Temperature dependence of O2 singlet photoluminescence in silica nanoparticles

The near infrared singlet emission and photoluminescence lifetime of O2 molecules embedded in silica nanoparticles are studied from room temperature down to 10 K. The area of the photoluminescence band under infrared excitation decreases for temperature above 100 K and the lifetime is shortened. These observations provide evidence of a thermally activated relaxation channel with activation energy of about 40 meV. This relaxation mechanism adds to the already known temperature independent electronic-to-vibrational coupling involving high energy vibrational modes of the host matrix or its impurities. The thermally activated process is suggested to consist in the breakage of the O2 molecule weak bonds with walls of the matrix interstices with ensuing molecular motion and collisional exchange of energy.