G. Hancock

Singlet methylene kinetics: Direct measurements of removal rates of ã1A1 and b̃1B1 CH2 and CD2

Abstract Rate constants for collisional removal of a 1 A 1 and b 1 B 1 CH 2 and CD 2 have been directly measured, using IR laser induced multiple photon dissociation to prepare the radicals, and time resolved laser induced fluorescence to observe them. For CH 2 (a 1 A 1 ) removal by He, Ne, Ar, Kr, Xe, N 2 , H 2 , O 2 , CO and CH 4 , rate constants of 3.1, 4.2, 6.0, 7.0, 16, 8.8, 130, 30, 56 and 73 × 10 −12 cm 3 molecule −1 s −1 were found respectively. These represent significant increases over the previously accepted values. Essentially no isotope effect is observed in the removal of CD 2 (a 1 A 1 ) by the rare gases. The rate determining step in removal by the rare gases and N 2 is thought to be singlet—triplet intersystem crossing controlled by long range attractive forces, and the results are discussed in terms of both isolated and mixed state theoretical models of these processes. For the other molecular collision partners, bimolecular chemical removal channels are possible, and may account for the relatively fast rates observed. Radiative lifetimes of five Σ vibronic levels of CH 2 (b 1 B 1 ) and three Σ vibronic levels of CD 2 (b 1 B 1 ) have been measured and found to lie in the range 2.5–6.0 μs, and collisional quenching rates for CH 2 (b 1 B 1 ) are found to be of the order of the gas kinetic collisional frequency.

Vector correlations in the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P)

The reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P) has been studied using translationally aligned oxygen atoms formed from the 355 nm polarized photodissociation of NO2. The nascent CO product was detected by laser‐induced fluorescence (LIF) with sub‐Doppler resolution in order to extract the pair correlations between the reagent and product relative velocities k and k’ and the product rotational angular momentum J’. Previous theories interpreting the Doppler profiles of photodissociation products in terms of vector correlations have been extended to the case of bimolecular reactions. The system studied was seen to yield a close to isotropic distribution of product velocities k’ about the k direction, and a rotational alignment of J’ with k close to zero. The CO molecule departs with its rotational angular momentum vector J’ aligned preferentially perpendicular to the product relative velocity k’, hence exhibiting a negative k’, J’ correlation. Further insight has been gained on these results by quasiclassical ...

Quenching of C2(a 3Πu) produced in an intense infrared laser field

Abstract C 2 (a 3 Π u ) has been produced in the multiple-photon dissociation of acrylonitrile, C 2 H 3 CN, by a TEA CO 2 laser. Laser-induced fluorescence has been used to determine time-resolved relative C 2 concentrations in the presence of added Ar and O 2 and second-order rate constants of 2 × 10 −15 and 3.4 × 10 −12 cm 3 molecule −1 5 −1 have been obtained for these two quenchers, respectively. Vacuum-ultraviolet chemiluminescence is observed in the quenching by O 2 .

Direct measurements of ã 1A1 CH2 removal rates

Abstract The rates of collisional removal of a 1A1 CH2 radicals were directly measured. IR multiple photon dissociation was used to produce the species and laser-induced fluorescence was used to monitor their decay. The rate constants for 1A1 CH2 removal by helium, argon, xenon, N2 and O2 were measured to be 2.8 × 10−12, 5.6 × 10−12, 16 × 10−12, 7.9 × 10−12 and 27 × 10−12 cm3 molecule−1 s−1 respectively. Possible processes contributing to these decay rates are discussed.

Product rotational alignment for the reaction O(3P)+ CS(X 1Σ+)→ CO(X 1Σ+)+ S(3P)

Rotational alignment of the CO(X 1Σ+, v′= 14) product of the O(3P)+ CS(X 1Σ+) reaction has been measured relative to the velocity vector k of the reagents. O(3P) atoms were produced with k aligned in the laboratory frame by pulsed laser photolysis of NO2, and the CO product was detected by polarised laser-induced fluorescence. Transformation of the measured laboratory-frame rotational alignments to the required values of the alignment parameter 〈P2(J′·k)〉 were carried out using previously determined values of the translational anisotropy for the photodissociation of NO2, making allowances for both the thermal distribution of CS radicals and the spread of recoil energies of the O-atom fragment. Values of 〈P2(J′·k)〉 were measured for J′ between 12 and 35, and found to be close to zero to within the range 0 ± 0.25, with the mean value being slightly positive. Measurements of the Doppler profiles of the transitions are in qualitative agreement with those predicted for an isotropic distribution of product velocities about the k direction. These preliminary results illustrate the scope of laser based methods of extracting quantum-state-resolved data on scattering dynamics under experimental conditions which do not involve the use of molecular beam methods.

Cavity-enhanced absorption spectroscopy of methane at 1.73 μm

Abstract The integrated cavity output spectroscopy technique has been applied to the study of methane near 1.73 μm using a tunable diode laser. We have shown that this simple approach produces accurate high resolution spectra and have achieved a detection sensitivity of 1.8×10 −7 cm −1 for mirror reflectivities of 99.84%.

Laser intensity effects in the IR multiple-photon absorption of OsO4

Abstract The visible luminescence resulting from multiple-photon absorption of CO 2 laser radiation in OsO 4 depends upon the laser intensity as well as fluence. The use of single-mode laser pulses, shaped by electro-optic crystal switching, has enabled this intensity dependence to be determined quantitatively.

Vibrational relaxation of NCO(X̃) by rare gases, and rate constant measurement of the NCO + NO reaction

Abstract Vibrational relaxation of the v2 mode of NCO(X) by rare gases takes place at rates which are in the order Ne

LASER-INDUCED FLUORESCENCE SPECTROSCOPY OF THE CD 2 (a 1 A 1 ) RADICAL - RENNER-TELLER EFFECT IN CH 2 AND CD 2

CD2 (a 1 A 1) radicals have been produced through infrared multiple photon dissociation and observed by laser induced fluorescence following dye laser excitation of the [btilde] 1 B 1-a 1 A 1 transition. Excitation spectra and radiative lifetimes for nine vibronic levels of the [btilde] 1 B 1 state have been measured. The spectral data is compared with that previously reported in the literature for CH2 and is discussed in terms of model Renner-Teller calculations which demonstrate the transferability of the potential functions for the two isotopic forms.

A chemometric study on human breath mass spectra for biomarker identification in cystic fibrosis

Alveolar breath samples from a small case-control study population have been collected and measured via ion-molecule reaction mass spectrometry, and a constructive statistical approach to the identification of volatile biomarkers has been formulated by applying multivariate statistical methods on the mass spectra. The nature of the data is such that the number of variables largely exceeds the observations, representing a typical experimental scenario when breath analysis is conducted using mass spectrometry. Principal components analysis has been performed on the high dimensional dataset of molecular abundances, providing evidence of case separation and reducing the number of functional discriminators by almost 90%. Afterwards, a deductive approach based on a binary regression was conducted on the reduced dataset, providing an entirely reliable case discrimination model exclusively depending on the concentrations in the breath mixture of 3 out of a total of 97 metabolites.