Gus Hancock

Spatially and temporally resolved laser‐induced fluorescence measurements of CF2 and CF radicals in a CF4 rf plasma

Laser‐induced fluorescence has been used to examine the temporal behavior of ground‐state CF2 and CF radicals in a CF4 plasma etching reactor. We also report the measured spatial dependencies of the radical concentrations, and develop a rigorous model for their interpretation. The results indicate that for CF2 and CF, wall removal processes are dominant at low pressures of the order of 50 mTorr, and that CF is not produced by electron impact dissociation of CF2.

Vibrational excitation of CO in the reaction: O + CS → CO + S

Abstract The relative rates at which O + CS → CO + S populates individual vibrational levels of CO have been determined (a) from infrared chemiluminescence experiments, and (b) by using a cw CO lawer to measure CO vibrational distributions produced when mixtures of CS 2 + O 2 are flash photolysed.

Quantum yields for production of O(1D) in the ultraviolet photolysis of ozone: Recommendation based on evaluation of laboratory data

[1] The quantum yield for O( 1 D) production in the photolysis of ozone in the ultraviolet region as a function of wavelength and temperature is a key input for modeling calculations in the atmospheric chemistry. To provide the modeling community with the best possible information, the available data are critically evaluated, and the best possible recommendations for the quantum yields are presented. Since the authors of this paper are the principal investigators of the groups which have provided most of the recent experimental data for the O( 1 D) quantum yields, the basic assumptions made by each group, the input parameters used in obtaining the quantum yields, and possible sources of systematic errors are well examined. The fitting expression of the O( 1 D) yield as a function of photolysis wavelength λ and temperature Tis presented in the ranges of 306 nm < X < 328 nm and 200 K < T < 300 K. The recommendation values of the O( 1 D) quantum yield for 290 nm < X < 306 nm and 328 nm < λ <350 nm are also presented. The formation mechanisms of O( 1 D) in the photolysis of ozone which result in the wavelength and temperature dependence of the O( 1 D) yields are interpreted.

Infrared chemiluminescence from the reaction O + CS → CO + S

Abstract Vibrationally excited CO is produced in the reaction sequence: O + CS2 → SO + CS; O + CS → CO + S. and gives rise to infrared emission. At least 50% of the energy of the secondary reaction is converted to vibrational energy. and it is suggested that this mechanism causes stimulated emission in the CS2 - O2 flash photolysis laser.

Direct and wavelength modulation spectroscopy using a cw external cavity quantum cascade laser

A continuous wave external cavity quantum cascade laser (EC-QCL) operating between 1872 and 1958 cm−1 has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the ν2 band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of 8.4×10−4 cm−1 Hz−1/2 was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of 23 to 3.7×10−5 cm−1 Hz−1/2.

The relative quantum yields of O2(a¹Δg) from the photolysis of ozone in the wavelength range 270nm ≤ λ ≤ 329nm

The relative quantum yield of O 2 (a 1 Δ g ) produced by the photolysis of ozone has been measured over the wavelength range 270nm≤λ≤329nm. The O 2 (a 1 Δ g ) fragment was detected by [2+1] resonance enhanced multiple photon ionisation (REMPI). The quantum yield of O 2 (a 1 Δ g ) is compared to that recommended for O( 1 D) and is found to follow it closely except at the longer wavelengths where O 2 (a 1 Δ g ) is still produced with a yield of 10% or more

Laser induced fluorescence detection of CF and CF2 radicals in a CF4/O2 plasma

Laser induced fluorescence has been used to detect ground‐state CF and CF2 radicals in a CF4/O2 plasma etching reactor. Measurements are reported of the spatial variation of CF2 concentrations, of the CF radical rotational state distributions, and of the variation of both species with O2 content of the plasma.

The relative quantum yields of O2(a¹Δg) from the photolysis of ozone at 227K

The relative quantum yields of O 2 (a 1 Δ g ) from the photolysis of ozone have been measured at 227(±2)K over the wavelength range 300nm≥λ≥322 nm, and are compared to those measured previously at 298K [Ball et al., 1993]. the yields at the two temperatures show the same intitial wavelength dependence in the fall-off region, in contrast to literature measurements of the O( 1 D) yields which show a shift in the fall-off curve to shorter wavelengths at low temperature. For λ>309 nm, the yield at 227K falls more rapidly than at 298K, confirming that internal energy mediated dissociation of ozone provides the major source of O 2 (a 1 Δ g ) at wavelength just beyond the 310nm threshold for the spin allowed production of the two singlet products. However, the yields of O 2 (a 1 Δ g ) at 227K and 298K were found to ve approximately equal for photolysis wavelengths λ≥320nm, and substantially larger than O( 1 D) yield predicted from modelling currently available for the photosis of vibrationally excited ozone. These results suggest the existence of a spin forbidden dissociation channel for O 2 (a 1 Δ g ) with a quantum yield of Φ≃0.1at wavelengths near to 320nm

Following interfacial kinetics in real time using broadband evanescent wave cavity-enhanced absorption spectroscopy: a comparison of light-emitting diodes and supercontinuum sources

A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 micros signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

A direct measurement of the O(1D) quantum yields from the photodissociation of ozone between 300 and 328 nm

Abstract The relative quantum yields of O( 1 D) produced in the photolysis of ozone at 298 K between 300 and 328 nm have been measured directly by resonance-enhanced multiple photon ionisation of the fragment. Clear indications are seen of a tail in the quantum yield extending to the long-wavelength limit of the experiments. Comparisons with previous studies are made to assess the importance of the photolysis of vibrationally excited molecules and spin-forbidden dissociation at wavelengths beyond the 310 nm thermodynamic threshold for the formation of both atomic and molecular oxygen in their singlet states.