Clusters of atoms or molecules have been extensively studied by a variety of spectroscopies because of their unusual properties. Experiments with van der Waals clusters of defined sizes are not easily possible because nozzle beam expansions used in their production yield broad size distributions. Moreover, being weakly bound they readily fragment in the commonly used electron impact-ionization mass spectrometer detectors. Here it is shown that light fragile clusters of He, H(2), and D(2) can be selected and identified nondestructively by diffraction from a transmission grating. The method is universally applicable also to heavier species and well suited for spectroscopic studies.

A molecular beam consisting of small helium clusters is diffracted from a 100 nm period transmission grating. The relative dimer intensities have been measured out to the 7th order and are used to determine the reduction of the effective slit width resulting from the finite size of the dimer. From a theoretical analysis of the data which also takes into account the van der Waals interaction with the grating bars, the bond length (mean internuclear distance) and the binding energy are found to be = 52+/-4 A and |E(b)| = 1. 1+0.3/-0.2 mK.

The immediate environment of a molecule can have a profound influence on its properties. Benzocaine, the ethyl ester of para-aminobenzoic acid that finds an application as a local anesthetic, is found to adopt in its protonated form at least two populations of distinct structures in the gas phase, and their relative intensities strongly depend on the properties of the solvent used in the electrospray ionization process. Here, we combine IR-vibrational spectroscopy with ion mobility-mass spectrometry to yield gas-phase IR spectra of simultaneously m/z and drift-time-resolved species of benzocaine. The results allow for an unambiguous identification of two protomeric species: the N- and O-protonated forms. Density functional theory calculations link these structures to the most stable solution and gas-phase structures, respectively, with the electric properties of the surrounding medium being the main determinant for the preferred protonation site. The fact that the N-protonated form of benzocaine can be found in the gas phase is owed to kinetic trapping of the solution-phase structure during transfer into the experimental setup. These observations confirm earlier studies on similar molecules where N- and O-protonation have been suggested.

Molecular beams of rare gas atoms and D2 have been diffracted from 100 nm period SiNx transmission gratings. The relative intensities of the diffraction peaks out to the 8th order depend on the diffracting particle and are interpreted in terms of effective slit widths. These differences have been analyzed by a new theory which accounts for the long-range van der Waals −C3/l 3 interaction of the particles with the walls of the grating bars. The values of the C3 constant for two different gratings are in good agreement and the results exhibit the expected linear dependence on the dipole polarizability. where l is the distance from the surface. This potential plays an important role in understanding virtually all static (thermodynamical) and dynamical aspects of gas adsorption phenomena. Despite its importance, very few experimental determinations of C 3 have so far been reported and most of our present knowledge is based on theoretical estimates [2]. The pioneering experiments by Raskin and Kusch on the deflection of Cs atoms from a conducting metal surface [3] have recently been extended to alkali atoms in high Rydberg states by measuring the transmission through 8 mm long narrow (2 − 9 µm) channels as a function of their principal quantum number n [4]. Similar techniques have also been applied to the interaction of alkali atoms in their ground state [5,6] or in low excited states [7]. Although the scattering of many different atoms and molecules from solid single crystal surfaces has been extensively studied, the reflection coefficients are relatively insensitive to the weak long range attractive forces since the collisions are largely determined by the reflection from the hard repulsive wall close to the surface [8].Here, a new atom optical technique using transmission grating diffraction [9,10] of molecular beams is employed. The van der Waals force causes a change in the diffraction intensities just as a smaller slit width would. A newly developed theory makes it possible to interpret measurements over a range of different beam energies in terms of the potential constant C 3 . For an incident plane wave the diffraction peak heights depend on the number of illuminated slits N , as N 2 . With N = 100 slits the gain in sensitivity is about four orders of magnitude over previous experiments.The measurements were made with a previously described [10] molecular beam diffraction apparatus. The beams are produced by a free jet expansion of the purified gas through a 5 µm diameter, 2 µm long orifice from a source chamber at a temperature T 0 , into vacuum of about 2 × 10 −4 mbar. At T 0 = 300 K the source pressure P 0 was 140 bar for He, Ne, Ar and D 2 and 50 bar for Kr. At lower source temperature P 0 was reduced to avoid cluster formation. The atomic beams are characterized by narrow velocity distributions with ∆v/v ≈ 2.1 % (He), 5 % (Ne), 7.6 % (D 2 ), 7.7 % (Ar), and 10 % (Kr) at T 0 = 300 K, where ∆v and v denote the full half width and the mean value, respectively. After passing through the 0.39 mm diameter skim...

Quantum theory dictates that upon weakening the two-body interaction in a three-body system, an infinite number of three-body bound states of a huge spatial extent emerge just before these three-body states become unbound. Three helium atoms have been predicted to form a molecular system that manifests this peculiarity under natural conditions without artificial tuning of the attraction between particles by an external field. Here we report experimental observation of this long predicted but experimentally elusive Efimov state of 4 He3 by means of Coulomb explosion imaging. We show spatial images of an Efimov state, confirming the predicted size and a typical structure where two atoms are close to each other while the third is far away. One Sentence Summary:We report experimental discovery of a gigantic molecule that consists of three helium atoms and is bound solely by a universal feature of quantum mechanics called "Efimov effect".Ever since the early days of celestial mechanics, the three-body problem posed a major challenge to physicists. In the early 20th century the failure of finding a stable solution for the classical helium atom (2 electrons and a nucleus) heralded the demise of Niels Bohr's program of semiclassical atomic physics (1). Quantum mechanics then added yet another surprising twist to the three-body problem when in 1970 Vitaly Efimov predicted the appearance of an infinite series of stable three-body states of enormous spatial extents (2). These Efimov states are predicted to exist for short-range interactions like the van der Waals force between atoms or the strong force between nucleons. When the potential becomes so shallow that the last two-body bound state is at the verge of becoming unbound or is unbound, then three particles stick together to form Efimov states. Intriguingly, this three-body behavior does not depend on the details of the underlying two-body interactions. This makes the Efimov effect a universal phenomenon, with important applications in particle, nuclear (3, 4), atomic (4), condensed matter (5) and biological physics (6).Figure 1 summarizes two facets of Efimov's prediction, namely the energy spectrum and the structure of an Efimov state. Figure 1A shows how the two-and three-body binding energies (the binding energy of an atomic cluster is defined as the energy needed to separate all constituents of the cluster to infinite distances) change as the depth of the two-body potential is increased. As 2 indicated by the arrow above Figure 1A, the depth of the two-body potential increases along the horizontal axis. As the depth increases, the s-wave scattering length a changes from negative values to infinitely large values to positive values. Negative a values correspond to the domain where shallow two-body bound states do not exist. For positive a, a shallow two-body bound state, the dimer (see the blue solid line), exists. Bound three-body states (called trimers) exist in the green-shaded area. The extremely weakly-bound three-body states close to threshold (see...

Amyloidogenic peptides and proteins play a crucial role in a variety of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. These proteins undergo a spontaneous transition from a soluble, often partially folded form, into insoluble amyloid fibrils that are rich in β-sheets. Increasing evidence suggests that highly dynamic, polydisperse folding intermediates, which occur during fibril formation, are the toxic species in the amyloid-related diseases. Traditional condensed-phase methods are of limited use for characterizing these states because they typically only provide ensemble averages rather than information about individual oligomers. Here we report the first direct secondary-structure analysis of individual amyloid intermediates using a combination of ion mobility spectrometry-mass spectrometry and gas-phase infrared spectroscopy. Our data reveal that oligomers of the fibril-forming peptide segments VEALYL and YVEALL, which consist of 4-9 peptide strands, can contain a significant amount of β-sheet. In addition, our data show that the more-extended variants of each oligomer generally exhibit increased β-sheet content.

A transmission grating is used to nondestructively analyze a low source temperature (6–60 K) beam of helium for small clusters. He2 and He3 are clearly resolved in first order diffraction. The relative ionization and fragmentation probabilities are measured and lend support to a recent mass spectrometer experiment claiming detection of He2.

The charge distribution in a molecule is crucial in determining its physical and chemical properties. Aminobenzoic acid derivatives are biologically active small molecules, which have two possible protonation sites: the amine (N-protonation) and the carbonyl oxygen (O-protonation). Here, we employ gas-phase infrared spectroscopy in combination with ion mobility-mass spectrometry and density functional theory calculations to unambiguously determine the preferred protonation sites of p-, m-, and o-isomers of aminobenzoic acids as well as their ethyl esters. The results show that the site of protonation does not only depend on the intrinsic molecular properties such as resonance effects, but also critically on the environment of the molecules. In an aqueous environment, N-protonation is expected to be lowest in energy for all species investigated here. In the gas phase, O-protonation can be preferred, and in those cases, both N- and O-protonated species are observed. To shed light on a possible proton migration pathway, the protonated molecule-solvent complex as well as proton-bound dimers are investigated.