Michael A. Duncan

Laser production of supersonic metal cluster beams

Cold beams of metal clusters are produced by combining a laser vaporization technique with pulsed supersonic nozzle technology. (AIP)

Infrared spectroscopy to probe structure and dynamics in metal ion-molecule complexes

Vibrational spectroscopy measurements are described for mass-selected metal cation-molecular complexes using the technique of infrared resonance-enhanced photodissociation (REPD) spectroscopy. We discuss the mechanism of the REPD process in the infrared and how multiphoton techniques or rare gas tagging can be employed to facilitate dissociation processes in strongly bound complexes. Spectra are reported for Fe+(CO2)n complexes that demonstrate the formation of coordination spheres around the metal cation and for Mg+(CO2)n complexes, where the interaction with theory makes it possible to obtain structures for small clusters. Both of these studies investigate the asymmetric stretch region of CO2. Ni+(C2H2)n complexes are studied in the C-H stretch region, demonstrating evidence for intracluster cyclization reactions. Finally, experiments are described for metal ion-benzene (M = V, Al) complexes excited in the benzene ring distortions and C-H bending region. These experiments confirm the ~ -bonded configuration of these complexes.

Frontiers in the spectroscopy of mass-selected molecular ions

Abstract Recent developments have made it possible to produce a variety of novel ionic molecular species and to study their spectroscopy. In this article, new ion production schemes and/or modified ion sources for these experiments are discussed, as are various mass spectrometry configurations used for mass selection. New kinds of ions and new forms of sensitive and selective spectroscopy in the ultraviolet, visible, and infrared wavelength regions for these ions are also reviewed.

Two‐color photoionization of naphthalene and benzene at threshold

We observe the resonant two‐photon ionization (R2PI) spectrum of naphthalene and benzene at and just above the first ionization potential in a supersonic molecular beam. For naphthalene, the spectrum consists of direct ionization step functions indicating the relative Franck–Condon factors for transitions into the ground state ion. The spacings observed between these steps measure the ν8 vibrational frequency in the naphthalene ion. The electric field dependence of these steps is found to be consistent with a field ionization red shift of the thresholds. In benzene we observe similar direct ionization. Superimposed on the threshold steps, however, are broad features (∼100 cm−1) due to vibrational autoionization of Rydberg states at this same energy. Implications of this data on the observation of high Rydbergs in such large organic molecules are discussed.

Resonance enhanced two‐photon ionization studies in a supersonic molecular beam: Bromobenzene and iodobenzene

The 1B2(ππ*)←1A1 absorptions of two monosubstituted halobenzenes have been investigated using resonance enhanced two‐photon ionization in a pulsed supersonic molecular beam. Detection of the photoions was accomplished by means of a time‐of‐flight mass spectrometer. The 1B2←1A1 system of bromobenzene has been observed with good sensitivity using this technique, even though the total decay rate of the 1B2 state is greater than 1×1011 sec−1. No ion signal was observed when the same transition was probed in iodobenzene, allowing us to place a lower limit on its decay rate of 4×1013 sec−1.

Photoionization Dynamics and Abundance Patterns in Laser Vaporized Tin and Lead Clusters

Tin and lead clusters are produced by laser vaporization in a pulsed nozzle source and studied with laser photoionization mass spectroscopy. ‘‘Magic numbers’’ are observed in both cluster size distributions under a variety of laser wavelength and power conditions which can be understood in terms of ionization thresholds, relative ionization cross sections, and multiphoton‐induced fragmentation. After investigation of the photoionization dynamics, relative abundances of different sized clusters are estimated. Abundance patterns of tin and lead clusters are compared to those reported previously for other group IV elements (C, Si, Ge) to investigate the role of periodicity in cluster growth and bonding properties. Especially abundant 10‐atom cluster species are observed for both tin and lead, as has been observed previously for both silicon and germanium. Other features not observed for silicon and germanium, such as abundance patterns characteristic of atom closepacking geometries, are observed to a limited...

Efficient multiphoton ionization of metal carbonyls cooled in a pulsed supersonic beam

Abstract Laser excitation of Fe(CO)5, Cr(CO)6 and Mo(CO)6 in the region of the charge transfer bands around 2800 A is found to produce Fe+, Cr+ and Mo+ with near unit efficiency in a laser flux as low as 108 W cm−2. The uncomplexed metal ion continues to dominate the photoion yield down to near threshold at a laser flux of 3 × 105 W cm−2. Excitation spectra of this photoion production indicate the absorption in this region is intrinsically diffuse.

Reflectron time‐of‐flight mass spectrometer for laser photodissociation

We describe a new reflectron time‐of‐flight mass spectrometer configuration for laser photodissociation of mass‐selected ions and the initial performance characteristics observed for this instrument. Ions are produced by laser photoionization within the acceleration region of the instrument or by laser vaporization in an external pulsed‐nozzle cluster ion source. Mass selection is accomplished with pulsed deflection plates at the end of an initial drift section. Laser photodissociation of selected ions takes place at the turning point in the ion trajectory in the reflectron. The transit time through a second drift section defines the fragment ion masses. Optimized operating conditions and the role of mass discrimination in this instrument are discussed.

Invited Review Article: Laser vaporization cluster sourcesa)

The laser vaporization cluster source has been used for the production of gas phase atomic clusters and metal-molecular complexes for 30 years. Numerous experiments in the chemistry and physics of clusters have employed this source. Its operation is simple in principle, but there are many subtle design features that influence the number and size of clusters produced, as well as their composition, charge state, and temperature. This article examines all aspects of the design of these cluster sources, discussing the relevant chemistry, physics, and mechanical aspects of experimental configurations employed by different labs. The principles detailed here provide a framework for the design and implementation of this source for new applications.

Photodissociation spectroscopy of Mg+–H2O and Mg+–D2O

Mg+–H2O ion–molecule complexes are produced in a pulsed supersonic nozzle cluster source. These complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time‐of‐flight mass spectrometer system. An electronic transition assigned as 2B2←X 2A1 is observed with an origin at 28 396 cm−1. The spectrum has a prominent progression in the metal‐H2O stretching mode with a frequency (ω’e) of 518.0 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D’0) at 15 787 cm−1. The corresponding ground state value (D‘0) is 8514 cm−1 (24.3 kcal/mol). The solvated bending mode, and symmetric and asymmetric stretching modes of water are also active in the complex, as are the magnesium bending modes. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30 267 cm−1 and a metal stretch frequency for Mg+–H2O of 488.5 cm−1 (ΔG1/2). Spectra of both excited states are also observed for Mg+–D2O. Partially resolved rotational struc...