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.

IR spectroscopy in catalysis

Femtochemistry at Metal Surfaces: Nonadiabatic Reaction Dynamics

The reversible rotation of a single isolated acetylene molecule between two diagonal sites on the Cu(100) surface at 8 K was induced and monitored with tunneling electrons from a scanning tunneling microscope (STM). Excitation of the C---H (C---D) stretch mode of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ ( ${\mathrm{C}}_{2}{\mathrm{D}}_{2}$) at 358 meV (266 meV) led to a 10-fold (60-fold) increase in the rotation rate. This increase is attributed to energy transfer from the C---H (C---D) stretch mode to the hindered rotational motion of the molecule. Inelastic electron tunneling spectroscopy with the STM provides the energies of the stretch modes and allows a quantitative determination of the inelastic tunneling and coupling probabilities.

Coupling of Vibrational Excitation to the Rotational Motion of a Single Adsorbed Molecule

Singlet-oxygen chemiluminescence in peroxide reactions.

Infrared absorption spectra of carbon monoxide in rare gas matrices

Medium and high resolution infrared spectra of CO trapped in solid Ne, Ar, Kr and Xe are presented. Spectra of low optical density samples are identical to those previously observed but a different assignment is proposed. Monomeric absorption is clearly identified by double doping experiments and by sample deposition through a nitrogen cold trap. In addition other broad weak bands are observed on the spectra of high optical density samples. These bands are assigned to combinations between internal vibration, libration and lattice vibrations. The barrier hindering rotation due to site distortion is estimated to 30–50 cm −1 . Librational motion is strongly perturbed by coupling with lattice motion. Carbon monoxide is also an efficient probe to study the dynamical properties of rare gas lattices.

Infrared spectra of carbon monoxide trapped in solid argon. Double-doping experiments with H2O, NH3 and N2

The infrared emission of CO trapped in solid Ne and Ar is observed at low temperature. The first vibrational level of 12 C 16 O is excited by a Q -switched frequency doubled CO 2 laser. The emission spectrum consists of several lines arising from upper vibrational levels of 12 C 16 O and also of 13 C 16 O and 12 C 18 O which are present in natural abundance. An interpretation is proposed which is based on the assumption that long range dipole—dipole interaction is the main physical process involved in these experiments. Resonance energy transfer produces an energy migration among 12 C 16 O molecules without any change in vibrational populations. Phonon assisted energy transfer takes place between vibrational levels of the various isotopic species present in the solution. In order to satisfy the resonance condition a phonon is emitted or absorbed whose energy compensates for the energy mismatch between the transitions in each interacting molecule due to isotopic effect and or vibrational anharmonicity. The range of this process is greatly extended by energy migration. At the low phonon bath temperature phonon emission is much more probable than phonon absorption. So a strong excitation of upper vibrational levels with in some cases population inversions is observed. Molecular impurities act as efficient quenching centers even at very low concentration. When highly purified samples are used, the fluorescence decay time is found to be 20.6 ms in Ne and 14.5 ms in Ar and does not significantly depend upon concentration and temperature. It is concluded that radiationless relaxation is unimportant.

Laser studies of vibrational energy transfer and relaxation of CO trapped in solid neon and argon

Vibrational spectroscopy of imperfect CO/Pd(111) surfaces obtained by adsorption between 150 and 230 K

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Infrared absorption spectra of carbon monoxide in rare gas matrices

Infrared absorption spectra of carbon monoxide in rare gas matrices

Infrared spectra of carbon monoxide trapped in solid argon. Double-doping experiments with H2O, NH3 and N2

Laser studies of vibrational energy transfer and relaxation of CO trapped in solid neon and argon

Vibrational spectroscopy of imperfect CO/Pd(111) surfaces obtained by adsorption between 150 and 230 K