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.

An overview is presented of experimental and theoretical research in the field of physics and engineering of singlet delta oxygen (SDO) production in low-temperature plasma of various electric discharges. Attention is paid mainly to the SDO production with SDO yield adequate for the development of an electric discharge oxygen–iodine laser (DOIL). The review comprises a historical sketch describing the main experimental results on SDO physics in low-temperature plasma obtained since the first detection of SDO in electric discharge in the 1950s and the first attempt to launch a DOIL in the 1970s up to the mid-1980s when several research groups started their activity aimed at DOIL development, stimulated by success in the development of a chemical oxygen–iodine laser (COIL). A detailed analysis of theoretical and experimental research on SDO production in electric discharge from the mid-1980s to the present, when the first DOIL has been launched, is given. Different kinetic models of oxygen low-temperature plasma are compared with the model developed by the authors. The latter comprises electron kinetics based on the accompanying solution of the electron Boltzmann equation, plasma chemistry including reactions of excited molecules and numerous ion–molecular reactions, thermal energy balance and electric circuit equation. The experimental part of the overview is focused on the experimental methods of SDO detection including experiments on the measurements of the Einstein coefficient for SDO transition and experimental procedures of SDO production in self-sustained and non-self-sustained discharges and analysis of different plasma-chemical processes occurring in oxygen low-temperature plasma which brings limitation to the maximum SDO yield and to the lifetime of the SDO in an electric discharge and its afterglow. Quite recently obtained results on gain and output characteristics of DOIL and some projects aimed at the development of high-power DOIL are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/40/2/R01/pdf

Physics and engineering of singlet delta oxygen production in low-temperature plasma

This communication reports on the experimental observation of the shortening of the induction zone length in a premixed mode of combustion in a subsonic H2–O2 low pressure flow due to the presence of oxygen molecules excited to the singlet a 1Δg electronic state. The low pressure electric glow discharge was used to produce singlet oxygen molecules. The analysis showed that even a small number of O2(a 1Δg) molecules (~1%) in the H2–O2 mixture allows one to noticeably reduce the ignition delay length and to ignite the mixture at a lower temperature. The results obtained exhibit the possibility to intensify the combustion of a hydrogen–oxygen mixture by means of excitation of O2 molecules by electrical discharge at low pressure (P = 10–20 Torr).

https://iopscience.iop.org/article/10.1088/0022-3727/41/19/192001/pdf

On the influence of electronically excited oxygen molecules on combustion of hydrogen–oxygen mixture

Multireference configuration interaction (MRCI) calculations have
been performed for the dipole transition moments of the Noxon band,
b
 1
Σ
 g

 +
–a
 1

Δ
 g
, and for the red and IR atmospheric emission
bands,
b
 1
Σ
 g

 +
–X
 3

Σ
 g

 -
 and
a
 1
Δ
 g
–X
 3
Σ
 g


 -
, in O
 2
 in collision complexes with
different solvent molecules. The spin–orbit coupling between the
b
 1
Σ
 g

 +
 and
X
 3
Σ
 g

 -
(M
 S

=0) states does not change after collision so the a–X
transition borrows intensity from the collision-induced Noxon band
b–a, and the b–X band borrows electric dipole transition
probability from the collision-induced difference dipole moments of
the b and X states. The calculations show that the b–a,
a–X and b–X transition probabilities are enhanced by
ca. 10
 5
, 10
 3
 and 1.2 times by
O
 2
+H
 2
 collisions. An additional order of
magnitude enhancement for all three transitions is possible for
solvent molecules with larger polarizability than that of molecular
hydrogen. The collision-induced b–a transition dipole moment
depends not only on the intermolecular distance, but also on the
internuclear O–O distance. Since a–X and b–a
transition moments are directly connected
(D
 a,X
=0.013iD
 b,a
), the strong
dependence on the O
 2
 bond length leads to an additional
enhancement of the (0,1) vibronic band intensity of the
a
 1
Δ
 g
–X
 3
Σ
 g


 -
 transition. The dipole moments of the b and X
states are also shown to depend on the O
 2
 bond length in
the collision complex, which leads to additional collision-induced
enhancement of the (0,1) band of the b–X system.

Collision-inducedb1Σg+–a1Δg,b1Σg+–X3Σg- anda1Δg–X3Σg- transition probabilities in molecularoxygen

Collision-induced b 1 Σ g + –a 1 Δ g , b 1 Σ g + –X 3 Σ g - and a 1 Δ g –X 3 Σ g - transition probabilities in molecular oxygen

Emission of the 0–0 and 0–1 bands of the a 1Δ → X 3Σg− system of molecular oxygen has been studied over the pressure range 0.5–25 Torr in a flow system using a Fourier-transform spectrometer. At high oxygen pressures and on the addition of NO, continuous emissions of O2(a): M collision complexes were found to appear in both bands beneath the discrete rotational lines of the magnetic dipole transitions. In both cases, the continua are distinctly broader than the unresolved low-pressure a → X bands. In the presence of Ar, Kr, Xe, N2, CO2 and SF6 narrower continua were found, but only in the 0–0 band. Bimolecular rate constants for the collision-induced emission processes were derived from the ratios of the intensities of the continua and of the discrete lines. For O2 as collision partner, the rate constants at 298 K for the 0–0 and 0–1 bands were found to be (3.4 ± 0.3) × 10−23 and (1.4 ± 0.1) × 10−23 cm3 s−1, while for NO they were (1.72 ± 0.03) × 10−21 and (9.2 + - 0.7) × 10−23 cm3 s−1.

Collision-induced emission of O2 (a 1Δg → X 3Σg−) in the gas phase

The collision-induced radiation of O2(a 1Δg)

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Collision-induced emission of O2 (a 1Δg → X 3Σg−) in the gas phase

The collision-induced radiation of O2(a 1Δg)