy: a ghost imaging approach to ultrafast absorption spectroscopy

The recent demonstration of isolated attosecond pulses from an X-ray free-electron laser (XFEL) opens the possibility for probing ultrafast electron dynamics at X-ray wavelengths. An established experimental method for probing ultrafast dynamics is X-ray transient absorption spectroscopy, where the X-ray absorption spectrum is measured by scanning the central photon energy and recording the resultant photoproducts. The spectral bandwidth inherent to attosecond pulses is wide compared to the resonant features typically probed, which generally precludes the application of this technique in the attosecond regime. In this paper we propose and demonstrate a new technique to conduct transient absorption spectroscopy with broad bandwidth attosecond pulses with the aid of ghost imaging, recovering sub-bandwidth resolution in photoproduct-based absorption measurements.

physics.chem-ph↗

arXiv2019

Electron Correlation Effects in Attosecond Photoionization of CO$_{2}$

A technique for measuring photoionization time delays with attosecond precision is combined with calculations of photoionization matrix elements to demonstrate how multi-electron dynamics affect photoionization time delays in carbon dioxide. Electron correlation is observed to affect the time delays through two mechanisms: autoionization of molecular Rydberg states and accelerated escape from a continuum shape resonance.

physics.optics↗

arXiv2019

Overcoming the absorption limit in high-harmonic generation from crystals

Since the new millennium coherent extreme ultra-violet and soft x-ray radiation has revolutionized the understanding of dynamical physical, chemical and biological systems at the electron's natural timescale. Unfortunately, coherent laser-based upconversion of infrared photons to vacuum-ultraviolet and soft x-ray high-order harmonics in gaseous, liquid and solid targets is notoriously inefficient. In dense nonlinear media, the limiting factor is strong re-absorption of the generated high-energy photons. Here we overcome this limitation by allowing high-order harmonics generated from a periodic array of thin one-dimensional crystalline silicon ridge waveguides to propagate in the vacuum gaps between the ridges, thereby avoiding the high absorption loss of the bulk nonlinear material and resulting in a ~ 100-fold increase in propagation length. As the grating period is varied, each high-harmonic shows a different and marked modulation, indicating the onset of coherent addition which is otherwise suppressed in absorption-limited emission. By beating the absorption limit, our results pave the way for bright coherent short-wavelength sources and their implementation in nano-photonic devices.

physics.optics↗

arXiv2017

Filming non-radiative population transfer: Time-resolved x-ray scattering near an avoided crossing

We show that time-resolved x-ray scattering from molecules prepared in a superposition of electronic states moving through an avoided crossing has new features not found in diffraction from the corresponding classical mixed state. Photoabsorption in molecular iodine at 520 nm produces a superposition of two dipole-allowed nearly degenerate electronic states, which interact due to non-adiabatic coupling. We show experimental evidence that the mixing of the nuclear wavepackets from the two electronic states at the avoided crossing leads to ultrafast changes in the angular composition of the scattering pattern. This provides a novel means to study transitions in excited molecular systems. We reconstruct a movie of the nuclear probability density arising from this interference.

physics.chem-ph↗

arXiv2017

Enhanced High-Harmonic Generation from an All-Dielectric Metasurface

The recent observation of high-harmonic generation from solids creates a new possibility for engineering fundamental strong-field processes by patterning the solid target with subwavelength nanostructures. All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to control high-harmonics and other high-field processes at nanoscales. Here we report enhanced non-perturbative high-harmonic emission from a Si metasurface that possesses a sharp Fano resonance resulting from a classical analogue of electromagnetically induced transparency. Harmonic emission is enhanced by more than two orders of magnitude compared to unpatterned samples. The enhanced high harmonics are highly anisotropic with excitation polarization and are selective to excitation wavelength due to its resonant feature. By combining nanofabrication technology and ultrafast strong-field physics, our work paves the way for designing new compact ultrafast photonic devices that operate under high intensities and short wavelengths.

physics.optics↗

arXiv2017

Geometric Dependence of Strong Field Enhanced Ionization in D$_{2}$O

We have studied strong-field enhanced dissociative ionization of D$_{2}$O in 40 fs, 800 nm laser pulses with focused intensities of $< 1 - 3 \times 10^{15}W/cm^2$ by resolving the charged fragment momenta with respect to the laser polarization. We observe dication dissociation into OD$^{+}$/D$^{+}$ dominates when the polarization is out of the plane of the molecule, whereas trication dissociation into O$^{+}$/D$^{+}$/D$^{+}$ is strongly dominant when the polarization is aligned along the D-D axis. Dication dissociation into O/D$^{+}$/D$^{+}$, and O$^{+}$/D$_2$$^{+}$ is not seen, nor is there any significant fragmentation into multiple ions when the laser is polarized along the C$_{2v}$ symmetry axis of the molecule. Even below the saturation intensity for OD$^{+}$/D$^{+}$, the O$^{+}$/D$^{+}$/D$^{+}$ channel has higher yield. By analyzing how the laser field is oriented within the molecular frame for both channels, we show that enhanced ionization is driving the triply charged three body breakup, but is not active for the doubly charged two body breakup. We conclude that laser-induced distortion of the molecular potential suppresses multiple ionization along the C$_{2v}$ axis, but enhances ionization along the D-D direction.

physics.chem-ph↗

arXiv2017

Imaging the Breakdown of Molecular-Frame Dynamics through Rotational Uncoupling

We demonstrate the breakdown of molecular-frame dynamics induced by the uncoupling of molecular rotation from electronic motion in molecular Rydberg states. We observe this non-Born-Oppenheimer regime in the time domain through photoelectron imaging of a coherent molecular Rydberg wave packet in $\textrm{N}_2$. The photoelectron angular distribution shows a radically different time evolution than that of a typical molecular-frame-fixed electron orbital, revealing the uncoupled motion of the electron as it precesses around the $averaged$ anisotropic potential of the rotating ion-core.

physics.atm-clus↗

arXiv2016

The attosecond regime of impulsive stimulated electronic Raman excitation

We have calculated the resonant and nonresonant contributions to attosecond impulsive stimulated electronic Raman scattering (SERS) in regions of autoionizing transitions. Comparison with Multiconfiguration Time-Dependent Hartree-Fock (MCTDHF) calculations find that attosecond SERS is dominated by continuum transitions and not autoionizing resonances. These results agree quantitatively with a rate equation that includes second-order Raman and first-and second-order photoionization rates. Such rate models can be extended to larger molecular systems. Our results indicate that attosecond SERS transition probabilities may be understood in terms of two-photon generalized cross sections even in the high-intensity limit for extreme ultraviolet wavelengths.

physics.atom-ph↗

arXiv2016

Coherent control using kinetic energy and the geometric phase of a conical intersection

Conical intersections (CI) between molecular potential energy surfaces with non-vanishing non-adiabatic couplings generally occur in any molecule consisting of at least three atoms. They play a fundamental role in describing the molecular dynamics beyond the Born-Oppenheimer approximation and have been used to understand a large variety of effects, from photofragmentation and isomerization to more exotic applications such as exciton fission in semiconductors. However, few studies have used the features of a CI as a tool for coherent control. Here we demonstrate two modes of control around a conical intersection. The first uses a continuous light field to control the population on the two intersecting electronic states in the vicinity of a CI. The second uses a pulsed light field to control wavepackets that are subjected to the geometric phase shift in transit around a CI. This second technique is likely to be useful for studying the role of nuclear dynamics in electronic coherence phenomena.

quant-ph↗

arXiv2015

Observation of Bloch oscillations in molecular rotation

The periodically kicked quantum rotor is known for non-classical effects such as quantum localisation in angular momentum space or quantum resonances in rotational excitation. These phenomena have been studied in diverse systems mimicking the kicked rotor, such as cold atoms in optical lattices, or coupled photonic structures. Recently, it was predicted that several solid state quantum localisation phenomena - Anderson localisation, Bloch oscillations, and Tamm-Shockley surface states - may manifest themselves in the rotational dynamics of laser-kicked molecules. Here, we report the first observation of rotational Bloch oscillations in a gas of nitrogen molecules kicked by a periodic train of femtosecond laser pulses. A controllable detuning from the quantum resonance creates an effective accelerating potential in angular momentum space, inducing Bloch-like oscillations of the rotational excitation. These oscillations are measured via the temporal modulation of the refractive index of the gas. Our results introduce room-temperature laser-kicked molecules as a new laboratory for studies of localisation phenomena in quantum transport.

quant-ph↗

arXiv2015

Dynamical Localization in Molecular Alignment of Kicked Quantum Rotors

The periodically $δ$-kicked quantum linear rotor is known to experience non-classical bounded energy growth due to quantum dynamical localization in angular momentum space. We study the effect of random deviations of the kick period in simulations and experiments. This breaks the energy and angular momentum localization and increases the rotational alignment, which is the analog of the onset of Anderson localization in 1-D chains.

physics.optics↗

arXiv2013

Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1,3-cyclohexadiene ring opening

Nonradiative energy dissipation in electronically excited polyatomic molecules proceeds through conical intersections, loci of degeneracy between electronic states. We observe a marked enhancement of laser-induced double ionization in the vicinity of a conical intersection during a non-radiative transition. We measured double ionization by detecting the kinetic energy of ions released by laser-induced strong-field fragmentation during the ring-opening transition between 1,3-cyclohexadiene and 1,3,5-hexatriene. The enhancement of the double ionization correlates with the conical intersection between the HOMO and LUMO orbitals.

physics.chem-ph↗

arXiv2013

Multiple orbital contributions to molecular high-harmonic generation in an asymmetric top

High-order harmonic generation (HHG) in aligned linear molecules can offer valuable information about strong-field interactions in lower-lying molecular orbitals, but extracting this information is difficult for three-dimensional molecular geometries. Our measurements of the asymmetric top SO2 show large axis dependencies, which change with harmonic order. The analysis shows that these spectral features must be due to field ionization and recombination from multiple orbitals during HHG. We expect that HHG can probe orbital dependencies using this approach for a broad class of asymmetric-top molecules.

physics.atom-ph↗

arXiv2013

Orientational decomposition of molecular high harmonic emission in three dimensions

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. 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 or orbitals 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 (SO2) 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.

physics.atom-ph↗

arXiv2011

Control of 1,3-Cyclohexadiene Photoisomerization Using Light-Induced Conical Intersections

We have studied the photo-induced isomerization from 1,3-cyclohexadiene to 1,3,5-hexatriene in the presence of an intense ultrafast laser pulse. We find that the laser field maximally suppresses isomerization if it is both polarized parallel to the excitation dipole and present 50 fs after the initial photoabsorption, at the time when the system is expected to be in the vicinity of a conical intersection that mediates this structural transition. A modified ab initio multiple spawning (AIMS) method shows that the laser induces a resonant coupling between the excited state and the ground state, i.e., a light-induced conical intersection. The theory accounts for the timing and direction of the effect.

physics.chem-ph↗