The photochemistry of formaldehyde: Absolute quantum yields, radical reactions, and NO reactions

Abstract

Articles you may be interested inUltraviolet photochemistry of buta-1,3-and buta-1,2-dienes: Laser spectroscopic absolute hydrogen atom quantum yield and translational energy distribution measurements Absolute quantum yields have been measured for the gas phase photolysis of H,CO and D,CO as a function of excitation energy. Over the spectral region examined, 290 nm ::;,,::; 360 nm, formaldehyde can dissociate into either molecular or free radical products: H,CO + h v-lH, + CO or H,CO+hv-lH+HCO. For H,CO the total photochemical quantum yield was <1>'0101= 1.0±0.1 for ,,::;326 nm and 0.68 at ,,= 353 nm. For D,CO, <I>'otal = 1.0 ±0.1 for,,::; 331 nm and drops to zero near the band origin. The mechanism of formyl radical recombination in pure formaldehyde was shown to be 2HCO-lH,CO+CO. Addition of NO to the photolysis mixture allowed determination of the radical/molecule branching ratio by introducing the scavenging reactions: HCO + NO-lHNO + CO, and H+NO+M-lHNO+M. The threshold for production of radicals was found to be between 339 nm and 330 nm for H,CO, and between 326 nm and 320 nm for D,CO. Radical quantum yields increase steeply at the threshold and remain constant near 0.4 at higher energies.1264

Spectroscopy of the transition of formaldehyde in the 30140–30790cm−1 range: The and rovibrational bands

“…Other experiments studied the quantum yield of the dissociation processes following the UV excitation [7,8,9,10,11,12]. These cross sections and quantum yields are important parameters for the description of photochemistry in the atmosphere induced by the sunlight as discussed in the introduction.…”

“…Formaldehyde is present in the atmosphere at concentration of ∼ 50 pptv (parts per trillion by volume) in clean tropospheric air [2] and up to 10-70 ppbv (parts per billion by volume) in the air in urban centers [3,4]. By excitation of theà 1 A 2 ←X 1 A 1 transition in the 260-360 nm wavelength range two dissociation channels [5,6] with high quantum yields [7,8,9,10,11,12] are open: (a) H 2 CO+hν → H 2 + CO and (b) H 2 CO+hν → H + HCO. The reaction channel (a) opens at wavelengths <360 nm, reaction channel (b) opens at wavelengths <330 nm.…”

Formaldehyde Photochemistry

“…The re is no long li ved in te rme di ate forme d in th e colli sion-fr ee de ca y of H2CO . For D2CO , the smalle r Sl -S o cou pl i ng ma tri x ele ments (2 9, 36, 39) and smalle r tu nneli ng ra te s (33) lea d to the primaril y radiative de ca y obse rved fo r the 4 0 and 4 1 le vels (47,58) as we ll as to the low photochem ic al quantum yi elds (20). Most of the en ergy re lea sed upon di ssocia ti on to H2 + CO (45-85%) (56) is fou nd in tra nsla tion.…”

A simple formulation of the CH&lt;sub&gt;2&lt;/sub&gt;O photolysis quantum yields

“…In particular, we hope to eventually construct expressions of the quantum yields for CHDO for which -apart from the threshold energies and a few isotope fractionation factors -no direct measurements exist. Our analysis of the quantum yields will be based on the data filed by JPL (Sander et al, 2011) andIUPAC (2006) omitting all measurements whose wavelength dependencies deviate strongly from the forms recommended by JPL or IU-PAC (e.g., McQuigg and Calvert, 1969;Clark et al, 1978;Tang et al, 1979, for rad ). Likewise, if measured data appear in several publications by the same authors, only the latest data were considered.…”