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Die CO-Oxidation, obwohl eine scheinbar einfache chemische Reaktion, dient als ein Modell, welches die Vielfalt und Attraktivitat der heterogenen Katalyse offenbart. Das Fritz-Haber-Institut ist ein Ort, an dem ein multidisziplinarer Forschungsansatz zum Studium einer solchen heterogenen Reaktion innerhalb eines Instituts entwickelt und umgesetzt werden kann. Dabei ist die Forschung am Institut primar durch Neugier getrieben, was sich auch in den funf Abschnitten dieses Aufsatzes widerspiegelt. Wir nutzen dabei mikroskopische Konzepte, um die Wechselwirkung einfacher Molekule mit wohldefinierten Materialien wie Clustern in der Gasphase oder festen Oberflachen zu untersuchen. Dieser Ansatz erfordert nicht selten die Entwicklung von neuen Methoden, Werkzeugen und Materialien zur Verifizierung dieser Konzepte, und dies ermoglicht eine methodologisch solide, breit aufgestellte Herangehensweise als ein Markenzeichen des Instituts.

Gas-phase infrared photodissociation spectra of the microhydrated bisulfate anions HSO4¯(H2O)n, with n = 1–16, are reported in the spectral range of 550–1800 cm–1. The spectra show extensive vibrational structure assigned to stretching and bending modes of the bisulfate core, as well as to water bending and librational modes. Comparison with electronic structure calculations suggests that the acidic proton of HSO4– is involved in the formation of a hydrogen bond from n ≥ 1 and that water–water hydrogen bonds form for n ≥ 2. The water network for the larger clusters forms hydrogen-bonded “bands” about the bisulfate core. The blue shifting of the SOH bending mode from 1193 (n = 1) to 1381 cm–1 (n = 12) accompanied by a dramatic decrease in its IR intensity suggests increased incorporation of the bisulfate hydrogen atom into the hydrogen-bonding network, the first step toward acid dissociation.

Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH(a¹Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X³Σ⁻ ground state is performed by driving the spin-forbidden A³Π ← a¹Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the ground- state NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.

Vibrational predissociation spectra of rare-gas-tagged [(CeO(2))(VO(2))(1-2)](+) and [(Ce(2)O(3))(VO(2))](+) clusters are measured in the 400-1200 cm(-1) region. Density functional theory (DFT) is used to determine the geometric and electronic structure of low-energy isomers of the partially reduced clusters. Comparison of experimental and simulated spectra provides evidence for the larger stability of Ce(+3)/V(+5) compared to that of Ce(+4)/V(+4), which confirms that the exceptionally high reducibility of Ce(+4) accounts for the promoting role of ceria in supported vanadium oxide catalysts.

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

The adsorption of carbon monoxide on neutral iridium clusters in the size range of n = 3 to 21 atoms is investigated with infrared multiple photon dissociation spectroscopy. For each cluster size only a single v(CO) band is present with frequencies in the range between 1962 cm-1 (n = 8) and 1985 cm-1 (n = 18) which can be attributed to an atop binding geometry. This behaviour is compared to the CO binding geometries on clusters of other group 9 and 10 transition metals as well as to that on extended surfaces. The preference of Ir for atop binding is rationalized by relativistic effects on the electronic structure of the later 5d metals.

Research on endohedral fullerenes has yielded unambiguous evidence for the existence of selected inside-the-cage complexes. Milligram quantities of purified samples of metallofullerenes have been produced, and characterized via techniques that selectively probe the endohedral guest. Experiments aimed at a detailed understanding of the molecular properties of the endohedral complexes as well as of the material properties of compounds containing endohedral fullerenes have just started.

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In the condensed phase, the peptide gramicidin S is often considered as a model system for a beta-sheet structure. Here, we investigate gramicidin S free of any influences of the environment by measuring the mid-IR spectra of doubly protonated (deuterated) gramicidin S in the gas phase. In the amide I (i.e., C=O stretch) region, the spectra show a broad split peak between 1580 and 1720 cm(-1). To deduce structural information, the conformational space has been searched using molecular dynamics methods and several structural candidates have been further investigated at the density functional level. The calculations show the importance of the interactions of the charged side-chains with the backbone, which is responsible for the lower frequency part of the amide I peak. When this interaction is inhibited via complexation with two 18-crown-6 molecules, the amide I peak narrows and shows two maxima at 1653 and 1680 cm(-1). A comparison to calculations shows that for this complexed ion, four C=O groups are in an antiparallel beta-sheet arrangement. Surprisingly, an analysis of the calculated spectra shows that these beta-sheet C=O groups give rise to the vibrations near 1680 cm(-1). This is in sharp contrast to expectations based on values for the condensed phase, where resonances of beta-sheet sections are thought to occur near 1630 cm(-1). The difference between those values might be caused by interactions with the environment, as the condensed phase value is mostly deduced for beta-sheet sections that are embedded in larger proteins, that interact strongly with solvent or that are part of partially aggregated species.

Die Forschung an kalten Molekulen hat sich in den letzten Jahren rasant entwickelt. Es gibt mittlerweile eine Reihe etablierter Methoden, um Molekule in der Gasphase auf Temperaturen im Millikelvin-Bereich zu kuhlen. Dennoch konzentriert sich ein Schwerpunkt der aktuellen Forschung darauf, neue Wege zu finden, um die Temperaturen der Molekule dem absoluten Nullpunkt noch naher zu bringen. Proben kalter Molekule bieten nicht nur wichtige Anwendungen fur die hochauflosende Spektroskopie, die von den verlangerten Wechselwirkungszeiten der langsamen Molekule mit der elektromagnetischen Strahlung profitiert. Sie versprechen auch einen Zugang zu einem exotischen Regime der chemischen Reaktivitat, in dem Phanomene wie Quantentunneln und ‒resonanzen vorherrschen. Dieser Aufsatz beginnt mit einer Einfuhrung in die Methoden, mit denen kalte Molekule hergestellt werden konnen, mit besonderem Schwerpunkt auf der Stark-Abbremsung und auf Molekulfallen. Ein wichtiger Teil des Aufsatzes ist neben den teilweise bereits realisierten auch den zukunftig moglichen Anwendungen kalter Molekule gewidmet, die beide mithilfe ausgewahlter Beispiele naher beleuchtet werden.

CO on a Chip Microfluidics technology has facilitated remarkable miniaturization of chemical synthesis platforms; through electrically gated solution flow and mixing, molecular reactions can be carried out on chips several centimeters across. When it comes to more fundamental dynamics studies, though, which involve probing gas-phase molecules in specific quantum mechanical states, the experiments still tend to require much larger interaction areas. Meek et al. (p. 1699 ) take a step toward miniaturization in this latter regime by demonstrating the isolation of a cold gas-phase beam of CO molecules just above a microelectrode-decorated chip. The technique relies on rapidly modulated electric fields that trap and then slow down the incoming molecules through dipole interactions. Once brought to a stop, the molecules can be held on the chip for a discrete period and then released to a detector.

Electrospray ionization (ESI) is the softest ionization method that is currently available and it is widely accepted, that ESI generated ions of proteins and protein assemblies at certain conditions retain characteristic aspects of their solution-state conformation. ESI mass spectrometry (MS) therefore evolved as a useful tool to obtain information on composition, stoichiometry, and dynamics of non-covalently associated protein complexes. While tertiary structure information of proteins can be obtained from ion mobility spectrometry (IMS), only a few techniques yield direct information on the secondary structure of gas-phase peptides and proteins. We present here the mid-IR spectroscopic secondary structural analysis of three de novo designed α-helical coiled coil model peptides and their non-covalently associated complexes in the gas-phase. The conformational stability of such coiled coil peptides in solution is primarily driven by aggregation. Isolated monomers usually remain unfolded. Two of the investigated peptides were designed to assemble into stable α-helical complexes in acidic solution, while the third one remains monomeric and unfolded at these conditions. Monomer ions of all three peptides show comparable photodissociation IR spectra and therefore suggest an unfolded conformation in the gas phase. In contrast, considerable C O stretch (amide-I) and N–H bend (amide-II) band shifts have been observed for the dimers which is consistent with an elevated H-bond content. These findings provide evidence that at least a fraction of the condensed phase α-helical structure is retained in the gas-phase coiled coil complexes.

We present infrared photodissociation spectra of the microhydrated nitrate ions NO(3)(-)(H(2)O)(1-6), measured from 600 to 1800 cm(-1). The assignment of the spectra is aided by comparison with calculated B3LYP/aug-cc-pVDZ harmonic frequencies, as well as with higher-level calculations. The IR spectra are dominated by the antisymmetric stretching mode of NO(3)(-), which is doubly degenerate in the bare ion but splits into its two components for most microhydrated ions studied here due to asymmetric solvation of the nitrate core. However, for NO(3)(-)(H(2)O)(3), the spectrum reveals no lifting of this degeneracy, indicating an ion with a highly symmetric solvation shell. The first three water molecules bind in a bidentate fashion to the terminal oxygen atoms of the nitrate ion, keeping the planar symmetry. The onset of extensive water-water hydrogen bonding is observed starting with four water molecules and persists in the larger clusters.

The gas-phase vibrational spectroscopy of bare and monohydrated suberate dianions, (-)OOC-(CH(2))(6)-COO(-) and (-)OOC-(CH(2))(6)-COO(-).H(2)O, is studied by infrared photodissociation aided by electronic structure calculations. To this end, the corresponding ion-Kr atom complexes are formed in a cooled buffer-gas-filled ion trap, and their infrared vibrational predissociation spectra are measured in the range from 660 to 3600 cm(-1). The water molecule binds to one of the two carboxylate groups in a bidentate fashion, characterized by the splitting of the carboxylate stretching bands, a substantially blue-shifted water bending band, and the presence of anomalously broadened bands in the O-H stretching and H(2)O rocking region. The C-C backbone structure remains unperturbed by the addition of a water molecule or a Kr atom. At 63 K, the all-trans isomer is the most abundant species, but evidence for dynamically interconverting conformers is also present from contributions to the absorption cross section on the low-energy tail of the C-H stretching region.

Here we report on a synchrotron for neutral molecules that consists of two hexapole focusers bent into a torus. By switching the voltages applied to the two half-rings, polar molecules can be confined in tight packets for up to a second as they revolve around the ring. In this Letter, the operation and trapping parameters of the molecular synchrotron are discussed. We plan to use a molecular synchrotron in the future as an arena for low-energy collision experiments. © 2009 Elsevier B.V. All rights reserved.

The recent demonstration of cooling and manipulation techniques for molecules offer newpossibilities for precision measurements in molecules. Here, we present the design of a molecularfountain based on a Stark decelerated molecular beam. In this fountain, ammonia molecules aredecelerated to a few meter per second, cooled to sub microKelvin temperatures and subsequentlylaunched. The molecules fly upwards some 30 cm before falling back under gravity, thereby passing amicrowave cavity twice – as they fly up and as they fall back down. The effective interrogationtime in such a Ramsey type measurement scheme includes the entire flight time between the twotraversals through the driving field, which is on the order of a 1/2 second. We present numericalsimulations of the trajectories through the decelerator and estimate the expected count rate. Wepresent an evaluation of the expected stability and accuracy for the inversion transition in15NH3 around 22.6 GHz. The estimated frequency instability is $7\times10^{-12}~\tau^{-1/2}$ , with τ being the measurement time in seconds. With a careful design ofthe interogation zone, systematic frequency shifts are kept below 10-14. Besides serving as aproof-of-principle, these measurements may be used as a test of the time-variation of fundamentalconstants using the sensitivity of the tunneling motion to a change of the proton-electron massratio.

The motion of neutral molecules in a beam can be manipulated with inhomogeneous electric and magnetic fields. Static fields can be used to deflect or focus molecules, whereas time-varying fields can be used to decelerate or accelerate beams of molecules to any desired velocity. We review the possibilities that this molecular-beam technology offers, ranging from ultrahigh-resolution spectroscopy using molecular fountains to novel crossed-beam scattering experiments. The ability to control the velocity of molecules using time-varying electrical and magnetic fields has led to a renewed interest in molecular beams. This article reviews the technology of these decelerators and discusses applications.

An experimental method is demonstrated that allows determination of the ratio between the electric ($E1$) and magnetic ($M1$) transition dipole moments in the $A\ensuremath{-}X$ band of OH, including their relative sign. Although the transition strengths differ by more than 3 orders of magnitude, the measured $M1$-to-$E1$ ratio agrees with the ratio of the ab initio calculated values to within 3%. The relative sign is found to be negative, also in agreement with theory.

We present the mid-IR (400-1800 cm(-1)) spectra of 9-ethyl guanine, guanosine, and 2-deoxyguanosine measured by IR-UV double-resonance spectroscopy. We compare the recorded mid-IR spectra with the spectra of the most stable structures obtained from RI-MP2 and RI-DFT-D calculations. The results confirm the enol form for all structures and demonstrate the efficacy of a new approach to DFT calculations that includes dispersion interactions.

Langsame und damit sehr kalte Molekule bieten eine ahnlich aufregende Perspektive wie ultrakalte Atome. Doch ihre Herstellung ist schwieriger, denn meist versagt die bei Atomen erfolgreiche Laserkuhlung. Eine Moglichkeit bietet der Stark-Verzogerer. Er arbeitet wie ein inverser Linearbeschleuniger und bremst uber den Stark-Effekt sogar elektrisch neutrale Molekule ab. Diese langsamen Molekule kann dann ein Speicherring aufnehmen oder eine elektrostatische Quadrupol-Falle fast zum Stillstand bringen. Kollisionsexperimente zwischen langsamen OH-Molekulen und Xe-Atomen haben jungst erste interessante Einblicke in diese Streuprozesse geliefert. Bei ihnen kann die Energie gerade noch ausreichen, um die quantisierten Rotationszustande der beteiligten Molekule zu andern.

We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X(2)Π, v = 0, j = 3/2 F1f level, in collisions with molecular hydrogen (H2 and D2). In a crossed beam experiment, the OH radicals were state selected and velocity tuned over the collision energy range 75-155 cm(-1) using a Stark decelerator. Relative parity-resolved state-to-state integral cross sections were determined for collisions with normal and para converted H2. These cross sections, as well as previous OH-H2 measurements at 595 cm(-1) collision energy by Schreel and ter Meulen [J. Chem. Phys. 105, 4522 (1996)], and OH-D2 measurements for collision energies 100-500 cm(-1) by Kirste et al. [Phys. Rev. A 82, 042717 (2010)], were compared with the results of quantum scattering calculations using recently determined ab initio potential energy surfaces [Ma et al., J. Chem. Phys. 141, 174309 (2014)]. Good agreement between the experimental and computed relative cross sections was found, although some structure seen in the OH(j = 3/2 F1f → j = 5/2 F1e) + H2(j = 0) cross section is not understood.

We report on the design and characterization of a new apparatus for performing quantum-state resolved surface scattering experiments. The apparatus combines optical state-specific molecule preparation with a compact hexapole and a Stark decelerator to prepare carrier gas-free pulses of quantum-state pure CO molecules with velocities controllable between 33 and 1000 m/s with extremely narrow velocity distributions. The ultrahigh vacuum surface scattering chamber includes homebuilt ion and electron detectors, a closed-cycle helium cooled single crystal sample mount capable of tuning surface temperature between 19 and 1337 K, a Kelvin probe for non-destructive work function measurements, a precision leak valve manifold for targeted adsorbate deposition, an inexpensive quadrupole mass spectrometer modified to perform high resolution temperature programmed desorption experiments and facilities to clean and characterize the surface.

We report on the production of a pulsed molecular beam of metastable NH ($a ^1\Delta$) radicals and present first results on the Stark deceleration of the NH ($a ^1\Delta, J=2, M\Omega=-4$) radicals from 550 m/s to 330 m/s. The decelerated molecules are excited on the spin-forbidden $A ^3\Pi \leftarrow a ^1\Delta$ transition, and detected via their subsequent spontaneous fluorescence to the $X ^3\Sigma^{-}, v"=0$ ground-state. These experiments demonstrate the feasibility of our recently proposed scheme [Phys. Rev. A 64 (2001) 041401] to accumulate ground-state NH radicals in a magnetic trap., Comment: 11 pages, 4 figures, v2: fixed author name for web-abstract, no changes to manuscript

▪ Abstract The motion of polar molecules can be controlled by time-varying inhomogeneous electric fields. In a Stark decelerator, this is exploited to accelerate, transport, or decelerate a fraction of a molecular beam. When combined with a trap, the decelerator provides a means to store the molecules for times up to seconds. Here, we review our efforts to produce cold molecules via this technique. In particular, we present a new generation Stark decelerator and electrostatic trap that selects a significant part of a molecular beam pulse that can be loaded into the trap. Deceleration and trapping experiments using a beam of OH radicals are discussed.

Contains fulltext : 103938.pdf (Publisher’s version ) (Closed access) Molecular scattering behavior has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable absolute velocity by passing the beam through a Stark decelerator. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in the range of 50 to 400 wavenumbers, with an overall energy resolution of about 13 wavenumbers. The behavior of the cross-sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled-channel calculations on ab initio computed potential energy surfaces.

Infrared spectra were recorded for a series of gas-phase Cr+ complexes using infrared multiphoton dissociation (IRMPD) in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The functionalized aromatic ligands (acetophenone, anisole, aniline, and dimethyl aniline) offer a choice of either aromatic ring-pi or n-donor-base binding sites. Use of the FELIX free electron laser light source allowed convenient, rapid scanning of the chemically informative wavelength range from approximately 500 to 1800 cm(-1), which in many cases characterized the preferred site of metal binding, as well as the electronic spin state of the complex. Mono-complex ions, Cr+(ligand), for anisole, aniline, and dimethyl aniline and bis-complex ions, Cr+(ligand)(2), for anisole, aniline, and acetophenone were produced by ligand attachment to laser-desorbed Cr+ ions in the FT-ICR cell. The photodissociation yields plotted as a function of wavelength were interpreted as approximations to the infrared absorption spectra and were compared with computed spectra of different possible geometries and spin states. Clear-cut diagnostic features in the spectra of the acetophenone, anisole, and aniline complexes showed the sites of Cr+ attachment to be the carbonyl oxygen site for acetophenone (bis-complex) and the ring-pi site for anisole and aniline (both mono- and bis-complexes). The bis-complexes of aniline and anisole are low-spin (probably doublet) states, while the mono-complexes of these same ligands are high-spin (sextet) states. The dimethyl aniline complex gave a cluttered spectrum in poor agreement with calculations, which may reflect a mixture of binding-site isomers in this case.

Ultimately, the resolution of any spectroscopic experiment is limited by the interaction time between the particles that are to be examined and the measuring device. The obtainable spectroscopic resolution in a molecular beam experiment can be considerably improved using samples of slow molecules, as produced, for example, in a Stark-decelerator. This is demonstrated here by measuring the inversion tunneling spectrum of ${}^{15}{\rm ND}_3$ using a pulsed molecular beam that has been decelerated to about 52 m/s. Hyperfine resolved inversion transitions in ${}^{15}{\rm ND}_3$ in the $\left\vert J,K\right > = \left\vert 1,1\right > $ state (around 1.43 GHz) are induced in a microwave region, and ${}^{15}{\rm ND}_3$ molecules that have undergone the transition are subsequently detected using a UV-laser based ionization detection scheme. To increase the signal intensity, the decelerated molecular beam is both transversally and longitudinally focused into the laser detection region. The observed spectral width of individual hyperfine transitions in the fully resolved spectrum is about 1 kHz, and the standard deviation of the best fit is 62 Hz.

Vanadium cluster cations with one, two or three adsorbed water molecules are investigated with infrared photodissociation spectroscopy in the region of the bending mode of water. In all of these complexes, the bending mode of adsorbed water is measured near the corresponding frequency of the isolated molecule. Dissociation processes are efficient, indicating that these resonances are characteristic of a substantial fraction of all complexes present. This indicates that water is adsorbed without significant dissociation on these clusters. Clusters with two or three water molecules have similar resonances near the bending mode of free water, indicating independent absorption without hydrogen bonding.

The adsorption of carbon monoxide on rhodium clusters in the size range of 3−15 atoms is studied in the gas phase using the frequency of the internal CO stretch, ν(CO), to probe the bonding situation of the CO. The IR absorption spectra of neutral, cationic, and anionic RhnCO complexes are measured in the frequency range of ν(CO), between 1650 and 2200 cm-1, using IR multiple photon dissociation spectroscopy. We find that for most clusters adsorption in an atop position (μ1) is preferred; however, for some clusters, CO in bridging (μ2) or hollow (μ3) sites can be identified as well. Comparison with DFT calculations carried out for the smallest cluster complexes RhnCO+/0/- (n = 3 and 4) shows that the experimentally identified CO adsorption sites correspond to the energetically favored positions.

Infrared spectra in the range of 530–1400 cm–1 are presented for tantalum oxide cluster cations % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeivaiaabg % gadaWgaaWcbaGaamiEaaqabaGccaqGpbWaa0baaSqaaiaadMhaaeaa % cqGHRaWkaaaaaa!3BB8! $$ {\rm{Ta}}_x {\rm{O}}_y^ + $$ (x = 3, 4 and 6). These spectra are obtained via multiple photon dissociation (MPD) spectroscopy of free cluster cations moving in a molecular beam. The IR spectra for % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeivaiaabg % gadaWgaaWcbaGaamiEaaqabaGccaqGpbWaa0baaSqaaiaadMhaaeaa % cqGHRaWkaaaaaa!3BB8! $$ {\rm{Ta}}_x {\rm{O}}_y^ + $$ are compared to spectra of % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOtaiaabk % gadaWgaaWcbaGaamiEaaqabaGccaqGpbWaa0baaSqaaiaadMhaaeaa % cqGHRaWkaaaaaa!3BB3! $$ {\rm{Nb}}_x {\rm{O}}_y^ + $$ as well as to the experimental data available for vanadium oxide cluster cations from IR-MPD spectroscopy. For % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeivaiaabg % gadaWgaaWcbaGaamiEaaqabaGccaqGpbWaa0baaSqaaiaadMhaaeaa % cqGHRaWkaaaaaa!3BB8! $$ {\rm{Ta}}_x {\rm{O}}_y^ + $$ and % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOtaiaabk % gadaWgaaWcbaGaamiEaaqabaGccaqGpbWaa0baaSqaaiaadMhaaeaa % cqGHRaWkaaaaaa!3BB3! $$ {\rm{Nb}}_x {\rm{O}}_y^ + $$ the IR spectra exhibit very strong similarities (including the band positions), whereas absorption bands in % MathType!MTEF!2!1!+- % feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOvamaaBa % aaleaacaWG4baabeaakiaab+eadaqhaaWcbaGaamyEaaqaaiabgUca % Raaaaaa!3AD6! $$ {\rm{V}}_x {\rm{O}}_y^ + $$ are relatively blue-shifted by about 20–40 cm–1.

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.

Infrared spectroscopy of a variety of interstellar sources shows strong mid-IR emission bands, which are generally attributed to emission from highly vibrationally excited polycyclic aromatic hydrocarbon molecules (PAHs) in the neutral and, particularly, cationic states. Over the past decade, various experimental methods have been developed to record the infrared spectra of cationic PAHs in the laboratory. In this paper, we discuss available experimental spectra obtained with matrix isolation spectroscopy (MIS), infrared multiple-photon dissociation of trapped ions (MPD), dissociation spectroscopy of ionic PAH van der Waals clusters (VDW), and infrared emission (IRE). Moreover, we compare these experimental spectra to density functional theory (DFT) calculations. The main body of experimental data relies on MIS and MPD spectra, and we present a detailed comparison of results from these methods, providing linear and multiple-photon absorption data, respectively. The effects of multiple-photon absorption, as encountered in MPD, and multiple-photon emission, occurring in interstellar spectra, are carefully assessed with the use of mathematical models, which include the effects of vibrational anharmonicity. We show that an analysis of the multiple-photon and linear data can provide important information on the anharmonicity parameters, which is otherwise difficult to attain. This is illustrated with a detailed comparison of the linear and multiple-photon absorption spectra of the naphthalene cation, yielding experimental anharmonicity parameters for the IR-active modes in the 500-1700 cm-1 range.

Inspired by the spectacular successes in the field of cold atoms, there is currently great interest in cold molecules. Cooling molecules is useful for various fundamental physics studies and gives access to an exotic regime in chemistry where the wave property of the molecules becomes important. Although cooling molecules has turned out to be considerably more difficult than cooling atoms, a number of methods to produce samples of cold molecules have been demonstrated over the last few years. This paper aims to review the application of cold molecules and the methods to produce them. Emphasis is put on the deceleration of polar molecules using time-varying electric fields. The operation principle of the array of electrodes that is used to decelerate polar molecules is described in analogy with, and using terminology from, charged-particle accelerators. It is shown that, by applying an appropriately timed high voltage burst, molecules can be decelerated while the phase-space density, i.e. the number of mol...

Resonant IR excitation of gas-phase molecules and clusters can lead to superhot species that thermally emit an electron. Monitoring the mass selected ion yield as a function of IR laser frequency yields the IR-REMPI (infrared resonance enhanced multiphoton ionization) spectrum of the molecule or cluster. Although this IR-REMPI spectrum is not the same as the linear absorption spectrum, it can be quite similar and it yields valuable information on the IR optical properties of the species investigated. In this article, the method and the necessary tools are presented. Results from experiments on fullerenes, metal carbide, metal oxide, and metal nitride clusters are shown.

We revisit the operation of the Stark decelerator and present a new, optimized operation scheme, which substantially improves the efficiency of the decelerator at both low and high final velocities, relevant for trapping experiments and collision experiments, respectively. Both experimental and simulation results show that this new mode of operation outperforms the schemes which have hitherto been in use. This new mode of operation could potentially be extended to other deceleration techniques.

We demonstrate the measurement and manipulation of the temperature of cold CO molecules in a microchip environment. Through the use of time-resolved spatial imaging, we are able to observe the phase-space distribution of the molecules, and hence deduce the corresponding temperature. We do this both by observing the expansion of the molecular ensemble in time and through the use of numerical trajectory simulations. Furthermore, we demonstrate the adiabatic cooling of the trapped molecular sample and discuss this process., Comment: 6 pages, 4 figures

The gas phase vibrational spectroscopy of Br−⋅(HBr)1,2,3 clusters has been studied between 6 and 16 μm (625 and 1700 cm−1) by multiphoton infrared photodissociation spectroscopy using the output of the free electron laser for infrared experiments. Infrared (IR) spectra were recorded by monitoring the mass-selected ion yield. In all three systems neutral HBr loss is found to be the dominant photofragmentation channel. BrHBr− exhibits a weak absorption band at 1558 cm−1 which is assigned to the overtone of the antisymmetric stretching mode ν3. A series of strong absorption bands was observed for Br−⋅(HBr)2 at energies in the 950–1450 cm−1 range. The Br−⋅(HBr)3 spectra reveal two absorption bands at 884 and 979 cm−1, which are assigned to two H-atom stretching modes. Evidence for the localization of the H atom and destruction of the symmetric BrHBr− hydrogen bond in the larger clusters is presented. Standard electronic structure calculations fail to reproduce the experimental IR spectra, indicating a breakdown of the harmonic approximation.

The Al–benzene complex is produced by laser vaporization in a pulsed nozzle source. It is ionized with an ArF excimer laser (193 nm), and the Al+(benzene) ions are stored in a quadrupole ion trap. Infrared excitation with a tunable free electron laser induces multiphoton photodissociation, and fragment ions are analyzed by a time-of-flight mass spectrometer. The infrared spectrum of Al+(benzene) is measured with resonance-enhanced multiphoton photodissociation (IR-REMPD) spectroscopy. Bands in the 600–1800 cm −1 region correspond to benzene vibrations shifted by the metal bonding. The spectrum indicates that Al+ binds in the symmetric η6π configuration on the benzene molecule.

A study of infrared laser-induced desorption of N 2 O condensed on NaCl(1 0 0) is presented. The comparison of the wavelength dependent desorption yield to the linear absorption spectrum shows that desorption of intact molecules occurs at the resonance frequencies of the symmetric stretching and the bending vibration of N 2 O. The analysis of the time-of-flight spectra yields translational temperatures that are well above the substrate temperature and depend strongly on the applied laser fluence. In addition, the dependence of the desorption signal on layer thickness and substrate temperature is investigated. The results are discussed in the context of different desorption mechanisms, and it is concluded that a resonant heating process can best explain the experimental observations.

Neutral (MgO)n clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via multiphoton absorption from either an ultraviolet excimer laser or a far-infrared free electron laser. While ultraviolet ionization produces mass spectra consistent with previous measurements, infrared ionization produces higher molecular weight ions from the same nascent source distribution. Ultraviolet ionization occurs by direct electronic excitation/ionization, while infrared ionization occurs by vibrational excitation followed by thermionic electron emission. In both cases, prominent masses are observed corresponding to cubic nanocrystals with near equal x:y:z dimensions. By tuning the IR wavelength while recording the mass-resolved ion yield, vibrational spectra are obtained revealing two resonances near 16 and 22 microns. Clusters up to 300 atoms in size are studied, and spectra exhibit a gradual variation with size, converging to positions near to, but not matching the bulk phonon frequencies. Structural implications of these vibrational spectra are investigated.

Neutral titanium carbide clusters are produced in the gas phase with a standard laser vaporization source and stored in an ion trap for 250 ms after ionization. During this time, the ions are irradiated with infrared light from a free electron laser. Multiphoton absorption and dissociation occurs when the IR light is in resonance with an infrared-allowed vibrational transition. Monitoring the amount of ionic fragments as a function of the IR wavelength yields the infrared spectrum of the parent, The infrared spectrum of the Ti8C12 cation is recorded and compared to the spectrum of the neutral species. (C) 2001 Elsevier Science B.V. All rights reserved.

Neutral vanadium-carbide clusters are produced in the gas phase with laser vaporization in a pulsed nozzle source and excited with a free electron laser in the 400-1000 cm(-1) region. Resonant multiphoton excitation induces thermionic electron emission in the isolated metal carbide nanocrystals. This process only occurs for stable neutral clusters which resist multiphoton dissociation. The species observed are cubic nanocrystals and truncated nanocrystals with carbon vacancies at their corners. Their wavelength spectra exhibit resonances associated with surface phonons. The resonance shows only a weak size dependence, shifting from 530 to 490 cm(-1) in going from V14C12 to V32C32 (C) 2001 Elsevier Science B.V. All rights reserved.

Infrared spectra of gas-phase cationic naphthalene, phenanthrene, anthracene, and pyrene are recorded in the 500-1600 cm(-1) range using multiphoton dissociation in an ion trap. Gas-phase polyaromatic hydrocarbons are photoionized by an excimer laser and stored in a quadrupole ion trap. Subsequent interaction with the intense infrared radiation of a free electron laser that is tuned in resonance with an infrared-allowed transition of the ion leads to sequential multiphoton absorption facilitated by rapid intramolecular vibrational redistribution. Absorption of more than 50-100 infrared photons raises the internal energy to above the dissociation threshold, leading eventually to fragmentation of the ion. Mass selective detection of the cationic species stored in the trap yields the infrared absorption spectrum of the parent ion.