▪ Abstract Laser ablation of in situ metals has recently made it possible to immerse a large number of different metal atoms and ions and small clusters of metal atoms in liquid helium (He) and thus study their absorption and emission spectra in the visible region. Atoms and molecules are readily picked up by large ( ≥ 103 atoms) He droplets, and their spectra are sensitively detected through the use of either beam depletion following absorption or laser-induced fluorescence. Within the past three years, a wide variety of molecules, ranging from OCS to large organic molecules such as amino acids and a number of van der Waals complexes and even large metal clusters, have been embedded in He droplets and studied either in infrared or in the visible region. These results are discussed here in detail, and the evidence for the effect of superfluidity on the spectral features is reviewed.

Spectroscopy of atoms and molecules in liquid helium

Near-Infrared Spectroscopy of Simple Hydrocarbons and Carbon Oxides Diluted in Solid N2and as Pure Ices: Implications for Triton and Pluto

The unique conditions forming atomic and molecular complexes and clusters using superfluid helium nanodroplets have opened up an innovative route for studying the physical and chemical properties of matter on the nanoscale. This review summarizes the specific characteristics of the formation of atomic clusters partly generated far from equilibrium in the helium environment. Special emphasis is on the optical response, electronic properties as well as dynamical processes which are mostly affected by the surrounding quantum matrix. Experiments include the optical induced response of isolated cluster systems in helium under quite different excitation conditions ranging from the linear regime up to the violent interaction with a strong laser field leading to Coulomb explosion and the generation of highly charged atomic fragments. The variety of results on the outstanding properties in the quantum size regime highlights the peculiar capabilities of helium nanodroplet isolation spectroscopy.

Formation and properties of metal clusters isolated in helium droplets

1. Electron Spin Resonance 2116 2. Infrared and Optical Absorption 2117 3. Laser-Induced Fluorescence 2119 III. Quantum Hosts 2122 IV. Matrix Isolation Spectroscopy of Molecular Ions 2124 A. Ion Sources for Matrix Isolation Spectroscopy 2124 B. Spectroscopic Methods 2126 1. Electron Spin Resonance 2126 2. Infrared and Optical Absorption 2126 3. Laser-Induced Fluorescence 2127 V. Deposition of Mass-Selected Species 2128 VI. Summary 2130

New Developments in Matrix Isolation Spectroscopy.

The relaxation of the higher-energy cis conformer of formic acid to the lower-energy trans form by a tunneling mechanism has been investigated in low-temperature rare gas matrices. In the temperature range 8–60 K, the tunneling takes place dominantly from the vibrational ground state of the cis form and the temperature dependence of the tunneling rate constant is influenced by the interactions with the environment. The temperature-dependent tunneling rates for HCOOH and DCOOH in solid Ar, Kr, and Xe are measured including data for molecules in different local environments within each host. It was found that the medium and the local environment has a significant influence on the tunneling rate.

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Cis-->trans conversion of formic acid by dissipative tunneling in solid rare gases: Influence of environment on the tunneling rate

Matrix-isolation spectroscopy, as a technique for studying unstable species, has had tremendous lasting powers. Since 1954, matrix-isolation has been able to assimilate new technology rather than being replaced by it. The matrix-isolation technique for producing and trapping new chemical species has been applied to an ever increasing range of chemical and physical problems since its inception. The last 12 years have seen a substantial number of new developments and applications of the matrix technique. The chapters in this book describe many of the contributions of matrix-isolation spectroscopy to chemistry and physics in the last decade, and as such predict the continued evolution of this technique over the next decade. Experimental techniques are generally closely related to the development of new apparatus, and each chapter has a section describing innovations in instrumentation such as the closed-cycle cryogenic cooler that has revolutionized the technique. In addition, each chapter describes many of the specialized methods used to prepare, trap and study particular new subject species.

Martin Moskovits

Papers

Chemistry and physics of matrix-isolated species