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The propyl + O 2 reactions are important prototype reactions for general alkyl + O 2 systems, possessing the most important reactive channels of larger species while remaining small enough for accurate quantum chemistry calculations and theoretical kinetics. Previous experiments [8] that characterized the production of OH in the propyl + O 2 reactions could be only qualitatively modeled by rigorous master-equation calculations, leaving a deficient validation of, for instance, the critical RO 2 ↔ QOOH isomerization pathways. This article revisits the combined experimental and modeling study of DeSain et al., using an improved experimental method to obtain time-resolved OH concentrations for pulsed-photolytic Cl-initiated oxidation of propane. The results are modeled with master-equation-based rate coefficients incorporated into a kinetic mechanism. Quantitative agreement can be obtained, providing experimental validation for the OH-producing pathways that are crucial for autoignition. The importance of the rigorous treatment of the kinetics for these highly pressure and temperature dependent reactions is emphasized, as well as the significance of the formally direct pathways such as chemical activation.

The photodissociation of vinyl iodide has been investigated at several wavelengths between 193 and 266 nm using three techniques: time-resolved Fourier transform emission spectroscopy, multiple pass laser absorption spectroscopy, and velocity-mapped ion imaging. The only dissociation channel observed is C-I bond cleavage to produce C2H3 (nu, N) + I (2P(J)) at all wavelengths investigated. Unlike photodissociation of other vinyl halides (C2H3X, X = F, Cl, Br), in which the HX product channel is significant, no HI elimination is observed. The angular and translational energy distributions of I atoms indicate that atomic products arise solely from dissociation on excited states with negligible contribution from internal conversion to the ground state. We derive an upper limit on the C-I bond strength of D0(C2H3-I) < or = 65 kcal mol(-1). The ground-state potential-energy surface of vinyl iodide is explored by ab initio calculations. We present a model in which the highest occupied molecular orbital in vinyl halides has increasing X(np) non-bonding character with increasing halogen mass. This change leads to reduced torsional force around the C-C bond in the excited state. Because the ground-state energy is highest when the CH2 plane is perpendicular to the CHX plane, a reduced torsional force in the excited state correlates with a lower rate for internal conversion compared to excited-state C-X bond fission. This model explains the gradual change in photodissociation mechanisms of vinyl halides from the dominance of internal conversion in vinyl fluoride to the dominance of excited-state dissociation in vinyl iodide.

This work reports measurements of absolute rate coefficients and Rice-Ramsperger-Kassel-Marcus (RRKM) master equation simulations of the C2H3+C2H4 reaction. Direct kinetic studies were performed over a temperature range of 300-700 K and pressures of 20 and 133 mbar. Vinyl radicals (H2C=CH) were generated by laser photolysis of vinyl iodide (C2H3I) at 266 nm, and time-resolved absorption spectroscopy was used to probe vinyl radicals through absorption at 423.2 nm. Measurements at 20 mbar are in good agreement with previous determinations at higher temperature. A weighted three-parameter Arrhenius fit to the experimental rate constant at 133 mbar, with the temperature exponent fixed, gives k=(7+/-1)x10(-14) cm3 molecule(-1) s(-1) (T/298 K)2 exp[-(1430+/-70) K/T]. RRKM master equation simulations, based on G3 calculations of stationary points on the C4H7 potential energy surface, were carried out to predict rate coefficients and product branching fractions. The predicted branching to 1-methylallyl product is relatively small under the conditions of the present experiments but increases as the pressure is lowered. Analysis of end products of 248 nm photolysis of vinyl iodide/ethylene mixtures at total pressures between 27 and 933 mbar provides no direct evidence for participation of 1-methylallyl.

The reactions of HCO and DCO with NO have been measured by the laser photolysis/continuous-wave (CW) laser-induced fluorescence (LIF) method from 296 to 623 K, probing the ({tilde B}{sup 2}A{prime} {l_arrow} {tilde X}{sup 2}A{prime}) HCO (DCO) system. The HCO + NO rate coefficient is (1.81 {+-} 0.10) x 10{sup -11} cm{sup 3} molecule{sup -1} s{sup -1} and the DCO + NO rate coefficient is (1.61 {+-} 0.12) x 10{sup -11} cm{sup 3} molecule{sup -1} s{sup -1} at 296 K. Both rate coefficients decrease with increasing temperature between 296 and 623 K. The kinetic isotope effect is k{sub H}/k{sub D} = 1.12 {+-} 0.09 at 296 K and increases to 1.25 {+-} 0.15 at 623 K. The normal kinetic isotope effect supports abstraction as the principal mechanism for the reaction, in agreement with recent computational results.

The pulsed laser photolysis/cw laser absorption technique is used to investigate the reaction of vinyl (C2H3) with NO in the temperature range from 295 to 700 K and pressures from 10 to 320 Torr (1.33 to 42.6 kPa). Vinyl radicals are generated by photolysis of vinyl iodide at 266 nm and detected by visible laser absorption in a vibronic band of the (A ← ) transition near 403 nm. The potential energy surface is explored with both quadratic configuration interaction and multi-reference configuration interaction ab initio calculations. These ab initio predictions are employed in RRKM theory based master equation simulations of the temperature and pressure dependent kinetics. At room temperature, the overall rate constant for removal of vinyl radical by NO is measured to be 1.6 ± 0.4 × 10−11 cm3 molecule−1 s−1, with negligible pressure dependence from 10 Torr (1.33 kPa) to 160 Torr (21.3 kPa) of helium. At constant pressure the rate constant decreases rapidly with temperature. At higher temperatures, a falloff of the rate constant to lower pressure is observed. The ab initio characterizations suggest a significant contribution from HCN + CH2O formation, with both isomerization transition states for the pathway leading to this product lying ∼15 kcal mol−1 (63 kJ mol−1) below the entrance channel. The master equation analysis provides a reasonably satisfactory reproduction of the observed kinetic data. The HCN + CH2O bimolecular channel, which proceeds from the addition complex through tight ring forming and opening transition states, has a negative temperature dependence and is the dominant channel for pressures of about 50 Torr (6.7 kPa) and lower. The theoretically predicted zero pressure rate coefficient is reproduced by the modified Arrhenius expression 5.02 × 10−11(T/298)−3.382exp(−516.3/T) cm3 molecule−1 s−1 (with T in K).

The reactions HCO ( DCO )+ O 2 have been measured by the laser photolysis/CW laser-induced fluorescence (LIF) method from 296 to 673 K, probing the ( B 2 A ′ ← X 2 A ′ ) HCO (DCO) system. The HCO ( DCO )+ O 2 rate coefficients are 5.63±0.31 and 5.61±0.23×10 −12 cm 3 molecule −1 s −1 , respectively, at 296 K; both are nearly independent of temperature between 296 and 673 K. The observed deuterium kinetic isotope effect is within the error estimate of previous measurements but is significantly smaller than recent theoretical predictions.

Using a frequency-chirped ring dye laser, we have stored and recalled as much as 48 bits of information with the copropagating stimulated-echo geometry on the (3)H(4)-(1)D(2) transition in Pr(3+):YAlO(3). Since the copropagating excitation scheme has all the excitation pulses spatially overlapping, this technique will be useful to the practical implementation of stimulated-echo memory because there is no need for alignment of beams. Our experiments also demonstrate that frequency chirping of the excitation pulses dramatically improves not only pulse-shape reproduction by the echo pulse but also the shot-to-shot stability of the amplitude of the recalled data.

We report what is to our knowledge the first two-photon-excited fluorescence spectroscopy of atomic fluorine. A doubled dye laser at 286 nm is Raman shifted in H(2) to 170 nm (sixth anti-Stokes order) to excite ground-state (2)P degrees (J) fluorine atoms to the (2)D degrees (J) level. The fluorine atoms are detected by one of two methods: observing the fluorescence decay to the (2)P(J) level or observing F(+) production through the absorption of an additional photo by the excited atoms. We have measured relative two-photon absorption cross sections to and the radiative lifetimes of the (2)D degrees (J) states.

Autoignition chemistry is central to predictive modeling of many advanced engine designs that combine high efficiency and low inherent pollutant emissions. This chemistry, and especially its pressure dependence, is poorly known for fuels derived from heavy petroleum and for biofuels, both of which are becoming increasingly prominent in the nation's fuel stream. We have investigated the pressure dependence of key ignition reactions for a series of molecules representative of non-traditional and alternative fuels. These investigations combined experimental characterization of hydroxyl radical production in well-controlled photolytically initiated oxidation and a hybrid modeling strategy that linked detailed quantum chemistry and computational kinetics of critical reactions with rate-equation models of the global chemical system. Comprehensive mechanisms for autoignition generally ignore the pressure dependence of branching fractions in the important alkyl + O{sub 2} reaction systems; however we have demonstrated that pressure-dependent 'formally direct' pathways persist at in-cylinder pressures.

Production of OH in the reaction of the neopentyl radical with O2 has been measured by a laser photolysis/cw absorption method for various pressures and oxygen concentrations at 673, 700, and 725 K. The MIT Reaction Mechanism Generator (RMG) was used to automatically generate a model for this system, and the predicted OH concentration profiles are compared to present and literature experimental results. Several reactions significantly affect the OH profile. The experimental data provide useful constraints on the rate coefficient for the formally direct chemical activation reaction of neopentyl radical with O2 to form OH (CH3)3CCH2 + O2 --OH + 3,3-dimethyloxetane (Rxn 1) At 673 K and 60 Torr, log k(1) (cm(3) molecule(-1) s(-1)) = -13.7 +/- 0.5. Absolute absorbance measurements on OH and I indicate that the branching ratio for R + O2 to OH is about 0.03 under these conditions. The data suggest that the ab initio neopentyl + O2 potential energy surface of Sun and Bozzelli is accurate to within 2 kcal mol(-1).

The absolute gas phase ultraviolet absorption spectra of trichlorovinylsilane and allyltrichlorosilane have been measured from 191 to 220 nm. Over this region the absorption spectra of both species are broad and relatively featureless, and their cross sections increase with decreasing wavelength. The electronic transitions of trichlorovinylsilane were calculated by ab initio quantum chemical methods and the observed absorption bands assigned to the A(1)A''-- X[combining tilde](1)A'' transition. The maximum absorption cross section in the region, at 191 nm, is sigma = (8.50 +/- 0.06) x 10(-18) cm(2) for trichlorovinylsilane and sigma = (2.10 +/- 0.02) x 10(-17) cm(2) for allyltrichlorosilane. The vinyl radical and the allyl radical are formed promptly from the 193 nm photolysis of their respective trichlorosilane precursors. By comparison of the transient visible absorption and the 1315 nm I atom absorption from 266 nm photolysis of vinyl iodide and allyl iodide, the absorption cross sections at 404 nm of vinyl radical ((2.9 +/- 0.4) x 10(-19) cm(2)) and allyl radical ((3.6 +/- 0.8) x 10(-19) cm(2)) were derived. These cross sections are in significant disagreement with literature values derived from kinetic modeling of allyl or vinyl radical self-reactions. Using these cross sections, the vinyl radical yield from trichlorovinylsilane was determined to be phi = (0.9 +/- 0.2) per 193 nm photon absorbed, and the allyl radical yield from allyltrichlorosilane phi = (0.7 +/- 0.2) per 193 nm photon absorbed.

Surface enhanced Raman spectroscopy has become a viable technique for the detection of single molecules. This highly sensitive technique is due to the very large (up to 14 orders in magnitude) enhancement in the Raman cross section when the molecule is adsorbed on a metal nanoparticle cluster. We report here SERS (Surface Enhanced Raman Spectroscopy) experiments performed by adsorbing analyte molecules on nanoscale silver particle clusters within the gelatin layer of commercially available holographic plates which have been developed and fixed. The Ag particles range in size between 5 - 30 nanometers (nm). Sample preparation was performed by immersing the prepared holographic plate in an analyte solution for a few minutes. We report here the production of SERS signals from Rhodamine 6G (R6G) molecules of nanomolar concentration. These measurements demonstrate a fast, low cost, reproducible technique of producing SERS substrates in a matter of minutes compared to the conventional procedure of preparing Ag clusters from colloidal solutions. SERS active colloidal solutions require up to a full day to prepare. In addition, the preparations of colloidal aggregates are not consistent in shape, contain additional interfering chemicals, and do not generate consistent SERS enhancement. Colloidal solutions require the addition of KCl or NaCl to increase the ionic strength to allow aggregation and cluster formation. We find no need to add KCl or NaCl to create SERS active clusters in the holographic gelatin matrix. These holographic plates, prepared using simple, conventional procedures, can be stored in an inert environment and preserve SERS activity after several weeks subsequent to preparation.

We report the first detection by optical means of highly vibrationally excited H2 X1Σ+g(vx=6–11). Vibrationally excited H2 was generated using a recently discovered hot‐wire effect in H2 gas, and was detected in 40 bands with 2+1 resonantly enhanced multiphoton ionization via the EF state (vEF=0–14). Rotational temperatures are in the range 200–650 K, well below that required for thermal excitation of the observed vibrational levels.

A coumarin laser dye has been found to produce efficiencies and lifetimes comparable to those of the rhodamines. Coumarin 521 (coumarin 334) produces up to 400 mW of single frequency light (frequency width

Chlorinated hydrocarbons (CHC) have a myriad of industrial applications including solvents, feedstocks, and the components of polymeric materials. The byproduct of these industrial uses is an enormous quantity of liquid waste. Incineration is an attractive treatment process for this hazardous waste, however, the current technology is costly and highly empirical. Better fundamental knowledge of incinerator chemistry and improved diagnostic techniques will improve incinerator design and help evaluate innovative CHC destruction schemes. The development of real-time monitors of the effluent stream is necessary to satisfy environmental concerns and insure compliance of emission regulations.

The reaction of trichlorosilane (HSiCl(3)) with atomic chlorine (Cl) has been investigated by using infrared kinetic spectroscopy of the HCl product. The overall second order rate constant for the reaction has been determined as a function of temperature by using pseudo-first-order kinetic methods. Formation of HCl (nu=0) was monitored on the (nu=1

Theory and measurements are presented for stimulated Brillouin gain spectroscopy in anisotropic media. The coupling between arbitrary pump and Stokes waves and the corresponding acoustic wave is formulated in terms of the elastic displacement vector and the photoelastic tensor. A propagation equation that fully includes transient effects is obtained for the Stokes wave. In the limit of small-signal gains this propagation equation can be solved analytically, yielding expressions that relate experimentally accessible quantities to material properties. Absolute Brillouin steady-state gain coefficients, linewidths, and frequency shifts are thereby determined in a number of optical materials at 532 nm. The Brillouin gain coefficient for fused silica is measured by three techniques, providing the reference for absolute gain measurements. Quantitative agreement is found between theory and experiment for transient effects on stimulated Brillouin scattering.

We report what is to our knowledge the first high-resolution Brillouin gain spectrum in a solid. Resonances corresponding to longitudinal (compressional) and transverse (shear) acoustic waves in fused silica are observed with good resolution and a high signal-to-noise ratio. Absolute gain coefficients, linewidths, and Brillouin frequency shifts are measured. The agreement with previously measured values is good.

Iso-C 3 H 7 S radicals were generated by 248 nm photodissociation of i -C 3 H 7 SH and laser-induced fluorescence of the A-X transition was studied in the 395–435 nm region. A prominent series of bands in the excitation spectrum is interpreted as transitions to levels of the C-S stretching mode in the excited state, which has a frequency of 347 ± 10 cm −1 . Based on an expected radiative lifetime of about 800 ns, the observed zero-pressure lifetime of ≈ 120 ns suggests that all of the vibrational levels of the A state are strongly predissociated. Electronic quenching of i -C 3 H 7 S(A) is observed for a variety of collision partners.

The laser-induced fluorescence technique has been used to study the reactions of the i-C/sub 3/H/sub 7/S radical with O/sub 2/, NO/sub 2/, and NO at 296 K. The i-C/sub 3/H/sub 7/S radicals were made by photodissociation of i-C/sub 3/H/sub 7/SH at 248 nm. The reactions with O/sub 2/ and NO/sub 2/ have rate coefficients of (1.1 +- 0.1) x 10/sup -14/ and (5.9 +- 0.6) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/, respectively. The reaction of i-C/sub 3/H/sub 7/S with NO involves a third body and has been shown to be in the transition region between the low- and high-pressure limits (although much closer to the latter). With the use of an expression developed by Troe to fit the results, values of k infinity = (4.0 +- 0.4) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ and k/sub 0/ = (1.7 /sub -0.6//sup +0.9) x 10/sup -26/ cm/sup 6/ molecule/sup -2/ s/sup -1/ have been obtained.

Photodissociation of NO has been investigated in the energy region between 8.89 eV, the energy required to produce N(2D)+O(3 P), and 9.26 eV, the ionization limit. Using the technique of resonance‐enhanced multiphoton ionization (REMPI), we have demonstrated that there are areas in this energy region in which there are large yields of N(2D), and others where the yields are small. Localized concentrations, from 2‐hν dissociation of NO at 270 nm, are an order of magnitude greater than can be obtained from a discharge in N2.

Long-lived (≈650 ns) S 2 (B → X) emission is observed in the 193 nm photodissociation of (CH 3 S) 2 . The same emission can also be produced by LIF pumping of S 2 (B ← X) following the photodissociation. The emission is enhanced by the presence of a buffer gas; He, Ar, N 2 , and SF 6 are similarly effective, and give the same lifetime. The long lifetime arises from collisional mixing with the slightly lower (790 cm −1 ) B'' 3 Π u state, which can serve as the long-lived precursor.

C 2 H 5 S radicals were generated by 248 nm photodissociation of C 2 H 5 SH and C 2 H 5 SSC 2 H 5 and laser-induced fluorescence on the A-X transition was studied. Prominent progressions are found in the C-S stretching mode which has a frequency of 408 ± 8 cm −1 in the excited state and 681 ± 15 cm −1 in the ground state. The 0 0 0 band head lies at 440.0 nm. All the bands are highly predissociated with the lifetime decreasing from 75 ns for the 0 0 0 band to ≈ 15 ns for the 3 0 6 band.

The laser-induced fluorescence technique has been used to study the reactions of the ethylthio radical (C/sub 2/H/sub 5/S) with NO, NO/sub 2/,and O/sub 2/ at 296 K. The reaction of C/sub 2/H/sub 5/S with NO involves a third body and has been shown to be in the transition region between the low- and high-pressure limits. Using an expression developed by Troe to fit the results, they have obtained values of the broadening factor F = 0.49 /sub /minus/0.06//sup /plus/0.04/, k/sub infinity/ = (5.2 /plus minus/ 0.5) /times/ 10/sup /minus/11/ cm/sup 3/ molecule/sup /minus/1/ s/sup /minus/1/, k/sub 0/(He,Ar) = (7.5/sub /minus/1.5//sup /plus/3.5/) /times/ 10/sup /minus/28/ cm/sup 6/ molecule/sup /minus/2/ s/sup /minus/1/, and k/sub 0/(SF/sub 6/) = (2.2/sub /minus/0.5//sup /plus/1.2/) /times/ 10/sup /minus/27/ cm/sup 6/ molecule/sup /minus/2/ s/sup /minus/1/. The reaction with NO/sub 2/ has a rate coefficient of (9.2 /plus minus/ 0.9) /times/ 10/sup /minus/11/ cm/sup 3/ molecule/sup /minus/1/ s/sup /minus/1/. No reaction could be found with O/sub 2/, for which an upper limit on the rate coefficient of 2 /times/ 10/sup /minus/17/ cm/sup 3/ molecule/sup /minus/1/ s/sup /minus/1/ was estimated.

HS radicals were generated by photodissociation of H2S at 193 nm and their disappearance monitored by LIF. The three‐body recombination of HS with NO has been studied over the temperature range 250–445 K using He, Ar, and N2 as diluent gases. The temperature dependences of the low‐pressure, three‐body rate coefficients are given by 10−(22.56±0.60)T−(3.28±0.27), 10−(23.27±0.50)T−(2.98±0.20), and 10−(24.43±0.94)T−(2.48±0.36) cm6 molecule−2 s−1 for He, Ar, and N2, respectively. A tentative value of (2.7±0.5)×10−11 cm3 molecule−1 s−1 for the high pressure limiting rate coefficient over the range 250–300 K is suggested on the basis of extrapolation. These kinetic data are evaluated in terms of unimolecular rate theory.

N(2D) can be generated by two‐photon dissociation of NO above the threshold wavelength of 279 nm. A comparison of the three‐photon NO+ ionization signal with the N(2D) excitation spectrum resulting from its detection by 2+1 resonance‐enhanced multiphoton ionization (REMPI) shows both similarities and differences. The NO Rydberg states that are generated in the initial two‐photon process either ionize or predissociate. The NO+ and N+ signals are indicative of predissociative lifetimes and channels, since an NO molecule that does not predissociate to N(2D) and O(3P) can separate to two lower atomic limits. In the 268–279 nm region there are three two‐photon absorption bands, 5sσ‐X 3‐0, 4dπ−‐X 2‐0, and 5dπ−‐X1‐0, which show very high N(2D) predissociative yields from the upper states. A fourth band, 4dδ‐X2‐0, behaves in a distinctly different manner from its 4dπ− complex partner. The presence of the valence G2Σ− state, which lies within the experimental energy range, is not in evidence from either the three‐photon NO+ spectrum, or from predissociation in the N(2D) channel. It appears that this state predissociates rapidly to the lower N(4S)+O(1D) or N(4S)+O(3P) limits, even above the N(2D)+O(3P) threshold.

CS radicals have been produced by photodissociation of CS 2 at 193 nm and their disappearance monitored by LIF. The vibrationally excited CS radicals rapidly relax to CS(ν = 0). At 298 K, the rate coefficients for CS(ν = 0) reactions with O 2 , O 3 and NO 2 are (2.9 ± 0.4) × 10 −19 , (3.0 ± 0.4) × 10 −16 and (7.6 ± 1.1) × 10 −17 cm 3 molecule −1 s −1 respectively. The quenching of CS(A 1 II) ν=0 by He has a rate coefficient of (1.3 ± 0.2) × 10 −12 cm 3 molecule −1 s −1 .

The technique of resonance-enhanced multiphoton ionization (REMPI) has been applied to the determination of the rate coefficient for the reaction N(/sup 2/D) + O(/sup 3/P) ..-->.. N(/sup 4/S) + O. N(/sup 2/D) is generated in a discharge flow system and detected by a 2 + 1 photon process at 269 nm. The rate coefficient is 3.4 (/sub -1.1//sup +1.5/) /times/ 10/sup /minus/11/ exp(-(145 /plus minus/ 107)/T) cm/sup 3/ molecule/sup /minus/1/ s/sup /minus/1/, measured over a temperature range of 196-465 K, and is a factor of 3-4 times larger than the value for the N(/sup 2/D) + O/sub 2/ interaction, measured in a similar manner. The new value for N(/sup 2/D) + O(/sup 3/P) exceeds earlier estimates by an order of magnitude, while current thermospheric models use a value for the N(/sup 2/D) + O(/sup 3/O) rate coefficient 40 times smaller than reported here. Application of the new data may have significant effects on deduced N(/sup 2/D) source terms in the atmosphere and on NO concentration profiles and resulting temperature profiles and may require reevaluation of the N(/sup 2/D) radiative lifetime.

S(3 3 P 0 ) atoms have been produced by quenching S( 1 D) atoms and, directly, in the photodissociation of CS 2 at 193 nm. Resonance-enhanced photoionization has been used to follow their decay back to a Boltzmann population. Rate coefficients for quenching S( 3 P 0 ) have been measured for Ar, He, N 2 , O 2 , CH 4 , N 2 O, OCS, SF 6 , CO 2 , and H 2 (in order of increasing rate coefficient) at 300 K.

F/sub 2/ (158 nm) and ArF (193 nm) excimer radiation have been used to photodissociate C/sub 2/N/sub 2/ and produce chemiluminescence in C/sub 2/N/sub 2/-O/sub 2/ mixtures. In addition, LIF measurements have followed the temporal behavior of the CN and NCO radicals. The behavior of CN is understood in terms of its reactions with O/sub 2/ and O. The NCO decay is controlled by radical or atom intermediates. This study was focused on the NO ..gamma..-band chemiluminescence, and evidence is presented to support the view that it arises from energy transfer from N/sub 2/(A) to NO and the N/sub 2/(A) is made by the reaction N(/sup 2/D) + NCO ..-->.. N/sub 2/(A) + CO. An estimate of approx. = 25% is obtained for the N/sub 2/(A) yield of this reaction.

Resonance-enhanced photoionization has been used to follow S(3P2) in the photodissociation of CS2 at 193 nm. The contributions from initial photodissociation and from S(1D) relaxation have been resolved and give a (15±5)% yield of S(1D). The possibility of secondary production of S(3Pj) by CS photodissociation with a second 193 nm photon is discussed. Although this might raise the S(1D) yield to (26±8)%, production of S(3PJ) is still the dominant photodissociation channel.

A flowing afterglow in a 1% N 2 /Ar mixture at 10 Torr has been used as a source of N( 2 D), and resonance-enhanced multiphoton ionization (REMPI) at 269 nm has been used to follow the removal of the N( 2 D) by C 2 N 2 and O 2 . The rate of removal by O 2 is consistent with previous work. The rate coefficient for N( 2 D)/C 2 N 2 interaction has been measured as (1.5±0.2) × 10 −11 cm 3 molecule −1 s −1 at 300 K, although the products have not yet been determined.

The technique of two-photon excited fluorescence (TPEF) was first demonstrated in 1981 for the light atomic radicals of oxygen [1], nitrogen [1] and hydrogen [2]. Of the light radical atoms, only fluorine has yet to be studied using this technique. We report here the first observation of two-photon excitation of atomic F at 170 nm followed by the detection of near ir fluorescence and ionization. In addition, we report the first Doppler-free spectra of atomic 0 at 226 nm using a single-frequency laser.

Brillouin scattering refers to the scattering of light by acoustic waves. Stimulated Brillouin scattering plays an important role in limiting optical power transmission for both high power applications, such as laser fusion, and long path applications, such as optical fiber communications.

Although 248-nanometer radiation falls 0.12 electron volt short of the energy needed to dissociate O(2) large densities of ozone (O(3)) can be produced from unfocused 248-nanometer KrF excimer laser irradiation of pure O(2). The process is initiated in some undefined manner, possibly through weak two-photon O(2) dissociation, which results in a small amount of O(3) being generated. As soon as any O(3) is present, it strongly absorbs the 248-nanometer radiation and dissociates to vibrationally excited ground state O(2) (among other products), with a quantum yield of 0.1 to 0.15. During the laser pulse, a portion of these molecules absorb a photon and dissociate, which results in the production of three oxygen atoms for one O(3) molecule destroyed. Recombination then converts these atoms to O(3), and thus O(3) production in the system is autocatalytic. A deficiency exists in current models of O(3) photochemistry in the upper stratosphere and mesosphere, in that more O(3) iS found than can be explained. A detailed analysis of the system as it applies to the upper atmosphere is not yet possible, but with reasonable assumptions about O(2) vibrational distributions resulting from O(3) photodissociation and about relaxation rates of vibrationally excited O(2) a case can be made for the importance of incuding this mechanism in the models.

A new method for absolute calibration of the detection efficiency of fluorescence collection systems is described for wavelengths covering the 180–2000-nm range. The radiation source for the technique is spontaneous Raman scattering in H2. An analytic expression is derived for the scattering cross section averaged over the solid angle of the fluorescence collection system. From this result, an optical system is calibrated at 845 nm, demonstrating that the technique is easy to use, is quick to implement, and gives accurate results.

The study of the electronic spectrum of atomic fluorine is extremely difficult since all the single photon transitions from the ground state require wavelengths that are below the LiF window cutoff at 106 nm. This situation presents an ideal application for the techniques of multiphoton excitation, followed by either fluorescence or ionization detection. We report here the first observation of three-photon resonant excitation of the 2 P1/2,3/2 excited states in atomic fluorine at 105,000 cm-1. This state is then ionized by the absorption of two additional photons forming a five-photon process. The experiment consists of focusing a doubled YAG-pumped dye laser at 286.3 nm into a flowing cell containing a mixture of F2 and He. The UV radiation completely dissociates the F2 in the focal volume and simultaneously ionizes the atomic fluorine. The ions are collected with a pair of biased electrodes and the charge is amplified. We estimate that ~104 ions are produced in the focal volume for a 1:1 signal/noise ratio. We also observe three-photon resonant four-photon ionization of molecular F2 in the same region. This is the first observation of the MPI spectrum of F2 and most of the observed features have been assigned using previous data.1 We have carefully measured the intensity of our laser in the focal volume and present estimates of the effective five-photon ionization cross section for detection applications.

Before multiphoton detection schemes for atoms and molecules can be placed on a quantitative footing, cross sections for the multiphoton transitions must be known. We have measured the relevant cross sections for atomic oxygen, an important species in flame diagnostics. The technique of two-photon excited fluorescence was used to measure the cross section for the 2p3p2 → 3p3p2,1,0 transition at 226 nm. Excitation was monitored by observing fluorescence at 845 nm from the 3p 3p2,1,0 → 3s 3S1 transition. The fluorescence collection system was calibrated absolutely using spontaneous Raman scattering from molecular hydrogen. The spatial and temporal profiles of the exciting dye laser pulses were carefully measured in a gently focused excitation geometry (peak intensity ~10 MW/cm2) allowing the two-photon cross section to be determined. The cross section for absorption of a third, identical photon (2 + 1 photoionization) was then determined by measuring the absolute number of ions produced per laser pulse. The measured cross sections are α = 2.5 × 10−28 cm4/W and σpi = 8.1 × 10−19 cm2. Using a square pulse in space and time, we calculate that ~ 1% ionization is possible for a laser fluence at 226 nm of 1 J/cm2 in a 10-ns long pulse.

Radiative lifetimes for CH3S(A 2A1) in the υ3 =0, 1, and 2 levels are 760±60, 810±60, and 197±15 ns, respectively. These results are in disagreement with previous work on the υ’3 =0 and 1 levels although in agreement for the υ3 =2 level. Since the previous work was in a beam, it is possible that loss from the viewing region contributed to the shorter lifetimes reported previously for the lower levels. Quenching by (CH3S)2 is fast for all three levels.