Laser induced fluorescence has been used to study the desorption dynamics of OH radicals produced by the reaction of NO2 and H2 over Pt(111), under UHV conditions. We measure an apparent Arrhenius activation energy for desorption that increases from 25 to 38 kcal/mole as the NO2 to H2 ratio in the dosing mixture, X, decreases from 2 to .05. Boltzmann‐like rotational energy distributions with relatively high rotational to surface temperature ratios (α=Tr/Ts=.69–.87) are reported for crystal temperatures between 1078 and 1433 K. α exhibits a small but definite dependence on both Ts and X. These results are compared with earlier data from the O2+H2 system and interpreted in terms of O‐coverage.

Rotational energy distributions of the desorbing NO product from the NH3+O2 reaction on a Pt(111) single crystal have been measured using the laser‐induced fluorescence technique in conjunction with an UHV apparatus. Over the surface temperature range 800–1300 K, the rotational temperature of NO was found to remain virtually constant near 400 K.

Abstractat substrate temp. in the range 300‐1200 K. By using multiphoton ionization mass spectrometry, the SiF2 radical is unambiguously detected and the apparent activation energy for its production is determined to be 8.9 and 22.1 kcal/mol for F2 and NF3, respectively.

We have recently demonstrated selective detection of SiF and SiF2 radicals produced in the reaction of F2 and NF3 with silicon in a flow system using multiphoton ionization mass spectrometry (MPI/MS)1. In this paper we report results for the spontaneous surface etching reaction of p-doped (110) and uncharacterized silicon single crystals by XeF2 over a wide range of temperatures (300-1000 K) and pressures (10-6 to 10-4 Torr). We have found that under single collison conditions SiF2 is the only gas phase radical produced in the surface reaction of XeF2 and silicon. The activation energy for producing gas phase SiF2, Ea[SiF2], is about 6.0 kcal/mol.

The spontaneous etching of silicon by xenon difluoride has been investigated under single collision conditions using multiphoton ionization mass spectrometry for the detection of radical products. SiF2 is the only gas phase radical observed with an apparent activation energy of 6.7±0.5 kcal/mol. Electron ionization techniques were used to determine an apparent activation energy for gas phase production of SiF4 of 5.6±0.8 kcal/mol. The identification of SiF2 as the sole reactive desorbing species during the steady-state etching process is useful for understanding many aspects of primary surface processes and secondary gas phase reactions in etching.

Abstract Catalytic ammonia denox reactions over a polycrystalline Pt surface in the temperature range 800–1330 K have been investigated by laser-induced-flourescence detection of the OH radical produced on and subsequently desorbed from the surface. For the NO 2 - NH 3 system, strong curvature is observed in the low temperature region of the Arrhenius plots, which could be attributed to the presence of different desorption processes. The two extracted apparent OH desorption energies increase from 30 to 49 kcal/mol and from a surprisingly low value of 2 to 27 kcal/mol as the NO 2 NH 3 reactant mixture ratio is decreased from 70 to 0.037. On the other hand, for the NO - NH 3 system, no such curvature in the Arrhenius plots is observed and the OH desorption energy decreases from 38 to 26 kcal/mol as the NO / NH 3 ratio is decreased from 70 to 0.4. The different trends in the apparent OH desorption energies as a function of the NO x / NH 3 ratio in the present two systems can be rationalized by the degree of coverage of chemisorbed O atoms relative to that in the earlier studied O 2 - NH 3 system.

The oxidation of benzene under fuel-lean conditions has been studied from 1600 to 2300 K in a shock tube using a stabilized CW CO laser to monitor CO production. The result of the kinetic modeling of the CO formation according to the global mechanism of Fujii and Asaba indicates that the rate of CO formation in the early stage of oxidatin depends very sensitively on the rate-limiting unimolecular decomposition of benzene: C6H6→1C6H5+H The rate constant obtained for the initiation reaction using a scheme consisting of a set of 25 reactions is given by the following expression: k1=1015.70±0.52 exp (−54, 300±2200/T) sec−1. for the pressure range of 1.9–2.7 atm. The result of an RRKM calculation for the reaction indicated that at temperatures above 2000 K, k1 becomes slightly pressure-dependent. The extrapolation of the individual data with the RRKM theory led to k1∞≌1016.76±0.50exp (−58,400 ± 2200/T) sec−1.

Abstract The decomposition of nitromethane (CH3NO2) in a shock tube has been studied by using a frequency-stabilized CW CO laser to measure the real-time production of NO and CO, two of the key decomposition products. Highly diluted CH3NO2/Ar mixtures (0·15 -0·75% CH3NO2) were used in incident shock experiments over the temperature range from 940 to 1520K and pressure range from 0·4 to 1·0 atm. A mechanism consisting of 37 chemical reactions was used to model the formation of these two products over the entire range of experimental conditions employed. All but one rate constant (i.e. CH3 + CH2O) were obtained either from the literature or from simple TST and RRKM calculations. The NO profiles could be quantitatively modeled over the entire temperature range, whereas those of CO could be accounted for only by increasing the values of the rate constant for CH3 + CH2O determined at low temperatures by more than a factor of 30. The reason for this adjustment is discussed. The combination of the evaluated rate constants with others for the CH3 + CH2O reaction covering the range of 350–1500K gave rise to the expression: Additionally, strong oscillatory behavior in the NO and CO profiles was observed when 2·5 – 3·0% CH3NO2 mixtures were shock-dissociated. A similar behavior was not detected in CH3ONO decomposition under the same conditions. The comparison of these two isomeric systems will be made.

Thermal decomposition of HCOOH has been studied from 1280 to 2030 K over a broad range of initial concentrations (0.07 to 1.6% in Ar) in a shock tube using a stabilized cw CO laser to monitor CO formation. The observed CO concentration profiles over the whole range of temperature and pressureemployed could be satisfactorily accounted for by the following two-channel molecular decomposition mechanism: HCOOH+M→1CO+H2O+MHCOOH+M→2CO2+H2O+M89-1 with the following apparent second order rate constants in the pressure range 0.75–2.8 atm. k1=2.3x1015exp(−50,000±1700/RT)ccmole−1sec−1k2=1.5x1016exp(−57,000±2800/RT)ccmole−1sec−1 The values of these two rate constants were evaluated by the non-linear least squares fitting of the observed CO concentration profiles. The possibility of an alternative decomposition route involving the initial production of H atoms, instead of H2, has also been considered. However, the rate constant evaluated from the kintic modeling of the CO formed at high temperatures cannot satisfactorily account for that observed at lower temperatures.

A cw CO laser has been used to probe the complete CO vibrational energy distributions in the reactions of CH2 radicals with O(3P) and O2 (3∑−g. The CO formed from the reaction with the atom was concluded to be produced via two equally important channels, O+CH2→CO+H2→CO+2H,, with an average vibrational energy of 19 kcal/mole, and an approximate vibrational temperature of ≈104K (for v≤12), which is sufficient to account for previously observed CO stimulated emission from O(3P)+CH2. The population inversion derives mainly from the molecular hydrogen elimination channel. In the reaction of CH2 with molecular oxygen, the CO formed is believed to be produced from both a molecular elimination path giving H2O+CO and from two possible atomic or radical production processes, yielding H+CO+OH. We find from analysis of the CO product vibrational energy distribution that 30% of the CO formed is produced via the former path, and 70% is formed from the latter path. This information is important to the detailed kinetic modeling of acetylene and other hydrocarbon combustion systems.

Because of the large fractional mass difference between hydrogen and deuterium, there exist many conventional methods of deuterium enrichment. One of the most efficient of these is the Girdler-Sulphide (GS) process which is based on the equilibrium isotope effect in the exchange between water and H2S. Economic projections for proposed new separation methods must compete favorably with the GS process before these new approaches can even begin to be seriously considered. This paper will discuss the projected energy requirements for a method of deuterium enrichment which is based on the selective vibrational enhancement of the reaction Cl + CH3D by a CW CO2 laser, while Cl + CH4 proceeds at essentially the thermal rate. The CH3D proposal is based on experiments reported earlier on CH3Br [1] and CH2D2 [2,3]. Experiments on CH3D itself are difficult to perform in the existing low-pressure apparatus because of the smaller absorption by this species as compared to CH2D2 which has accidental coincidences with the CO2 laser. At higher pressures typical of a production facility, pressure broadening should remove this obstacle and the absorption coefficients at CO2 laser wavelengths should reflect the low resolution spectra and may be acceptably large. Because of the enormous spectral differences between CH3D and CH4, there will be no loss of spectroscopic isotope resolution at any conceivable pressure.

The reactions of CS(X 1Σ+), CS2(X 1Σ+g) and OCS(X 1Σ+) with O(3P) were studied at 298 K by means of a CO laser resonance absorption technique. The CO(ν) population distribution produced from the reaction O(3P) + CS(X 1Σ+) studied in a quartz flash photolysis tube (λ>/ 200 nm) is similar to distributions observed previously for ν> 7. For ν < 7 an energetically colder vibrational population was observed which is attributed to the reaction of O(3P) atoms with undissociated CS2(X 1Σ+g). Subsequent experiments carried out in a Pyrex flash photolysis tube (λ>/ 300 nm) in which the O(3P) + CS2(X 1Σ+g) reaction is the only one which can occur confirmed that the colder population observed is attributable to this process. The branching ratio for the reaction channel O(3P) + CS2(X 1Σ+g) → CO(X 1Σ+) + S2(3Σ−g) has been measured. We find that 1.4 ± 0.2% of the O + CS2 reaction proceeds through this channel, and that the rate constant for this reaction channel is, k = 3.5 (±0.5) × 1010 cm3/mole s. Isotope labeled experiments using 18O atoms show that the O(3P) + OCS(X 1Σ+) reaction takes place by a direct stripping mechanism, wherein CO(ν) is produced exclusively from the parent OCS molecule. The CO(ν) formed in this reaction carries about 9% of the total available energy.

Prior to the work in this laboratory demonstrating successful Br isotope separation using a cw CO2 laser [1], the only two reports of isotope separation using cw IR lasers to excite the reagents have been those by MAYER et al. [2] and CHIEN and BAUER [3]. The chief reasons for this are the severe problems of VV interisotope energy exchange and bulk heating, both of which cause isotope scrambling. Recently, MANUCCIA and co-workers have devised the method of “VT competing deactivation” which overcomes these constraints and have demonstrated cw IR laser isotope separation employing a vibrationally enhanced bimolecular reaction [1]. Employing the technique, we have succeeded in enriching deuterium by as much as 72% in the products of the radical chain reaction of chlorine atoms with a mixture of CH2D2 and CH4 by exciting the CH2D2 molecules with a cw CO2 laser. A conservative energy cost estimate based on natural abundance methane feed (preliminary results with a CH3D-CH4 mixture have shown comparable deuterium enrichment) indicates that it is within a factor of 3–4 of that for the current commercial process.

Chemischer InformationsdienstVolume 9, Issue 32 Reviews ChemInform Abstract: KINETICS AND MECHANISMS OF REACTIONS OF FLUOROMETHYLIDYNE, MONOFLUOROMETHYLENE, AND DIFLUOROMETHYLENE RADICALS D. S. Y. HSU, D. S. Y. HSUSearch for more papers by this authorM. E. UMSTEAD, M. E. UMSTEADSearch for more papers by this authorM. C. LIN, M. C. LINSearch for more papers by this author D. S. Y. HSU, D. S. Y. HSUSearch for more papers by this authorM. E. UMSTEAD, M. E. UMSTEADSearch for more papers by this authorM. C. LIN, M. C. LINSearch for more papers by this author First published: August 8, 1978 https://doi.org/10.1002/chin.197832351AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume9, Issue32August 8, 1978 RelatedInformation