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

Coherent Radiative Decay of Molecular Rotations: A Comparative Study of Terahertz-Oriented versus Optically Aligned Molecular Ensembles

The decay of field-free rotational dynamics is experimentally studied by two complementary methods: laser-induced molecular alignment and terahertz-field-induced molecular orientation. A comparison between the decay rates of different molecular species at various gas pressures reveals that oriented molecular ensembles decay faster than aligned ensembles. The discrepancy in decay rates is attributed to the coherent radiation emitted by the transiently oriented ensembles and is absent from aligned molecules. The experimental results reveal the dramatic contribution of coherent radiative emission to the observed decay of rotational dynamics and underline a general phenomenon expected whenever field-free coherent dipole oscillations are induced.

Rotational Dynamics of Quantum State-Selected Symmetric-Top Molecules in Nonresonant Femtosecond Laser Fields

Rotational dynamics of quantum state selected and unselected CH₃I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrödinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state |1, ±1, ∓1⟩, the transitions can be assigned to ΔJ = ±1, ±2, while for thermally populated molecules, the transitions are ΔJ = ±2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state-selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.

Impulsive laser-induced alignment of OCS molecules at FERMI

OCS full rotational revival dynamics induced by impulsive NIR alignment monitored by Coulomb explosion correlated fragments after S 2p excitation.

Adsorbed molecules in external fields: Effect of confining potential

We study the rotational excitation of a molecule adsorbed on a surface. As is well known the interaction potential between the surface and the molecule can be modeled in number of ways, depending on the molecular structure and the geometry under which the molecule is being adsorbed by the surface. We explore the effect of change of confining potential on the excitation, which is largely controlled by the static electric fields and continuous wave laser fields. We focus on dipolar molecules and hence we restrict ourselves to the first order interaction in field-molecule interaction potential either through permanent dipole moment or/and the molecular polarizability parameter. It is shown that confining potential shapes, strength of the confinement, strongly affect the excitation. We compare our results for different confining potentials.

Rotational Control of Asymmetric Molecules: Dipole- versus Polarizability-Driven Rotational Dynamics

We experimentally study the optical- and terahertz-induced rotational dynamics of asymmetric molecules in the gas phase. Terahertz and optical fields are identified as two distinct control handles over asymmetric molecules, as they couple to the rotational degrees of freedom via the molecular dipole and polarizability selectively. The distinction between those two rotational handles is highlighted by different types of quantum revivals observed in long-duration (>100  ps) field-free rotational evolution. The experimental results are in excellent agreement with random phase wave function (RPWF) simulations [Phys. Rev. A 91, 063420 (2015)] and provide verification of the RPWF as an efficient method for calculating asymmetric molecular dynamics at ambient temperatures, where exact calculation methods are practically not feasible. Our observations and analysis pave the way for orchestrated excitations by both optical and terahertz fields as complementary rotational handles that enable a plethora of new possibilities in three-dimensional rotational control of asymmetric molecules.

Quantum Nuclear Dynamics Pumped and Probed by Ultrafast Polarization Controlled Steering of a Coherent Electronic State in LiH

The quantum wave packet dynamics following a coherent electronic excitation of LiH by an ultrashort, polarized, strong one-cycle infrared optical pulse is computed on several electronic states using a grid method. The coupling to the strong field of the pump and the probe pulses is included in the Hamiltonian used to solve the time-dependent Schrodinger equation. The polarization of the pump pulse allows us to control the localization in time and in space of the nonequilibrium coherent electronic motion and the subsequent nuclear dynamics. We show that transient absorption, resulting from the interaction of the total molecular dipole with the electric fields of the pump and the probe, is a very versatile probe of the different time scales of the vibronic dynamics. It allows probing both the ultrashort, femtosecond time scale of the electronic coherences as well as the longer dozens of femtoseconds time scales of the nuclear motion on the excited electronic states. The ultrafast beatings of the electronic coherences in space and in time are shown to be modulated by the different periods of the nuclear motion.

Pair-eigenstates and mutual alignment of coupled molecular rotors in a magnetic field

We examine the rotational states of a pair of polar (2)Σ molecules subject to a uniform magnetic field. The electric dipole-dipole interaction between the molecules creates entangled pair-eigenstates of two types. In one type, the Zeeman interaction between the inherently paramagnetic molecules and the magnetic field destroys the entanglement of the pair-eigenstates, whereas in the other type it does not. The pair-eigenstates exhibit numerous intersections, which become avoided for pair-eigenstates comprised of individual states that meet the selection rules ΔJi = 0, ± 1, ΔNi = 2n (n = 0, ±1, ±2,…), and ΔMi = 0, ± 1 imposed by the electric dipole-dipole operator. Here Ji, Ni and Mi are the total, rotational and projection angular momentum quantum numbers of molecules i = 1, 2 in the absence of the electric dipole-dipole interaction. We evaluate the mutual alignment of the pair-eigenstates and find it to be independent of the magnetic field, except for states that undergo avoided crossings, in which case the alignment of the interacting states is interchanged at the magnetic field corresponding to the crossing point. We present an analytic model which provides ready estimates of the pairwise alignment cosine that characterises the mutual alignment of the pair of coupled rotors.

Extracting conformational structure information of benzene molecules via laser-induced electron diffraction

We have measured the angular distributions of high energy photoelectrons of benzene molecules generated by intense infrared femtosecond laser pulses. These electrons arise from the elastic collisions between the benzene ions with the previously tunnel-ionized electrons that have been driven back by the laser field. Theory shows that laser-free elastic differential cross sections (DCSs) can be extracted from these photoelectrons, and the DCS can be used to retrieve the bond lengths of gas-phase molecules similar to the conventional electron diffraction method. From our experimental results, we have obtained the C-C and C-H bond lengths of benzene with a spatial resolution of about 10 pm. Our results demonstrate that laser induced electron diffraction (LIED) experiments can be carried out with the present-day ultrafast intense lasers already. Looking ahead, with aligned or oriented molecules, more complete spatial information of the molecule can be obtained from LIED, and applying LIED to probe photo-excited molecules, a "molecular movie" of the dynamic system may be created with sub-Ångström spatial and few-ten femtosecond temporal resolutions.

Retrieving transient conformational molecular structure information from inner-shell photoionization of laser-aligned molecules

We discuss a scheme to retrieve transient conformational molecular structure information using photoelectron angular distributions (PADs) that have averaged over partial alignments of isolated molecules. The photoelectron is pulled out from a localized inner-shell molecular orbital by an X-ray photon. We show that a transient change in the atomic positions from their equilibrium will lead to a sensitive change in the alignment-averaged PADs, which can be measured and used to retrieve the former. Exploiting the experimental convenience of changing the photon polarization direction, we show that it is advantageous to use PADs obtained from multiple photon polarization directions. A simple single-scattering model is proposed and benchmarked to describe the photoionization process and to do the retrieval using a multiple-parameter fitting method.

High-intracavity-power thin-disk laser for the alignment of molecules

We propose a novel approach for strong alignment of gas-phase molecules for experiments at arbitrary repetition rates. A high-intracavity-power continuous-wave laser will provide the necessary ac electric field of 10(10)-10(11) W/cm(2). We demonstrate thin-disk lasers based on Yb:YAG and Yb:Lu(2)O(3) in a linear high-finesse resonator providing intracavity power levels in excess of 100 kW at pump power levels on the order of 50 W. The multi-longitudinal-mode operation of this laser avoids spatial-hole burning even in a linear standing-wave resonator. The system will be scaled up as in-vacuum system to allow for the generation of fields of 10(11) W/cm(2). This system will be directly applicable for experiments at modern X-ray light sources, such as synchrotrons or free-electron lasers, which operate at various very high repetition rates. This would allow to record molecular movies through temporally resolved diffractive imaging of fixed-in-space molecules, as well as the spectroscopic investigation of combined X-ray-NIR strong-field effects of atomic and molecular systems.

Laser induced alignment of state-selected CH3I

Hexapole state selection is used to prepare CH3I molecules in the |JKM〉 = |1±1∓1〉 state. The molecules are aligned in a strong 800 nm laser field, which is linearly polarised perpendicular to the weak static extraction field E of the time of flight setup. The molecules are subsequently ionised by a second time delayed probe laser pulse. It will be shown that in this geometry at high enough laser intensities the Newton sphere has sufficient symmetry to apply the inverse Abel transformation to reconstruct the three dimensional distribution from the projected ion image. The laser induced controllable alignment was found to have the upper and lower extreme values of 〈P2(cos θ)〉 = 0.7 for the aligned molecule and -0.1 for the anti-aligned molecule, coupled to 〈P4(cos θ)〉 between 0.3 and 0.0. The method to extract the alignment parameters 〈P2(cos θ)〉 and 〈P4(cos θ)〉 directly from the velocity map ion images will be discussed.

Photoelectron diffraction from laser-aligned molecules with X-ray free-electron laser pulses

We report on the measurement of deep inner-shell 2p X-ray photoelectron diffraction (XPD) patterns from laser-aligned I₂ molecules using X-ray free-electron laser (XFEL) pulses. The XPD patterns of the I₂ molecules, aligned parallel to the polarization vector of the XFEL, were well matched with our theoretical calculations. Further, we propose a criterion for applying our molecular-structure-determination methodology to the experimental XPD data. In turn, we have demonstrated that this approach is a significant step toward the time-resolved imaging of molecular structures.

Effective doping of low energy ions into superfluid helium droplets

We report a facile method of doping cations from an electrospray ionization (ESI) source into superfluid helium droplets. By decelerating and stopping the ion pulse of reserpine and substance P from an ESI source in the path of the droplet beam, about 10(4) ion-doped droplets (one ion per droplet) can be recorded, corresponding to a pickup efficiency of nearly 1 out of 1000 ions. We attribute the success of this simple approach to the long residence time of the cations in the droplet beam. The resulting size of the doped droplets, on the order of 10(5)/droplet, is measured using deflection and retardation methods. Our method does not require an ion trap in the doping region, which significantly simplifies the experimental setup and procedure for future spectroscopic and diffraction studies.

Velocity map imaging with non-uniform detection: Quantitative molecular axis alignment measurements via Coulomb explosion imaging

We present a method for inverting charged particle velocity map images which incorporates a non-uniform detection function. This method is applied to the specific case of extracting molecular axis alignment from Coulomb explosion imaging probes in which the probe itself has a dependence on molecular orientation which often removes cylindrical symmetry from the experiment and prevents the use of standard inversion techniques for the recovery of the molecular axis distribution. By incorporating the known detection function, it is possible to remove the angular bias of the Coulomb explosion probe process and invert the image to allow quantitative measurement of the degree of molecular axis alignment.

Communication: Creation of molecular vibrational motions via the rotation-vibration coupling

Building on recent advances in the rotational excitation of molecules, we show how the effect of rotation-vibration coupling can be switched on in a controlled manner and how this coupling unfolds in real time after a pure rotational excitation. We present the first examination of the vibrational motions which can be induced via the rotation-vibration coupling after a pulsed rotational excitation. A time-dependent quantum wave packet calculation for the HF molecule shows how a slow (compared to the vibrational period) rotational excitation leads to a smooth increase in the average bond length whereas a fast rotational excitation leads to a non-stationary vibrational motion. As a result, under field-free postpulse conditions, either a stretched stationary bond or a vibrating bond can be created due to the coupling between the rotational and vibrational degrees of freedom. The latter corresponds to a laser-induced breakdown of the adiabatic approximation for rotation-vibration coupling.

Toward atomic resolution diffractive imaging of isolated molecules with X-ray free-electron lasers

We give a detailed account of the theoretical analysis and the experimental results of an X-ray-diffraction experiment on quantum-state selected and strongly laser-aligned gas-phase ensembles of the prototypical large asymmetric rotor molecule 2,5-diiodobenzonitrile, performed at the Linac Coherent Light Source [Phys. Rev. Lett.112, 083002 (2014)]. This experiment is the first step toward coherent diffractive imaging of structures and structural dynamics of isolated molecules at atomic resolution, i.e., picometers and femtoseconds, using X-ray free-electron lasers.

Cold and Controlled Molecular Beams: Production and Applications

The field of cold molecules has become an important source of new insight in fundamental chemistry and molecular physics. High-resolution spectroscopy benefits from translationally and internally cold molecules by increased interaction times and reduced spectral congestion. Completely new effects in scattering dynamics become accessible with cold and controlled molecules. Many of these experiments use molecular beams as a starting point for the generation of molecular samples. This review gives an overview of methods to produce beams of cold molecules, starting from supersonic expansions or effusive sources, and provides examples of applications in spectroscopy and molecular dynamics studies.

Two-state wave packet for strong field-free molecular orientation

We demonstrate strong laser-field-free orientation of absolute-ground-state carbonyl sulfide molecules. The molecules are oriented by the combination of a 485-ps-long nonresonant laser pulse and a weak static electric field. The edges of the laser pulse create a coherent superposition of two rotational states resulting in revivals of strong transient molecular orientation after the laser pulse. The experimentally attained degree of orientation ⟨cosθ⟩≈0.6 corresponds to the theoretical maximum for mixing of the two states. Switching off the dc field would provide the same orientation completely field free.

A new perspective: imaging the stereochemistry of molecular collisions

The concept of the steric effect plays a central role in chemistry. This Perspective describes how the polarization of reactant molecules in space can be used to probe directly the steric effect, and highlights some of the new measurements that are made possible by coupling reactant orientation and alignment with ion imaging techniques.

Complete Quantum State Selectivity in Cold Molecular Beams Using Deflection-Resistant Dark States in a STIRAP Configuration

One of the main goals of chemical dynamics is the creation of molecular beams composed of a single (vibrational, rotational, and magnetic) quantum state of choice. In this Letter, we propose a method to achieve complete quantum state selectivity by producing resistance to electromagnetically induced deflection (EID) and that the state to be selected can be "dialed in" at will. We illustrate the method by showing in detail how to purify thermal beams of the LiRb and IF molecules to yield molecular beams composed of a variety of prechosen single internal quantum states and/or superpositions of such states. We expect that this method will be implemented in all subsequent explorations of the fundamentals of chemical reactions and their control and the use of cold molecules as a vehicle for studying some of the most profound issues of quantum dynamics.

Low-energy photoelectrons in strong-field ionization by laser pulses with large ellipticity

The 3D photoelectron momentum distributions created by the strong-field ionization of argon atoms and naphthalene molecules with intense, large ellipticity (∼0.7) femtosecond laser pulses are studied. The experiment reveals the presence of low-energy electrons for randomly oriented naphthalene, but not for argon. Our theory shows that the induced dipole part of the cationic potential facilitates the creation of the low-energy electrons. We establish the conditions in terms of laser pulse parameters and molecular properties for which this type of low-energy electrons can be observed and point to applications thereof.

Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.

Two-pulse field-free orientation reveals anisotropy of molecular shape resonance

We report the observation of macroscopic field-free orientation, i.e., more than 73% of CO molecules pointing in the same direction. This is achieved through an all-optical scheme operating at high particle densities (>10(17)  cm(-3)) that combines one-color (ω) and two-color (ω+2ω) nonresonant femtosecond laser pulses. We show that the achieved orientation solely relies on the hyperpolarizability interaction as opposed to an ionization-depletion mechanism, thus, opening a wide range of applications. The achieved strong orientation enables us to reveal the molecular-frame anisotropies of the photorecombination amplitudes and phases caused by a shape resonance. The resonance appears as a local maximum in the even-harmonic emission around 28 eV. In contrast, the odd-harmonic emission is suppressed in this spectral region through the combined effects of an asymmetric photorecombination phase and a subcycle Stark effect, generic for polar molecules, that we experimentally identify.

Multipulse three-dimensional alignment of asymmetric top molecules

We show, by computation and experiment, that a sequence of nonresonant and impulsive laser pulses with different ellipticities can effectively align asymmetric top molecules in three dimensions under field-free conditions. By solving the Schrödinger equation for the evolution of the rotational wave packet, we show that the 3D alignment of 3,5 difluoroiodobenzene molecules improves with each successive pulse. Experimentally, a sequence of three pulses is used to demonstrate these results, which extend the multipulse schemes used for 1D alignment to full 3D control of rotational motion.

Terahertz-induced field-free orientation of rotationally excited molecules

We have used picosecond THz pulses to induce transient field-free orientation of OCS molecules. Coherent optical Raman excitation prepares the molecules in rotational superposition states prior to THz irradiation, substantially enhancing the degree of orientation. The time-dependent alignment and orientation are characterized via Coulomb explosion in an intense probe laser. The degree of OCS orientation is an order of magnitude larger than previously observed following THz irradiation and is achieved with a significantly smaller THz field.The field-free orientation level is comparable to that generated using pulsed, two-color laser fields but is obtained with negligible target ionization.

Spatial separation of molecular conformers and clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

FIELD-FREE MOLECULAR ORIENTATION IN DISSIPATIVE MEDIA BY A COMBINATION OF FEMTOSECOND AND THz LASER PULSES

We present a theoretical scheme for achieving the field-free molecular orientation in dissipative media by a combination of femtosecond and THz laser pulses. Numerical calculations are performed by solving the quantum Liouville equation based on multilevel Bloch model. The molecular orientation degree is sensitive to the carrier-envelope phase of the THz pulse and the delay time between the two pulses. The orientation and the rotational population of CO molecules in dissipative environment are computed at different pressures and temperatures. The influence of pure decoherence on the molecular orientation is also discussed.