State-Resolved Probing of Attosecond Timescale Molecular Dipoles.

We report an experimental study of iodomethane attosecond transient absorption spectroscopy (ATAS) in the region of iodine 4d core-to-valence/Rydberg excitation. Similar to previous atomic experiments, extreme ultraviolet near-infrared (XUV-NIR) delay-dependent absorbance changes reflect a light-induced phase due to an NIR-field driven AC Stark shift of the excited states, as well as pathway interferences arising from couplings between neighboring states. As a novel aspect of molecular ATAS, we observe pronounced differences between the ATAS signatures of valence and Rydberg states. While the core-to-valence transitions carry the majority of the XUV oscillator strength, the core-to-Rydberg transitions are dominantly affected by a moderately strong, nonionizing NIR field. Our experimental findings are corroborated by ab initio calculations and ATAS simulations.

Extreme-ultraviolet refractive optics.

Refraction is a well-known optical phenomenon that alters the direction of light waves propagating through matter. Microscopes, lenses and prisms based on refraction are indispensable tools for controlling light beams at visible, infrared, ultraviolet and X-ray wavelengths1. In the past few decades, a range of extreme-ultraviolet and soft-X-ray sources has been developed in laboratory environments2-4 and at large-scale facilities5,6. But the strong absorption of extreme-ultraviolet radiation in matter hinders the development of refractive lenses and prisms in this spectral region, for which reflective mirrors and diffractive Fresnel zone plates7 are instead used for focusing. Here we demonstrate control over the refraction of extreme-ultraviolet radiation by using a gas jet with a density gradient across the profile of the extreme-ultraviolet beam. We produce a gas-phase prism that leads to a frequency-dependent deflection of the beam. The strong deflection near to atomic resonances is further used to develop a deformable refractive lens for extreme-ultraviolet radiation, with low absorption and a focal length that can be tuned by varying the gas pressure. Our results open up a route towards the transfer of refraction-based techniques, which are well established in other spectral regions, to the extreme-ultraviolet domain.

Few-femtosecond passage of conical intersections in the benzene cation.

Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system-the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.

XUV-induced reactions in benzene on sub-10 fs timescale: nonadiabatic relaxation and proton migration.

Unraveling ultrafast dynamical processes in highly excited molecular species has an impact on our understanding of chemical processes such as combustion or the chemical composition of molecular clouds in the universe. In this article we use short (<7 fs) XUV pulses to produce excited cationic states of benzene molecules and probe their dynamics using few-cycle VIS/NIR laser pulses. The excited states produced by the XUV pulses lie in an especially complex spectral region where multi-electronic effects play a dominant role. We show that very fast τ ≈ 20 fs nonadiabatic processes dominate the relaxation of these states, in agreement with the timescale expected for most excited cationic states in benzene. In the CH3+ fragmentation channel of the doubly ionized benzene cation we identify pathways that involve structural rearrangement and proton migration to a specific carbon atom. Further, we observe non-trivial transient behavior in this fragment channel, which can be interpreted either in terms of propagation of the nuclear wavepacket in the initially excited electronic state of the cation or as a two-step electronic relaxation via an intermediate state.

Communication: XUV transient absorption spectroscopy of iodomethane and iodobenzene photodissociation.

Time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy of iodomethane and iodobenzene photodissociation at the iodine pre-N4,5 edge is presented, using femtosecond UV pump pulses and XUV probe pulses from high harmonic generation. For both molecules the molecular core-to-valence absorption lines fade immediately, within the pump-probe time-resolution. Absorption lines converging to the atomic iodine product emerge promptly in CH3I but are time-delayed in C6H5I. We attribute this delay to the initial π → σ(*) excitation in iodobenzene, which is distant from the iodine reporter atom. We measure a continuous shift in energy of the emerging atomic absorption lines in CH3I, attributed to relaxation of the excited valence shell. An independent particle model is used to rationalize the observed experimental findings.

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation.

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20-26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

Nucleophilic substitution dynamics: comparing wave packet calculations with experiment.

The reactive collision of chloride anions and methyl iodide molecules forming iodide anions and methyl chloride is a typical example of a concerted bimolecular nucleophilic substitution (SN2) reaction. We present wave packet dynamics calculations to investigate quantum effects in the collinear gas phase reaction. A new type of reduced coordinate system is introduced to allow for an efficient solution of the time-dependent Schrödinger equation on an ab initio potential energy surface. The reduced coordinates were designed to study the direct rebound mechanism under the Walden inversion. Especially the suppressed direct rebound mechanism at low collision energies, the quantum effects of the initial state preparation and the influence of the CH3 inversion mode are addressed. The internal energy distributions of the molecular product are evaluated from the wave packet calculations and compared to experimental results obtained with crossed-beam velocity map ion imaging. The observed reactivity is discussed in light of a dynamical barrier, a concept that is illustrated by the wave packet dynamics.

Absolute photodetachment cross section measurements of the O(-) and OH(-) anion.

Absolute total photodetachment cross sections of O(-) and OH(-) anions stored in a multipole radio frequency trap have been measured using a novel laser depletion tomography method. For OH(-) the total cross sections of 8.5(1)(stat)(3)(syst) and 8.1(1)(stat)(7)(syst)x10(-18) cm(2), measured at 662 and 632 nm, respectively, were found constant in the temperature range of 8-300 K. The O(-) cross sections 5.9(1)(stat)(2)(syst) and 6.3(1)(stat)(2)(syst)x10(-18) cm(2) measured at 170 K at 662 and 532 nm, respectively, agree within error estimations with preceding experiments and increase the accuracy of the widely used calibration standard for relative photodetachment measurements of diverse atomic and molecular species.

Nonstandard behavior of a negative ion reaction at very low temperatures.

We have studied the negative ion reaction NH2-+H_{2}-->NH_{3}+H- in the temperature range from 300 to 8 K. We observe a strongly suppressed probability for proton transfer at room temperature. With decreasing temperature, this probability increases, in accordance with a longer lifetime of an intermediate anion-neutral complex. At low temperatures, a maximum in the reaction rate coefficient is observed that suggests the presence of a very small barrier at long range or a quantum mechanical resonance feature.

Inverse temperature dependent lifetimes of transient S(N)2 ion-dipole complexes.

The association and collisional stabilization of the S(N)2 entrance channel complex [Cl(-)...CH3Cl]* is studied in a low-temperature radiofrequency ion trap. The temperature dependence of the ternary rate coefficient is measured and a much stronger inverse temperature dependence than expected from a simple statistical calculation is found. From these data the lifetime of the transient S(N)2 complex has been derived as a function of temperature. It is suggested that the inverse temperature dependent rates of nonsymmetric S(N)2 reactions are related to the observed inverse temperature dependence of the transient ion-dipole complexes.

Imaging nucleophilic substitution dynamics.

Anion-molecule nucleophilic substitution (S(N)2) reactions are known for their rich reaction dynamics, caused by a complex potential energy surface with a submerged barrier and by weak coupling of the relevant rotational-vibrational quantum states. The dynamics of the S(N)2 reaction of Cl- + CH3I were uncovered in detail by using crossed molecular beam imaging. As a function of the collision energy, the transition from a complex-mediated reaction mechanism to direct backward scattering of the I- product was observed experimentally. Chemical dynamics calculations were performed that explain the observed energy transfer and reveal an indirect roundabout reaction mechanism involving CH3 rotation.

Evaporation of buffer-gas-thermalized anions out of a multipole rf ion trap.

We identify plain evaporation of ions as the fundamental loss mechanism out of a multipole ion trap. Using thermalized negative Cl- ions we find that the evaporative loss rate is proportional to a Boltzmann factor. This thermodynamic description allows us to extract the effective depth of the ion trap. As a function of the rf amplitude we find two distinct regimes related to the stability of motion of the trapped ions. For low amplitudes the entire trap allows for stable motion and the trap depth increases with the rf field. For larger rf amplitudes rapid energy transfer from the field to the ion motion can occur at large trap radii, which leads to a reduction of the effective trapping volume. In this regime the trap depth decreases again with increasing rf amplitude. We give an analytical parametrization of the trap depth for various multipole traps that allows predictions of the most favorable trapping conditions.

Photodetachment of cold OH- in a multipole ion trap.

The absolute photodetachment cross section of OH- anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational-state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH- anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.

Effects of molecular rotation in low-energy electron collisions of H3+.

Measurements on the energetic structure of the dissociative recombination rate coefficient in the millielectronvolt range are described for H3+ ions produced in the lowest rotational levels by collisional cooling and stored as a fast beam in the magnetic storage ring TSR (Test Storage Ring). The observed resonant structure is consistent with that found previously at the storage ring facility CRYRING in Stockholm, Sweden; theoretical predictions yield good agreement on the overall size of the rate coefficient, but do not reproduce the detailed structure. First studies on the nuclear spin symmetry influencing the lowest level populations show a small effect different from the theoretical predictions. Heating processes in the residual gas and by collisions with energetic electrons, as well as cooling owing to interaction with cold electrons, were observed in long-time storage experiments, using the low-energy dissociative recombination rate coefficient as a probe, and their consistency with the recent cold H3+ measurements is discussed.

Velocity map imaging of ion-molecule reactive scattering: the Ar(+)+ N(2) charge transfer reaction.

We present a velocity map imaging spectrometer for the study of crossed-beam reactive collisions between ions and neutrals at (sub-)electronvolt collision energies. The charge transfer reaction of Ar(+) with N(2) is studied at 0.6, 0.8 and 2.5 eV relative collision energies. Energy and angular distributions are measured for the reaction product N. The differential cross section, as analyzed with a Monte Carlo reconstruction algorithm, shows significant large angle scattering for lower collision energies in qualitative agreement with previous experiments. Significant vibrational excitation of N(+)(2) is also observed. This theoretically still unexplained feature indicates the presence of a low energy scattering resonance.

High-resolution dissociative recombination of cold H3+and first evidence for nuclear spin effects.

The energy-resolved rate coefficient for the dissociative recombination (DR) of H(3)(+) with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to 500 microeV by using a cryogenic GaAs photocathode. This and a previous cold- measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate H(3)(+) in the two lowest, (J,G) = (1,1) and (1,0) rotational states prevailing also in cold interstellar matter. The use of para-H(2) in the ion source, expected to enhance para-H(3)(+) in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

Action spectroscopy and temperature diagnostics of H+ 3 by chemical probing.

Infrared absorption spectroscopy of few hundred H+(3) ions trapped in a 22-pole ion trap is presented using chemical probing as a sensitive detection technique down to the single ion level. By exciting selected overtone transitions of the (v(1)=0,v(2) (l)=3(1))<--(0,0(0)) vibrational band using an external cavity diode laser an accurate diagnostics measurement of the effective translational and rotational temperatures of the trapped ions was performed. The absolute accuracy of the measured transition frequencies was improved by a factor of four compared to previous plasma spectroscopy measurements using velocity modulation [Ventrudo et al., J. Chem. Phys. 100, 6263 (1994)]. The observed buffer gas cooling conditions in the ion trap indicate how to cool trapped H+(3) ions into the lowest ortho and para rotational states. Future experiments will utilize such an internally cold ion ensemble for state-selected dissociative recombination experiments at the heavy ion storage ring Test Storage Ring (TSR).