Brad E. Forch

Collisional Transfer between and Quenching of the 3P 3p and 5p States of the Oxygen Atom

Abstract Collisional quenching and excitation transfer between the oxygen atom 3p 3P and 5P states have been investigated in a discharge flow apparatus with O2 and N2 collisional partners. The 3P state was excited by two-photon excitation at 225.6 nm from the 2p43P ground state, and temporal profiles of the 3P←3S and 5P← 5S emission at 844.7 and 777.5 nm, respectively, to the lower 3s manifold were recorded. From an analysis of 3P decay curves as a function of quencher pressure, bimolecular rate constants for the removal of the 3P state were determined: k3Q(O2) = (7.8±0.8) × 10−10 and k3Q(N2)= (5.87±0.15)× 10−10 molecule−1 cm3 s−1. The intercept of the O2 Stern-Volmer plot yielded k3r = (2.98± 0.15)×10 7 s−1 for the 3P radiative decay rate. The collisional quenching rate constant for the 5P state by O2 was obtained by analysis of 5P decay curves: k5Q(O2)= (10.8±1.8)× 10−10 molecule−1 cm3 s−1. Rate constants k35 for 3P← 5P excitation transfer were obtained from the ratio of the 5P←5S to 3P← 3S emission intensities as a function of quencher pressure: k35 (O2) ≈ 6× 10−11 and k35 (N2)≈2 ×10 −11 molecule−1 cm3 s−1.

Laser-based ignition of H2O2 and D2O2 premixed gases through resonant multiphoton excitation of H and D atoms near 243 nm☆

Abstract We have investigated the use of a tunable laser system that operates in the ultraviolet (UV) to ignite premixed reactive gaseous flows of H 2 O 2 and D 2 O 2 at atmospheric pressure. The amount of incident laser energy (ILE) required to ignite the premixed flows as a function of laser excitation wavelength shows two distinct minima. The spectral position of these minima correspond exactly to the location of the resonance, two-photon excitation wavelengths of atomic hydrogen and deuterium at 243.07 and 243.00 nm, respectively. The relative spacing between these minima at the energy level of the 1S–2S two-photon excited transition is 22 cm−1, which is in excellent agreement with the known value for HD deuterium isotope shift (22.4 cm−1). We believe that this is both the first report of a sensitive wavelength dependence on the laser energy required to ignite these mixtures through resonant multiphoton excitation of H and D atoms (produced from H2 and D2 photolysis) and the first report of a deuterium isotope-wavelength-effect in laser ignition. Measurement of the ILE required for ignition versus equivalence ratio (Φ) shows that the most efficient ignition occurred with ∼ 0.55 mJ ILE at Φ = 0.7 in the fuel-lean region. Strong experimental evidence is given that shows that ignition occurs through the efficient resonant formation of a well-controlled, laser-produced microplasma. An estimate of the laser power dependence for the photolysis, excitation, and ionization processes responsible for microplasma initiation was determined in a molecular-beam time-of-flight mass spectrometer. Threshold for microplasma formation (70 torr) was determined in a variable pressure flow cell. These new experimental results indicate that resonance enhancement in the formation of a microplasma is a well-controlled ignition method that appears to alleviate the problems associated with the sharp thresholds encountered in the well-known, nonresonant laser-produced spark (gas breakdown) process.

Coriolis interaction and intramolecular vibrational redistribution: A high-resolution spectroscopic probe of the rotational contribution to vibrational

Abstract We present here high-resolution fluorescence excitation spectra of the 12 0 2 band of pyrimidine in a molecular beam, which provide compelling ev

A Novel Detector for Gas Chromatography Based on UV Laser-Produced Microplasmas

The use of UV laser-produced microplasmas for the detection of analyte molecules in the effluent gases of a gas chromatograph is described. The microplasmas are formed when a carbon-containing analyte is present in the carrier gas flow but not in the carrier gas alone. The microplasmas are produced by using the 193-nm output of the ArF excimer laser, with only modest pulse energies (10 mJ) required. Three means for detecting the presence of the microplasmas have been effected and are compared: optogalvanic (plasma electron) detection, photoacoustic (blast wave) detection, and photometric (plasma emission) detection, particularly of electronically excited carbon atoms at 248 nm. The relative responses of these three techniques have been determined for microliter injection of acetylene into the helium flow. Present limits for acetylene with the use of the optogalvanic, photoacoustic, and photometric techniques are 500, 500, and 10 ng, respectively. Optimization of these techniques is expected to improve the detection limits by 2–4 orders of magnitude. The virtues of this detector include the fact that it requires no flame and that it is sensitive to carbon-containing species such as CO and CO2.

Ultraviolet laser microplasma–gas chromatography detector: detection of species-specific fragment emission

Characteristic laser-produced microplasma emissions from various simple carbon-containing vapors entrained in a He carrier gas have been observed and compared. A focused ArF (193-nm) excimer laser is used to induce microplasmas with modest pulse energies (15 mJ or less) in the effluent region of a gas chromatography capillary column. Strong atomic (C, H, O, Cl, and F) as well as molecular (C(2), CH, and CCI) emissions are observed with very high SNRs. A plasma emission survey indicates that different classes of molecule show unique spectra which make it relatively easy to distinguish one chemical class from another. These results suggest that a laser microplasma gas chromatography detector (LM-GCD) should offer additional discrimination/resolution for unknown sample gas mixture analysis. In addition, the LM-GCD exhibits a significant advantage over certain other GC detectors, like the widely used flame ionization detector, by readily detecting nonresponsive gases such as CO, CO(2), CCl(4) and Freons.

Coriolis interactions and intramolecular vibrational redistribution in jet-cooled pyrimidine

Abstract Evidence is presented which indicate that the second-order Coriolis interaction play an important role in intramolecular vibrational redistribution.

“Channel-three-like” behavior of photoexcited isoquinoline vapor: a model for electronic and vibrational relaxation

Abstract Evidence is presented which indicates that the channel-three-like behavior of photoexcited isoquinoline vapor is due to direct internal conversion, triggered by intramolecular vibrational relaxation. It is suggested that the same mechanism may account for the similar behavior in other azaaromatics.

Coriolis effects on “intermediate case” of intramolecular vibrational relaxation: Rotational-temperature dependence of energy- resolved fluorescence from jet-cooled perylene with 776–975 cm−1 excess energies

Abstract The rotational-temperature dependence of the energy-resolved fluorescence, combined with the calculated level density, indicates that the large variations in the rate of the intramolecular vibrational relaxation over a small excess energy in S 1 perylene is due to the fluctuations in the density of accessible vibrational levels, resulting from the vibrational selection rules for second-order Coriolis coupling.

Photoinduced vibrational predissociation of pyrimidine clusters in a supersonic molecualr beam: Evidence for statistical energy partitioning

Abstract Evidence is presented which indicates that the vibrational predissociation of pyrimidine clusters leads primarily to the production of rotationally very hot S 1 pyrimidines in their zero-point vibrational levels. The generation of vibrationless S 1 pyrimidine is an indication of statistical energy partioning, and it contrasts the pattern of energy partitioning observed in the vibrational predissociation of tetrazine-argon van der Waals complexes. The tendency of the pyrimidine clusters to follow statistical-like energy partitioning is attributed to the greater numbers and reduced frequencies of van der Waals modes of the clusters relative to those of the complex.

Preparation of elusive S1(nπ*) states by photodissociation of van der waals molecules: π* → n Fluorescence of isoquinoline☆

Abstract Evidence is presented which indicates that photodissociation of van der Waals molecules may be utilized to produce elusive excited states of bare molecules.