arXiv subjects

Henrik Stapelfeldt

Publications and source records attributed to Henrik Stapelfeldt.

At least 37 records · Page 2Linked to original sources

arXiv2020

X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame

We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV ($λ\approx130 \text{pm}$) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the CSPAD detector and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to within 5 %. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data which should be readily available at upcoming high-repetition-rate facilities.

physics.atm-clus↗

arXiv2019

Far-from-equilibrium dynamics of angular momentum in a quantum many-particle system

We use laser-induced rotation of single molecules embedded in superfluid helium nanodroplets to reveal angular momentum dynamics and transfer in a controlled setting, under far-from-equilibrium conditions. As an unexpected result, we observe pronounced oscillations of time-dependent molecular alignment that have no counterpart in gas-phase molecules. Angulon theory reveals that these oscillations originate from the unique rotational structure of molecules in He droplets and quantum-state-specific transfer of rotational angular momentum to the many-body He environment on picosecond timescales. Our results pave the way to understanding collective effects of macroscopic angular momentum exchange in solid state systems in a bottom-up fashion.

physics.atm-clus↗

arXiv2019

Structure determination of the tetracene dimer in helium nanodroplets using femtosecond strong-field ionization

Dimers of tetracene molecules are formed inside helium nanodroplets and identified through covariance analysis of the emission directions of kinetic tetracene cations stemming from femtosecond laser-induced Coulomb explosion. Next, the dimers are aligned in either one or three dimensions under field-free conditions by a nonresonant, moderately intense laser pulse. The experimental angular covariance maps of the tetracene ions are compared to calculated covariance maps for seven different dimer conformations and found to be consistent with four of these. Additional measurements of the alignment-dependent strong-field ionization yield of the dimer narrows the possible conformations down to either a slipped-parallel or parallel-slightly-rotated structure. According to our quantum chemistry calculations, these are the two most stable gas-phase conformations of the dimer and one of them is favorable for singlet fission.

physics.atm-clus↗

arXiv2019

Molecular movie of ultrafast coherent rotational dynamics

Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. We present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single-quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, $\langle \cos^{2}{θ_{2D}}\rangle=0.96$ ($\langle \cos^{2}θ\rangle=0.94$) was achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalised the observed very high degree of alignment.

physics.chem-ph↗

arXiv2019

Atomic-resolution imaging of carbonyl sulfide by laser-induced electron diffraction

Measurements on the strong-field ionization of carbonyl sulfide molecules by short, intense, 2~\um wavelength laser pulses are presented from experiments where angle-resolved photoelectron distributions were recorded with a high-energy velocity map imaging spectrometer, designed to reach a maximum kinetic energy of 500~eV. The laser-field-free elastic-scattering cross section of carbonyl sulfide was extracted from the measurements and is found in good agreement with previous experiments, performed using conventional electron diffraction. By comparing our measurements to the results of calculations, based on the quantitative rescattering theory (QRS), the bond lengths and molecular geometry were extracted from the experimental differential cross sections to a precision better than $\pm5$~pm and in agreement with the known values.

physics.chem-ph↗

arXiv2019

Gas-phase structural isomer identification by Coulomb explosion of aligned molecules

The gas-phase structures of four difluoroiodobenzene and two dihydroxybromobenzene isomers were identified by correlating the emission angles of atomic fragment ions created following femtosecond laser-induced Coulomb explosion. The structural determinations were facilitated by confining the most polarizable axis of each molecule to the detection plane prior to the Coulomb explosion event using one-dimensional laser-induced adiabatic alignment. For a molecular target consisting of two difluoroiodobenzene isomers, each constituent structure could additionally be singled out and distinguished.

physics.chem-ph↗

arXiv2019

Alignment of the CS$_2$ Dimer Embedded in Helium Droplets Induced by a Circularly Polarized Laser Pulse

Dimers of carbon disulfide (CS$_2$) molecules embedded in helium nanodroplets are aligned using a moderately intense, 160ps, non-resonant, circularly polarized laser pulse. It is shown that the intermolecular carbon-carbon (C-C) axis aligns along the axis perpendicular to the polarization plane of the alignment laser pulse. The degree of alignment, quantified by $\langle \cos^2(θ_\text{2D}) \rangle$, is determined from the emission directions of recoiling CS$_2$$^+$ fragment ions, created when an intense 40fs probe laser pulse doubly ionizes the dimers. Here, $θ_\text{2D}$ is the projection of the angle between the C-C axis on the 2D ion detector and the normal to the polarization plane. $\langle \cos^2(θ_\text{2D}) \rangle$ is measured as a function of the alignment laser intensity and the results agree well with $\langle \cos^2(θ_\text{2D}) \rangle$ calculated for gas-phase CS$_2$ dimers with a rotational temperature of 0.4K.

physics.chem-ph↗

arXiv2018

Femtosecond Laser Induced Coulomb Explosion Imaging of Aligned OCS Oligmers inside Helium Nanodroplets

Dimers and trimers of carbonyl sulfide (OCS) molecules embedded in helium nanodroplets are aligned by a linearly polarized 160 ps long moderately intense laser pulse and Coulomb exploded with an intense 40 fs long probe pulse in order to determine their structures. For the dimer, recording of 2D images of OCS$^+$ and S$^+$ ions and covariance analysis of the emission directions of the ions allow us to conclude that the structure is a slipped-parallel shape similar to the structure found for gas phase dimers. For the trimer, the OCS$^+$ ion images and corresponding covariance maps reveal the presence of a barrel-shaped structure (as in gas phase) but also other structures not present in the gas phase, most notably a linear chain structure.

physics.chem-ph↗

arXiv2018

Long-lasting field-free alignment of large molecules inside helium nanodroplets

Molecules with their axes sharply confined in space, available through laser-induced alignment methods, are essential for many current experiments, including ultrafast molecular imaging. Most of these applications require both that the aligning laser field is turned-off, to avoid undesired perturbations, and that the molecules remain aligned sufficiently long that reactions and dynamics can be mapped out. Presently, this is only possible for small, linear molecules and for times less than 1 picosecond. Here, we demonstrate strong, field-free alignment of large molecules inside helium nanodroplets, lasting tens of picoseconds. Molecular alignment in either one or three dimensions is created by a slowly switched-on laser pulse, made field-free through rapid pulse truncation, and retained thanks to the impeding effect of the helium environment on molecular rotation. We illustrate the opportunities that field-free aligned molecules open by measuring the alignment-dependent strong-field ionization yield of a thiophene oligomer. Our technique will enable molecular-frame experiments, including ultrafast excited state dynamics, on a variety of large molecules and complexes.

physics.chem-ph↗

arXiv2018

Hyperfine-Structure-Induced Depolarization of Impulsively Aligned $\rm I_2$ Molecules

A moderately intense $450$ fs laser pulse is used to create rotational wave packets in gas phase $\rm{I_2}$ molecules. The ensuing time-dependent alignment, measured by Coulomb explosion imaging with a delayed probe pulse, exhibits the characteristic revival structures expected for rotational wave packets but also a complex non-periodic substructure and decreasing mean alignment not observed before. A quantum mechanical model attributes the phenomena to coupling between the rotational angular momenta and the nuclear spins through the electric quadrupole interaction. The calculated alignment trace agrees very well with the experimental results.

physics.chem-ph↗

arXiv2018

Switched Wave Packets with Spectrally Truncated Chirped Pulses

A new technique for obtaining switched wave packets using spectrally truncated chirped laser pulses is demonstrated experimentally and numerically by one-dimensional alignment of both linear and asymmetric top molecules. Using a simple long-pass transmission filter, a pulse with a slow turn on and a rapid turn off is produced. The degree of alignment, characterized by $\langle\cos^2θ_\text{2D}\rangle$ rises along with the pulse intensity and reaches a maximum at the peak of the pulse. After truncation $\langle\cos^2θ_\text{2D}\rangle$ drops sharply but exhibits pronounced half and full revivals. The experimental alignment dynamics trace agrees very well with a numerically calculated trace based on solution of the time-dependent Schrödinger equation. However, the extended periods of field-free alignment of asymmetric tops following pulse truncation reported previously is not reproduced in our work.

physics.chem-ph↗

arXiv2018

Probing the UV-Induced Photodissociation of CH$_\text{3}$I and C$_\text{6}$H$_\text{3}$F$_\text{2}$I with Femtosecond Time-Resolved Coulomb Explosion Imaging at FLASH

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from the FLASH free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267\,nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon--iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.

physics.atom-ph↗

arXiv2017

Coulomb explosion imaging of concurrent CH$_{2}$BrI photodissociation dynamics

The dynamics following laser-induced molecular photodissociation of gas-phase CH$_{2}$BrI at 271.6 nm were investigated by time-resolved Coulomb explosion imaging using intense near-IR femtosecond laser pulses. The observed delay-dependent photofragment momenta reveal that CH$_{2}$BrI undergoes C-I cleavage, depositing 65.6% of the available energy into internal product states, and that absorption of a second UV photon breaks the C-Br bond of CH$_{2}$Br. Simulations confirm that this mechanism is consistent with previous data recorded at 248 nm, demonstrating the sensitivity of Coulomb explosion imaging as a real-time probe of chemical dynamics.

physics.chem-ph↗

arXiv2017

Alignment, Orientation, and Coulomb Explosion of Difluoroiodobenzene Studied with the Pixel Imaging Mass Spectrometry (PImMS) Camera

Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C6H3F2I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with the Pixel Imaging Mass Spectrometry (PImMS) camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one- and three-dimensional alignment and mixed- field orientation, and compare the Coulomb explosion process induced at both wavelengths.

physics.chem-ph↗

arXiv2017

Strongly aligned molecules inside helium droplets in the near-adiabatic regime

Iodine (I$_2$) molecules embedded in He nanodroplets are aligned by a 160 ps long laser pulse. The highest degree of alignment, occurring at the peak of the pulse and quantified by $\langle \cos^2 θ_{2D} \rangle$, is measured as a function of the laser intensity. The results are well described by $\langle \cos^2 θ_{2D} \rangle$ calculated for a gas of isolated molecules each with an effective rotational constant of 0.6 times the gas-phase value, and at a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to describe rotating molecules in superfluid helium rationalizes why the alignment mechanism is similar to that of isolated molecules with an effective rotational constant. A major advantage of molecules in He droplets is that their 0.4 K temperature leads to stronger alignment than what can generally be achieved for gas phase molecules -- here demonstrated by a direct comparison of the droplet results to measurements on a $\sim$ 1 K supersonic beam of isolated molecules. This point is further illustrated for more complex system by measurements on 1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species studied the highest values of $\langle \cos^2 θ_{2D} \rangle$ achieved in He droplets exceed 0.96.

physics.chem-ph↗

arXiv2017

Laser-induced rotation of iodine molecules in He-nanodroplets: revivals and breaking-free

Rotation of molecules embedded in He nanodroplets is explored by a combination of fs laser-induced alignment experiments and angulon quasiparticle theory. We demonstrate that at low fluence of the fs alignment pulse, the molecule and its solvation shell can be set into coherent collective rotation lasting long enough to form revivals. With increasing fluence, however, the revivals disappear -- instead, rotational dynamics as rapid as for an isolated molecule is observed during the first few picoseconds. Classical calculations trace this phenomenon to transient decoupling of the molecule from its He shell. Our results open novel opportunities for studying non-equilibrium solute-solvent dynamics and quantum thermalization.

physics.chem-ph↗

arXiv2015

Strongly aligned gas-phase molecules at Free-Electron Lasers

We demonstrate a novel experimental implementation to strongly align molecules at full repetition rates of free-electron lasers. We utilized the available in-house laser system at the coherent x-ray imaging beamline at the Linac Coherent Light Source. Chirped laser pulses, i. e., the direct output from the regenerative amplifier of the Ti:Sa chirped pulse amplification laser system, were used to strongly align 2,5-diiodothiophene molecules in a molecular beam. The alignment laser pulses had pulse energies of a few mJ and a pulse duration of 94 ps. A degree of alignment of $\left<\cos^2\!θ_{2D}\right>$ = 0.85 was measured, limited by the intrinsic temperature of the molecular beam rather than by the available laser system. With the general availability of synchronized chirped-pulse-amplified near-infrared laser systems at short-wavelength laser facilities, our approach allows for the universal preparation of molecules tightly fixed in space for experiments with x-ray pulses.

physics.atm-clus↗

arXiv2014

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.

physics.atom-ph↗