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We report on the Stark deceleration and electrostatic trapping of $^{14}$NH ($a ^1\Delta$) radicals. In the trap, the molecules are excited on the spin-forbidden $A ^3\Pi \leftarrow a ^1\Delta$ transition and detected via their subsequent fluorescence to the $X ^3\Sigma^-$ ground state. The 1/e trapping time is 1.4 $\pm$ 0.1 s, from which a lower limit of 2.7 s for the radiative lifetime of the $a ^1\Delta, v=0,J=2$ state is deduced. The spectral profile of the molecules in the trapping field is measured to probe their spatial distribution. Electrostatic trapping of metastable NH followed by optical pumping of the trapped molecules to the electronic ground state is an important step towards accumulation of these radicals in a magnetic trap., Comment: replaced with final version, added journal reference

In this paper, we review recent developments in our efforts to get complete control over the velocity of a molecular beam. Central to these experiments is the use of a Stark decelerator, with which the velocity of a beam of neutral polar molecules can be continuously varied, using techniques akin to methods used in charged particle accelerator physics. These techniques allow for confining isolated gas-phase molecules in traps, but can also be used, to advantage, in a variety of molecular beam experiments where the velocity of the beam is an important parameter. Particularly, molecular beam inelastic scattering experiments can be performed with a variable collision energy and with a high intrinsic energy resolution.

Publisher Summary This chapter explains vibrational spectroscopy of gas-phase clusters and complexes. The chapter starts with a description of the experimental methods, beginning with the mechanism of IR multiple photon excitation (MPE), followed by a short discussion of the processes that can follow MPE and are used within the work to detect resonant IR-MPE, namely IR resonance enhanced multiple photon ionization (REMPI) and IR multiple photon dissociation (MPD). An alternative photodissociation (PD) technique is also described in the chapter. This technique is based on the formation of cluster–messenger atom complexes and is particularly suited for the measurement of IR-PD spectra at longer wavelength and/or of smaller clusters. Details of the IR light source FELIX and of the experimental setups are also explained. In addition it also presents an overview on the studies on metal-containing clusters and is divided into four parts: metal carbide clusters, metal oxide clusters, transition metal clusters, and complexes of transition metal complexes. The chapter ends with an outlook summarizing potential future developments in the field.

We present gas-phase infrared spectra of tantalum cluster cations containing 6–20 atoms. Infrared multiple photon dissociation of their complexes with argon atoms is used to obtain vibrational spectra in the region between 90 and 305 cm-1. Many spectra have features in common with the vibrational spectra of the lighter homologs, vanadium and niobium, pointing to a common cluster growth mechanism.

A linear AC trap for polar molecules in high-field seeking states has been devised and implemented, and its characteristics have been investigated both experimentally and theoretically. The trap is loaded with slow 15ND₃ molecules in their ground state (para-ammonia) from a Stark decelerator. The trap's geometry offers optimal access as well as improved loading. We present measurements of the dependence of the trap's performance on the switching frequency, which exhibit a characteristic structure due to nonlinear resonance effects. The molecules are found to oscillate in the trap under the influence of the trapping forces, which were analyzed using 3D numerical simulations. On the basis of expansion measurements, molecules with a velocity and a position spread of 2.1 m/s and 0.4 mm, respectively, are still accepted by the trap. This corresponds to a temperature of 2.0 mK. From numerical simulations, we find the phase-space volume that can be confined by the trap (the acceptance) to be 50 mm³ (m/s)³.

Far-infrared absorption spectra of small neutral and cationic niobium clusters containing 5 to 9 Nb atoms have been obtained by multiple photon dissociation spectroscopy of their argon complexes. The experimental far-IR spectra are recorded in the 85–600 cm-1 region and cover the range of the structure-specific vibrational fundamentals, i.e., the finger-print range, for these metal clusters. The experiments are accompanied by quantum chemical calculations employing density-functional theory. A comparison of the experimental and calculated far-IR spectra spectra allows to identify the cluster structures. Although the experimental spectra for clusters containing 5, 6, 8, and 9 Nb atoms are very different for cationic and neutral clusters, the comparison with theory reveals that, nevertheless, the overall geometries for cations and neutrals are very similar, except for Nb60/+

We present a combined experimental and theoretical study on the radiative lifetime of CO in the $a^3\Pi_{1,2}, v=0$ state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be $2.63\pm0.03$ ms for the $a^3\Pi_1, v=0, J=1$ level. From spin-orbit coupling between the $a^3\Pi$ and the $A^1\Pi$ state a 20% longer radiative lifetime of 3.16 ms is calculated for this level. It is concluded that coupling to other $^1\Pi$ states contributes to the observed phosphorescence rate of metastable CO., Comment: replaced with final version, added journal reference

We report on an element-dependent critical size for argon physisorption at 80 K on transition-metal-doped silicon clusters. Argon does not attach to elemental silicon clusters but only to surface-located transition-metal atoms. Thus physisorption provides structural information. Specifically, the minimal cluster size for the formation of endohedral singly metal-doped silicon cages has been determined. The observed critical size for doubly doped silicon clusters indicates that larger caged molecules can be formed, eventually leading to the growth of metal-doped silicon nanorods.

Polar molecules in high-field seeking states cannot be trapped in static traps as Maxwell's equations do not allow a maximum of the electric field in free space. It is possible to generate an electric field that has a saddle point by superposing an inhomogeneous electric field to an homogeneous electric field. In such a field, molecules are focused along one direction, while being defocused along the other. By reversing the direction of the inhomogeneous electric field the focusing and defocusing directions are reversed. When the fields are being switched back and forth at the appropriate rate, this leads to a net focusing force in all directions. We describe possible electrode geometries for creating the desired fields and discuss their merits. Trapping of ND315 ammonia molecules in a cylindrically symmetric ac trap is demonstrated. We present measurements of the spatial distribution of the trapped cloud as a function of the settings of the trap and compare these to both a simple model assuming a linear force and to full three-dimensional simulations of the experiment. With the optimal settings, molecules within a phase-space volume of 270 mm3 (m/s) 3 remain trapped. This corresponds to a trap depth of about 5 mK and a trap volume of about 20 mm3. © 2006 The American Physical Society.

We report the successful buffer-gas cooling and magnetic trapping of chromium atoms with densities exceeding $10^{12}$ atoms per cm$^{3}$ at a temperature of 350 mK for the trapped sample. The possibilities to extend the method to buffer-gas cool and magnetically trap molecules are discussed. To minimize the most important loss mechanism in magnetic trapping, molecules with a small spin-spin interaction and a large rotational constant are preferred. Both the CrH ($^6\Sigma^+$ ground state) and MnH ($^7\Sigma^+$) radicals appear to be suitable systems for future experiments., Comment: 9 pages, 4 Figures

Beams of polar molecules can be focused using an array of electrostatic lenses in alternating gradient (AG) configuration. They can also be accelerated or decelerated by applying an appropriate high voltage switching sequence to the lenses. AG focusing is applicable to molecules in both low-field and high-field-seeking states and is particularly well suited to the problem of decelerating heavy molecules and those in their ground rotational state. We describe the principles of AG deceleration and set out criteria to be followed in decelerator design, construction and operation. We calculate the longitudinal and transverse focusing properties of a decelerator, and exemplify this by 2D-imaging studies of a decelerated beam of metastable CO molecules., fix author names

We demonstrate feedback control optimization for the Stark deceleration and trapping of neutral polar molecules using evolutionary strategies. In a Stark-decelerator beamline pulsed electric fields are used to decelerate OH radicals and subsequently store them in an electrostatic trap. The efficiency of the deceleration and trapping process is determined by the exact timings of the applied electric field pulses. Automated optimization of these timings yields an increase of 40 % of the number of trapped OH radicals., Comment: 7 pages, 4 figures (RevTeX) (v2) minor corrections (v3) no changes to manuscript, but fix author list in arXiv abstract

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V2O5)n-1(VTiO5)- (n=2–4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V2O5)n clusters. Upon systematic exchange of V by Ti atoms in V4-nTinO-10 (n=1–4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti5O-10.

Stark deceleration relies on time-dependent inhomogeneous electric fields which repetitively exert a decelerating force on polar molecules. Fourier analysis reveals that such fields, generated by an array of field stages, consist of a superposition of partial waves with well-defined phase velocities. Molecules whose velocities come close to the phase velocity of a given wave get a ride from that wave. For a square-wave temporal dependence of the Stark field, the phase velocities of the waves are found to be odd-fraction multiples of a fundamental phase velocity $\ensuremath{\lambda}∕\ensuremath{\tau}$, with $\ensuremath{\lambda}$ and $\ensuremath{\tau}$ the spatial and temporal periods of the field. Here we study explicitly the dynamics due to any of the waves as well as due to their mutual perturbations. We first solve the equations of motion for the case of single-wave interactions and exploit their isomorphism with those for the biased pendulum. Next we analyze the perturbations of the single-wave dynamics by other waves and find that these have no net effect on the phase stability of the acceleration or deceleration process. Finally, we find that a packet of molecules can also ride a wave which results from an interference of adjacent waves. In this case, small phase stability areas form around phase velocities that are even-fraction multiples of the fundamental velocity. A detailed comparison with classical trajectory simulations and with experiment demonstrates that the analytic ``wave model'' encompasses all the longitudinal physics encountered in a Stark decelerator.

Infrared/UV hole-burning spectroscopy is performed on individual conformers of the protected dipeptide Z-Aib-Pro-NHMe. The extended IR range probed in this study allows one to elucidate both the H-bonding motif (5-7 microm) as well as the backbone structure (7-10 microm). Comparison with DFT calculations shows that the backbone is locally constrained to an alpha-conformation by Aib and to a gamma-turn by Pro. The gamma-turn motif observed here is intriguing since the condensed phase structure is known to be a beta-turn. This is the first actual observation of such a discrepancy, and it emphasizes the subtle balance between intra- and intermolecular forces, which is responsible for the relative stability of the different secondary structure motifs.

Resonantly enhanced infrared laser-induced desorption of methane condensed on a single-crystal NaCl(100) surface is observed after excitation with the widely tunable infrared laser output of the free-electron laser at the free-electron laser for infrared experiments facility using mass spectroscopic detection and time-of-flight analysis. Desorption of methane is observed only when the exciting light is in resonance with an internal vibrational mode of the molecule. Different intramolecular modes of the three methane isotopologues under study--CH(4), CD(4), and CD(3)H--are excited; the degenerate deformation mode nu(4) is observed for CH(4) and CD(4) at 7.69 and 10.11 microm, respectively, as well as the nu(2) and nu(4) modes of CD(3)H at 7.79, 9.75, and 9.98 microm. The desorption signals for the pure layers of these different methane isotopologues as well as for different mixtures of two of these are investigated as a function of the infrared wavelength and the laser fluence. The desorption behavior for pure and mixed layers is compared and the underlying desorption mechanism is discussed.

Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag(+) and Zn(2+) with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag](+) the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn(2+) forms a deprotonated dimer complex with Phe, [Zn+Phe(2)-H](+), in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly supporting this as the predominant binding geometry. This most stable calculated complex is characterized by N/ O/ring metal chelation with a tetrahedral-type coordination core of Zn(2+) to N and O of both ligands. Another similar tightly chelated structure shows a square-planar-type coordination core, but this structure is computed to be less stable and gives a less satisfactory match to the experimental spectrum. This preference for the tetrahedral geometry of the Lewis-basic atomic ligands parallels the common Zn(II) coordination geometry in proteins. The number of clearly identifiable peaks resolved in the IR-MPD spectra as well as the much-improved matches between the observed spectra and the DFT-calculated spectra of the most stable geometries compared to previous studies are noteworthy for systems of this size and complexity. These results demonstrate that IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment. In addition, we present a novel ion-capturing method for Fourier transform ion cyclotron resonance mass spectrometry which removes the necessity of a buffer gas pulse, while allowing ion trapping at moderate voltages with apparently reduced collisional excitation of the ions.

The concept of phase stability in a Stark decelerator ensures that polar molecules can be accelerated, guided, or decelerated without loss; molecules within a certain position and velocity interval are kept together throughout the deceleration process. In this paper the influence of the transverse motion on phase stability in a Stark decelerator is investigated. For typical deceleration experiments---i.e., for high values of the phase angle ${\ensuremath{\phi}}_{0}$---the transverse motion considerably enhances the region in phase space for which phase stable deceleration occurs. For low values of ${\ensuremath{\phi}}_{0}$, however, the transverse motion reduces the acceptance of a Stark decelerator and unstable regions in phase space appear. These effects are quantitatively explained in terms of a coupling between the longitudinal and transverse motion. The predicted longitudinal acceptance of a Stark decelerator is verified by measurements on a beam of OH $(X\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Pi}_{3∕2},J=3∕2)$ radicals passing through a Stark decelerator.

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

The merging of molecular beam methods with those of accelerator physics has yielded new tools to manipulate the motion of molecules. Over the last few years, decelerators, lenses, bunchers, traps, and storage rings for neutral molecules have been demonstrated. Molecular beams with a tunable velocity and with a tunable width of the velocity distribution can now be produced, and are expected to become a valuable tool in a variety of physical chemistry and chemical physics experiments. Here we present a compact molecular beam machine, capable of producing 3D spatially focused packets of state-selected accelerated or decelerated molecules. © the Owner Societies 2006.

A four electrode electrostatic trap geometry is demonstrated that can be used to combine a dipole, quadrupole and hexapole field. A cold packet of 15ND3 molecules is confined in both a purely quadrupolar and hexapolar trapping field and additionally, a dipole field is added to a hexapole field to create either a double-well or a donut-shaped trapping field. The profile of the 15ND3 packet in each of these four trapping potentials is measured, and the dependence of the well-separation and barrier height of the double-well and donut potential on the hexapole and dipole term are discussed., Comment: submitted to pra; 7 pages, 9 figures

The mid-infrared vibrational spectra of the cationic and protonated forms of the molecule indazole (C 7 N 2 H 6 ) are recorded in the gas phase, applying the method of free electron laser induced multiple photon dissociation spectroscopy in a quadrupole ion trap. The spectra are compared to density functional theory calculations, which for the protonated species suggests that the proton attaches to the pyridine-like nitrogen atom. The spectrum of the indazole cation raises the question whether indazole undergoes an N1–N2 H atom shift upon ionization. The spectra for the charged species are further discussed in comparison with the spectrum of neutral indazole. Although the spectral range probed in this study, 600–1800 cm −1 , does not cover the hydrogen stretching modes, the spectra are found to be very distinct, indicating how a subtle change in electron distribution can have major effects on the vibrational spectrum of a conjugated system.

The vibrational spectra of Ag(3) and Ag(4) are recorded in the far-infrared between 100 and 220 cm(-1) using multiple photon dissociation spectroscopy of their complexes with Ar atoms. For Ag(3)-Ar two IR active bands are found at 113 and 183 cm(-1), for Ag(4)-Ar one band at 163 cm(-1) and very weak IR activity at 193 cm(-1) are observed. This, together with recent theoretical studies, allows for a reassignment of the controversial vibrational data reported earlier for the bare Ag(3) cluster. The influence of the number of Ar atoms in the complexes on the frequency of the IR active modes is found to be minor. However, the low-frequency IR-active band of Ag(3) shifts with increasing Ar coverage from 113 cm(-1) for Ag(3)-Ar to about 120 cm(-1) for Ag(3)-Ar(4), the value known for Ag(3) embedded in rare gas matrices.

In this review, we present an overview of our recent work on the infrared spectroscopy of mass-selected gas-phase molecular ions. The ions are stored and isolated in a quadrupole ion trap where they are investigated with infrared multiple photon dissociation spectroscopy using a free electron laser (FEL). The wide and continuous tunability of FELIX, the FEL for Infrared eXperiments at the FOM institute for Plasma Physics in The Netherlands, allows us to explore the interesting mid-infrared range between 500 and 2000 cm −1 , sometimes referred to as the fingerprint region. The experiments have focused on polycyclic aromatic hydrocarbons because of their hypothesized occurrence in the interstellar medium, both in neutral and cationic forms. Furthermore, several related species of fundamental chemical interest have been investigated, such as molecular fragment ions and protonated species. The spectra are compared to theoretical spectra calculated at the density functional theory level. The effects of multiple photon excitation and dissociation dynamics on the appearance of the spectrum are extensively discussed. A mathematical model to account for the effects of anharmonicity is described. It explains for instance, qualitatively, how the observed spectral response can vary (slightly) with the fragment channel, which allows one to “direct” dissociation reactions in a non-coherent fashion by the choice of the infrared excitation wavelength.

A high vacuum gate valve for skimmed molecular beam experiments is described. It is designed with a very short extent of only 10 mm along the molecular beam axis to minimize the distance between the molecular beam source and the experiment to provide the maximum molecular flux to the experiment. At the same time it provides free space on both sides of the skimmer to not disturb the supersonic expansion in front of the skimmer, to give optical access to the full distance between beam source and skimmer, and to allow for placing electrostatic devices very close behind the skimmer. The gate valve allows to maintain high vacuum conditions (10^{-8} mbar) in the experimental chamber while the source chamber is brought up to atmospheric pressure for modifications or maintenance. The valve can be operated from outside the vacuum chamber while maintaining vacuum conditions in all chambers., RevTeX; 3 pages, 3 figures

The UV laser-induced desorption of CO from an epitaxial film of NiO(1 0 0) on Ni(1 0 0) yields a linear dependence of the desorption signal on the laser intensity with a cross-section of (8.3 ± 0.9) × 10 −18 cm 2 . Rotational and vibrational temperatures of 870 K and 1340 K, respectively, are observed. State-resolved velocity distributions are observed, where one part can be described by a Maxwellian flux distribution with 〈 E kin 〉/2 k ranging from 1000 to 1800 K. In addition a faster component is present which gains in importance for increasing J ″. A distinct rotation–translational coupling is found.

We report the vibrational spectra of the carbonyl complexes of anionic gold clusters in the range of the CO stretching frequency as measured in the gas phase using IR multiple photon dissociation spectroscopy. The investigated complexes contain between 3 and 14 Au atoms and up to 7 CO ligands. Special attention is given to the complexes that exhibit saturation CO coverage as well as to the monocarbonyl species. In conjunction with data from the corresponding cationic complexes we quantify how the CO stretching frequency varies with the charge state of the gold cluster. Our results provide a size- and charge-dependent basis to interpret values of the CO stretching frequency measured for CO on deposited gold clusters in terms of the charge states of the clusters.

Anharmonic vibrational calculations for the benzoic acid monomer and dimer in the mid-IR regime (500...1800 cm-1) are reported. Harmonic frequencies and intensities are obtained at the DFT/B3LYP level of theory employing D95(d,p) and cc-pVTZ basis sets. Anharmonic corrections obtained from standard perturbation theory lead to red shifts of 1...3 %. In almost all cases, the resulting frequencies deviate by less than 1 % from previous measurements [ Bakker et al., J. Chem. Phys. 119, 11180 (2003)]. Calculated intensities are in qualitative agreement with the absorption experiment, with the cc-pVTZ values being superior to the D95(d,p) ones for a few modes of the dimer. The antisymmetric out-of-plane bending mode of the dimer, which is strongly blue-shifted with respect to the monomer frequency, represents a remarkable exception: The harmonic frequencies obtained for the two basis sets differ notably from each other, and the anharmonically corrected frequencies deviate from the experimental value by 8 % (D95(d,p)) or 3 % (cc-pVTZ). Non-perturbative calculations in reduced dimensionality reveal that the relatively small total anharmonic shift (few tens of cm-1) comprises of partly much larger contributions (few hundreds of cm-1) which are mostly canceling each other. Many of the individual anharmonic couplings are beyond the validity of second order perturbation theory based on cubic and semi-diagonal quartic force constants only. This emphasizes the need for high-dimensional, non-perturbative anharmonic calculations at high quantum chemical level when accurate frequencies of H-atom vibrations in double hydrogen bonds are sought for.

Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived excited state and stored in a trap. The long observation time afforded by the trap can then be exploited to measure the radiative lifetime of this state by monitoring the temporal decay of the population in the trap. This method is demonstrated here and used to benchmark the Einstein $A$-coefficients in the Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is Stark decelerated and loaded into an electrostatic quadrupole trap. The radiative lifetime of the upper $\Lambda$-doublet component of the $X ^2\Pi_{3/2}, v=1, J=3/2$ level is determined as $59.0 \pm 2.0$ ms, in good agreement with the calculated value of $57.7 \pm 1.0$ ms., Comment: 4 pages, 3 figures, submitted to Phys. Rev. Lett

To understand the intrinsic properties of peptides, which are determined by factors such as intramolecular hydrogen bonding, van der Waals bonding and electrostatic interactions, the conformational landscape of isolated protein building blocks in the gas phase was investigated. Here, we present IR-UV double-resonance spectra of jet-cooled, uncapped peptides containing a tryptophan (Trp) UV chromophore in the 1000-2000 cm(-1) spectral range. In the series Trp, Trp-Gly and Trp-Gly-Gly (where Gly stands for glycine), the number of detected conformers was found to decrease from six (Snoek et al., PCCP, 2001, 3, 1819) to four and two, respectively, which indicates a trend to relaxation to a global minimum. Density functional theory calculations reveal that the O-H in-plane bending vibration, together with the N-H in-plane bend ing and the peptide C=O stretching vibrations, is a sensitive probe to hydrogen bonding and, thus, to the folding of the peptide backbone in these structures. This enables the identification of spectroscopic fingerprints for the various conformational structures. By comparing the experimentally observed IR spectra with the calculated spectra, a unique conformational assignment can be made in most cases. The IR-UV spectrum of a Trp-containing nonapeptide (Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu) was recorded as well and, although the IR spectrum is less well-resolved (and it probably results from different isomers), groups of amide I (peptide C=O stretching) and amide II (N-H in-plane bending) bands can still be recognised, in agreement with predictions at the AM1 level.

We report on the interaction of carbon monoxide with cationic gold clusters in the gas phase. Successive adsorption of CO molecules on the Au(n)(+) clusters proceeds until a cluster size specific saturation coverage is reached. Structural information for the bare gold clusters is obtained by comparing the saturation stoichiometry with the number of available equivalent sites presented by candidate structures of Au(n)(+). Our findings are in agreement with the planar structures of the Au(n)(+) cluster cations with nor = 7 that are suggested by ion mobility experiments [Gilb, S.; Weis, P.; Furche, F.; Ahlrichs, R.; Kappes, M. M. J. Chem. Phys. 2001, 116, 4094]. By inference we also establish the structure of the saturated Au(n)(CO)(m)(+) complexes. In certain cases we find evidence suggesting that successive adsorption of CO can distort the metal cluster framework. In addition, the vibrational spectra of the Au(n)(CO)(m)(+) complexes in both the CO stretching region and in the region of the Au-C stretch and the Au-C-O bend are measured using infrared photodepletion spectroscopy. The spectra further aid in the structure determination of Au(n)(+), provide information on the structure of the Au(n)(+)-CO complexes, and can be compared with spectra of CO adsorbates on deposited clusters or surfaces.

A pulsed beam of ground state OH radicals is slowed down using a Stark decelerator and is subsequently loaded into an electrostatic trap. Characterization of the molecular beam production, deceleration and trap loading process is performed via laser induced fluorescence detection inside the quadrupole trap. Depending on details of the trap loading sequence, typically $10^5$ OH ($X^2\Pi_{3/2}, J=3/2$) radicals are trapped at a density of around $10^7$ cm$^{-3}$ and at temperatures in the 50-500 mK range. The 1/e trap lifetime is around 1.0 second., Comment: 4 pages, 3 figures

The first infrared hole burning spectrum of a gas-phase neutral peptide not containing an aromatic amino acid is presented. In the model peptide Z-Pro-NHMe, the amide I and II bands in the 1500-1800 cm(-1) region appear to be a clear diagnostic for the secondary structure of the backbone, while the analysis of a series of coupled CH bending modes in the 1000-1500 cm(-1) region allows to distinguish between different possible orientations of the chromophore. The geometry of the peptide is strongly constrained by the proline and only one conformation of the backbone is observed, which is identified as a hydrogen bonded gamma-turn.

Infrared multiple-photon dissociation spectroscopy is effected on the K(+) tagged aromatic amino acids tyrosine and phenylalanine, as well as the K(+) tagged peptides bradykinin fragment 1-5 and [Leu]-enkephalin. The fingerprint (800-1800 cm(-1)) infrared spectra of these species are compared to density-functional theory (DFT) calculated spectra to determine whether the complex is in the charge solvation (CS) or salt bridge (SB) (i.e. zwitterionic) configuration. For the aromatic amino acids the CS structure is favored and the tridentate N/O/ring structure is found to be the preferred binding geometry for K(+). The experimental and theoretical evidence for bradykinin fragment 1-5 tagged with K(+) suggests that the SB structure is favored; the calculations indicate a head-to-tail looped structure stabilized by a salt bridge between the protonated guanidine group and the deprotonated C-terminus, which allows K(+) to sit in a binding pocket with five C=O electrostatic interactions. For K(+) tagged [Leu]-enkephalin the spectroscopic evidence is not as clear. While the calculations clearly favor a CS structure and the observation of a weak carboxylic acid C=O stretching band in the infrared spectrum matches this finding, the prominence of a band at 1600 cm(-1) renders the analysis more ambiguous, and hence the presence of some salt bridge ions cannot be excluded. Another striking feature in the [Leu]-enkephalin spectrum is the high infrared activity of the tyrosine side-chain modes, which can be clearly identified from comparison to the [Tyr + K](+) experimental spectrum, but which is not reproduced by the DFT calculations.

The conformational structure of short peptide chains in the gas phase is studied by laser spectroscopy of a series of protected dipeptides, Ac-Xxx-Phe-NH(2), Xxx=Gly, Ala, and Val. The combination of laser desorption with supersonic expansion enables us to vaporize the peptide molecules and cool them internally; IR/UV double resonance spectroscopy in comparison to density functional theory calculations on Ac-Gly-Phe-NH(2) permits us to identify and characterize the conformers populated in the supersonic expansion. Two main conformations, corresponding to secondary structures of proteins, are found to compete in the present experiments. One is composed of a doubly gamma-fold corresponding to the 2(7) ribbon structure. Topologically, this motif is very close to a beta-strand backbone conformation. The second conformation observed is the beta-turn, responsible for the chain reversal in proteins. It is characterized by a relatively weak hydrogen bond linking remote NH and CO groups of the molecule and leading to a ten-membered ring. The present gas phase experiment illustrates the intrinsic folding properties of the peptide chain and the robustness of the beta-turn structure, even in the absence of a solvent. The beta-turn population is found to vary significantly with the residues within the sequence; the Ac-Val-Phe-NH(2) peptide, with its two bulky side chains, exhibits the largest beta-turn population. This suggests that the intrinsic stabilities of the 2(7) ribbon and the beta-turn are very similar and that weakly polar interactions occurring between side chains can be a decisive factor capable of controlling the secondary structure.

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.

Infrared spectra of a 104 amino-acid protein in the gas phase as a function of its charge state are presented. The spectra contain clearly resolvable bands in the amide I and II spectral regions, as well as a band at 1483 cm(-1), which is not observed in solution phase spectroscopy and is especially prominent for the higher charge states. Compared to solution, the amide I band is blue-shifted and the amide II band red-shifted, as expected for species in an environment with reduced hydrogen bonding. The band positions are suggestive of a mostly alpha-helical structure of the protein and their widths are comparable to those in solution, suggesting a similar conformational distribution.

Deceleration and trapping of polar molecules using time-varying electric fields is described. Decelerated ND3 and OH molecules are trapped in an electrostatic trap, slow beams of ammonia are injected in a storage ring and have also been used for testing micro-structured optical elements and for high-resolution spectroscopy. Alternate gradient deceleration and AC electric trapping of ground-state molecules is discussed.

The motion of polar molecules can be controlled by time-varying inhomogeneous electric fields. In a Stark decelerator, this is exploited to select a fraction of a molecular beam that is accelerated, transported, or decelerated. Phase stability ensures that the selected bunch of molecules is kept together throughout the deceleration process. In this paper an extended description of phase stability in a Stark decelerator is given, including higher-order effects. This analysis predicts a wide variety of resonances that originate from the spatial and temporal periodicity of the electric fields. These resonances are experimentally observed using a beam of OH ({sup 2}{pi}{sub 3/2},v=0,J=3/2) radicals passing through a Stark decelerator.

The far-infrared vibrational spectra for charged vanadium clusters with sizes of 3-15 atoms have been measured using infrared multiple photon dissociation of Vn+Ar--Vn(+)+Ar. Using density-functional theory calculations, we calculated the ground state energy and vibrational spectra for a large number of stable and metastable geometries of such clusters. Comparison of the calculated vibrational spectra with those obtained in the experiment allows us to deduce the cluster size specific atomic structures. In several cases, a unique atomic structure can be identified, while in other cases our calculations suggest the presence of multiple isomers.

We report the first gas-phase spectroscopic study of a three-residue model of a peptide chain, Ac-Phe-Gly-Gly-NH2 (Ac = acetyl), using the IR/UV double resonance technique. The existence of at least five different conformers under supersonic expansion conditions is established, most of them exhibiting rather strong intramolecular H-bonds. One of the most populated conformers, however, exhibits a different H-bonding network characterized by two weak H-bonds. Comparison of the amide A and I/II experimental data with density functional theory calculations carried out on a series of selected conformations enables us to assign this conformer to two successive beta-turns along the peptide chain, the two H-bonds being of C10 type, i.e., each of them closing a 10-atom ring in the molecule. The corresponding form is found to be more stable than the 310 helix secondary structure (not observed), presumably because of specific effects due to the glycine residues.

The issue of the influence of the side chain/backbone interaction on the local conformational preferences of a phenylalanine residue in a peptide chain is addressed. A synergetic approach is used, which combines gas-phase UV spectroscopy as well as gas-phase IR/UV double-resonance experiments with DFT and post Hartree-Fock calculations. N-Acetyl-Phe-amide was chosen as a model system for which three different conformers were observed. The most stable conformer has been identified as an extended beta(L) conformation of the peptide backbone. It is stabilized by a weak but significant NH-pi interaction bridging the aromatic ring on the residue (i) with the NH group on residue (i+1), with the aromatic side chain being in an anti conformation. This stable conformation corresponds to the common NH(i+1)-aromatic(i) interaction encountered in proteins for the three aromatic residues (phenylalanine, tyrosine, and tryptophan), which illustrates the relevance of gas-phase investigations to structural biology issues. The two other less abundant conformers have been assigned to two gamma-folded backbone conformations that differ by the orientation of the side chain. In all cases, the IR data provided spectroscopic fingerprints of these interactions. Finally, the strong conformational dependence of the fluorescence yield found for N-acetyl-Phe-amide illustrates the role of the environment on the excited-state dynamics of these species, which is often exploited by biochemists to monitor protein structural changes from tryptophan lifetime measurements.

Summary form only given. There is currently a rapidly growing interest in the physics of cold molecules in general, and in the physics of cold polar gases in particular. This interest stems, for instance, from the anticipated importance of the anisotropic interaction between electric dipoles on the formation of a molecular Bose Einstein condensate, from the possibility to use single component cold polar gases for the observation of the superfluid transition, and from the possibility to use arrays of trapped polar molecules for quantum computation. There are now three techniques via which samples of trapped cold molecules have been produced. Photo association of trapped alkali atoms has been used to produce translationally cold alkali dimers, which have subsequently been trapped in a far off resonance optical trap and, more recently, in a quadrupole magnetic trap. Magnetic trapping of paramagnetic molecules thermalized in a He buffer gas has been demonstrated as well. In the third method, explored in our laboratory and detailed in this presentation, time varying electric fields are used to first decelerate pulsed beams of polar molecules, after which bunches of these slow molecules are electrostatically confined. Inthe most recent experiments, it is demonstrated that in a single deceleration and loading cycle, both (bosonic) 14ND3 and (fermionic) 15ND3 ammonia isotopomers can be trapped, even simultaneously, at densities higher than 10 cm/sup 3/ and at temperatures of around 25 mK.

The vibrational spectroscopy of the mono- and divanadium oxide cluster cations VO(1-3)+ and V2O(2-6)+ is studied in the region from 600 to 1600 wave numbers by infrared photodissociation of the corresponding cluster cation-helium atom complexes. The comparison of the experimental depletion spectra with the results of density functional calculations on bare vanadium oxide cluster cations allows for an unambiguous identification of the cluster geometry in most cases and, for VO(1-3)+ and V2O(5,6)+, also of the electronic ground state. A common structural motif of all the studied divanadium cluster cations is a four-membered V-O-V-O ring, with three characteristic absorption bands in the 550-900 wave number region. For the V-O-V and V=O stretch modes the relationship between vibrational frequencies and V-O bond distances follows the Badger rule.

By miniaturizing electrode geometries high electric fields can be produced using modest voltages. A planar array of $20\text{ }\ensuremath{\mu}\mathrm{m}$ wide gold electrodes, spaced $20\text{ }\ensuremath{\mu}\mathrm{m}$ apart, is made on a sapphire substrate. A voltage difference of up to 350 V is applied to adjacent electrodes, generating an electric field that decreases exponentially with distance from the substrate. This microstructured array can be used as a mirror for polar molecules and can be rapidly switched on and off. This is demonstrated by retro\penalty1000\-\allowhyphens{}reflecting a beam of state-selected ammonia molecules with a forward velocity of about $30\text{ }\text{ }\mathrm{m}/\mathrm{s}$.

A decelerated beam of neutral ammonia molecules is injected into an electrostatic storage ring. Electrostatic lenses are used to map the emittance of the decelerator onto the acceptance of the ring. The tangential velocity spread of the package of molecules in the ring is set to less than 1 m / s. The package of molecules can be observed for more than 50 distinct round trips, corresponding to 40 m in circular orbit and almost 0.5 s storage time, sufficiently long for a first investigation of its transversal motion in the ring. Getting full control over both the internal and external degrees of freedom of molecules has been an important goal in molecular physics during the past decade. This control can be achieved, in principle, with appropriately tailored electromagnetic fields [1,2]. Radiation fields can be used, for instance, to prepare molecules in a single selected quantum state, thereby controlling their electronic, vibrational, and rotational degrees of freedom. Static inhomogeneous electric and magnetic fields have been used for a long time to (de)focus polar molecules in a beam, thereby manipulating their transversal position and velocity and controlling their orientation [3]. Time-varying electric fields can be used to efficiently change the forward(longitudinal) position and velocity of these molecules as well [4,5]. In the experiments reported here, a package of ammonia molecules in a single rovibrational state, spatially oriented, is decelerated and focused and subsequently forced in circular orbits. The absolute velocity, as well as the width of the velocity distribution, of the package of molecules is under computer control, and these experiments thereby serve as a demonstration of the level of control over neutral molecules that can nowadays be