Graham Hancock

Time-resolved pulsed FTIR emission studies of atom-radical reactions: Product chemiluminescence from the O(3P)+CF2(X̃ 1A1) reaction

Abstract A time-resolved interferometer has been used to study the product vibrational infrared chemiluminescence from atom-radical reactions. The entire temporal evolution of the emission spectrum is obtained from a single interferometric scan. Results are given for the O( 3 P) + CF 2 (X 1 A 1 ) reaction, which exhibits strong emission from highly vibrationally excited CO 2 , formed by reaction of O atoms with FCO, the major primary product of the title reaction.

CHF(X1A′) radical kinetics. Part 1.—Reaction with NO and O2

Ground-state CHF radicals, formed in the gas phase by infrared multiple photon dissociation, and detected by laser-induced fluorescence, react with NO and O2 with bimolecular rate constants of (7.0 ± 0.4)× 10–12 and <5 × 10–16 cm3 molecule–1 s–1, respectively, at 295 K. NCO (X2Πi) is detected as an important product of the CHF + NO reaction, and is formed at the same rate as that of CHF removal. The zero-pressure lifetime of the first excited state of NCO, A2Σ+(000) was measured to be 350 ± 30 ns.

Preparation of CHF (1A′) by infrared multiphoton dissociation and reactions with alkenes

A study of the vibrational, and translational energies of the CHF (X1 A′) radical prepared by infrared multiphoton dissociation, IRMPD, is presented. The vibrational and rotational temperatures measured near the CO2 laser pulse peak depend on the delay, nature and pressure of the buffer gas. For pure precursor (20 mTorr) and at delays of 0.4 and 3 µs the measured rotational temperatures were 900 and 600 K, respectively. Vibrational temperatures at 3 µs delay for samples of pure precursor (20 m Torr) and of precursor with 1 Torr of Ar were 790 and 630 K.The transient migration method was applied to measure the diffusion coefficient, and hence the collisional diameter, of CHF (X1A′) in Ar. Attempts to measure the average translational energy of the CHF fragment by the same method at low pressures produced extremely low temperatures, and forced a re-examination of the validity of the technique. The average vibrational relaxation rate constant, as determined by the same method and confirmed by direct measurements, gives kVT 10–10 cm3 molecule–1 s –1.The bimolecular rate constants for removal of CHF (X1A′) with several alkenes are reported to be (/10–12 cm3 molecule–1 s–1) : kethene= 5.4 ± 0.3; kpropene= 13.0 ± 1.0; kbutene= 9.5 ± 2.0; kisobutene= 17.0 ± 2; kbutadiene= 22.0 ± 3.0. Some of the reactions are CO2 laser fluence dependent, and, in addition, butadiene gives a considerable luminescence. A method is suggested for extrapolation of the apparent kinetic constants to zero fluence to obtain meaningful results.

Collisional behaviour with Ar of the Λ doublets of CH(X2Π)N″= 15 produced in the two-photon dissociation of CH2CO at 279.3 nm

Relaxation of the N″= 15 Λ doublets of CH(X 2Π) produced in the two-photon dissociation of ketene at 279.3 nm has been observed in the presence of Ar. An initially equilibrated nascent Λ doublet population is seen to be removed in such a way that the component of A″ symmetry (π orbital perpendicular to the plane of rotation) dominates. The application of recent theory describing collisions of 2Π diatomics with closed shell atoms is briefly discussed.

Rate constant for reaction of CH (X2Π) with ketene

A bimolecular rate constant for the reactive removal of CH (X 2Π) by ketene (CH2CO) has been measured under pseudo-first-order conditions using multiphoton dissociation of CH2CO at 308 nm to generate the radicals and laser induced fluorescence to detect them. A value of (2.4 ± 0.2)× 10–10 cm3 molecule–1 s–1 was obtained at room temperature (295 ± 2 K).

Infrared chemiluminescence from the O + CF2 reaction: part 1.—Kinetics of the emission near 2000 cm–1

Time-resolved infrared chemiluminescence has been observed from the reaction of ground-state oxygen atoms with CF2(X1A1) radicals. FTIR measurements were first used to establish the time dependence of the emission spectrum, and kinetic measurements were then carried out using a series of narrowband IR filters. IR emission observed between 1840 and 2350 cm–1 was assigned to CO2 formed in the reaction sequence O + CF2→ FCO + F, O + FCO → CO2+ F and emitting in the Δv3=–1 bands. Total IR emission intensities were correlated with CF2 concentrations as measured by laser-induced fluorescence. Changes in the kinetics and intensities of the IR emission as a function of wavenumber and reagent pressures were consistent with this formation scheme followed by quenching of vibrationally excited CO2, mainly by collisions with O atoms. An upper limit of 0.06 was estimated for the ratio of vibrationally excited CO to CO2 produced in the system.

Production of CH(X2Π) from the multiphoton dissociation of CH2CO at wavelengths of 279.3 and 308 nm

CH(X2Π) has been observed by laser-induced fluorescence (LIF) spectroscopy as the product of the two-photon dissociation of ketene (CH2CO) at 279.3 and 308 nm. The nascent distribution of rotational levels is Gaussian in profile, consistent with a ‘rotational reflection’ principle in the dissociation. Thermodynamic arguments imply a fragmentation pathway to CH + HCO following an initial one-photon absorption to the 1A″ excited state of ketene, a second photon absorption, and dissociation following rearrangement via a formylmethylene isomer. Analysis of Λ doublets in the LIF spectra shows no orbital alignment of CH produced on photolysis at 279.3 nm, but some propensity for Π(A′) symmetry alignment (π orbital of CH parallel to the plane of rotation) for photolysis at 308 nm.

Time-Resolved Pulsed FT-IR Emission Studies of Photochemical Reactions

A time-resolved Fourier transform emission spectrometer, operating in the stop-scan mode, is demonstrated as an inexpensive and versatile instrument for observation of infrared vibrational chemiluminescence. The entire evolution of an emission spectrum is obtained from a single scan of the interferometer, with a spectral and temporal resolution of 2 cm−1 and 10 ns, respectively. Results are presented for a number of radical-radical reactions studied by this technique, where emission from highly excited CO, HF, CO2, and N2O is observed. Measurements include nascent vibrational distributions, quantum yields for branching into different product channels, and bimolecular rate constants for the production and vibrational relaxation of product species. Experiments at low total pressure enable nascent vibrational and rotational distributions to be found for the HF fragment of the CO2 laser photolysis of 1,1-chlorofluoroethylene. In addition, time-resolved spectra of HF, CO, CO2, CF4, and CHF3 are demonstrated for infrared emission observed from a reactive ion plasma etching chamber.

Removal rates of CHF (Ã 1A″ (0, 0, 0)) by alkenes

Abstract Absolute removal rates of CHF (A 1 A″ (0, 0, 0)) by ethene (C 2 H 4 ), propene (C 3 H 6 ), 1-butene (1-C 4 H 8 ), isobutene( i -C 4 H 8 ), 1,3-butadiene (C 4 H 6 ), difluoromethane (CH 2 F 2 ), nitric oxide (NO) and argon (Ar) have been measured at room temperature. CHF in the A 1 A″ state was produced by infrared multiphoton dissociation of CH 2 F 2 forming the CHF (X 1 A′) state and further pumping to the A 1 A″ state by absorption of a visible dye laser pulse. Removal processes were found to be second order with the following rate constants in units of 10 −10 cm 3 molecule −1 s −1 : k (C 2 H 4 ) = 0.9 ± 0.2. k (C 3 H 6 ) = 1.0 ± 0.2; k (1−1C 4 H 8 ) = 1.1 ± 0.2; k ( i -C 4 H 8 ) = 1.1 ± 0.2; k (C 4 H 6 ) = 1.0 ± 0.2; k (Ar) = 0.27 ± 0.02; k (NO) = 0.8 ± 0.1; k (CH 2 F 2 ) = 1.3 ± 0.1. The Parmenter—Seaver correlation for collisional removal of A 1 A″ CHF is discussed.

Rate-constant measurement of the O(3P)+ CF2(X1A1) reaction

The kinetics of the bimolecular reaction of O(3P) atoms with CF2(X1A1) radicals have been investigated. Ground state CF2 was generated by infrared multiple photon dissociation of CF2HCl in the presence of an excess of O atoms, and its decay was measured by time-resolved laser-induced fluorescence. The rate constant at 295 K for the process O(3P)+ CF2(X1A1)→ products was measured as (1.75 ± 0.35)× 10–11 cm3 molecule–1 s–1.