Photochemistry of Methyl Bromide on the α-Cr2O3(0001) Surface

Journal Article · Thu Sep 30 04:00:00 EDT 2010 · Surface Science, 604(19-20):1800-1807

DOI:https://doi.org/10.1016/j.susc.2010.07.008· OSTI ID:990528

Henderson, Michael A

The photochemical properties of the Cr-terminated α-Cr2O3(0001) surface were explored using methyl bromide (CH3Br) as a probe molecule. CH3Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr2O3(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH3Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~130 K. The CH3Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr3+ sites and near complete removal of CH3Br TPD states above 150 K. The photochemistry of chemisorbed CH3Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH3Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~10-22 cm2. Parent PSD was indicative of molecular photodesorption, but CH3 was also detected in PSD and Br atoms were left on the surface, both reflective of photo-induced CH3-Br bond dissociation. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr2O3, occurring at photon energies <3 eV, are not responsible for photodecomposition of 2 adsorbed CH3Br. It is unclear to what extent band-to-band versus direct CH3Br photolysis play in CH3-Br bond dissociation initiated by more energetic photons.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 990528
• Report Number(s): PNNL-SA-73192; 35219; KC0302010
• Journal Information: Surface Science, 604(19-20):1800-1807, Journal Name: Surface Science, 604(19-20):1800-1807 Journal Issue: 19-20 Vol. 604; ISSN 0039-6028
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ATOMS
CROSS SECTIONS
DESORPTION
DISSOCIATION
Environmental Molecular Sciences Laboratory
IRRADIATION
LIGHT BULBS
METHYL BROMIDE
OXYGEN
PHOTOCHEMISTRY
PHOTOLYSIS
PHOTONS
PLASMA
PROBES
PYROLYSIS
REMOVAL
SATURATION