G.J. von Helden

Do small fullerenes exist only on the computer? Experimental results on C=/−20 and C+/−24

Abstract Experimental results on the structure of carbon cluster anions and cations with 20 and 24 atoms are presented. Recent quantum chemical calculations predict either cage or graphitic structures to be lowest in energy for C20 and C24. However experimentally we find C+20 and C−20 are dominated by monocyclic ring structures, with C−20 exhibiting minor amounts of planar bicyclic structures and a linear structure. For C+24 and C−24 both monocyclic rings and planar bicycle rings are observed. No fullerene or “graphitic” structures are observed for these clusters. A number of possible bicyclic structures for C24 are discussed.

Structure determination of small vanadium clusters by density-functional theory in comparison with experimental far-infrared spectra

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.

Mid-IR spectra of different conformers of phenylalanine in the gas phase

The experimental mid- and far-IR spectra of six conformers of phenylalanine in the gas phase are presented. The experimental spectra are compared to spectra calculated at the B3LYP and at the MP2 level. The differences between B3LYP and MP2 IR spectra are found to be small. The agreement between experiment and theory is generally found to be very good, however strong discrepancies exist when -NH2 out-of-plane vibrations are involved. The relative energies of the minima as well as of some transition states connecting the minima are explored at the CCSD(T) level. Most transition states are found to be less than 2000 cm(-1) above the lowest energy structure. A simple model to describe the observed conformer abundances based on quasi-equilibria near the barriers is presented and it appears to describe the experimental observation reasonably well. In addition, the vibrations of one of the conformers are investigated using the correlation-corrected vibrational self-consistent field method.

Structure determination of gas phase aluminum oxide clusters

Neutral aluminum oxide clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via (multi-) photon absorption from either an ultraviolet excimer laser or from a far-infrared free electron laser. Ultraviolet (multi-) photon ionization produces sparse mass spectra with only relatively light aluminum oxide clusters, while infrared ionization produces a smooth distribution of higher molecular weight ions from the same nascent source distribution. Tuning the IR wavelength, multiphoton infrared spectra are recorded pointing to the γ-Al2O3 structure for a whole series of AlO·(Al2O3)n clusters, n≤34.

Experimental study of gas phase titanium and aluminum oxide clusters

We present an experimental study of the vibrational properties of gas phase titanium oxide and aluminum oxide clusters. The titanium and aluminum oxide clusters have a stoichiometry of (Ti2O3)x-(TiO2)y (with (x, y) from (2, 4) to (11, 29)) and AlO-(Al2O3)n (5 ≤ n ≤ 70). The vibrational properties of the clusters are obtained using infrared resonance enhanced multi- photon ionization (IR-REMPI) spectroscopy. Titanium oxide clusters have a strong vibrational band at ∼13.5 µm, suggesting that their structure is close to the rutile bulk phase of TiO2. Aluminum oxide clusters seem to have a structure comparable to the bulk γ-Al2O3; their IR-REMPI spectra exhibit a vibrational band at ∼11 µm and another band at ∼15 µm which appears in the spectra of clusters containing more than 7-8 Al atoms and becomes more intense as the cluster size increases. As hot neutral clusters are observed to evaporate more easily electrons than neutral fragments, one can conclude that they are very stable and thus very good nucleation seeds for dust growth.

Thermal bimolecular reactions of size selected transition metal cluster ions: Nb+n + O2, n = 1–6

Abstract Niobium cluster ions, Nb + n , are generated using a new laser desorption source. This source, floated at 5 kV, is coupled to a high resolution reverse geometry mas spectrometer. Mass selected cluster ions exiting the mass spectrometer are decelerated to several electronvolts and spatially focused on a small hole in the entrance plate to a high pressure (0–7 Torr), variable temperature (80–600K) drift reactor. The cluster ions gently drift through this 4 cm long cell under the influence of small electric fields. A helium buffer gas is used to thermalize the ions. Trace amounts of O 2 are added for the reaction studies. Ions exiting the drift reactor are analyzed using a quadrupole mass filter and ion counting techniques. The quadrupole mass filter has an upper mass range of about 600 dalton, limiting the current maximum cluster size to n = 6. All reactions of Nb + n with O 2 are found to be fast, proceeding at 40 ± 10% of the collision rate. For n = 2–6, the major (90%) product results from loss of NbO from the Nb + n · O 2 collision complex. For n = 3–6, other (small) competing channels include loss of NbO 2 and Nb atoms from the collision complex. Fast sequential reactions of the reaction products with O 2 are observed. An analysis of the arrival time distribution of Nb + at the detector indicates laser desorption forms ≈ 80% ground state (4d 4 , 5 D) and 20% excited state (4d 3 5s 1 , 5 F). Collisional deactivation by He occurs at a rate constant ⪡ 1 × 10 −15 cm 3 s −1 .

Infrared resonance-enhanced multiphoton ionization spectroscopy of magnesium oxide clusters

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 no...

The infrared spectrum of al+-benzene in the gas phase

Abstract 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.

Excitation of C 60 using a chirped free electron laser

Gas phase C60 is resonantly excited using picosecond infrared (IR) pulses from a free electron laser. The excitation can be very high, reaching levels where the thermal emission of electrons from C60 is observed. The excitation is much more efficient when the IR radiation is chirped to lower frequencies during the excitation process. The excitation process is modeled and the results are compared to the experiment.

INFRARED RESONANCE ENHANCED MULTI-PHOTON IONIZATION SPECTROSCOPY OF C84

Abstract Gas-phase C84 is resonantly excited using infrared (IR) radiation from a free electron laser. At specific wavelengths, strong ion signals are observed, resulting from thermionic emission of electrons by the fullerenes. The mass-selected ion yield is recorded as a function of IR frequency and the resulting spectrum yields information on the electronic ground state of the gas-phase molecule.