Johanna L. Miller

Cavity ring-down spectroscopy measurements of single aerosol particle extinction. I. The effect of position of a particle within the laser beam on extinction

A continuous wave distributed feedback diode laser operating in the near infrared at wavelengths close to 1650 nm has been used to measure the extinction of light by single aerosol particles. The technique of optical feedback cavity ring-down spectroscopy (CRDS) was used for measurement of CRDS events at a repetition rate of 1.25 kHz. This very high repetition rate enabled multiple measurements of the extinction of light by single aerosol particles for the first time and demonstrated the dependence of light scattering on the position of a particle within the laser beam. A model is proposed to explain quantitatively this phenomenon. The minimum detectable dimensionless extinction coefficient epsilonmin was determined to be 3x10(-6). Extinction values obtained for single spherical polymer beads from a monodisperse sample of particles of diameter of 4 microm are in near-quantitative agreement with the values calculated by the Mie scattering theory. The deviations from the Mie theory expected for measurement of extinction by CRDS using a continuous wave laser are discussed in the companion paper.

Cavity ring-down spectroscopy measurement of single aerosol particle extinction. II. Extinction of light by an aerosol particle in an optical cavity excited by a cw laser

The authors present an analytical derivation of the scattered power from a spherical, homogeneous, nonabsorbing particle in a plane standing wave. The scattered power changes significantly with the position of the particle with respect to the peaks and nodes of the standing wave, even for particles whose diameters are many times the wavelength of the light. The analysis is applicable to continuous-wave cavity ring-down spectroscopy on aerosol particles, and the structure of the standing wave is expected to affect both the measured ring-down time and the shape of the ring-down trace. The dependence of the extinction on the phase of the standing wave at the location of the particle is captured in a parameter zeta which connects the current treatment to standard Mie scattering theory. Methods for calculating zeta are presented.

Dissociation of the ground state vinoxy radical and its photolytic precursor chloroacetaldehyde: Electronic nonadiabaticity and the suppression of the H+ketene channel

This work is a study of the competition between the two unimolecular reaction channels available to the vinoxy radical (CH(2)CHO), C-H fission to form H+ketene, and isomerization to the acetyl radical (CH(3)CO) followed by C-C fission to form CH(3) + CO. Chloroacetaldehyde (CH(2)ClCHO) was used as a photolytic precursor to the vinoxy radical in its ground state; photodissociation of chloroacetaldehyde at 193 nm produces vinoxy radicals with internal energies spanning the G3//B3LYP calculated barriers to the two available unimolecular reaction channels. The onset of the CH(3) + CO channel, via isomerization to the acetyl radical, was found to occur at an internal energy of 41 +/- 2 kcal/mol, agreeing well with our calculated isomerization barrier of 40.8 kcal/mol. Branching to the H+ketene channel was too small to be detected; we conclude that the branching to the H+ketene channel must be at least a factor of 200 lower than what is predicted by a RRKM analysis based on our electronic structure calculations. This dramatic result may be explained in part by the presence of a conical intersection at planar geometries along the reaction coordinate leading to H+ketene, which results in electronically nonadiabatic recrossing of the transition state.

193-nm photodissociation of acryloyl chloride to probe the unimolecular dissociation of CH2CHCO radicals and CH2CCO

The work presented here uses photofragment translational spectroscopy to investigate the primary and secondary dissociation channels of acryloyl chloride (CH2==CHCOCl) excited at 193 nm. Three primary channels were observed. Two C-Cl fission channels occur, one producing fragments with high kinetic recoil energies and the other producing fragments with low translational energies. These channels produced nascent CH2CHCO radicals with internal energies ranging from 23 to 66 kcal/mol for the high-translational-energy channel and from 50 to 68 kcal/mol for the low-translational-energy channel. We found that all nascent CH2CHCO radicals were unstable to CH2CH + CO formation, in agreement with the G3//B3LYP barrier height of 22.4 kcal/mol to within experimental and computational uncertainties. The third primary channel is HCl elimination. All of the nascent CH2CCO coproducts were found to have enough internal energy to dissociate, producing CH2C: + CO, in qualitative agreement with the G3//B3LYP barrier of 39.5 kcal/mol. We derive from the experimental results an upper limit of 23 +/- 3 kcal/mol for the zero-point-corrected barrier to the unimolecular dissociation of the CH2CHCO radical to form CH2CH + CO.

Photodissociation dynamics of ethyl ethynyl ether: A new ketenyl radical precursor

The work presented here investigates the dynamics of the photodissociation of ethyl ethynyl ether at 193.3 nm with photofragment translational spectroscopy and laser-induced fluorescence. The data from two crossed laser-molecular beam apparatuses, one with vacuum ultraviolet photoionization detection and one with electron bombardment detection, showed that only cleavage of the C–O bond to form a C2HO radical and a C2H5 (ethyl) radical occurs. We observed neither cleavage of the other C–O bond nor molecular elimination to form C2H4+CH2CO (ketene). The C2HO radical is formed in two distinct product channels, with 37% of the radicals formed from a channel with recoil kinetic energies extending from about 10 to 70 kcal/mole and the other 63% formed from a channel with lower average recoil energies ranging from 0 to 40 kcal/mole. The measurements using photoionization detection reveal that the C2HO radical formed in the higher recoil kinetic-energy channel has a larger ionization cross section for photon energ...