C. Wittig

The 266 nm photolysis of ICN: Recoil velocity anisotropies and nascent E,V,R,T excitations for the CN+I(2P3/2) and CN+I(2P1/2) channels

We report the detection of nascent CN(X 2Σ+, v‘=0) following the 266 nm photodissociation of 300 K ICN, using sub‐Doppler resolution laser‐induced fluorescence, and polarized photolysis and probe lasers. When monitoring a particular CN internal state, the translational energies of the I+CN and I*+CN channels differ by the iodine spin‐orbit splitting 7603 cm−1. This is used to determine the separate contributions from each channel. For I+CN, high N‘ are selectively produced, with little population below N‘=20 (〈Erot〉 =3300±300 cm−1), whereas the I*+CN channel is associated with a distribution peaked sharply at low N‘(〈Erot〉 =355±35 cm−1). It is clear that the low and high N‘ derive from linear and bent exit channel geometries, respectively. The spatial anisotropy is high (βI =1.3±0.2; βI* =1.6±0.2) and initial excitation is via a parallel transition(s), probably to a state which begins correlating with I*+CN in the linear configuration. Nascent spin‐rotation states (F1 and F2) are also resolved for each ch...

Nascent product excitations in unimolecular reactions: The separate statistical ensembles method

The unimolecular reaction of a vibrationally excited molecule having low rotational excitation often leads to nascent products in which the vibrational degrees of freedom appear ‘‘hotter’’ than the rotation, translation (R,T) degrees of freedom. We show that this can derive from parent vibrations being ‘‘hot’’ while parent rotations remain ‘‘cold,’’ since the parentage of product vibration is parent vibration, while product R,T excitations are obtained from parent vibration as well as rotation. Calculations are performed for reactions having loose transition states and no reverse barriers, in which an ensemble of 3N–6 degrees of freedom are used to equilibrate parent vibrations, thereby providing a statistical distribution of product vibrational excitations. For each set of product vibrational states, all R,T excitations are then apportioned statistically using the phase space theory of unimolecular reactions (PST). The results indicate that for those energies above reaction threshold (E‡) which exceed th...

Orienting reactants using van der Waals precursors: OCO ⋅ HBr+hν → {OCO ← H}+Br → CO+OH+Br

We report the results of an experimental study in which the reaction of atomic hydrogen with carbon dioxide is examined under conditions wherein the reagents are highly oriented relative to one another. Orientation is achieved using nozzle expansions to prepare a precursor of the form CO2 ⋅ HBr, which is best described as a weakly bound van der Waals or hydrogen bonded complex. The weak bond insures that the CO2 and HBr moieties are separable insofar as electronic excitations and HBr dissociation are concerned. Translationally hot H atoms are produced by the 193 nm photolysis of the HBr constituent of the nearly linear CO2 ⋅ HBr complex. Dissociation is direct, and the initial H‐atom velocity is directed primarily along the HBr axis. Because the HBr is complexed with the CO2, the initial conditions for the ensuing reaction are quite restricted relative to cases in which there is no selective orientation, i.e., bulk or molecular beam conditions. OH(X2Π) is detected under experimental conditions which minim...

Kinetic and internal energy distributions via velocity‐aligned Doppler spectroscopy: The 193 nm photodissociation of H2S and HBr

We report center‐of‐mass kinetic energy distributions for the 193 nm photodissociation of H2S and HBr using the method of velocity–aligned Doppler spectroscopy. Nascent H atoms are detected by sequential two‐photon photoionization via Lyman‐α (121.6 nm + 364.7 nm), and internal SH(X 2Π) and Br excitations are observed directly in the H‐atom kinetic energy distributions. The kinetic energy resolution is much better than in ‘‘conventional’’ sub‐Doppler resolution spectroscopy and results from detecting spatially selected species whose velocities are aligned with the wave vector of the probe radiation, kprobe, thereby providing a kinetic energy distribution for a specific laboratory direction. This improved resolution is achieved in the present experiments by using pulsed, collimated, and overlapped photolysis and probe beams, but the vital aspect of the technique involves increasing the delay between the two lasers in order to discriminate against species having velocity components perpendicular to kprobe. ...

Subpicosecond resolution studies of the H+CO2→CO+OH reaction photoinitiated in CO2–HI complexes

Time resolved studies of the title reaction have been carried out by photodissociating the HI moiety within weakly bound CO2–HI complexes. The HOCO° intermediate decomposes via a unimolecular decomposition mechanism, and the emerging hydroxyl radicals are monitored with subpicosecond temporal resolution by using laser‐induced fluorescence. The measured rates are in good agreement with several theoretical predictions: Rice, Ramsperger, Kassel, and Marcus (RRKM calculations; classical trajectory simulations on the best available potential energy surface; and recent quantum scattering calculations.

Probing the NO2→NO+O transition state via time resolved unimolecular decomposition

Time resolved, subpicosecond resolution measurements of photoinitiated NO2 unimolecular decomposition rates are reported for expansion cooled and room temperature samples. The molecules are excited by 375–402 nm tunable subpicosecond pulses having bandwidths ≥20 cm−1 to levels which are known to be thorough admixtures of the 2B2 electronically excited state and the 2A1 ground electronic state. Subsequent decomposition is probed by a 226 nm subpicosecond pulse that excites laser‐induced fluorescence (LIF) in the NO product. When increasing the amount of excitation over the dissociation threshold, an uneven, ‘‘step‐like’’ increase of the decomposition rate vs energy is observed for expansion cooled samples. The steps are spaced by ∼100 cm−1 and can be assigned ad hoc to bending at the transition state. Relying on experimental estimates for the near threshold density of states, we point out that simple transition state theory predictions give rates that are consistent with these measured values. The rates ar...

The use of van der Waals forces to orient chemical reactants: The H+CO2 reaction

The 193 nm excitation of the CO2 ⋅ HBr van der Waals complex causes HBr to dissociate rapidly, thereby promoting the endothermic reaction of atomic hydrogen with the adjacent CO2. The reactants are highly oriented by the CO2 ⋅ HBr equilibrium geometry.

NCNO → CN+NO: Complete NO(E, V, R) and CN(V, R) nascent population distributions from well‐characterized monoenergetic unimolecular reactions

We report detailed vibrational, rotational, and electronic (V, R, E) distributions of nascent NO(X 2Π) deriving from monoenergetic unimolecular reactions of jet‐cooled NCNO. Excitation is via the A 1A″ ← X 1A’ system above dissociation threshold (17 085±5 cm−1), and vibrational predissociation occurs following radiationless decay of the initially excited A 1A″ state. These results are combined with data on the corresponding CN(X 2Σ+) nascent V, R distributions, thereby providing a complete description of the energy partitioning into the various degrees of freedom of both products. The data presented here support our previous conclusion that dissociation is ‘‘statistical.’’ All the V, R distributions of both products can be predicted accurately using a modification of the phase space theory of unimolecular reactions (PST), which we call the separate statistical ensembles (SSE) method; it is expected that this method will have quite general applicability. NO spin‐orbit excitation is ‘‘cold’’ relative to ...

The kinetics of free radicals generated by IR laser photolysis. II. Reactions of C2(X 1Σ+g), C2(a 3Πu), C3(X̄ 1Σ+g) and CN(X 2Σ+) with O2☆

Abstract The 300 K reactions of O 2 with C 2 (X 1 Σ + g ), C 2 (a 3 Π u ), C 3 (X 1 Σ + g ) and CN(X 2 Σ + ), which are generated via IR multiple photon dissociation (MPD), are reported. From the spectrally resolved chemiluminescence produced via the IR MPD of C 2 H 3 CN in the presence of O 2 , CO molecules in the a 3 Σ + , d 3 Δ i , and e 3 Σ − states were identified, as well as CH(A 2 Δ) and CN(B 2 Σ + ) radicals. Observation of time resolved chemiluminescence reveals that the electronically excited CO molecules are formed via the single-step reactions C 2 (X 1 Σ + g , a 3 Π u ) + O 2 → CO(X 1 Σ + + CO(T), where T denotes are electronically excited triplet state of CO. The rate coefficients for the removal of C 2 (X 1 Σ + g ) and C 2 (a 3 Π u ) by O 2 were determined both from laser induced fluorescence of C 2 (X 1 Σ + g ) and C 2 (a 3 Π u ), and from the time resolved chemiluminescence from excited CO molecules, and are both (3.0 ± 0.2)10 −12 cm 3 molec −1 s −1 . The rate coefficient of the reaction of C 3 with O 2 , which was determined using the IR MPD of allene as the source of C 3 molecules, is −14 cm 3 molec −1 s −1 . In addition, we find that rate coefficients for C 3 reactions with N 2 , NO, CH 4 , and C 3 H 6 are all −14 cm 3 molec −1 s −1 . Excited CH molecules are produced in a reaction which proceeds with a rate coefficient of (2.6 ± 0.2)10 −11 cm 3 molec −1 s −1 . Possible reactions which may be the source of these radicals are discussed. The reaction of CN with O 2 produces NCO in vibrationally excited states. Radiative lifetime of the Ā 2 Σ state of NCo and the Ā 1 Π u (000) state of C 3 are reported.

Optically pumped molecular lasers in the 11–17‐μm region

A large number of laser transitions in the 11–17‐μm region have been obtained by optically pumping CF4, NOCl, CF3I, and NH3 with a single line‐tunable CO2 TEA laser. These lasers have respectable conversion efficiency and can be scaled to high energies in a spectral region where the selective excitation of molecules is prerequisite to performing selective photodissociation and laser‐induced chemistry experiments.