Photolysis of Acetone at 266 nm: Emission of CO(v = 1 ¨C 6)detected by Time-Resolved Fourier Transform Spectroscopy.

MAXIMILIANO A. BURGOS PACI ; MARK SAUNDERS; GUS HANCOCK

Ciudad de Tandil, Pcia. de Bs. As. Argentina

Congreso; XV Congreso Argentino de Fisicoqu¨ªmica y Qu¨ªmica Inorg¨¢nica; 2007

Universidad de General Sarmiento y Universidad de Buenos Aires

Upon photolysis of acetone in the gas phase, infra-red emission of CO (¦Í <6) in the spectralregion 1900 ¨C 2300 cm-1 was detected with a step-scan time-resolved Fourier transformspectrometer (TR-FTIR).A Nd:YAG laser (Spectron SL803-10) was employed to generate ~ 800 mJ per pulse 1064 nmradiation, which was frequency doubled twice by two temperature controlled KD*P crystals toobtain ~ 40 mJ of 266 nm radiation. The 266 nm emission, which pulsed at a frequency of 10 Hzand had a typical beam of area ~ 1 cm2 , was multi-passed in a Welsh cell in order to photolyseacetone, typically at pressures of 0.5 ¨C 5 Torr, in the presence of excess Ar. The IR emissionfrom vibrational excited products was collected with Welsh cell optics and then focused into acommercial step-scan FTIR spectrometer (Bruker, IFS 66 S). Emission in the region 1800 ¨C4000 cm-1 was detected using a liquid nitrogen cooled InSb detector (Graseby Infrared IS-2, D*= 2.26 x 10 cm Hz1/2 W-1 ) which had a response time of approximately 2 ¦Ìs.At 266 nm energy, the accessible transition for acetone is ¦°* ¡û n which yields:      CH3C(O)CH3              ¡ú                         CH3C(O) + CH3   We observed no emission near 3000 cm-1 from CH3 or CH3CO radicals vibrationally excited. It islikely that the primary reaction product, CH3CO absorbs a 266 nm photon and dissociates to COand CH3, the CO fragment being vibrationally excited, with emission observed for CO(v = 1 ¨C 6)above the level of noise.      CH3C(O)                     ¡ú                          CH3 + COThe measured vibrational distribution was found to decrease monotonically as function ofincreasing vibrational quantum number. The distribution is considerably hotter than thatpreviously determined for photolysis at 193 nm. We determine a Boltzman temperature of (8300¡À 800) K, which is much higher than the typical Boltzman temperature of (2100 ¡À 600) K for thephotolysis at 193 nm. It should be mentioned that at 193 nm, acetone is completely photolysedto CO and CH3 in one pulse with a quantum yield > 0.95. The energy excess at 193 nmphotolysis is distributed into vibrations of CO and CH3. More energy but lower Boltzmanntemperature in CO at 193 nm than at 266 nm is perhaps evidence that different mechanismsoperates in both cases.