L31. UV Absorption Spectrum of Formyl Radical, HCO
Evgeni N. Chesnokov and Lev N. Krasnoperov
Department of Chemical Engineering
Chemistry and
Environmental Science
New Jersey Institute of Technology
University Heights
Newark, NJ 07102
E-mail: Chesnok@ns.kinetics.nsc.ru
E-mail: Krasnoperov@adm.njit.edu
Formyl radical, HCO, plays important roles in hydrocarbon combustion and atmospheric chemistry. UV absorption spectroscopy is widely used in the determination of the structure and reactivity of free radicals.1 Quantitative determination of the free radical-radical reaction rates requires reliable absorption cross-sections. In this work, UV absorption cross-sections were measured using pulsed excimer laser photolysis combined with the transient UV absorption spectroscopy.
The reactions were studied at ambient temperature, 298 ± 3 K, and the buffer gas (He) pressure of 1 bar in a heatable flow reactor. Limited additional data on the UV absorption and the rate constants were obtained at elevated temperatures. Formyl radicals were generated by pulsed photolysis of acetaldehyde at 308 nm (XeCl excimer laser). Formation of HCO and CH3 is the major channel (95%) for the photolysis of acetaldehyde at 308 nm:2
CH3CHO + hn(308nm) |
--> CH3 + HCO | (1a) |
| --> CH4 + CO | (1b) |
|
|
Figure 1. Absorption cross-sections of formyl radical (298 K, 1 bar He) |
The minor channel produces methane and carbon monoxide, which do not lead to any kinetic interference under the experimental conditions of the study. The initial concentrations of formyl radicals were calculated based on the measured energy density of the laser radiation (Molectron EM400), the absorbed energy and the quantum yield of formyl radicals (0.95). While the cross-section of acetaldehyde at 308 nm was also measured, s308(CH3CHO) = (3.2 ± 0.2)x10-20 cm2 molecule-1, 298 K), it was not used for the determination of the free radical concentrations. UV absorption spectrum of HCO is shown in Fig. 1 together with the only measurement available in the literature.3
In addition, rate constant of self reaction of formyl radicals and reaction of formyl and methyl radicals are being investigated. To separate these two processes, additional methyl radicals are produced by pulsed photolysis of acetone at 193 nm (ArF excimer laser). Production of two methyl radicals is the major channel (98%):4
| (CH3)2CO + hn(193nm) | --> 2 CH3 + CO | (2) |
|
Figure 2. Sample absorption profile of methyl radicals after photolysis of a mixture containing acetaldehyde and acetone by two consecutive pulses from XeCl (308 nm) and ArF (193 nm) lasers. |
The 193 nm pulse is applied with a delay with respect to the 308 nm pulse which allows reliable determination of the concentration of methyl radicals (Fig. 2). This determination is based on the measured cross-sections of formyl radical and on the equal yields of methyl and formyl radicals in the photolysis of acetaldehyde. Temporal profiles of formyl and methyl radicals are recorded via UV absorption (230 nm and 216.5 nm, respectively).Typical concentrations of the reactant are: [CH3CHO] = (1.0 - 3.0)x1017, [(CH3)2CO] = (4.0 - 12) x1014 molecule cm-3, laser energy fluxes 5 mJ cm-2 and 2 mJ cm-2 for 308 and 193 nm, respectively. The initial concentrations of the free radicals used are high ([HCO]0 = (1.5 – 5.0)x1013; [CH3]0 = (1.5 – 15)x1013 molecule cm-3 to ensure that the radical-radical processes 3,4 and 5 are the major reactions in the consumption of the free radicals:
| HCO + HCO | --> products | (3) |
| HCO + CH3 | --> products | (4) |
| CH3 + CH3 | --> C2H6 | (5) |
Preliminary data on the rate constants of reactions 3 and 4 are obtained, the experiments are in progress.
1.Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978.
2. Horowitz, Kersher, and Calvert. J. Phys. Chem, 1982, 86, 3105.
3. Hochanadel, C.J.; Sworski, T.J.; Ogren, P.J. J. Phys. Chem., 1980, 84, 231.4. Lightfoot, P. D.; Kirwan, S. P.; Pilling, M. J. J. Phys. Chem. 1988, 92, 4938.
