Activation of oxygen molecules by 1070  nm laser radiation in aerated solvents

Population of the chemically active singlet ¹Δ_g(0) state of molecular oxygen occurring due to direct laser excitation of the ¹Δ_g(1)←³Σg-(0) transition has been observed for the first time, to the best of our knowledge, in oxygen molecules dissolved in organic solvents saturated with air under natural conditions (room temperature and normal atmospheric pressure). The data were obtained in 1 cm spectrophotometric cells due to the application of a set of high-power IR fiber and diode lasers. The rate of laser generation of the singlet (¹Δ_g(0)) states in oxygen molecules was monitored by a chemical trapping method. It was found that the action spectra of singlet oxygen generation have one distinct band with a maximum at 1070 nm and half-width of ∼10nm. The absorption coefficients at 1070 nm were shown to be 100-110-fold lower than those at the main oxygen absorption peak (1273 nm) corresponding to the ¹Δ_g(0)←³Σg-(0) transition. Under excitation at 810-1061 nm, very low trapping rates were observed, which did not depend on excitation wavelengths being probably caused by thermal effects. There was no reliable increase in the trapping rate under irradiation at 810 and 920 nm corresponding to the ¹Δ_g(2,3)←³Σg-(0) transitions. This fact suggests that absorbance corresponding to these transitions is much lower than that at 1070 nm. The obtained results are important for both spectroscopy of oxygen and mechanistic studies of biological and therapeutic action of laser radiation.