Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.15 K (3He droplets), lower than possible in most solid matrices. Thus the helium-droplet technique combines the benefits of both the gas phase and the classical matrix-isolation techniques. Most important, the superfluid helium facilitates binary encounters, and absorbs the released binding energy upon recombination. Thus the droplet can be viewed as an isothermal nanoscopic reactor, which isolates single molecules, clusters, or even a single reactive encounter at ultralow temperatures.

Superfluid Helium Droplets: A Uniquely Cold Nanomatrix for Molecules and Molecular Complexes

Electron Transfer Past and Future (R Marcus) Electron Transfer Reactions in Solution: A Historical Perspective (N Sutin) Electron Transfer--From Isolated Molecules to Biomolecules (M Bixon & J Jortner) Charge Transfer in Bichromophoric Molecules in the Gas Phase (D Levy) Long--Range Charge Separation in Solvent--Free Donor--Bridge--Acceptor Systems (B Wegewijs & J Verhoeven) Electron Transfer and Charge Separation in Clusters (C Dessent, et al) Control of Electron Transfer Kinetics: Models for Medium Reorganization and Donor--Acceptor Coupling (M Newton) Theories of Structure--Function Relationships for Bridge--Mediated Electron Transfer Reactions (S Skourtis & D Beratan) Fluctuations and Coherence in Long--Range and Multicenter Electron Transfer (G Iversen, et al) Lanczos Algorithm for Electron Transfer Rates in Solvents with Complex Spectral Densities (A Okada, et al) Spectroscopic Determination of Electron Transfer Barriers and Rate Constants (K Omberg, et al) Photoinduced Electron Transfer Within Donor--Spacer--Acceptor Molecular Assemblies Studied by Time--Resolved Microwave Conductivity (J Warman, et al) From Close Contact to Long--Range Intramolecular Electron Transfer (J Verhoeven) Photoinduced Electron Transfers Through sigma Bonds in Solution (N--C Yang, et al) Indexes

Electron transfer : from isolated molecules to biomolecules

▪ 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

A rotationally resolved infrared spectrum has been obtained for S${\mathrm{F}}_{6}$ molecules captured inside of helium clusters of approximately 4000 atoms. The spectrum is interpreted in terms of a spherical top Hamiltonian, although the resulting rotational constant is considerably smaller than that of free S${\mathrm{F}}_{6}$. This can be attributed to the interaction between the molecule and the surrounding helium liquid. The results indicate that the S${\mathrm{F}}_{6}$ is located inside the helium cluster and a fit to the spectrum yields a rotational temperatures for the perturbed S${\mathrm{F}}_{6}$ of $0.37\ifmmode\pm\else\textpm\fi{}0.05$ K.

Rotationally Resolved Spectroscopy of SF6 in Liquid Helium Clusters: A Molecular Probe of Cluster Temperature.

Helium nanodroplets have emerged as a new and exciting medium for studying the structure and dynamics of both this quantum solvent and impurities that can be doped into (onto) and grown inside (on the surface) of the droplets. Spectroscopic studies of these molecular impurities can provide detailed information on helium as a solvent and its interaction with the solute. This is particularly important given that helium is completely transparent to photons below 20 eV, making the direct spectroscopic study of liquid helium problematic. Since liquid helium is an extremely weak solvent, the corresponding perturbations to the spectrum of the solute molecules are often minor; really only evident because of the high resolution that is often achieved in such studies. As a result, helium nanodroplet spectra often resemble the corresponding gas-phase results. Indeed, for the case of rotational spectroscopy, the gas-phase Hamiltonian is often sufficient to describe the system, with the effects of the solvent being to...

Infrared spectroscopy of helium nanodroplets: novel methods for physics and chemistry

The scattering of He atoms from liquid Hen-clusters withn=10 to 1000 at energies between 0.01 and 5.0 meV has been investigated by calculating integral cross sections for elastic scattering, absorption scattering and vibrational excitation of the liquid drop vibrations using a hard core potential, a transparent core potential, a black core and an optical model potential. The implications for planned scattering experiments are discussed.

Liquid drop and optical model calculations of elastic, vibrationally inelastic and absorptive scattering of 4 He atoms from 4 He n -clusters

The embedding of single SF 6 molecules in large liquid 4 He clusters (N≃3×10 3 atoms) and their ion molecule reactions with He + within the clusters are studied in a molecular beam apparatus equipped with a sensitive mass spectrometer. After passing the He cluster beam (stagnation conditions: P 0 =80 bar, T 0 =24 K) through a scattering chamber filled with SF 6 , the electron impact mass spectra reveal essentially only the ion fragments SF 5 + and SF 6 + in addition to the usual He cluster ion fragments

Anomalies in the reactions of helium(1+) with sulfur hexafluoride embedded in large helium-4 clusters

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Liquid drop and optical model calculations of elastic, vibrationally inelastic and absorptive scattering of 4 He atoms from 4 He n -clusters

Anomalies in the reactions of helium(1+) with sulfur hexafluoride embedded in large helium-4 clusters