Photodissociation of singlet oxygen in the UV region

Photodissociation of singlet oxygen, O-2 a(1)Delta(g), by ultraviolet radiation in the region from 200 to 240 nm has been investigated using velocity map imaging of the atomic oxygen photofragments. Singlet oxygen molecules are generated in a pulsed discharge and studied by one-laser photodissociation and detection around 226 nm as well as two color photodissociation at various wavelengths in the range from 200 to 240 nm. A simple model of the discharge on and off signal indicates efficient conversion of O-2 X-3 Sigma(-)(g) (v = 0) in the parent beam to O-2 a(1)Delta(g)(v = 0-2). Minute amounts of highly excited vibrational levels of ground state O-2 X-3 Sigma(-)(g) (v > 0) are detected but no evidence is found for production of the O-2 B-1 Sigma(+)(g) state. Over the decreasing wavelength range 240-200 nm the a(1)Delta(g)-state signal relative to the X-3 Sigma(-)(g)(v = 0) signal decreases strongly. Around 226 nm the a(1)Delta(g)(v = 0-2) states averaged branching ratio percentage for O(P-3(j) j = 2 : 1 : 0) is found to be 56 : 36 : 8 (+/- 5%), respectively. The anisotropy parameter for photodissociation of a(1)Delta(g)(v = 0-2) averages to beta = 1.3 +/- 0.4. The a(1)Delta(g)(v = 0) photodissociation cross section is found to 3-10 times stronger than theory predicts. Furthermore, the photodissociation image shows a strong parallel character, (i.e., transition moment parallel to the molecular axis) while theory predicts a predominantly negative character.