Detection of the Fraunhofer band B (690 nm) in the absorption spectra of oxygen in aerated solvents

IR phosphorescence (1270 nm) and chemical trapping of singlet 1Δg(0) molecular oxygen have been studied in aerated organic solvents under laser irradiation at 690 nm and 765 nm, in order to reveal the Fraunhofer band B [1Σg+(1)←3Σg−(0) transition] in the absorption spectra of dissolved oxygen molecules at normal conditions and compare its parameters with those of the previously observed Fraunhofer band A [1Σg+(0)←3Σg−(0) transition]. Chemical trapping experiments were found to be inappropriate for solution of this problem because complexes of the traps with oxygen were revealed, which were more photoactive under irradiation by red laser light than oxygen molecules themselves. Phosphorescence of singlet oxygen was reliably detected under laser irradiation of aerated solvents at 690 nm. It was about one order weaker than under excitation at 765 nm. However, in both cases, phosphorescence linearly depended on excitation power, equally increased after oxygen purging and built up in the solvents containing heavy atoms (bromine and iodine). Similar phosphorescence decay times were observed under pulsed laser excitation at both 690 and 770 nm. The data provide compelling evidence that phosphorescence arising under irradiation at 690 nm is due to excitation of the Fraunhofer band B in the oxygen absorption spectrum. The molar absorption coefficient at the peak of this band was estimated to be ≈5×10−5M−1cm−1 in the aerated solvents lacking heavy atoms. Thus, for the first time, absorption coefficients corresponding to the Fraunhofer band B of dissolved oxygen were estimated in aerated solvents under natural conditions. This information is important for both spectroscopy of oxygen and mechanistic studies of biological and therapeutic action of laser radiation.