Direct measurements of absolute concentration and lifetime of singlet oxygen in the gas phase by electron paramagnetic resonance

doi:10.1016/j.cplett.2008.04.031

Chemical Physics Letters

Volume 457, Issues 4–6, 27 May 2008, Pages 312-314

https://doi.org/10.1016/j.cplett.2008.04.031 Get rights and content

Abstract

Electron paramagnetic resonance (EPR) spectroscopy was employed to determine the lifetime and absolute concentration of singlet oxygen, O₂(¹Δ_g), in the gas phase. O₂(¹Δ_g) molecules were generated by gas-phase photosensitization with octafluoronaphthalene. The absolute concentration of O₂(¹Δ_g) at total pressure of 0.3 Torr was estimated to be 3 × 10⁻⁶ mol dm⁻³ at 305 K under our conditions using the ground-state oxygen molecule, O₂( $^{3} \sum_{g}^{-}$ ), as a quantitative standard. The observed lifetime of O₂(¹Δ_g) is 7 s at 0.6 Torr. This is the first report of the gas-phase lifetime of O₂(¹Δ_g) observed by EPR.

Graphical abstract

EPR spectroscopy was employed to determine the lifetime and absolute concentration of singlet oxygen, O₂(¹Δ_g), in the gas phase.

Introduction

The lowest electronic configuration of molecular oxygen allows various combinations of orbital and spin angular momenta of the two outermost electrons. According to Hund’s rule, the electronic ground state is X $^{3} \sum_{g}^{-}$ , while a ¹Δ_g and b $^{1} \sum_{g}^{+}$ states lie 0.98 and 1.63 eV above the ground state. The lowest excited singlet state of oxygen molecule, O₂(¹Δ_g), plays a key role in many photochemical and photobiological processes. Researchers in several areas of science have studied the physical and chemical properties of O₂(¹Δ_g) for many years [1].

It is important to measure the concentration and lifetime of O₂(¹Δ_g) for most applications requiring O₂(¹Δ_g) as a reactive intermediate. In the case of the chemical oxygen–iodine laser (COIL), the O₂(¹Δ_g) generator is a key part of the COIL and the concentration of O₂(¹Δ_g) in the O₂(¹Δ_g) generator influences crucially the gain of COIL [2]. Various methods have been developed to measure the concentration of O₂(¹Δ_g) in the gas phase and in condensed phases [1]. The method most widely employed is the measurement of the 1.27 μm IR emission (a $^{1} Δ_{g} \to X^{3} \sum_{g}^{-}$ transition) [3], [4]. Although the transition a $^{1} Δ_{g} \to X^{3} \sum_{g}^{-}$ is strictly forbidden in the electronic dipole approximation, both by orbital and spin selection rules, it has been observed in the near-IR region. However, this method needs to be calibrated by some absolute method. On the other hand, O₂(¹Δ_g) is paramagnetic owing to its orbital angular momentum. Electron paramagnetic resonance (EPR) is the most reliable technique for determining O₂(¹Δ_g) concentration because O₂( $^{3} \sum_{g}^{-}$ ) is recommended as a quantitative standard [5], [6], [7], [8].

The lifetime of O₂(¹Δ_g) varies many orders of magnitude depending on its environment [1]. There are several methods for measuring O₂(¹Δ_g) lifetime. A new method for measuring O₂(¹Δ_g) lifetime in the liquid phase based on the time-resolved EPR detection of the spin polarization has been reported [9], [10], [11], [12], [13], [14]. To the best of our knowledge, the O₂(¹Δ_g) lifetime measurement in the gas phase by EPR has not been reported.

In the present work, we observed the EPR spectra of O₂(¹Δ_g) in the gas phase. The absolute concentration of O₂(¹Δ_g) was determined by using O₂( $^{3} \sum_{g}^{-}$ ) as a quantitative standard. We also observed the time dependence of the EPR signals of O₂(¹Δ_g) and O₂( $^{3} \sum_{g}^{-}$ ) to determine the lifetime of O₂(¹Δ_g). The preliminary results of EPR experiments to measure the relative concentration of O₂(¹Δ_g) have been given in a previous Letter [15].

Section snippets

Experimental

Octafluoronaphthalene (Tokyo Kasei) was used without further purification. The EPR spectra were measured at room temperature by a JEOL JES-FA200 X band spectrometer with 100 kHz magnetic field modulation. A JEOL ES-UCX2 universal cavity (≈9.2 GHz) and a Varian V-4535 large sample access cylindrical cavity (≈8.9 GHz) equipped with a homemade 100 kHz power amplifier were used as resonant cavities. The static magnetic field was calibrated with an Echo Electronics EFM-2000AX proton NMR gauss meter.

O₂(¹Δ

Absolute concentration of O₂(¹Δ_g)

Fig. 1 shows the EPR spectra of O₂(¹Δ_g) and O₂( $^{3} \sum_{g}^{-}$ ) observed at low pressures. As is clearly seen in Fig. 1, the characteristic four-line EPR spectra with relative intensity approximately 2:3:3:2 were observed during excitation. The basic features of the EPR spectrum for O₂(¹Δ_g) can be described in terms of an electronic orbital angular momentum along the internuclear axis, Λ. Together with the rotational angular momentum, they form the total angular momentum, J, which is quantized along the

Acknowledgements

The authors wish to express their thanks to the Instrumental Analysis Center, Yokohama National University, for the use of the EPR spectrometer. They also wish to thank Ms. Mayu Hirano, Mr. Masahito Moriya and Mr. Rensuke Koshiba of our laboratory for their help. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 15550009) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Direct measurements of absolute concentration and lifetime of singlet oxygen in the gas phase by electron paramagnetic resonance

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Absolute concentration of O2(1Δg)

Acknowledgements

Chem. Phys. Lett.

Chem. Phys. Lett.

J. Photochem. Photobiol. C

J. Mol. Spectrosc.

Chem. Phys.

J. Mol. Spectrosc.

Chem. Phys. Lett.

Chem. Rev.

Rev. Sci. Instrum.

Rev. Sci. Instrum.

J. Chem. Phys.

J. Am. Chem. Soc.

Absolute concentration of O₂(¹Δ_g)