Free Electron Laser For Infrared Experiments Research Articles

Spectroscopic Detection of Cyano-Cyclopentadiene Ions as Dissociation Products upon Ionization of Aniline.

The H-loss products (C6H6N+) from the dissociative ionization of aniline (C6H7N) have been studied by infrared predissociation spectroscopy in a cryogenic ion trap instrument at the free electron laser for infrared experiments (FELIX) laboratory. Broadband and narrow line width vibrational spectra in the spectral fingerprint region of 550–1800 cm–1 have been recorded. The comparison to calculated spectra of the potential isomeric structures of the fragment ions reveals that the dominant fragments are five-membered cyano-cyclopentadiene ions. Computed C6H7N•+ potential energy surfaces suggest that the dissociation path leading to H loss starts with an isomerization process, following a similar trajectory as the one leading to HNC loss. The possible presence of cyano-cyclopentadiene ions and related five-membered ring species in Titan’s atmosphere and the interstellar medium are discussed.

Infrared action spectroscopy of fundamental nitrilium ions: Protonated vinyl- and ethyl cyanide

Infrared action spectra of the protonated forms of two fundamental organic nitriles, vinyl cyanide, C2H3CN, and ethyl cyanide, C2H5CN, have been observed using the FELion 22-pole ion trap apparatus and the Free Electron Laser for Infrared eXperiments (FELIX) at Radboud University, Nijmegen (The Netherlands). Vibrational bands of both ions were observed in the wavenumber regime between 300 and 2500cm−1 and spectroscopically assigned based on results from anharmonic force field calculations performed here at the coupled cluster level of theory. As the action spectroscopy scheme used in the present study probes the Ne-tagged weakly bound complexes, C2H3CNH+−Ne and C2H5CNH+−Ne, corresponding quantum-chemical calculations of these systems were also performed. Significant blueshifts observed for the strong -CNH+ bending modes indicate that the neon atom is attached to the proton bound to the nitrile group in both molecular ions.

Mid-infrared spectroscopic signatures of dibenzopyrene cations – The effect of symmetry on PAH IR spectroscopy

In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (C2h, Cs and C1). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.

Descendant of the X-ogen carrier and a ‘mass of 69’: infrared action spectroscopic detection of HC3O+ and HC3S+

ABSTRACT The carbon chain ions HCO and HCS – longer variants of the famous ‘X-ogen’ line carrier HCO – have been observed for the first time using two cryogenic 22-pole ion trap apparatus (FELion, Coltrap) and two different light sources: the Free Electron Laser for Infrared eXperiments (FELIX), which was operated between 460 and 2500 cm, and an optical parametric oscillator operating near 3200 cm; signals from both experiments were detected by infrared predissociation action spectroscopy. The majority of vibrational fundamentals were observed for both ions and their wavenumbers compare very favourably with results from high-level anharmonic force field calculations performed here at the coupled-cluster singles and doubles level augmented by a perturbative treatment of triple excitations, CCSD(T). As the action scheme employed here probes the Ne-tagged weakly bound variants, Ne–HCO and Ne–HCS, corresponding calculations of these systems were also performed. Differences in the structures and molecular force fields between the bare ions and their Ne-tagged complexes are found to be very small.

Protoisomerization of Indigo and Isoindigo Dyes Confirmed by Gas-Phase Infrared Ion Spectroscopy.

Gas-phase infrared multiple-photon dissociation (IRMPD) spectra are recorded for the protonated dye molecules indigo and isoindigo by using a quadrupole ion trap (QIT) mass spectrometer coupled to the free electron laser for infrared experiments (FELIX). From their fingerprint IR spectra (600—1800 cm–1) and comparison with quantum-chemical calculations at the density functional level of theory (B3LYP/6-31++G(d,p)), we derive their structures. We focus particularly on the question of whether trans-to-cis isomerization occurs upon protonation and transfer to the gas phase. The trans-configuration is energetically favored in the neutral forms of the dyes in solution and in the gas phase. Instead, the cis-isomer is lower in energy for the protonated forms of both species, but indigo is also notorious for not undergoing double-bond trans-to-cis isomerization, in contrast to many other conjugated systems. The IR spectra suggest that protoisomerization from trans to cis indeed occurs for both dyes. To estimate the extent of isomerization, on-resonance kinetics are measured on diagnostic and common vibrational frequencies to determine the ratio of cis-to-trans isomers. We find ratios of 65–70% cis and 30–35% trans for indigo versus 75–80% cis and 20–25% trans for isoindigo. Transition-state calculations for the isomerization reactions have been carried out, which indeed suggest a lower barrier for protonated isoindigo, qualitatively explaining the more efficient isomerization.

Effect of molecular structure on the infrared signatures of astronomically relevant PAHs

Emission bands from polycyclic aromatic hydrocarbons (PAHs) dominate the mid-infrared spectra of a wide variety of astronomical sources, encompassing nearly all stages of stellar evolution. Despite their similarities, details in band positions and shapes have allowed a classification of PAH emission to be developed. It has been suggested that this classification is in turn associated with the degree of photoprocessing of PAHs. Over the past decade, a more complete picture of the PAH interstellar life-cycle has emerged, in which a wide range of PAH species are formed during the later stages of stellar evolution. After this they are photoprocessed, increasing the relative abundance of the more stable (typically larger and compact) PAHs. For this work we have tested the effect of the symmetry, size, and structure of PAHs on their fragmentation pattern and infrared spectra by combining experiments at the free electron laser for infrared experiments (FELIX) and quantum chemical computations. Applying this approach to the cations of four molecular species, perylene (C20H12), peropyrene (C26H14), ovalene (C32H14) and isoviolanthrene (C34H18), we find that a reduction of molecular symmetry causes the activation of vibrational modes in the 7–9 μm range. We show that the IR characteristics of less symmetric PAHs can help explain the broad band observed in the class D spectra, which are typically associated with a low degree of photoprocessing. Such large, nonsymmetrical irregular PAHs are currently largely missing from the NASA Ames PAH database. The band positions and shapes of the largest more symmetric PAH measured here, show the best resemblance with class A and B sources, representative of regions with high radiation fields and thus heavier photoprocessing. Furthermore, the dissociation patterns observed in the mass spectra hint to an enhanced stability of the carbon skeleton in more symmetric PAHs with respect to the irregular and less symmetric species, which tend to loose carbon containing units. Although not a direct proof, these findings are fully in line with the grandPAH hypothesis, which claims that symmetric large PAHs can survive as the radiation field increases, while their less symmetric counterparts are destroyed or converted to symmetric PAHs.

Spectroscopic Characterization of the Product Ions Formed by Electron Ionization of Adamantane.

A structural characterization of the products formed in the dissociative electron ionization of adamantane (C10H16) is presented. Molecular structures of product ions are suggested based on multiple‐photon dissociation spectroscopy using the Free Electron Laser for Infrared eXperiments (FELIX) in combination with quantum‐chemical calculations. Product ions are individually isolated in an ion trap tandem mass spectrometer and their action IR spectra are recorded. Atomic hydrogen loss from adamantane yields the 1‐adamantyl isomer. The IR spectrum of the C8H11 + product ion is best reproduced by computed spectra of 2‐ and 4‐protonated meta‐xylene and ortho‐ and para‐protonated ethylbenzenes. The spectrum of the product ion at m/z 93 suggests that it is composed of a mixture of ortho‐protonated toluene, para‐protonated toluene and 1,2‐dihydrotropylium, while the spectrum of the m/z 79 ion is consistent with the benzenium ion. This study thus suggests that adamantane is efficiently converted into aromatic species and astrophysical implications for the interstellar medium are highlighted.

Laboratory Gas-phase Infrared Spectra of Two Astronomically Relevant PAH Cations: Diindenoperylene, and Dicoronylene,

Abstract The first gas-phase infrared spectra of two isolated astronomically relevant and large polycyclic aromatic hydrocarbon (PAH) cations—diindenoperylene (DIP) and dicoronylene (DC)—in the 530–1800 cm−1 (18.9−5.6 μm) range—are presented. Vibrational band positions are determined for comparison to the aromatic infrared bands. The spectra are obtained via infrared multiphoton dissociation spectroscopy of ions stored in a quadrupole ion trap using the intense and tunable radiation of the free electron laser for infrared experiments (FELIX). DIP+ shows its main absorption peaks at 737 (13.57), 800 (12.50), 1001 (9.99), 1070 (9.35), 1115 (8.97), 1152 (8.68), 1278 (7.83), 1420 (7.04), and 1550 (6.45) cm−1(μm), in good agreement with density functional theory (DFT) calculations that are uniformly scaled to take anharmonicities into account. DC+ has its main absorption peaks at 853 (11.72), 876 (11.42), 1032 (9.69), 1168 (8.56), 1300 (7.69), 1427 (7.01), and 1566 (6.39) cm−1(μm), which also agree well with the scaled DFT results presented here. The DIP+ and DC+ spectra are compared with the prominent infrared features observed toward NGC 7023. This results both in matches and clear deviations. Moreover, in the 11.0–14.0 μm region, specific bands can be linked to CH out-of-plane (oop) bending modes of different CH edge structures in large PAHs. The molecular origin of these findings and their astronomical relevance are discussed.

Geometrical Structures of Partially Oxidized Rhodium Cluster Cations, Rh6Om+ (m = 4, 5, 6), Revealed by Infrared Multiple Photon Dissociation Spectroscopy.

Infrared multiple photon dissociation (IRMPD) spectra of Rh6Om+ (m = 4-10) are obtained in the 300-1000 cm-1 spectral range using the free electron laser for infrared experiments (FELIX) via dissociation of Rh6Om+ or Rh6Om+-Ar complexes. The spectra are compared with the calculated spectra of several stable geometries obtained by density functional theory (DFT) structural optimization. The spectrum for Rh6O4+ shows prominent bands at 620 and 690 cm-1 and is assigned to a capped-square pyramidal Rh atom geometry with three bridging O atoms and one O atom in a hollow site. Rh6O5+ displays bands at 460, 630, 690, and 860 cm-1 and has a prismatic Rh geometry with three bridging O atoms and two O atoms in a hollow site. Rh6O6+ shows three intense bands around 600-750 cm-1 and multiple weak bands in the range of 350-550 cm-1. This species has a prismatic Rh geometry with four bridging O atoms and two O atoms in a hollow site. Considering that Rh6Om+ (m ≤ 3) adopts tetragonal bipyramidal Rh6 structures, the change at m = 4 to capped bipyramidal and at m = 5 to prismatic geometries results in a reduction of the number of triangular hollow sites. Since NO preferentially binds on a triangular hollow site through the N atom, the geometry change lowers the possibility of NO dissociative adsorption.

Infrared driven CO oxidation reactions on isolated platinum cluster oxides, PtnOm+

This collaboration has recently shown that infrared excitation can drive decomposition reactions of molecules on the surface of gas-phase transition metal clusters. We describe here a significant extension of this work to the study of bimolecular reactions initiated in a similar manner. Specifically, we have observed the infrared activated CO oxidation reaction (CO(ads) + O(ads) --> CO2(g)) on isolated platinum oxide cations, Pt(n)O(m)+. Small platinum cluster oxides Pt(n)O(m)+ (n = 3-7, m = 2, 4), have been decorated with CO molecules and subjected to multiple photon infrared excitation in the range 400-2200 cm(-1) using the Free Electron Laser for Infrared eXperiments (FELIX). The Pt(n)O(m)CO+ clusters have been characterised by infrared multiple photon dissociation spectroscopy using messenger atom tagging. Evidence is observed for isomers involving both dissociatively and molecularly adsorbed oxygen on the cluster surface. Further information is obtained on the evolution of the cluster structure with number of platinum atoms and CO coverage. In separate experiments, Pt(n)O(m)CO+ clusters have been subjected to infrared heating via the CO stretch around 2100 cm(-1). On all clusters investigated, the CO oxidation reaction, indicated by CO2 loss and production of Pt(n)O(m) = 1+, is found to compete effectively with the CO desorption channel. The experimental observations are compared with the results of preliminary DFT calculations in order to identify both cluster structures and plausible mechanisms for the surface reaction.

Infrared Spectroscopy of Discrete Uranyl Anion Complexes

The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO22+) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO2A(S)n]+, where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n = 0-3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO2A]+) were as low or lower than measurements for [UO2]2+ ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30-40 cm(-1) higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO2A]+ produced [UO2AS]+, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO2AS]+ were in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO2AS]+ species having different solvent neutrals, values decreased with increasing neutral nucleophilicity.

Reaction products in mass spectrometry elucidated with infrared spectroscopy

Determining the structure and dynamics of large biologically relevant molecules is one of the key challenges facing biology. Although X-ray crystallography (XRD) and nuclear magnetic resonance (NMR) yield accurate structural information, they are of limited use when sample quantities are low. Mass spectrometry (MS) on the other hand has been very successful in analyzing biological molecules down to atto-mole quantities and has hence begun to challenge XRD and NMR as the key technology in the life sciences. This trend has been further assisted by the development of MS techniques that yield structural information on biomolecules. Of these techniques, collision-induced dissociation (CID) and hydrogen/deuterium exchange (HDX) are among the most popular. Despite advances in applying these techniques, little direct experimental evidence had been available until recently to verify their proposed underlying reaction mechanisms. The possibility to record infrared spectra of mass-selected molecular ions has opened up a novel avenue in the structural characterization of ions and their reaction products. On account of its high pulse energies and wide wavelength tunability, the free electron laser for infrared experiments (FELIX) at FOM Rijnhuizen has been shown to be ideally suited to study trapped molecular ions with infrared photo-dissociation spectroscopy. In this paper, we review recent experiments in our laboratory on the infrared spectroscopic characterization of reaction products from CID and HDX, thereby corroborating some of the reaction mechanisms that have been proposed. In particular, it is shown that CID gives rise to linear fragment ion structures which have been proposed for some time, but also yields fully cyclical ring structures. These latter structures present a possible challenge for using tandem MS in the sequencing of peptides/proteins, as they can lead to a scrambling of the amino acid sequence information. In gas-phase HDX of an amino acid it is shown that the structure can be changed from a charge solvated to a zwitterionic structure, thereby demonstrating that HDX can be an invasive technique, in fact changing the structure of the analyte. These results emphasize that more fundamental work is required in order to understand the underlying mechanisms in two of the most important structural techniques in MS.

Modeling paraxial wave propagation in free-electron laser oscillators

Modeling free-electron laser (FEL) oscillators requires calculation of both the light-beam interaction within the undulator and the light propagation outside the undulator. We have developed a paraxial optical propagation code that can be combined with various existing models of gain media, for example, Genesis 1.3 for FELs, to model oscillators with full paraxial wave propagation within the resonator. A flexible scripting interface is used both to describe the optical resonator and to control the codes for propagation and amplification. To illustrate its capabilities, we numerically investigate two significantly different FEL oscillators: the free-electron laser for infrared experiments (FELIX) system and the vacuum-ultraviolet (VUV)-FEL oscillator of the proposed high-gain fourth generation light source. For the FELIX system, we find that diffraction losses are a considerable part of the single-pass cavity loss (at a wavelength of 40 μm). We also demonstrate that a resonator with hole coupling may be a viable alternative to a standard resonator with transmissive optics for the high gain VUV-FEL oscillator.

Far-Infrared spectroscopy of isolated transition metal clusters

The vibrational far-infrared (IR) spectra of isolated metal clusters in the gas phase can be measured by performing photo dissociation spectroscopy of their rare gas complexes. For these experiments an intense and widely tunable source of far-IR radiation is required, and the Free Electron Laser for Infrared eXperiments (FELIX) is ideally suited for this. Vibrational spectra are obtained for vanadium cluster cations as well as for neutral and cationic niobium clusters. The comparison of the experimental vibrational spectra with theoretically calculated spectra allows for the determination of the structure of the metal clusters.

Probing a strong hydrogen bond with infrared spectroscopy: Vibrational predissociation of BrHBr−⋅Ar

The gas phase vibrational spectroscopy of BrHBr−, a prototypical strongly hydrogen-bonded species, has been studied between 6 and 17 μm (590 and 1670 cm−1) by infrared vibrational predissociation of the BrHBr−⋅Ar ion. Infrared excitation was accomplished using the output of the free electron laser for infrared experiments (FELIX). Predissociation spectra were recorded by monitoring depletion of mass-selected BrHBr−⋅Ar ions as a function of excitation wavelength. Four prominent absorption bands are observed at 733, 890, 1048, and 1201 cm−1. They are assigned to the fundamental of the hydrogenic stretching mode ν3 and a sequence of ν3+nν1 combinations (n=1–3). Additional features to the blue of these bands spaced by ∼21 cm−1 are attributed to combination bands involving motion of the Ar messenger atom. Differences in the relative intensities of the ν3+nν1 combinations bands in comparison to previous matrix experiments are rationalized on the basis of the underlying dissociation dynamics.

Picosecond time‐resolved studies of excited state lifetime of Be acceptor in GaAs/AlAs multiple quantum wells

AbstractThe influence of the quantum well potential on the carrier dynamics of holes bound to the acceptors in Be δ‐doped AlAs/GaAs multiple quantum wells is investigated by far‐infrared time‐resolved spectroscopy using the Free‐Electron Laser for Infrared eXperiments (FELIX). The lifetime of the 2p excited state of acceptors was measured as a function of the width of the quantum well and the temperature. It is found that the lifetime decreases monotonically with decreasing well width and is independent of temperature. We suggest that the relaxation process of the 2p → 1s is non‐radiative and mediated via confined acoustic phonon modes, in particular, zone‐folded acoustic phonon modes which arise from the periodic nature of the multiple quantum well samples. For one particular well width the lifetime was measured across the inhomogeneously broadened D‐line as 30, 30, 70, and 105 ps for 43, 44, 46, and 47 μm wavelengths, respectively. We believe the latter is due to diffusion of the Be δ‐doped atomic layer in the quantum wells.

Mid-infrared (275–60-μm) second-harmonic generation in LiInS_2

Phase-matched second-harmonic generation is obtained in various LiInS(2) crystals by use of the tunable picosecond output of the free-electron laser for infrared experiments (FELIX) as the pump source in the mid-IR range from 2.75 to 6.0 microm. Deviations from the phase-matching curve calculated from Boyd's refractive-index data are observed. Furthermore, the optical damage threshold of the crystals has been measured to be 1.1. J/cm(2)(>6 GW/cm(2)) at the 5-microm wavelength. LiInS(2) holds great promise for parametric interaction in the 1-13-microm range.

A GaAs/AlAs superlattice autocorrelator for picosecond THz radiation pulses

We report on a GaAs/AlAs, wide-miniband, superlattice autocorrelator for picosecond THz radiation pulses (operated at room temperature); the autocorrelator is based on the THz radiation-induced reduction of current through the superlattice. THz radiation (frequency 7.2 THz) from the FELIX (free-electron laser for infrared experiments) was coupled into the superlattice with an antenna system. We measured the current reduction for two time-delayed pulses and found that the signal decreased when the time delay was smaller than the pulse duration. With this superlattice autocorrelator we were able to resolve laser pulses that had a duration of a few picoseconds.

Limitations on quantum ladder climbing in complex systems

The use of cesium Rydberg atoms as a test system for quantum ladder climbing with chirped laser pulses is described. Quantum ladders can be used in the coherent control of chemical reactions; for instance, by exciting a specific molecular bond by climbing its vibrational ladder. A Rydberg atom is an easy test system because of the large transition dipole moments between Rydberg states. The Rydberg states are easy to access and to analyze. Picosecond far-infrared laser pulses around a wavelength of 50 \ensuremath{\mu}m, with peak intensities of ${10}^{9}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ from the free-electron laser for infrared experiments (FELIX) at Nieuwegein, The Netherlands, were used. It is found that the complexity of the system puts severe limits on the allowed intensities that are used to drive the ladder. At high intensity the ladder is distorted due to coupling with states outside the ladder. This distortion frustrates efficient climbing.

Mid-Ir Fel User Facility Felix

The relative merit of the use of the free electron laser for infrared experiments (FELIX) for spectroscopy is discussed and illustrated with some examples.