For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

We report observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom–silicon clusters. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and F2 laser photoionization time‐of‐flight mass spectrometry. Three group‐VIB transition metals and copper have been investigated. The dominant product cluster peaks observed in the mass spectra obtained for all three group VIB metals corresponds to identical but remarkable cluster stoichiometries. The dominant product peaks have formulas given by MSin where n=16. Copper results are different than the other three metals, indicating the importance of the metal valence electronic structure to the chemistry. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interfa...

Mixed metal–silicon clusters formed by chemical reaction in a supersonic molecular beam: Implications for reactions at the metal/silicon interface

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.

Invited Review Article: Laser Vaporization Cluster Sources

,\\ . \\ Krlshnan Baiasukamanian has been an Assmiate Professw of CblIWty slna, 1987 at Arkma State UntverSHy. Tempe. AZ. He was born in Bangalore. Indla. in 1956. He received his MSc. (Hawrs) desee from the &! a 1e of Techndogy and sdence. Pilani. India. in 1977 and his M.A. and Ph.D. degrees from The Johns Hopkims LhiverSny in 1980. He was a pcstdoctaai assodate and visiting lectuer in ttm Depamnent of Chemishy and Lawrence Berkeley Laboratwy. University of California. Berkeley. from 1980 to 1983. From 1983 1

Spectroscopic constants and potential-energy curves of heavy p-block dimers and trimers.

Some aspects of the field evaporation behaviour of GaSb

Neutral and singly charged anion or cation clusters of antimony and bismuth are produced by laser vaporization in a pulsed nozzle cluster source. Neutral clusters are photoionized near threshold with a fluorine excimer laser, while ionized clusters are sampled directly from the laser vaporization plasma. All species are mass analyzed with a time‐of‐flight spectrometer. Abundance patterns in these systems exhibit ‘‘magic numbers’’ that are strikingly charge dependent. Compared at the same charge, corresponding neutral and cation spectra of these metals are quite similar, but significant differences arise in the two anion cluster distributions. Neutral cluster growth in these systems ends abruptly at five atoms while the ionized distributions fall off gradually out to beyond 12–15 atoms. These unique patterns are examined in the context of cluster growth dynamics and suspected cluster stability based on the predictions of simple electron counting models.

Neutral and ionic clusters of antimony and bismuth: A comparison of magic numbers

Antimony and bismuth cluster cations are produced by laser vaporization in a pulsed nozzle source and probed with mass spectroscopy and mass selected photofragmentation at 248 nm. Mass spectral abundances are interpreted to reflect cation stability patterns. Fragmentation products and branching ratios are consistent with the production of stable neutral and cation molecular fragments, suggesting a statistical dissociation mechanism.

M. E. Geusic

Papers

Neutral and ionic clusters of antimony and bismuth: A comparison of magic numbers

Photofragmentation of antimony and bismuth cluster cations at 248 nm