Oxidation of Organic Films by Beams of Hydroxyl Radicals

Abstract

We have studied the oxidation of self-assembled monolayers (SAMs) of alkanes and alkenes with a thermal beam of OH radicals. The target films were produced by bonding alkane thiols and alkene thiols to a gold surface and the SAMs are mounted in a vacuum chamber at a base pressure of 10-9 Torr. Hydroxyl radicals were produced by a corona discharge in an Ar/H2O2/water mixture. The resultant molecular beam was scanned by an electrostatic hexapole and the OH radicals [4 (+/- 1) x 1011 OH radicals cm-2 sec-1] were focused onto the target SAM. All of the hydroxyl radicals impinging on the SAM surface are rotationally (J' ' </= 5/2) cold. The vibrational temperature of the radicals is estimated to be 1700-3400 K which implies that between 5% and 22% of the hydroxyl radical beam is OHv=1 and the remainder (95% to 78%) is OHv=1. The collision energy of the beam with the SAM is 333 cm-1 corresponding to a 485 K thermal beam. We employed reflection/absorption infrared spectroscopy (RAIRS) to monitor the reactivity of OH with an alkane and an alkene SAM. RAIRS demonstrated that a 10 min dose of OH radicals largely destroys the CH3- groups at the interface. This corresponds to a deposition of 2.4 x 1014 OH cm-2 or about 60% of the SAM monolayer. Oxidation of an undec-10-ene-1-thiol (HS(CH2)9HC=CH2) SAM with OH radicals proceeded more quickly with all the terminal alkenes, -CH=CH2, eliminated within 5 min following deposition of 1.2 x 1014 radicals. We believe that the OH radicals initiate a radical-induced polymerization of the alkene film.