Dynamics of Singlet Oxygen Molecule Trapped in Silica Glass Studied by Luminescence Polarization Anisotropy and Density Functional Theory

Abstract

The lowest excited electronic state of the O2 molecule, a 1 g, the "singlet oxygen", is of utmost importance for photochemistry and photobiology. For O2 trapped in silica glass, the lifetime of this state and the associated a 1 g ⟶ X 3 Σg − photoluminescence (PL) is the longest known for O2 in any condensed medium at room temperature. We studied the temperature dependence, decay kinetics and polarization anisotropy of this PL with 1064 nm excitation to the a 1 g (v=1) state, as well as with excitation to higher energies. PL at this excitation shows non-zero polarization anisotropy at 295 K, which increases with cooling to 14 K. At variance, excitation to higher energies yields depolarized PL. Polarization data indicate weak electric dipole character of the emission of the spin-and parityforbidden a 1 g ⟶ X 3 Σg − transition, enabled by O2-SiO2 cage interactions. Density functional theory calculations indicate that at low temperatures the rotation of O2 is partially or fully frozen even in large silica voids. As the temperature increases, PL is increasingly depolarized by libration movement of O2 molecules. Analysis of O2 optical absorption in optical fibers allows one to obtain the absorption cross sections of X⟶a and X⟶b transitions of O2 in SiO2 glass and to evaluate both radiative and non-radiative rates of a⟶X luminescence. * The 1st index (capital H or V) denotes the excitation polarization direction, the 2nd one (lower case h or v)-the PL emission polarization direction.

Optical Absorption of Excimer Laser‐Induced Dichlorine Monoxide in Silica Glass and Excitation of Singlet Oxygen Luminescence by Energy Transfer from Chlorine Molecules

“…The PL intensity was stable in time, its photobleaching or enhancement under 355 nm laser irradiation was not observed. Furthermore, this PL band was observed also with 385 nm (3.22 eV) CW LED excitation (trace 3) and—as expected—with the previously reported (e.g., the studies by Bregnhøj et al and Skuja et al [ 18,19 ] ) “direct” 766 nm (1.618 eV) laser excitation of O 2 (trace 2).The slight differences in the PL band fwhm and peak positions evident in Figure 6 are not meaningful at the present accuracy level. The larger measured halfwidth (fwhm) of PL excited at 355 nm (trace 1) is probably due to lower spectral resolution in this case.…”

“…This PL band is a fingerprint of widely studied species, an excited metastable singlet state O 2 molecule (see, e.g., the article by Bregnhøj et al [ 18 ] for overview and references). It is much studied in silica as well (e.g., the article by Skuja et al [ 19 ] ). Its excitation in 300–400 nm spectral region was unexpected, since O 2 molecule has no electronic states there.…”

“…Studies of direct excitation to the two lowest electronic excited states (optical transitions at 0.97V and 1.62 eV) are less numerous (see review [ 28 ] ). In the case of interstitial O 2 in silica the situation is reverse: in numerous past studies of bulk glass (see the article by Skuja et al [ 19 ] for references) or nanoparticles, [ 29 ] only the direct photoexcitation of O 2 has been reported for photon energies below 6.4 eV. However, 1 O 2 PL in silica is excited also in processes involving excitons or electron–hole recombination, caused by electron‐ [ 30 ] or X‐ray [ 31 ] irradiation, and in photolysis of interstitial O 3 molecules.…”

“…The possibility that both Cl 2 and O 2 molecules can initially reside in a single large void, can be additionally examined by PL decay measurements (Figure 8). The decay of singlet O 2 PL in “dry” silica (decay constant τ ≈850 ms) at room temperature is the slowest among observations of O 2 PL in any condensed matter matrix [ 19 ] because the rate of 1 O 2 nonradiative relaxation in SiO 2 matrix is record‐low. All other matrices cause higher nonradiative rates.…”

“…Sample "A" was irradiated in vacuum for 120 s with 157 nm (7.9 eV) photons using F 2 excimer laser (Lambda Physik LPF-210) emitting ≈20ns long pulses with power density 19.6 mJ/cm 2 at repetition rate 50 pulses/s. That yielded the cumulative fluence of 11.74 J/cm 2 or 9.27×10 19 photons/cm 2 . To remove the slight surface contamination caused by photolysis of residual vapors in the vacuum chamber, the sample was etched in diluted (1%) HF for 30 s. Optical absorption spectra were obtained using Hamamatsu 10082CAH CCD spectrograph with spectral resolution 1.5 nm and deuterium lamp (Ocean Optics).…”