John R. Wiesenfeld

Photodissociation of alkyl iodides

The branching ratios for formation of I (5 2P1/2) and I (5 2P3/2) following broad‐band photolysis of a number of alkyl iodides and their fluorinated and deuterated analogs have been directly determined using time‐resolved atomic absorption spectroscopy. The yields of ground and electronically excited atoms are strongly dependent on the structure of the alkyl group, those possessing secondary or tertiary α‐carbon atoms displaying the lowest yields. The observed correlation between alkyl group structure and branching ratio is discussed using simple symmetry arguments to assess the role of spin–orbit coupling in the photodissociative excited state.

Full characterization of OH product energetics in the reaction of O(1D2) with hydrocarbons

The energetics of the OH(X 2Π, 0≤ν‘≤4) product arising from the reaction of O(1D2) with the hydrocarbons CH4, C2H6, C3H8, and C(CH3)4 was fully characterized using laser‐induced fluorescence (LIF). The product distribution is in sensible accord with earlier more limited LIF and infrared chemiluminescence studies, and the overall yield of OH decreases dramatically in the case of the heavier hydrocarbons as would be expected if dissociation of the collision intermediate was dominated by rupture of the relatively weak C–C bond. The energetics of the O(1D2)/CH4 reaction suggest that it proceeds via an insertion/elimination reaction, while that of O(1D2) with the heavier hydrocarbons appears to involve two parallel mechanisms. The major channel yields vibrationally and rotationally cool OH; by comparison with abstraction of hydrogen by O(3PJ) which preferentially yields vibrationally excited OH, this channel is associated with dissociation of a long‐lived complex. The highly excited component of OH population ...

Collisional deactivation of O(2 1D2) by the atmospheric gases

The thermal rate constants for collisional deactivation of electronically excited oxygen atoms, O(2 1D2), by N2, O2, N2O, CO2, CH4, and H2O have been measured at 295 K. The optically metastable atoms were produced by pulsed photolysis of O3 at 248 nm using a KrF excimer laser and monitored by following the increase in O(2 3PJ) concentration following quenching of O(2 1D2) through the use of time‐resolved atomic resonance spectroscopy. This method of analysis obviated difficulties associated with the evaluation of the curve‐of‐growth law which were encountered in previous attempts to measure the removal of O(2 1D2) using a similar technique. Rate constants reported here are in excellent agreement with those measured by observation of the weak O(2 1D2) →O(2 3PJ) emission at 630 nm. In addition, it was possible to determine the extent of physical quenching in the deactivation of O(2 1D2) by H2O, CO2, CH4, and N2O.

Reaction dynamics of O(1D2)+H2, HD, D2: OH, OD(X 2Πi) product internal energy distributions

Partial internal energy distributions of the hydroxyl reaction products of O(1D)+H2, HD, and D2 reactions are presented. Inverted rotational distributions, preferential population of the π+ lambda doubling sublevels, and statistical population of the spin sublevels are observed. A slight preferential formation of the OD vs OH reaction products observed is measured for the reaction of O(1D)+HD. Surprisal analysis of these results indicates both dynamical and kinematic constraints on the reaction dynamics. Comparison of these results with published model calculations suggest that an insertion mechanism to form a highly energetic collision complex dominates the reaction dynamics.

Dynamics of the reaction O(1D2)+H2→OH(X 2Π,v″=2,3)+H: Full characterization of product energetics

The product OH(X 2Π) resulting from the subject reaction has been detected in v‘=2 and v‘=3 with full resolution of N‘, f‘, λ‘ sublevels using LIF spectroscopy in the off‐diagonal Δv=−2 sequence bands in the region 385–409 nm. As was noted previously for OH in the v‘=0 and 1 states, strongly inverted rotational distributions were found; in the present case, efficient OH production was observed up to the available exoergicity of the reaction. Production of the π+ component was again seen to be significantly more probable than that of the π−. The ratio of summed populations in the two observed vibrational levels P(v‘=3)/P(v‘=2)=0.39±0.02 is smaller than that observed by other methods; experimental uncertainties in all methods used to date are discussed. The current observations are consistent with a mechanism in which O(1D2) inserts into the H–H bond to form a highly excited H–O–H complex which then dissociates. No evidence was obtained for a parallel process in which an H atom is directly abstracted by the...

Multiphoton ionization of O2 X 3Σ−g, a 1Δg, and b 1Σ+g via the two‐photon resonant nsσg, ndσg, and ndπg Rydberg levels

Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ−g, a 1Δg, and b 1Σ+g states of O2. The 0–0 band of the C 1Πg state is reported for the first time. Measurements of other vibrational bands terminating in the C 3Πg and d 1Πg states are in good agreement with determinations by other groups. Several vibrational levels (v’=0–5) of the 3dπg Rydberg complex have been assigned on the basis of (1) an analysis of the spin–orbit couplings between the (Λ,S) basis‐set states, (2) spectral simulation, and (3) the behavior of the states when the excitation radiation is changed from linear to circular polarization.

Direct observation of O(3PJ) in the photolysis of O3 at 248 nm

The photodissociation of O3 at 248 nm has been studied following photolysis with a KrF excimer laser. The results of the current experiments, which were carried out in absorption using improved high‐speed detection electronics, are in good agreement with previous direct observations of significant O(3PJ) photofragment yield following photolysis at 266 nm. A quantitative estimate of the quantum yield for O(1D2) production at 248 nm is 0.85±0.02. The implications of this observation for earlier studies of O(1D2) reactivity are discussed.

Product energy distribution of hydroxyl radicals in O(1D2) + HCl → OH + Cl

Abstract Reaction of electronically excited oxygen atoms with HCl yields highly vibrationally and rotationally excited OH displaying the propensity for Π (A′) λ sublevel production characteristic of reactions in which the rotation plane of the OH product coincides with the plane of a triatomic intermediate complex. Because the rotational distribution differs significantly from that expected from direct HLH ′ abstraction, the reaction probably proceeds via the HOCl( 1 A′) surface that favors formation of the OH diatomic. The vibrational distribution agrees with that previously observed by infrared chemiluminescence; earlier observation of OH (X 2 Π) using LIF could not be confirmed.

Electronic quenching of highly rotationally excited OH(A 2Σ, ν′=0, 1) by H2O

Abstract Population of specific high-lying rotational levels of OH (A 2 Π) in ν′=0 and 1 is achieved by tunable dye laser excitation of the A←X transition of the OH product formed in O( 1 D)+H 2 O→2 OH(ν′=0, 1). Observation with a fast transient recorder/signal averager of the spontaneous A→X emission temporal profile as a function of H 2 O pressure yields the rate constant, k H 2 O , for collisional quenching following excitation of specific OH(A 2 Π, ν′, N ′, F ′) states. This rate decreased with increasing rotational excitation in sensible agreement with a model in which long-range attractive forces on a highly directional potential surface dominate collisional quenching. Extrapolation of the observed emission lifetimes to P H 2 O =0 yields optical lifetimes in good agreement with previously reported values.

Time‐resolved resonance fluorescence studies of O(1D2) yields in the photodissociation of O3 at 248 and 308 nm

The yield of electronically excited oxygen atoms O(1D2) following excimer laser photolysis of ozone at 248 and 308 nm was directly measured using time‐resolved resonance fluorescence emission at ≊130 nm. The result obtained at 248 nm is in good agreement with a previous resonance fluorescence measurement in which the yield of ground‐state O(3PJ) was determined in the presence of gases which either quenched or reacted with O(1D2). The relative yield determined at 308 nm 0.79±0.02 is the first such determination in the region of weak Hartley band absorption near the thermochemical threshold for production of O(1D2)+O2(a1Δg). When taken in conjunction with the previously measured relative yields of O(1D2) at this wavelength, this result suggests that the yield of electronically excited oxygen atoms at 300 nm is 0.96±0.02.