Laser-induced alignment and orientation of quantum-state-selected large molecules

Laser-Induced Alignment of Nanoparticles and Macromolecules for Coherent-Diffractive-Imaging Applications

Laser-induced alignment of particles and molecules was long envisioned to support three-dimensional structure determination using "single-molecule diffraction" with X-ray free-electron lasers [PRL 92, 198102 (2004)]. However, the alignment of isolated macromolecules has not yet been demonstrated also because quantitative modeling is very expensive. We computationally demonstrated that the alignment of nanorods and proteins is possible with a standard laser technology. We performed a comprehensive analysis on the dependence of the degree of alignment on molecular properties and experimental details, e.g., particle temperature and laser-pulse energy. Considering the polarizability anisotropy of about 150,000 proteins, our analysis revealed that most of these proteins can be aligned using realistic experimental parameters.

Controlled molecule injector for cold, dense, and pure molecular beams at the European x-ray free-electron laser

A permanently available molecular-beam injection setup for controlled molecules (COMO) was installed and commissioned at the small quantum systems (SQS) instrument at the European x-ray free-electron laser (EuXFEL). A b-type electrostatic deflector allows for pure state-, size-, and isomer-selected samples of polar molecules and clusters. The source provides a rotationally cold (T ≈ 1 K) and dense (ρ ≈ 108 cm-3) molecular beam with pulse durations up to 100 µs generated by a new version of the Even-Lavie valve. Here, a performance overview of the COMO setup is presented along with characterization experiments performed both with an optical laser at the Center for Free-Electron-Laser Science and with x rays at EuXFEL under burst-mode operation. COMO was designed to be attached to different instruments at the EuXFEL, in particular, the SQS and single particles, clusters, and biomolecules (SPB) instruments. This advanced controlled-molecules injection setup enables x-ray free-electron laser studies using highly defined samples with soft and hard x-ray FEL radiation for applications ranging from atomic, molecular, and cluster physics to elementary processes in chemistry and biology.

Collisional alignment and molecular rotation control the chemi-ionization of individual conformers of hydroquinone with metastable neon

The relationship between the shape of a molecule and its chemical reactivity is a central tenet in chemistry. However, the influence of the molecular geometry on reactivity can be subtle and result from several opposing effects. Here, using a crossed-molecular-beam experiment in which individual rotational quantum states of specific conformers of a molecule are separated, we study the chemi-ionization reaction of hydroquinone with metastable neon atoms. We show that collision-induced alignment of the reaction partners caused by geometry-dependent long-range forces influences reaction pathways, which is, however, countered by molecular rotation. The present work provides insights into the conformation-specific stereodynamics of complex polyatomic systems and illustrates the capability of advanced molecule-control techniques to unravel these effects.

Alignment transport between ultracold polar molecules

We propose an array of ultracold polar molecules as a platform to study alignment transport between molecules. We envision a polar molecule being aligned with an intense off-resonant laser field whose alignment migrates to a nearby molecule due to dipole-dipole interactions. Our results show that the transport of the alignment is due to a complex interplay between electric field-driven excitations and dipole-dipole interactions. All mechanisms for alignment transfer are elucidated and analyzed. Using NaCs as a prototype molecule, we find that the time for alignment transfer is (10 μs), which makes the phenomena readily observable in the lab.

Alignment of ND3 molecules in dc-electric fields

The control of movement and orientation of gas-phase molecules has become the focus of many research areas in molecular physics. Here, ND3 molecules are polarized in a segmented, curved electrostatic guide and adiabatically aligned inside a rotatable mass spectrometer (MS). Alignment is probed by photoionization using a linearly polarized laser. Rotation of the polarization at fixed MS orientation has the same effect as the rotation of the MS at fixed polarization, proving that the molecular alignment adiabatically follows the MS axis. Polarization-dependent ion signals reveal state-specific populations and allow for a quantification of the aligned sample in the space-fixed reference frame.

Intermolecular interactions probed by rotational dynamics in gas-phase clusters

The rotational dynamics of a molecule is sensitive to neighboring atoms or molecules, which can be used to probe the intermolecular interactions in the gas phase. Here, we real-time track the laser-driven rotational dynamics of a single N2 molecule affected by neighboring Ar atoms using coincident Coulomb explosion imaging. We find that the alignment trace of N-N axis decays fast and only persists for a few picoseconds when an Ar atom is nearby. We show that the decay rate depends on the rotational geometry of whether the Ar atom stays in or out of the rotational plane of the N2 molecule. Additionally, the vibration of the van der Waals bond is found to be excited through coupling with the rotational N-N axis. The observations are well reproduced by solving the time-dependent Schrödinger equation after taking the interaction potential between the N2 and Ar into consideration. Our results demonstrate that environmental effects on a molecular level can be probed by directly visualizing the rotational dynamics.

Preparation of Tautomer-Pure Molecular Beams by Electrostatic Deflection

Tautomers are ubiquitous throughout chemistry and typically considered inseparable in solution. Yet (bio)chemical activity is highly tautomer-specific, with common examples being the amino and nucleic acids. While tautomers exist in an equilibrium in solution, in the cold environment of a molecular beam the barrier to tautomerization is typically much too high for interconversion, and tautomers can be considered separate species. Here we demonstrate the first separation of tautomers within a molecular beam and the production of tautomerically pure gas-phase samples. We show this for the 2-pyridone/2-hydroxypyridine system, an important structural motif in both uracil and cytosine. Spatial separation of the tautomers is achieved via electrostatic deflection in strong inhomogeneous fields. We furthermore collect tautomer-resolved photoelectron spectra using femtosecond multiphoton ionization. This paves the way for studying the structure-function-dynamic relationship on the level of individual tautomers, using approaches that typically lack the resolution to do so, such as ultrafast dynamics experiments.

Enhanced EMC-Advantages of partially known orientations in x-ray single particle imaging

Single particle imaging of proteins in the gas phase with x-ray free-electron lasers holds great potential to study fast protein dynamics, but is currently limited by weak and noisy data. A further challenge is to discover the proteins' orientation as each protein is randomly oriented when exposed to x-rays. Algorithms such as the expand, maximize, and compress (EMC) exist that can solve the orientation problem and reconstruct the three-dimensional diffraction intensity space, given sufficient measurements. If information about orientation were known, for example, by using an electric field to orient the particles, the reconstruction would benefit and potentially reach better results. We used simulated diffraction experiments to test how the reconstructions from EMC improve with particles' orientation to a preferred axis. Our reconstructions converged to correct maps of the three-dimensional diffraction space with fewer measurements if biased orientation information was considered. Even for a moderate bias, there was still significant improvement. Biased orientations also substantially improved the results in the case of missing central information, in particular in the case of small datasets. The effects were even more significant when adding a background with 50% the strength of the averaged diffraction signal photons to the diffraction patterns, sometimes reducing the data requirement for convergence by a factor of 10. This demonstrates the usefulness of having biased orientation information in single particle imaging experiments, even for a weaker bias than what was previously known. This could be a key component in overcoming the problems with background noise that currently plague these experiments.

Quantum control of field-free molecular orientation

Generating field-free (non-stationary) orientation of molecules in space has been a longstanding goal in the field of quantum control of molecular rotation, which has significant applications in physical chemistry, chemical physics, strong-field physics, and quantum information science. In this Perspective, we review and examine several representative control schemes developed in recent years and implemented in theoretical and experimental areas for generating field-free orientation of molecules. By conducting numerical simulations of different control schemes on the same molecular system, we demonstrate that quantum coherent control, specifically targeting a limited number of the lowest-lying rotational levels to achieve an optimal superposition, can result in a high degree of orientation. To this end, we provide an overview of our latest developed analytical method, which enables the precise design of terahertz field parameters through resonant excitation. This design approach facilitates the attainment of desired field-free orientations by optimizing the amplitudes and phases of rotational wave functions for the selected rotational levels. Finally, we outlook the significance of such progress in multiple frontier research fields, highlighting its potential applications in ultracold physics, quantum computation, quantum simulation, and quantum metrology.

Enhanced molecular orientation via NIR-delay-THz scheme: Experimental results at room temperature

Light-induced orientation of gas phase molecules is a long-pursued goal in physics and chemistry. Here, we experimentally demonstrate a six-fold increase in the terahertz-induced orientation of iodomethane (CH₃I) molecules at room temperature, provided by rotational pre-excitation with a moderately intense near-IR pulse. The paper highlights the underlying interference of multiple coherent transition pathways within the rotational coherence manifold and is analyzed accordingly. Our experimental and theoretical results provide desirable and practical means for all-optical experiments on oriented molecular ensembles.

Molecular orientation echoes via concerted terahertz and near-IR excitations

A new and efficient method for orientation echo spectroscopy is presented and realized experimentally. The excitation scheme utilizes concerted rotational excitations by both ultrashort terahertz and near-IR pulses and its all-optical detection is enabled by the molecular orientation-induced second harmonic method [J. Phys. Chem. A126, 3732 (2022)10.1021/acs.jpca.2c03237]. This method provides practical means for orientation echo spectroscopy of gas phase molecules and highlights the intriguing underlying physics of coherent rotational dynamics induced by judiciously-orchestrated interactions with both resonant (terahertz) and nonresonant (NIR) fields.

Ultrafast Imaging of Molecular Chirality with Photoelectron Vortices

Ultrafast imaging of molecular chirality is a key step toward the dream of imaging and interpreting electronic dynamics in complex and biologically relevant molecules. Here, we propose a new ultrafast chiral phenomenon exploiting recent advances in electron optics allowing access to the orbital angular momentum of free electrons. We show that strong-field ionization of a chiral target with a few-cycle linearly polarized 800 nm laser pulse yields photoelectron vortices, whose chirality reveals that of the target, and we discuss the mechanism underlying this phenomenon. Our Letter opens new perspectives in recollision-based chiral imaging.

Internal Electric Field-Induced Formation of Exotic Linear Acetonitrile Chains

The application of external electric and magnetic fields is a powerful tool for aligning molecules in a controlled way, if the thermal fluctuations are small. Here we demonstrate that the same holds for internal electric fields in a molecular cluster. The electric field of a single molecular dipole, HCl, is used to manipulate the aggregation mechanism of subsequently added acetonitrile molecules. As a result, we could form exotic linear acetonitrile (CH₃CN) chains at 0.37 K, as confirmed by infrared spectroscopy in superfluid helium nanodroplets. These linear chains are not observed in the absence of HCl and can be observed only when the internal electric field created by an HCl molecule is present. The accompanying simulations provide mechanistic insights into steric control, explain the selectivity of the process, and show that non-additive electronic polarization effects systematically enhance the dipole moment of these linear chains. Thus, adding more CH₃CN monomers even supports further quasi-linear chain growth.

Optimization of the double-laser-pulse scheme for enantioselective orientation of chiral molecules

We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses' parameters, a significant (~ 10%) degree of enantioselective orientation can be achieved at zero and at five kelvin rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments may open new avenues for discriminating and separating molecular enantiomers.

Vibrationally resolved photoelectron angular distributions of ammonia

We present a theoretical study of vibrationally resolved photoelectron angular distributions for ammonia in both laboratory and molecular frames, in the photon energy range up to 70 eV, where only valence and inner-valence ionization is possible. We focus on the band resulting from ionization of the 3a₁ HOMO orbital leading to NH₃⁺ in the electronic ground state, , for which the dominant vibrational progression corresponds to the activation of the umbrella inversion mode. We show that, at room temperature, the photoelectron angular distributions for randomly oriented molecules or molecules whose principal C₃ symmetry axis is aligned along the light polarization direction are perfectly symmetric with respect to the plane that contains the intermediate D_3h conformation connecting the pyramidal structures associated with the double-well potential of the umbrella inversion mode. These distributions exhibit symmetric, nearly perfect two-lobe shapes in the whole range of investigated photon energies. In contrast, for molecules where the initial vibrational state is localized in one of the two wells, a situation that can experimentally be achieved by introducing an external electric field, the molecular-frame photoelectron angular distributions (MFPADs) are in general asymmetric, but the degree of asymmetry of the two lobes dramatically changes and oscillates with photoelectron energy. We also show that, at ultracold temperatures, where all aligned molecules initially lie in the delocalized ground vibrational state, the photoelectron angular distributions are perfectly symmetric, but the two-lobe shape is only observed when the final vibrational state of the resulting NH₃⁺ cation has even parity. When the latter vibrational state has odd parity, the angular distributions are much more involved and, at photoelectron energies of ∼10 eV, they directly reflect the bi-pyramidal geometry of the molecule in its ground vibrational state. These results suggest that, in order to obtain structural information from MFPADs in ammonia and likely in other molecules containing a similar double-well potential, one could preferably work at ultracold temperatures, which is not the case for most molecules.

Laser-Induced Coulomb Explosion Imaging of Aligned Molecules and Molecular Dimers

We discuss how Coulomb explosion imaging (CEI), triggered by intense femtosecond laser pulses and combined with laser-induced alignment and covariance analysis of the angular distributions of the recoiling fragment ions, provides new opportunities for imaging the structures of molecules and molecular complexes. First, focusing on gas phase molecules, we show how the periodic torsional motion of halogenated biphenyl molecules can be measured in real time by timed CEI, and how CEI of one-dimensionally aligned difluoroiodobenzene molecules can uniquely identify four structural isomers. Next, focusing on molecular complexes formed inside He nanodroplets, we show that the conformations of noncovalently bound dimers or trimers, aligned in one or three dimensions, can be determined by CEI. Results presented for homodimers of CS₂, OCS, and bromobenzene pave the way for femtosecond time-resolved structure imaging of molecules undergoing bimolecular interactions and ultimately chemical reactions. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Trends in angle-resolved molecular photoelectron spectroscopy

In this perspective article, main trends of angle-resolved molecular photoelectron spectroscopy in the laboratory up to the molecular frame, in different regimes of light-matter interactions, are highlighted with emphasis on foundations and most recent applications.

High density molecular jets of complex neutral organic molecules with Tesla valves

Supersonic jets of gas-phase atoms and small molecules have enabled a variety of ultrafast and ultracold chemical studies. However, extension to larger, more complex neutral molecules proves challenging for two reasons: (i) Complex molecules, such as cis-stilbene, exist in a liquid or solid phase at room temperature and ambient pressure and (ii) a unidirectional flow of high-density gaseous beams of such molecules to the interaction region is required. No delivery system currently exists that can deliver dense enough molecular jets of neutral complex molecules without ionizing or exciting the target for use in gas-phase structural dynamics studies. Here, we present a novel delivery system utilizing Tesla valves, which generates more than an order-of-magnitude denser gaseous beam of molecules compared to a bubbler without Tesla valves at the interaction region by ensuring a fast unidirectional flow of the gaseous sample. We present combined experimental and flow simulations of the Tesla valve setup. Our results open new possibilities of studying large complex neutral molecules in the gas-phase with low vapor pressures in future ultrafast and ultracold studies.

Orientation and Alignment dynamics of polar molecule driven by shaped laser pulses

We review the theoretical status of intense laser induced orientation and alignment-a field of study which lies at the interface of intense laser physics and chemical dynamics and having potential applications such as high harmonic generation, nano-scale processing and control of chemical reactions. The evolution of the rotational wave packet and its dynamics leading to orientation and alignment is the topic of the present discussion. The major part of this article primarily presents an overview of recent theoretical progress in controlling the orientation and alignment dynamics of a molecule by means of shaped laser pulses. The various theoretical approaches that lead to orientation and alignment such as static electrostatic field in combination with laser field(s), combination of orienting and aligning field, combination of aligning fields, combination of orienting fields, application of train of pulses etc. are discussed. It is observed that the train of pulses is quite an efficient tool for increasing the orientation or alignment of a molecule without causing the molecule to ionize. The orientation and alignment both can occur in adiabatic and non-adiabatic conditions with the rotational period of the molecule taken under consideration. The discussion is mostly limited to non-adiabatic rotational excitation (NAREX) i.e. cases in which the pulse duration is shorter than the rotational period of the molecule. We have emphasised on the so called half-cycle pulse (HCP) and square pulse (SQP). The effect of ramped pulses and of collision on the various laser parameters is also studied. We summarize the current discussion by presenting a consistent theoretical approach for describing the action of such pulses on movement of molecules. The impact of a particular pulse shape on the post-pulse dynamics is also calculated and analysed. In addition to this, the roles played by various laser parameters including the laser frequency, the pulse duration and the system temperature etc. are illustrated and discussed. The concept of alignment is extended from one-dimensional alignment to three-dimensional alignment with the proper choice of molecule and the polarised light. We conclude the article by discussing the potential applications of intense laser orientation and alignment.

Waveform control of molecular dynamics close to a conical intersection

Conical intersections are ubiquitous in chemical systems but, nevertheless, extraordinary points on the molecular potential energy landscape. They provide ultra-fast radiationless relaxation channels, their topography influences the product branching, and they equalize the timescales of the electron and nuclear dynamics. These properties reveal optical control possibilities in the few femtosecond regime. In this theoretical study, we aim to explore control options that rely on the carrier envelope phase of a few-cycle IR pulse. The laser interaction creates an electronic superposition just before the wave packet reaches the conical intersection. The imprinted phase information is varied by the carrier envelope phase to influence the branching ratio after the conical intersection. We test and analyze this scenario in detail for a model system and show to what extent it is possible to transfer this type of control to a realistic system like uracil.

Field-free molecular orientation by delay- and polarization-optimized two fs pulses

Unless the molecular axis is fixed in the laboratory frame, intrinsic structural information of molecules can be averaged out over the various rotational states. The macroscopic directional properties of polar molecules have been controlled by two fs pulses with an optimized delay. In the method, the first one-color laser pulse provokes molecular alignment. Subsequently, the molecular sample is irradiated with the second two-color laser pulse, when the initial even-J states are aligned, and the odd-J states are anti-aligned in the thermal ensemble. The second pulse selectively orients only the aligned even-J states in the same direction, which results in significant enhancement of the net degree of orientation. This paper reports the results of simulations showing that the two-pulse technique can be even more powerful when the second pulse is cross-polarized. This study shows that the alignment and orientation can be very well synchronized temporally because the crossed field does not disturb the preformed alignment modulation significantly, suggesting that the molecules are very well confined in the laboratory frame. This cross-polarization method will serve as a promising technique for studying ultrafast molecular spectroscopy in a molecule-fixed frame.

Perspectives on single particle imaging with x rays at the advent of high repetition rate x-ray free electron laser sources

X-ray free electron lasers (XFELs) now routinely produce millijoule level pulses of x-ray photons with tens of femtoseconds duration. Such x-ray intensities gave rise to the idea that weakly scattering particles-perhaps single biomolecules or viruses-could be investigated free of radiation damage. Here, we examine elements from the past decade of so-called single particle imaging with hard XFELs. We look at the progress made to date and identify some future possible directions for the field. In particular, we summarize the presently achieved resolutions as well as identifying the bottlenecks and enabling technologies to future resolution improvement, which in turn enables application to samples of scientific interest.

Absolute carrier-envelope-phase dependences of single and double ionization of methanol in a near-IR few-cycle laser field

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH₃OH) in an intense near-IR few-cycle laser field (2.1 × 10¹⁴ W/cm²) by the asymmetry in the ejection direction of CH₃ ⁺ for the non-hydrogen migration channels and CH₂ ⁺ for the hydrogen migration channels created through the C-O bond breaking after the ionization. Based on the absolute CEP values at the laser-molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1-053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition.

Controlled beams of shock-frozen, isolated, biological and artificial nanoparticles

X-ray free-electron lasers promise diffractive imaging of single molecules and nanoparticles with atomic spatial resolution. This relies on the averaging of millions of diffraction patterns of identical particles, which should ideally be isolated in the gas phase and preserved in their native structure. Here, we demonstrated that polystyrene nanospheres and Cydia pomonella granulovirus can be transferred into the gas phase, isolated, and very quickly shock-frozen, i.e., cooled to 4 K within microseconds in a helium-buffer-gas cell, much faster than state-of-the-art approaches. Nanoparticle beams emerging from the cell were characterized using particle-localization microscopy with light-sheet illumination, which allowed for the full reconstruction of the particle beams, focused to < 100   μ m , as well as for the determination of particle flux and number density. The experimental results were quantitatively reproduced and rationalized through particle-trajectory simulations. We propose an optimized setup with cooling rates for particles of few-nanometers on nanosecond timescales. The produced beams of shock-frozen isolated nanoparticles provide a breakthrough in sample delivery, e.g., for diffractive imaging and microscopy or low-temperature nanoscience.

Quantum-assisted diamagnetic deflection of molecules

We measure the diamagnetic deflection of anthracene and adamantane in a long-baseline matter-wave interferometer.

How reactant polarization can be used to change the effect of interference on reactive collisions

It is common knowledge that integral and differential cross sections (DCSs) are strongly dependent on the spatial distribution of the molecular axis of the reactants. Hence, by controlling the axis distribution, it is possible to either promote or hinder the yield of products into specific final states or scattering angles. This idea has been successfully implemented in experiments by polarizing the internuclear axis before the reaction takes place, either by manipulating the rotational angular distribution or by the Stark effect in the presence of an orienting field. When there is a dominant reaction mechanism, characterized by a set of impact parameters and angles of attack, it is expected that a preparation that helps the system to reach the transition state associated with that mechanism will promote the reaction, whilst a different preparation would generally impair the reaction. However, when two or more competing mechanisms via interference contribute to the reaction into specific scattering angles and final states, it is not evident which would be the effect of changing the axis preparation. To address this problem, throughout this article we have simulated the effect that different experimental preparations have on the DCSs for the H + D2 reaction at relatively high energies, for which it has been shown that several competing mechanisms give rise to interference that shapes the DCS. To this aim, we have extended the formulation of the polarization dependent DCS to calculate polarization dependent generalized deflection functions of ranks greater than zero. Our results show that interference is very sensitive to changes in the internuclear axis preparation, and that the shape of the DCS can be controlled exquisitely.

Robust and Accurate Computational Estimation of the Polarizability Tensors of Macromolecules

Alignment of molecules through electric fields minimizes the averaging over orientations, e.g., in single-particle-imaging experiments. The response of molecules to external ac electric fields is governed by their polarizability tensor, which is usually calculated using quantum chemistry methods. These methods are not feasible for large molecules. Here, we calculate the polarizability tensor of proteins using a regression model that correlates the polarizabilities of the 20 amino acids with perfect conductors of the same shape. The dielectric constant of the molecules could be estimated from the slope of the regression line based on the Clausius-Mossotti equation. We benchmark our predictions against the quantum chemistry results for the Trp cagemini protein and the measured dielectric constants of larger proteins. Our method has applications in computing laser alignment of macromolecules, for instance, benefiting single-particle imaging, as well as for estimation of the optical and electrostatic characteristics of proteins and other macromolecules.

Nonresonant Photons Catalyze Photodissociation of Phenol

Phenol represents an ideal polyatomic system for demonstrating photon catalysis because of its large polarizability, well-characterized excited-state potential energy surfaces, and nonadiabatic dissociation dynamics. A nonresonant IR pulse (1064 nm) supplies a strong electric field (4 × 10⁷ V/cm) during the photolysis of isolated phenol (C₆H₅OH) molecules to yield C₆H₅O + H near two known energetic thresholds: the S₁/S₂ conical intersection and the S₁ - S₀ origin. H-atom speed distributions show marked changes in the relative contributions of dissociative pathways in both cases, compared to the absence of the nonresonant IR pulse. Results indicate that nonresonant photons lower the activation barrier for some pathways relative to others by dynamically Stark shifting the excited-state potential energy surfaces rather than aligning molecules in the strong electric field. Theoretical calculations offer support for the experimental interpretation.

All-optical field-free three-dimensional orientation of asymmetric-top molecules

Orientation and alignment of molecules by ultrashort laser pulses is crucial for a variety of applications and has long been of interest in physics and chemistry, with the special emphasis on stereodynamics in chemical reactions and molecular orbitals imaging. As compared to the laser-induced molecular alignment, which has been extensively studied and demonstrated, achieving molecular orientation is a much more challenging task, especially in the case of asymmetric-top molecules. Here, we report the experimental demonstration of all-optical field-free three-dimensional orientation of asymmetric-top molecules by means of phase-locked cross-polarized two-color laser pulse. This approach is based on nonlinear optical mixing process caused by the off-diagonal elements of the molecular hyperpolarizability tensor. It is demonstrated on SO2 molecules and is applicable to a variety of complex nonlinear molecules.

Spatially Separating the Conformers of a Dipeptide

Atomic-resolution-imaging approaches for single molecules, such as coherent X-ray diffraction at free-electron lasers, require the delivery of high-density beams of identical molecules. However, even very cold beams of biomolecules typically have multiple conformational states populated. We demonstrate the production of very cold (T_rot ≈2.3 K) molecular beams of intact dipeptide molecules, which were then spatially separated into the individual populated conformational states. This is achieved using the combination of supersonic expansion laser-desorption vaporization with electrostatic deflection in strong inhomogeneous fields. This represents the first demonstration of a conformer-separated and rotationally cold molecular beam of a peptide, which enables the investigation of conformer-specific chemistry using inherently non-conformer-specific techniques. It furthermore represents a milestone toward the direct structural imaging of individual biological molecules with atomic resolution by ultrafast diffractive-imaging methods.

Coherent Control of the Rotation Axis of Molecular Superrotors

The control of ultrafast molecular rotational motion has benefited from the development of innovative techniques in strong-field laser physics. Here, we theoretically demonstrate a novel type of coherent control by inducing rotation of an asymmetric-top molecule about two different molecular axes. An optical centrifuge is applied to the hydrogen sulfide (H₂S) molecule to create a molecular superrotor, an object performing ultrafast rotation about a well-defined axis. Using two distinct pulse envelopes for the optical centrifuge, we show that H₂S can be excited along separate pathways of rotational states. This leads to stable rotation about two entirely different molecular axes while ensuring rotation is about the propagation direction of the centrifuge, i.e., the laboratory-fixed Z-axis. The presented scheme to control the angular momentum alignment of a molecule will, for instance, be useful in studies of molecule-molecule or molecule-surface scattering, especially due to the large amounts of energy associated with superrotors, which can even be controlled by changing the duration of the optical centrifuge pulse.

Rotational Friction and Diffusion of Quantum Rotors

We present the Markovian quantum master equation describing rotational decoherence, friction, diffusion, and thermalization of planar, linear, and asymmetric rotors in contact with a thermal environment. It describes how an arbitrary initial rotation state decoheres and evolves toward a Gibbs-like thermal ensemble, as we illustrate numerically for the linear and the planar top, and it yields the expected rotational Fokker-Planck equation of Brownian motion in the semiclassical limit.

Imaging of rotational wave-function in photodissociation of rovibrationally excited HCl molecules

We demonstrate a visualization of quantum mechanical phenomena with the velocity map imaging (VMI) technique, combining vibrationally mediated photodissociation (VMP) of a simple diatomic HCl with the VMI of its H-photofragments. Free HCl molecules were excited by a pump infrared (IR) laser pulse to particular rotational J levels of the v = 2 vibrational state, and subsequently a probe ultraviolet laser photodissociated the molecule at a fixed wavelength of 243.07 nm where also the H-fragments were ionized. The molecule was aligned by the IR excitation with respect to the IR laser polarization, and this alignment was reflected in the angular distribution of the H-photofragments. In particular, the highest degree of molecular alignment was achieved for the J=1←0 transition, which exclusively led to the population of a single rotational state with M = 0. The obtained images were analyzed for further details of the VMP dynamics, and different J states were studied as well. Additionally, we investigated the dynamic evolution of the excited states by changing the pump-probe laser pulse delay; the corresponding images reflected dephasing due to a coupling between the molecular angular momentum and nuclear spin. Our measurements confirmed previous observation using the time-of-flight technique by Sofikitis et al. [J. Chem. Phys. 127, 144307 (2007)]. We observed a partial recovery of the originally excited state after 60 ns in agreement with the previous observation.

Time-dependent analysis of the mixed-field orientation of molecules without rotational symmetry

We present a theoretical study of the mixed-field orientation of molecules without rotational symmetry. The time-dependent one-dimensional and three-dimensional orientation of a thermal ensemble of 6-chloropyridazine-3-carbonitrile molecules in combined linearly or elliptically polarized laser fields and tilted dc electric fields is computed. The results are in good agreement with recent experimental results of one-dimensional orientation for weak dc electric fields [J. L. Hansen, J. Chem. Phys. 139, 234313 (2013)]. Moreover, they predict that using elliptically polarized laser fields or strong dc fields, three-dimensional orientation is obtained. The field-dressed dynamics of excited rotational states is characterized by highly non-adiabatic effects. We analyze the sources of these non-adiabatic effects and investigate their impact on the mixed-field orientation for different field configurations in mixed-field-orientation experiments.

Stereodynamics of Ne(3P2) reacting with Ar, Kr, Xe, and N2

Stereodynamics experiments of Ne(³P₂) reacting with Ar, Kr, Xe, and N₂ leading to Penning and associative ionization have been performed in a crossed molecular beam apparatus. A curved magnetic hexapole was used to state-select and polarize Ne(³P₂) atoms which were then oriented in a rotatable magnetic field and crossed with a beam of Ar, Kr, Xe, or N₂. The ratio of associative to Penning ionization was recorded as a function of the magnetic field direction for collision energies between 320 cm^-1 and 500 cm^-1. Reactivities are obtained for individual states that differ only in Ω, the projection of the neon total angular momentum vector on the inter-particle axis. The results are rationalized on the basis of a model involving a long-range and a short-range reaction mechanism. Substantially lower probability for associative ionization was observed for N₂, suggesting that predissociation plays a critical role in the overall reaction pathway.

Photophysics of indole upon X-ray absorption

A photofragmentation study of gas-phase indole (C₈H₇N) upon single-photon ionization at a photon energy of 420 eV is presented.

Photoinduced Ultrafast Charge Transfer and Charge Migration in Small Gold Clusters Passivated by a Chromophoric Ligand

Because the development of attopulses, charge migration induced by short optical pulses has been extensively investigated. We report a computational purely electronic dynamical study of ultrafast few femtoseconds (fs) charge transfer and charge migration in realistic passivated stoichiometric Au₁₁ and Au₂₀ gold nanoclusters functionalized by a bipyridine ligand. We show that a net significant amount of electronic charge (0.1 to 0.4 |e| where |e| is the electron charge) is permanently transferred from the bipyridine chromophore to the gold cluster during the short 5-6 fs UV-vis strong pulse. This electron transfer to the metallic core is induced by the optical excitation of electronic states with a partial charge transfer character involving the chromophore before the onset of nuclei motion. In addition, the photoexcitation by the strong fs pulse builds a nonequilibrium electronic density that beats between the chromophore and the metallic core around the average of the transferred value. Modular systems made of a donor chromophore that can be photoexcited in the UV-vis range coupled to an efficient acceptor that could trap the charge are of interest for applications to nanodevices. Our study provides understanding on the very early, purely electronic dynamics built by the fs optical excitation and the initial charge separation step.