Leonard E. Jusinski

Temperature dependence and deuterium kinetic isotope effects in the HCO (DCO) + O2 reaction between 296 and 673 K

Abstract 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.

Temperature-dependent kinetics of the vinyl radical (C2H3) self-reaction.

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.

Efficient and stable operation of an Ar+-pumped continuous-wave ring laser from 505–560 nm using a coumarin laser dye

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 <1 MHz) at 520 nm in a ring laser when excited with 2.4 W of 457.9 nm light from an Ar+ ion laser. Operation with a 488 nm pump shows slightly lower efficiency. Dye lifetime to 75% full power has exceeded 1000 W h over a period of several months. Methanol and ethylene glycol are the only solvents required. This dye operates in a portion of the tuning range of a continuous-wave ring dye laser which is otherwise extremely difficult to access using an argon ion laser as the pump source. Used in conjunction with an external buildup cavity, 8–12 mW of 258 nm light is reliably produced for weeks at a time.

Ultraviolet photodissociation of vinyl iodide: understanding the halogen dependence of photodissociation mechanisms in vinyl halides

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.

Kinetics of the reaction of vinyl radicals with NO: Ab initio theory, master equation predictions, and laser absorption measurements

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).

Reaction of chlorine atom with trichlorosilane from 296to473K

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<--0) R(2) line at 2944.914 cm(-1) and that of HCl (nu=1) on the (nu=2<--1) R(2) line at 2839.148 cm(-1). The overall second order rate constant was determined to be (2.8+/-0.1)x10(-11) cm(3) molecule(-1) s(-1) at 296 K. The rate constant shows no pressure dependence and decreases slightly with increased temperature [k=(2.3+/-0.2)x10(-11)e((66+/-3)/T) cm(3) molecule(-1) s(-1)]. Substantial vibrational excitation is measured in the HCl product, with the fraction of HCl (nu=1)/HCl (total)=0.41+/-0.08. These observations are consistent with the reaction being a barrierless hydrogen abstraction reaction. The experimental results are supported by ab initio quantum chemical calculations that show the transition state for abstraction to lie below the energy of the reactants, in disagreement with previously published calculations.

Frequency-resolved optical gating using cascaded second-order nonlinearities

We demonstrate a method that yields intuitive traces, unambiguous pulse measurement, and sufficient sensitivity to measure unamplified pulses, and it is simple to implement. It uses FROG cascaded X(2) effects for the optical nonlinearity, in this case, upconversion followed by downconversion.

Surface enhanced Raman spectroscopy (SERS) from a molecule adsorbed on a nanoscale silver particle cluster in a holographic plate

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.