HCO production, vibrational relaxation, chemical kinetics, and spectroscopy following laser photolysis of formaldehyde

Abstract

Formaldehyde vapor was photolyzed with a tunable pulsed uv laser. Flash kinetic absorption spectra of the HCO produced were recorded by intracavity dye laser spectroscopy with a time resolution of 1 μs. The energy threshold for radical production was confirmed to be at 86±1 kcal/mole. Photolysis at 294.1 nm produced HCO in its ground vibronic state (∼2/3) and with one quantum of vibrational excitation in either the bending (∼1/3) or CO stretching (10−1–10−2) vibrations. Observation of the CO stretching hot band absorptions allowed that frequency to be determined as 1868.4±1 cm−1. Quantitative, state-resolved measurements of concentration vs time were made in pure H2CO and in mixtures with O2, NO, or Ar. The vibrational relaxation rate for the bending vibration of HCO in collisions with H2CO was (4.3±1) ×10−12 cm3 molecule−1 s−1. Reaction rates for HCO+NO→HNO+CO and HCO+O2→HO2+CO were measured as (1.4±0.2) ×10−11 and (4.0±0.8) ×10−12 cm3 molecule−1 s−1, respectively. Approximate rates were determined for the radical–radical reactions H+HCO→H2+CO and 2HCO→H2CO+CO as 10−9.26±0.3 and 10−10.2±0.5, respectively.

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.…”

Combined quantum chemical and modeling study of CO hydrogenation on water ice

“…Gas-phase experiments devoted to studying reaction (1) (Reilly et al 1978;Dane et al 1988;Rumbles et al 1990;Sappey et al 1990) indicate, however, that only relatively energetic H-atoms produced by photolysis or radical-induced dissociation of H 2 CO may react with CO to produce HCO. The activation energy was also obtained through an Arrhenius plot between 260−340 K using fast Lyman-α spectrophotometry to be about 8.3 ± 2 kJ mol −1 (Wang et al 1973), whereas energy barriers calculated using quantum chemical methods span the 15−21 kJ mol −1 range, depending on the adopted method and level of theory (Woon 1996(Woon , 2002Marenich & Boggs 2003;Goumans et al 2007Goumans et al , 2008Peters et al 2013).…”

Laser Synthesis of Organic Materials in the Gas Phase: A Noble Method to Prepare Nanoparticles

“…The product yield increased with increasing partial pressure of H2CO (3 and 6 Torr) and with increasing laser intensity. As was expected, the results showed that the nucleation reaction of the aerosol particles was initiated through one-photon decomposition of H2CO molecule, although one-photon energy of N2 laser light is still a little lower than the threshold energy (86 kcal/mol) for the decomposition into H and HCO radicals [30][31][32].…”