Singlet O2 Formation by O-atom Recombination on Fused Quartz Surfaces

Abstract

Experimental results are presented that demonstrate O2(a 1g) ( = 0) formation by catalytic O-atom surface recombination on the walls of a room-temperature fused-quartz flow-tube reactor. Resonance-enhanced multiphoton ionization (REMPI) is used to detect O2(a 1g) ( = 0) downstream of a nitrogen discharge flow titrated with nitric oxide to introduce oxygen atoms. A calibration procedure based on ozone photodissociation is developed to quantify O2(a 1g) ( = 0) REMPI signals. Partial pressures of O2(a 1g) ( = 0) in the range of 2.9 to 14 Torr are measured in the REMPI cell for O-atom partial pressures of 1.4to 2.9-mTorr atomic oxygen introduced at the titration port. O2(a 1g) ( = 1) could not be detected; an upper limit for the O2(a 1g) ( = 1) partial pressure is 1/5 of the O2(a 1g) ( = 0) partial pressure. A simple chemical kinetics model demonstrates that measured O2(a 1g) pressures cannot be explained by gas-phase chemistry alone and must involve O atoms participating in surface reactions. We find that collisional deactivation of O2(a 1g) on the tube walls must be included to satisfactorily model the experimentally observed pressures and trends. Modeling results also suggest that the O2(a 1g) surface production yield is 10% or more.