Volume 214, Issue 4 - 4

The pressure dependence of the OH yield in the reaction O( 1 D) + CH 4 has been investigated at room temperature in the pressure range from 4 to 560 bar of helium. O( 1 D)-atoms were generated by pulsed laser photolysis (PLP) of N 2 O and OH-radicals were detected by saturated laser induced fluorescence (SLIF). The OH-yield was determined relative to the OH yield in the reaction of O( 1 D) with H 2 which is assumed to be unity and pressure independent. Within the pressure range of our experiments we found a constant OH-yield of (71±5)% in the reaction of O( 1 D) with methane. From the OH fluorescence lifetime at the highest pressures near 500 bar of helium we obtained the rate constant for the collisional deactivation of electronically OH radicals of k q (He) = (3±0.3)× 10 -15 cm 3 s -1 , corresponding to a cross section of σ q (He) = (2.1±0.2)× 10 -20 cm 2 . The rate constant for collisional deactivation of vibrationally excited OH radicals in the electronic ground state OH(v=1) + He → OH(v=0) + He (reaction (5)) was determined to be k 5 (He) = (1.2±0.2)× 10 -16 cm 3 s -1 .

In high-pressure flames that occur in many practical combustion devices such as industrial furnaces, rocket propulsion and internal engine combustion, the assumption of an ideal gas is not appropriate. The present paper presents a model that includes modifications of the equation of state, transport and thermodynamic properties. The model is implemented into a Fortran program that was developed to simulate numerically one-dimensional planar premixed flames. The influence of the modifications for the real gas behavior on the laminar flame speed and on flame structure is illustrated for stoichiometric H 2 -air flames at initial pressures ranging from 0.1 to 100 MPa.

New configurations of hydrazine (N 2 H4) n clusters have been calculated from dimer to tetramer which contain one monomer with one inverted NH 2 group. This procedure generates new chiral isomers of the clusters. The calculated lineshifts of the asymmetric NH 2 wag mode lead for the dimer and the trimer to a much better agreement with the experimental results than obtained in previous calculations without the chiral isomers. For the tetramer the spectra do not differ for the two configurations.

The reactions of 3-pentoxy radicals with O 2 and their unimolecular decomposition were investigated using time-resolved and simultaneous detection of NO 2 and OH in the laser flash initiated oxidation of bromopentane combined with numerical simulation of the experimentally obtained concentration-time profiles. NO 2 was monitored by cw-LIF, whereas OH was detected by laser long-path absorption at 308 nm. 3-pentoxy radicals were produced selectively by the excimer-laser photolysis of 3-bromopentane at 248 nm and subsequent reaction of the 3-pentyl radicals with O 2 in the presence of NO. All experiments were performed at a temperature of 293±3 K and 50 mbar total pressure. The following rate coefficients for reaction with oxygen (kO 2 ) and for thermal decomposition (k decomp ) of 3-pentoxy radicals were obtained: k O2 = (7.2±3.5)× 10 -15 cm 3 molecule -1 s-1 and k decomp = (5.0±2.5)× 10 3 s -1 . In addition to the experimental investigation, the unimolecular reaction channels of 3-pentoxy radicals were also analyzed by a combined ab initio /RRKM treatment. From the ab initio calculations structures of the 3-pentoxy radical and its decomposition transition states as well as energy barriers were determined. This information was used to solve the J-independent Master Equation which allowed thermal decomposition rate coefficients to be determined. The calculated rate constants are in good agreement with those determined experimentally.

Detailed modeling of soot formation was carried out using different complex models. Main effort was put on modeling particle inception. The experimental data base consisted of soot volume fraction measurements observed in laminar premixed ethene flames for pressures between 5 and 20 bar and behind reflected shock waves for the pressure range of 4.5 to 55 bar with the fuel ethene and benzene, respectively. Due to the observation time scale in the shock wave experiments (a few milliseconds) induction period and initial soot formation zone are temporally well resolved. The present study investigates the formation and growth of the polycyclic aromatic hydrocarbon (PAH) and the adjacent section of particle formation by PAH coagulation. For the models the same relatively large reaction gas phase mechanism up to the first aromatic ring structure was used. PAH formation was described either by applying linear lumping of the well known HACA sequence (model I) or by using a detailed chemical kinetic model for species ranging from benzene to coronene (model II). Particle inception and growth was described by applying a standard mechanistic model which comprises coagulation and surface growth. The modeling study showed that growth of soot precursors and particle inception predicted by the PAH reaction mechanism (model II) gives better agreement with measured soot volume fration profiles than the HACA mechanism alone (model I). From the study it was also deduced that a simplified PAH growth model based on reactive coagulation of a relevant growth species (model III) gives similar results. From the comparison with the high pressure flame experiments it was also shown that the modified soot model is able to predict soot formation and oxidation in the post flame zone.

Doppler-free two-photon absorption is demonstrated for cold molecules in a supersonic beam with a spectral resolution of 10 MHz. It was realized using a frequency stabilized tunable external concentric resonator placed within the vacuum chamber and crossed by the molecular beam. The achieved high resolution allows us to resolve single rotational lines in the S 1 ← S 0 , 14 1 0 two photon vibronic band of benzene molecules. The theoretical analysis of the symmetric top rotational line structure of the isotropic Q-branch in a totally symmetric two-photon band leads to accurate rotational constants and a value for the rotational temperature achieved at various distances from the nozzle orifice in the cold molecular beam. The smallest linewidth of the rotational lines is 8.5 MHz and given by the flight time of the molecules in the supersonic beam through the laser beam waist in the resonator and the laser bandwidth.

Rate coefficients for reactions of the C 2 radical in its a 3 Π u electronic state with H, N and O atoms and with C 3 O 2 , C 4 F 6 , H 2 , NO, N 2 O, C 2 H 2 , C 2 H 4 , C 3 H 4 and C 6 H 6 were determined. This work represents the first study of reactions of C 2 (a 3 Π u ) radicals with the atoms investigated at room temperature and 4 Torr total pressure. The bimolecular rate constants obtained for the atom reactions were k C2+O = (9.8±1.0)× 10 -11 , k C2+N = (2.8±1.0)× 10 -11 and k C2+N <6× 10 -14 , in units of cm 3 s -1 . In addition, the reaction C 2 + O was found to be independent of total pressure in the range 2-60 Torr. For the reaction C 2 (a 3 Π u ) + ethene (C 2 H 4 ) a temperature and pressure independent rate constant of (9.5±1.2)× 10 -11 cm 3 s -1 was obtained in the temperature range 298-1000 K at 100 Torr total pressure and in the pressure range 5-100 Torr at 298 K. The following rate constants were determined at room temperature and a total pressure of 4 Torr for the reactions of C 2 (a 3 Π u ) radicals with benzene (C 6 H 6 ), acetylene (C 2 H 2 ) and allene (C 3 H 4 ): k C2+C6H6 = (4.9±0.1)× 10 -10 , k C2+C2H2 = (1.0±0.1)× 10 -10 and k C2+C3H4 = (1.9±0.3)× 10 -10 , in units of cm 3 s -1 . The reaction C 2 + NO was investigated at room temperature and 100 Torr total pressure, a rate constant k C2+NO = (6.8±0.3)× 10 -11 cm 3 s -1 was obtained. The reaction C 2 + N 2 O was studied at 4 Torr total pressure in the temperature range 300-700 K for which a temperature independent rate constant k C2+N2O = (3.1±0.4)× 10 -14 cm 3 s -1 was determined.