Vacuum Ultraviolet Laser-induced Fluorescence Spectroscopy Research Articles

Probing the kinetic energy-release dynamics of H-atom products from the gas-phase reaction of O(3P) with vinyl radical C2H3.

The gas-phase radical-radical reaction dynamics of ground-state atomic oxygen O((3)P) with vinyl radicals C2H3 has been studied by combining the results of vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed beam configuration with ab initio calculations. The two radical reactants O((3)P) and C2H3 were produced by photolysis of NO2 and supersonic flash pyrolysis of C2H3I, respectively. Doppler profile analysis of the kinetic energy release of the nascent H-atom products from the title reaction O((3)P) + C2H3→ H((2)S) + CH2CO (ketene) revealed that the average translational energy of the products and the average fraction of the total available energy were 7.03 ± 0.30 kcal mol(-1) and 7.2%. The empirical data combined with CBS-QB3 level ab initio theory and statistical calculations demonstrated that the title oxygen-hydrogen exchange reaction is a major reaction channel, through an addition-elimination mechanism involving the formation of a short-lived, dynamical complex on the doublet potential energy surface. On the basis of systematic comparison with several exchange reactions of hydrocarbon radicals, the observed kinetic energy release can be explained in terms of the weak impulse at the moment of decomposition in the loose transition state with a product-like geometry and a small reverse barrier along the exit channel.

Laser-induced fluorescence study of spin–orbit quenching of Cl(2P1/2) by H2, D2, and HD gases

Employing ultraviolet laser flash photolysis in combination with pulsed vacuum ultraviolet laser induced-fluorescence spectroscopy room-temperature rate constants kQ,H2=(5.8±0.6)×10−11cm3s−1, kQ,D2=(5.6±0.6)×10−12cm3s−1, and kQ,HD=(2.7±0.3)×10−11cm3s−1 for electronically non-adiabatic quenching of spin–orbit excited Cl(2P1/2) atoms in collisions with H2, D2, and HD gases were determined, which allow for comparison with results of recent quantum-scattering calculations [14,15].

A comparative study of hydrogen-atom release dynamics in radical–radical reactions

This paper presents a comparative study of the atomic hydrogen release dynamics in the oxidation reactions of saturated and unsaturated hydrocarbon radicals. The prototypal radical–radical reactive scattering processes were examined by applying a combination of crossed beams and high-resolution vacuum-ultraviolet laser-induced fluorescence spectroscopy. With the aid of ab initio and statistical calculations, remarkable dynamic features were uncovered in the Doppler profile analysis of the atomic hydrogen products. The difference in the reactivity of saturated and unsaturated hydrocarbon radicals could be rationalized in terms of the characteristic geometry of the transition state and activation barriers along the exit channel on the doublet potential energy surface.

Crossed-beam radical-radical reaction dynamics of O(P3)+C3H3→H(S2)+C3H2O

The radical-radical oxidation reaction, O(3P)+C3H3 (propargyl)-->H(2S)+C3H2O (propynal), was investigated using vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed-beam configuration, together with ab initio and statistical calculations. The barrierless addition of O(3P) to C3H3 is calculated to form energy-rich addition complexes on the lowest doublet potential energy surface, which subsequently undergo direct decomposition steps leading to the major reaction products, H+C3H(2)O (propynal). According to the nascent H-atom Doppler-profile analysis, the average translational energy of the products and the fraction of the average transitional energy to the total available energy were determined to be 5.09+/-0.36 kcal/mol and 0.077, respectively. On the basis of a comparison with statistical prior calculations, the reaction mechanism and the significant internal excitation of the polyatomic propynal product can be rationalized in terms of the formation of highly activated, short-lived addition-complex intermediates and the adiabaticity of the excess available energy along the reaction coordinate.

Rate constants of the O( 1D) reactions with N 2, O 2, N 2O, and H 2O at 295 K

Using a technique of laser flash photolysis coupled with vacuum ultraviolet laser-induced fluorescence spectroscopy, the rate coefficients of O( 1D) reactions with N 2, O 2, N 2O, and H 2O at 295 ± 2 K have been determined to be k N 2 = ( 3 . 29 ± 0 . 27 ) × 10 - 11 , k O 2 = ( 4 . 06 ± 0 . 24 ) × 10 - 11 , k N 2 O = ( 1 . 35 ± 0 . 08 ) × 10 - 10 and k H 2 O = ( 2 . 07 ± 0 . 18 ) × 10 - 10 cm 3 molecule - 1 s - 1 . The quoted uncertainties include estimated errors and are the 95% confidence level. The k N 2 and k N 2 O values obtained are larger than the current NASA/JPL recommendations by 26% and 16%, respectively, although they are still within the error limits associated with the recommendations.

Laser-induced fluorescence study of the quenching of Cl( 2P 1/2) in collisions with N 2 molecules and rare gas atoms

The rate constants of physical quenching of Cl( 2P 1/2) in collisions with N 2 molecules and rare gas atoms (Rg = He, Ne and Ar) at 295 ± 2 K were investigated using a technique of vacuum ultraviolet laser-induced fluorescence spectroscopy. The rate constants determined are (7.6 ± 0.8) × 10 −15, (5.7 ± 0.3) × 10 −14, ⩽8 × 10 −16 and ⩽5 × 10 −16 cm 3 molecule −1 s −1 for collision partners of N 2, He, Ne and Ar, respectively.

Quantum Yield for O(1D) Production from Ozone Photolysis in the Wavelength Range of 193−225 nm

The quantum yield values for O(1D) production from the photodissociation reaction of O3 between 193 and 225 nm at 298 ± 2 K are reported. The O(1D) photofragments have been detected directly using a technique of vacuum ultraviolet laser-induced fluorescence spectroscopy at 115.22 nm which is associated with the electronic transition of O(3s1D° − 2p1D). It has been found that the O(1D) quantum yield values decrease monotonically as the photolysis wavelength becomes shorter from 0.90 ± 0.12 (225 nm) to 0.48 ± 0.03 (193 nm). Photodissociation processes of O3 around 210 nm and its atmospheric implications have been discussed.

Formation of O(3P) Atoms in the Photolysis of N2O at 193 nm and O(3P) + N2O Product Channel in the Reaction of O(1D) + N2O

The O(3P) atom produced in the 193 nm photolysis of N2O has been detected by a technique of vacuum-ultraviolet laser-induced fluorescence spectroscopy around 130 nm. The quantum yield value of the O(3P) atoms produced directly in the photolysis of N2O at 193 nm at room temperature has been determined to be 0.005 ± 0.002. The O(3P) atom formation process in the reaction of O(1D) + N2O is also studied, and the channel branching ratio of O(3P) + N2O has been determined to be 0.04 ± 0.02 among the product channels, 2NO, N2 + O2, and O(3P) + N2O. Photodissociation processes of N2O at 193 nm and reaction processes of O(1D) + N2O system have been discussed on the basis of the experimental results. Because of importance of the O(1D) + N2O reaction in the stratosphere, impact of the experimental result of O(3P) formation from the O(1D) + N2O reaction on the stratospheric chemistry is also studied by one-dimensional atmospheric model calculations.

Determination of the heat of formation of O3 using vacuum ultraviolet laser-induced fluorescence spectroscopy and two-dimensional product imaging techniques

Two different techniques, vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy and two-dimensional (2D) ion counting product imaging, have been used to determine the bond energy for the dissociation of jet-cooled O3 into O(1D)+O2(a 1Δg). The photofragment excitation (PHOFEX) spectrum for O(1D) products is recorded by detecting the VUV-LIF signal associated with the 3s 1D0–2p 1D transition at 115.22 nm while scanning the photolysis laser wavelength between 305 and 313 nm. A clear cut-off corresponding to the appearance threshold into O(1D)+O2(a 1Δg) is observed in this PHOFEX spectrum. The 2D image of the O(1D) products from the O3 photolysis near 305 nm is measured using an ion-counting method, with the detection of O(1D) atoms by [2+1] resonance enhanced multiphoton ionization (REMPI) at 205.47 nm. The kinetic-energy distribution obtained from the 2D image shows rotational structure due to the O2(a 1Δg,v″=0) fragment. The bond energy into O(1D)+O2(a 1Δg) has been obtained from the rotational assignments in the kinetic-energy distribution. The two different experimental approaches give consistent results and an accurate value of the bond dissociation energy into O(1D)+O2(a 1Δg) is found to be 386.59±0.04 kJ/mol. The standard heat of formation of O3, ΔfH0(O3)=−144.31±0.14 kJ/mol, has also been calculated from the bond energy obtained, in conjunction with thermochemical data for O2 molecule and O atom. The uncertainty for the ΔfH0(O3) value obtained in the present study is smaller than the previous value which has been used widely.

Stimulated-emission pumping for state-selective initiation of unimolecular reactions

Stimulated-emission pumping is successfully applied to the study of unimolecular dissociation dynamics of HFCO → HF + CO in a pulsed jet. Dissociative levels of the ground electronic state are prepared by stimulated-emission pumping, and the CO fragments formed from these levels are probed by vacuum-ultraviolet laser-induced fluorescence spectroscopy. The rotational energy distribution of the CO fragments indicates modest differences between the dissociation dynamics of HFCO and those of H2CO. However, the Doppler width of CO fragments from HFCO indicates that the total kinetic energy of the fragments from HFCO is ~65% of total available energy, the same fraction as in the dissociation of H2CO. The linewidths of the dissociative levels are also measured with high-resolution fluorescence-dip-detected stimulated-emission pumping spectroscopy.

Vacuum ultraviolet laser spectroscopy: Radiative lifetimes of interacting 2Δ states of NO

Interactions between the B′ 2Δ non-Rydberg state and nearby 2Δ Rydberg states of NO have been studied by vacuum ultraviolet laser-induced fluorescence spectroscopy. By using tunable, coherent radiation in the 60 000–70 000 cm−1 range, NO molecules in a supersonic jet were excited and their radiative decay was monitored by time-resolved fluorescence detection. In this way accurate, collision-free radiative lifetimes were measured for several levels of the B′ 2Δ, F 2Δ, N 2Δ, and D 2Σ+ excited states of NO. Using these lifetimes the oscillator strengths for the B′–X and F–X transitions were calculated. The radiative lifetimes for the perturbed states were quite different from the unperturbed lifetimes, and these changes are discussed in terms of the known interactions between states in this spectral region. For the unperturbed vibrational levels of the B′ 2Δ state, radiative lifetimes between 129–139 ns were measured, while for the F 2Δ state, a radiative lifetime of 30 ns was measured. For the perturbed bands of the F2Δ and B′ 2Δ states, the radiative lifetimes were accurately described by a two-state homogeneous perturbation model, but for the B′(v=7)∼N(v=0) mixed states, the lifetimes were dramatically different from those expected on the basis of two interacting states. This is caused by a third-interacting state whose identity is not known. The B′(v=8,9) states, while having unperturbed line positions, have shorter radiative lifetimes, indicating the onset of predissociation in those levels.