D. W. Setser

Rate constants and quenching mechanisms for the metastable states of argon, krypton, and xenon

Rate constants have been measured by the flowing afterglow technique at 300 °K for the quenching of Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2) by a large number of small molecules. For the same reagent, the magnitudes of the cross‐sections usually increase in the series Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2). The Ar(3P2) and Ar(3P0) data are compared to results in the literature for these states and to data for Ar(3P1) and Ar(1P1). The set of thermal quenching cross sections are used to test the correlations between the magnitudes of the cross sections and properties of the reagents as predicted by the orbiting, absorbing‐sphere, golden rule, and curve‐crossing mechanisms for quenching. The best correlation is between the cross sections and the C6 coefficient. The analysis supports the proposition that the orbiting‐controlled, curve‐crossing model is the general mechanism governing the magnitude of the thermal cross sections for quenching of the metastable states. This model explains the very large quenching ...

Quenching cross sections for electronic energy transfer reactions between metastable argon atoms and noble gases and small molecules

Reaction rate constants for the quenching of electronic energy in metastable argon (3P0,2) by Kr, Xe, and a number of simple molecules have been measured. A hollow, cold‐cathode discharge excites the metastables in a flow apparatus. The concentration of metastables was followed by absorption spectroscopy as a function of time and of quenching molecule concentration. Quenching of Ar*(3P2) by Kr, CO, N2, CF4, and H2(D2) proceeds at rates between 0.6 and 7 × 10−11 cm3 molecule −1 · sec−1. Except for Kr, Xe, N2, CO, and CH4, the 3P0 metastable level is quenched slightly more rapidly than the 3P2 level. With the aid of data in the literature, the contribution from the product channels (Penning and associative ionization) are considered for quenching by NO and C2H2. These channels appear not to be of major importance for quenching since the ionization efficiency of these two reactions is low: ∼ 0.2 for NO and ∼ 0.1 for C2H2. The quenching mechanism is discussed in terms of both a curve crossing and a ``golden r...

Quenching rate constants for metastable argon, krypton, and xenon atoms by fluorine containing molecules and branching ratios for XeF* and KrF* formation

The flowing afterglow technique was used to study the reactions of Xe(3P2), Kr(3P2), and Ar(3P2) metastable atoms with small fluorine containing molecules. Fluorides from Groups III through VIII (XeF2) were examined. Although all the fluorides have large quenching rate constants, only F2, XeF2, some interhalogen fluorides, and small molecules with the OF or NF bond have high branching ratios for XeF* or KrF* excimer formation. The branching ratio measurements were made via comparison to the XeCl* and KrCl* emission intensities from reaction of Xe(3P2) and Kr(3P2) with Cl2, which were adopted as reference reactions. Within experimental error, the branching ratios are unity for Kr(3P2) and Xe(3P2) with Cl2, F2, and OF2. Increasing the argon pressure from 1–40 torr gives extensive vibrational relaxation but no electronic quenching of the lowest energy excimer state of XeF*, XeCl*, KrCl*, or KrF*. Increasing pressure also reduces the intensity of the secondary emission system of KrCl* and XeCl* which implies ...

Chemical Applications of Metastable Argon Atoms. IV. Excitation and Relaxation of Triplet States of N2

An argon flow system containing about 0.01% of metastable argon atoms (3P2,0) in the absence of other energy carriers is described and characterized. The interaction of these metastable atoms with nitrogen gives radiative transitions of N2 from C(3Πu) → B(3Πg) → A(3Σu+) and finally A(3Σu+) → X(1Σg+). Absolute‐intensity measurements, high‐resolution spectra, and pressure‐dependence data show that independent channels exist for production of C(3Πu) and B(3Πg). Unusual nonequilibrium populations of the rotational levels, the three spin sublevels, and the λ‐doublet levels were found for the N2(C3Πu) state. Possible mechanisms responsible for producing these populations are discussed. Study of the dependence of these populations upon pressure permitted evaluation of collisional relaxation within the various levels of N2(C3Πu). It was shown that rotational relaxation mainly occurs by ΔK = ± 1 with a rate constant of 7.4 × 1013 cm3 mol−1·sec−1. The rate constant for electronic quenching of N2(C3Πu) by argon is l...

HF infrared chemiluminescence: Energy disposal and the role of the radical fragment in the abstraction of hydrogen from polyatomic molecules by F atoms

HF infrared chemiluminescence has been utilized to study the energy disposal for the abstraction of hydrogen by fluorine atoms from polyatomic molecules which yield radical fragments with large stabilization energies. The prototype systems selected for study, methyl benzenes, phenol, and acetonitrile, are cases which yield resonance stabilized radicals as products. Comparison is made to the energy disposal from the reaction of F with the primary C–H bonds of aliphatic hydrocarbons, which have smaller radical stabilization energies. In general the radical stabilization energy, which is associated with major changes in geometry of the radical relative to the parent molecules, was not available to the HF product. The reactions of F + benzene and ethylene also were studied to provide reference data for different types of C–H bonds. The HF vibrational energy distributions have been interpreted using an extension of the information theory which previously has been applied to three body reactions. Vibrational su...

Penning ionization optical spectroscopy: Metastable helium (He 2 3S) atoms with nitrogen, carbon monoxide, oxygen, hydrogen chloride, hydrogen bromide, and chlorine

The generation of triplet metastable helium atoms (19.8 eV) by a hollow cathode discharge coupled to a flow apparatus is described. With this apparatus the Penning ionization reactions of He(2 3S) with six diatomic molecules were investigated at pressures of 1–5 torr of helium and less than 1 mtorr of reagent at 300, 225, and 77°K. The reactions were monitored by analysis of the visible and ultraviolet emission spectra. The following band systems were observed: N2+ (B 2Σu+‐X 2Σg+, A 2Πu−X 2Σg+); CO+(B 2Σ+− X 2Σ+, B 2Σ+−A 2Πi, A 2Πi−X 2Σ+); O2+(A 2Πu−X 2Πg, b 4Σg−−a 4Πu); HCl+(A 2Σ+−X 2Π); HBr+(2Σ+−X 2Π) and Cl2+(A 2Πu−X 2Πg). For the last four cases atomic emissions were observed indicating the presence of reaction channels in competition with Penning ionization. The vibrational populations of O2+(A 2Πu), HCl+(A 2Σ+) and HBr+(A 2Σ+) were not in accord with populations expected for vertical transitions from the molecular ground state. However, the N2+(B 2Σu+), CO(A 2Πi), and O2+(b 4Σg−) vibrational populat...

Reactive quenching studies of Xe (6s, 3P2) metastable atoms by chlorine containing molecules

The XeCl* emission spectrum has been used to study the reactions of Xe  (6s, 3P2) metastable atoms with 20 chlorine containing reagents. Although the total quenching rate constants are large, only Cl2, the mixed halogens and Group VI chlorides have high product branching fractions for XeCl* excimer formation. From analysis of the low pressure XeCl* emission spectra, the ratios for XeCl  (III,1/2 or B,1/2) and XeCl  (II,3/2 or C,3/2) formation were determined. Using results presented in the following paper, the vibrational energy distributions of the XeCl* molecules also can be estimated. A wide difference, depending upon reagent, is observed for the vibrational energy disposal. The short wavelength limit of the XeCl  (B–X) emission can be used to assign upper limits to D°0  (R–Cl) and this work gives D°0  (SCl–Dl) ?44.7; D°0  (ClS2–Cl) ?46.2, D°0  (SOCl–Cl) ?53.9; D°0  (SO2–Cl) ?58.5, and D°0  (PCl2–Cl) ?77.7 kcal mole−1. For purposes of comparison, the XeCl* emission spectra also were obtained from the r...

Rate constants for specific product channels from metastable Ar(3P2,0) reactions and spectrometer calibration in the vacuum ultraviolet

The product channels from interaction of metastable argon atoms (3P2,0) with a series of chlorine containing molecules (Cl2, NOCl, ClO, HCl, CCl4, PCl3, and SnCl4), as well as some other molecules (Br2, N2O, NO2, H2S, and COS), have been investigated by emission spectroscopy. The rate constants for product channels were established by comparison of the emission intensities from the excited state products to the emission intensities from excited state Kr atoms, which have known rate constants for excitation by metastable argon atoms. Comparison of the individual product rate constants with previously determined total quenching rate constants of Ar(3P2,0) gave the branching ratios for emitting product channels. Although our measurements include the wavelength range from 120–800 nm, emphasis is placed upon the vacuum ultraviolet region and upon the ArCl*, ArBr*, and ArO* product channels. The highest ArCl* yield, ∼50%, was obtained for Cl2 and ClO. Quenching mechanisms for the halogen containing reagents are...

Chemical Applications of Metastable Argon Atoms. III. Production of Krypton and Xenon Metastable Atoms

An inexpensive experimental technique has been developed for the production of argon, krypton, and xenon metastable atoms using a discharge‐flow system. The reactions of these metastable atoms with N2, CO, and N2O were investigated in the pressure range 0.3–10 torr, and the emission spectra resulting from the reactions were identified. These spectra gave information about the collision event and also about some of the relaxation processes. Franck–Condon factors apparently do not control the molecular excitation processes in collisions of excited heavy atoms with light diatomic molecules. This discharge flow source of metastable atoms provides emission spectra of sufficient intensity from added CO and N2O for study of emission bands which currently are not well understood. Molecular emission from N2O has been observed, apparently for the first time.

Radiative lifetimes and two‐body deactivation rate constants for Ar(3p5, 4p) and Ar(3p5,4p′) states

The radiative lifetimes and two‐body deactivation rate constants of the Ar(3p54p and 4p′) levels have been measured by a time‐resolved laser‐induced fluorescence technique in a flowing afterglow apparatus. The flowing afterglow produced the Ar(3p54s,3P2) metastable atoms, which subsequently were pumped by the pulsed dye laser. The measured radiative lifetimes are in excellent agreement with the calculated transition probabilities based upon intermediate‐coupling theory. Radiative branching ratios of these states also were measured in order to assign the transition probabilities and the absorption oscillator strengths of the transitions between the 4s and 4p levels. The two‐body deactivation rate constants range from 1×10−10 to 2×10−11 cm3 atom−1 sec−1. Specific product states from the two‐body collisions between Ar(4p and 4p′) atoms and Ar were identified from the laser‐induced emission spectra and rate constants for individual product states were assigned for each level. Our results show that two‐body co...