Λ-doublet populations in CH(A2Δ) produced in the 193 nm multiphoton dissociation of (CH3)2CO, (CD3)2CO, (CH3)2S and CH3NO2

Unequal intensities of the Λ-doublet components were observed in the CH(A²Δ-X²Π) emission following the multiphoton dissociation of (CH₃)₂CO, (CH₃)₂S and CH₃NO₂ by an ArF laser (193 nm). The power dependence of the emission intensity was estimated to be cubic (3.1±0.2) when the laser power was below ≈ 8×10¹⁷ photons cm^?2 pulse^?1. The Λ-doublet populations depended on the rotational quantum number N′ and the preferred level changed at N′ = 20. A similar behavior was observed for the CD(A²Δ) from (CD₃)₂CO. Rotational distributions show bimodal behavior, having a hump around N′ = 13 in CH(A²Δ) and N′ = 11 in CD(A²Δ). These trends indicate that the CH(A²Δ) is produced through multiple processes where stepwise mechanisms are operative via either CH₂ or CH₃, or both radicals as intermediates.     

Laser photolysis of C2H2 and CF3C2H at 193 nm: Production of C2(d3Πg and CH(A2Δ) and their quenching by Xe

The 193 nm laser photodissociation of CH₂H₂ and CF₃C₂H has been studied. With the laser beam focused, C₂(d³Π_g) and CH(A²Δ) radicals were formed by multiphoton processes in both C₂H₂ and CF₃C₂H; however, the one-photon process forming C₂H is still predominant in CF₃C₂H photolysis. The production of C₂(d³Π_g) and CH(A²Δ) emissions is prompt,and the emission intensities show similar (less than quadratic) dependence on laser power whether the radicals are produced from C₂H₂ or CF₃C₂H. In addition, the vibrational distribution of the Swan system is nearly the same in CF₃C₂H as in C₂H₂. The results indicate that the overall photolytic processes are similar in two molecules. Both the C₂(d³Π_g) and CH(A²Δ) emissions are quenched by Xe with rate constants of 4.8×10^?11 and 1.8×10^?11 cm³ molecule^?1 s^?1, respectively.     

Laser induced photodissociation of Hn3 at 266 nm. II. Reactions of NH(1Δ) with HN3 HCl and hydrocarbon species

In the preceding paper it was shown that the 266 nm photodissociation of HN₃ gives rise to NH fragments exclusively in the vibrationless a(¹Δ) state with about 900 cm^?1 of rotational energy. These fragments collisionally react with HN₃ to produce NH₂(²A₁) in a chemiluminescent reaction. The time resolved chemiluminescence emission is used to determine the reaction rate for NH(¹Δ) + HN₃ → NH₂(²A₁) + N₃(²Π_g). The reaction rates of NH(¹Δ) with several other species, HCl, CH₄, C₂H₄, C₃H₆ and C₆H₁₂ are reported. Possible mechanisms for these reactions are considered. Condensed phase experiments are reported describing the addition reaction of NH(¹Δ) with cyclohexane.     

Laser induced photodissociation of Hn3 at 266 nm. I. Primary products,photofragment energy distributions and reactions of intermediates

A detailed analysis of the primary photodissociation products resulting from the 266 nm laser photolysis of HN₃ is reported. The major primary fragments are N₂(¹Σ_g⁺) and NH(¹Δ). The NH(¹Δ) fragment is formed ? 99.8% in the ν = 0 level with ≈ 900 cm^?1 of rotational energy and ? 5000 cm^?1 of translational energy for the axially scattered fragments. A new chemiluminescent reaction is reported: NH(¹Δ) + HN₃ (¹A′) → NH ₂(²A₁) + N₃(²Π_g), which appears to be a major reaction channel of the primary NH(¹Δ) fragment. A kinetic analysis of this reaction and several other NH(¹Δ) reactions are the subject of the following associated paper. A correlation study of the NH(¹Δ) and N₂(¹Σ_g⁺) products with the dissociating states of HN₃ is made which requires a reassignment of the lower-lying HN₃ transitions.     

Production of electronically excited NF by the reaction of fluorine atoms with HN3

Electronically excited NF in both the a¹Δ and b ¹Σ⁺ states hasbeen observed from the reaction of fluorine atoms with HN₃. The results suggest that fluorine atoms first abstract the hydrogen atom from HN₃, then react with the remaining N₃ to form NF^≠(a¹Δ). NF^*(b¹Σ⁺) is produced by a subsequent energy pooling reaction between NF^≠(a¹Δ) and vibrationally excited HF. The rate of the F + N₃ reaction is estimated to be ≈ 10¹² and ³ mole^?1 s^?1.     

The production of Cr(NH3)6 from the acid catalysed hydrolysis of Cr(NH3)5(NCO)

The rate of the reaction

has been investigated at 40–65°C with [HClO₄] varying from 0.04 to 0.6 M (μ = 0.6 M, NaClO₄). The observed rate law has the form: -d[Cr(NH₃)₅(NCO)²⁺]/dt = k_obs[Cr(NH₃)₅(NCO)²⁺] where k_obs = a[H+]²{1 + b[H⁺]²} and ^?1 at 55.0°C, a = 0.36 M^?1 s^?2 and b = 6.9 × 10^?3 M^?1 s^?1. The rate of loss of Cr(NH₃)₅(NCO)²⁺ increases with increasing acidity to a limiting value (at [H⁺] ～ 0.5 M) but the yield of Cr(NH₃)₆³⁺ decreases with increasing [H⁺] and increases with increasing temperature. In the kinetic studies the maximum yield of Cr(NH₃)₆³⁺ was 35% but a synthetic procedure has been developed to give a 60% yield.     

Laser induced fluorescence of CH2 (ã1A1) produced in the photodissociation of ketene at 337 nm. The CH2 (ã1A1-X̃3B1)energy separation

The photodissociation of ketene, CH₂CO(X?¹A₁) → CH₂ (ã¹A₁) + CO(X ¹Σ⁺) has been observed at 337 nm, using a pulsed nitrogen laser. The CH₂ (ã ¹A₁) radical has been detected by laser induced fluorescence with a tunable dye laser. A laser excitation spectrum has been obtained from CH₂(ã ¹A₁) over the wavelength interval from 588.9 to 595.6 nm in the Σ ← Π vibronic subband of the CH₂ (ã ¹A₁); υ″ = 0, 0, 0?b? ¹B₁; υ′ = 0, 14, 0) transition. For the CH₂(ã ¹A₁ ; υ′= 0, 0, 0?X? ³B₁; υ′' = 0, 0, 0) energy separation an upper limit of (6.3 ^± 0.8) kcal/mole has been found. The radiative lifetime τ and the rate constant k for the removal of the 0₀₀ rotational level of the Σ(0, 14, 0) vibronic state have been measured directly. The values are τ = (4.2 ^± 0.2) μs and k = (7.4 ± 0.3) × 10^?10 cm³ molecule^?1 s^?1, respectively.     

The production of H(2S) atoms in the energy-transfer reaction of O2(1Δg) with HO2(X̃2A″)

The reaction of O₂(¹Δg) with HO₂(X?) was studied in an isothermal flow reactor in the pressure range 7?p? 10.7 mbar at temperatures between 299?T? 423 K. H-atom production was observed in the reaction O₂(¹Δg) + HO₂(òA′) - H(²S)+ 2O₂ (³Σ_g^?). The rate of this reaction (k₁) is estimated to be k₁ = (1 ± 0.5) × 10¹⁴ CM³ Mol^?1 s^?1. The implications of this reaction to recent determinations of the rate of the reaction H + O₂(¹Δg) are discussed.     

CAS SCF Cl calculations for the 3Σ−g, 1Σ+g, 3Σ+u, and 5Δu states of Sc2

CAS SCF CI (SD) calculations have been carried out for the ³Σ^?_g, ¹Σ⁺_g, ³Σ⁺_u, and ⁵Δ_u states of Sc₂ using large gaussian basis sets. The ³Σ^?_g, ¹Σ⁺_g, and ³Σ⁺_u states arise from the ²D(4s² 3d¹) + ²D(4s² 3d¹) limit of Sc₂ and are found to be only weakly bound (D_c ≈ 0.06 eV and R_c ≈ 8.0a₀). The ⁵Δ_u state arises from the ²D(4s² 3d¹) + ⁴F(4s¹ 3d¹ 4p¹) atomic limit. This state is found to be strongly bound relative to its limits (D_c ≈ 0.8 eV and R_c ≈ 7.0a₀).     

Studies on the reaction N + N3 → N2(B 3Πg) + N2(X 1Σ+g)

Strongly enhanced N₂ first positive emission N₂(B ³Π_g → A ³Σ⁺_u) has been observed on addition of N atoms into a flowing mixture of Cl and HN₃. The dependence of the emission intensity on N atom concentration gave a rate constant for the reaction N + N₃ → N₂(B ³Π_g) + N₂(X ¹Σ⁺_g) of _i(1.6 ± 1.1) × 10^?11 cm³ molecule^?1 s^?1. That for the reaction Cl + HN₃ → HCl + N₃ is (8.9 ± 1.0) × 10^?13 cm³ molecule^?1 s^?1 from the decay of the emission. Comparison of the emission intensity in ClHN₃ with that in ClHN₃N gave the rate constant of the reaction N₃ + N₃ → N₂(B ³Π_g) + 2N₂(X ¹Σ⁺_g) as 1.4 × 10^?12 cm³ molecule^?1 s^?1 on the assumption that N + N₃ yields only N₂(B ³Π_g) + N₂(X ¹Σ⁺_g).     

La(Li1/3Ti2/3)O3: a new 1:2 ordered perovskite

A new 1:2 ordered perovskite La(Li_1/3Ti_2/3)O₃ has been synthesized via solid-state techniques. At temperature >1185°C, Li and Ti are randomly distributed on the B-sites and the X-ray powder patterns can be indexed in a tilted (b⁻b⁻c⁺) Pbnm orthorhombic cell (a=a_c√2=5.545 Å, b=a_c√2=5.561 Å, c=2a_c=7.835 Å). However, for T?1175°C, a 1:2 layered ordering of Li and Ti along 〈111〉_c yields a structure with a P2₁/c monoclinic cell with a=a_c√6=9.604 Å, b=a_c√2=5.552 Å, c=a_c3√2=16.661 Å, β=125.12°. While this type of order is well known in the A²⁺(B²⁺_1/3B⁵⁺_2/3)O₃ family of niobates and tantalates, La(Li_1/3Ti_2/3)O₃ is the first example of a titanate perovskite with a 1:2 ordering of cations on the B-sites.     

Non-exponential decay of the fluorescence from the 1Δg state in liquid oxygen at high excitation intensities

The ¹Δ_g state of liquid oxygen (natural O₂ and ¹⁸O₂) was excited by intense Nd: YAG laser pulses. The observed fluorescence decay is non-exponential and depends on pump laser intensity. Various decay channels of the ¹Δ_g state are discussed. The energy pooling reaction ¹Δ_g + ¹Δ_g → ¹Σ⁺_g + ³Σ^?_g is verified experimentally. Values f constant of liquid natural O₂ and ¹⁸O₂ are given.     

Vibrational emission of NO2 from the reaction of NO with O3

Vibrational chemiluminescence in the Δν₁ = Δν₃ = ?1 band of NO₂ is observed both in the O + NO and O₃ + NO reactions and shown to be emitted by molecules with up to 11 000 cm^?1 of vibrational energy. Quenching rate constants of NO₂³ are estimated ranging from about 6 × 10^?14 for Ar to about 3 × 10^?12 cm³ s^?1 for NO₂. The ratio of vibrational to electronic emission is 0.06 ± 0.03 for O + NO and 5.3 ± 1.0 for O₃ + NO. It is suggested that vibrationally excited NO₂ is a major product of that channel of the O₃ + NO reaction which forms ground-state NO₂(²A₁) directly.     

The energy balance and branching ratios associated with the chemiluminescent reaction Si(3P) + N2O(1Σ) → SiO*(a3Σ+, b3Π, A1Π) + N2(υ″ ⩾ 5) — possible formation of vibrationally excited N2

Silicon atoms react under single collision conditions with N₂O to yield chemiluminescent emission corresponding to the SiO a³Σ⁺?X¹Σ⁺ and b³Π?X¹Σ⁺ intercombination systems and the A¹Π?X¹Σ⁺ band system. A most striking feature of the SiN₂O reaction is the energy balance associated with the formation of SiO product molecules in the A¹Π and b³Π states. A significant energy discrepancy ( = 10000 cm^? = 1.24 eV) is found between the available energy to populate the highest energetically accessible excited-state quantum levels and the highest quantum level from which emission is observed. It is suggested that this discrepancy may result from the formation of vibrationally excited N₂ in a concerted fast SiN₂O reactive encounter. Emission from the SiO a³Σ⁺ (A¹Π) and b³Π(A¹Π, E¹Σ₀⁺) triplet-state manifold results primarily from intensity borrowing involving the indicated singlet states. Perturbation calculations indicate the magnitude of the mixing between the b³Π, A¹Π and E¹Σ₀⁺ states ranges between 0.5 and 2%. On the basis of these calculations, the branching ratio (excited triplet)/(excited singlet) is found to be well in excess of 500. An approximate vibrational population distribution is deduced for those molecules formed in the b³Π state. The present studies are correlated with those of previous workers in order to provide an explanation for diverse relaxation effects as well as observed changes in the ratio of a³Σ⁺ to b³Π emission as a function of pressure and experimental environment. Some of these effects are attributable to a strong coupling between the a³Σ⁺ and b³Π state. Based on the current results, there appears to be little correlation between either (1) the branching ratio for excited state formation or (2) the total absolute cross section for excited-state formation and (3) the measured quantum yield for the SiN₂O reaction. Implications for chemical laser development are considered.     

Possible production of O2(1Δg) and O2(1Σ+g) in the reaction of NO with O3

High-resolution spectra of the NO₂ continuum emission produced from the reaction NO + O₃ → NO₂ + O₂ have been investigated to detect any possible emission from O₂(¹Δ_g) at 1270 nm or O₂(¹Σ⁺_g) at 762 nm. The photolysis of O₃/O₂ mixtures at 253.7 nm, which produces both states of O₂ with known quantum efficiency, has been used as an internal standard. From the results it is concluded that less than 1/300 and 1/200 of the NO + O₃ reactive collissions result in production of O₂(¹Δ_g) or O₂(¹Σ⁺_g), respectively, at room temperature.     

New open-framework in the uranyl vanadates A3(UO2)7(VO4)5O (A=Li, Ag) with intergrowth structure between A(UO2)4(VO4)3 and A2(UO2)3(VO4)2O

New uranyl vanadates A₃(UO₂)₇(VO₄)₅O (M=Li (1), Na (2), Ag (3)) have been synthesized by solid-state reaction and their structures determined from single-crystal X-ray diffraction data for 1 and 3. The tetragonal structure results of an alternation of two types of sheets denoted S for _∞²[UO₂(VO₄)₂]⁴⁻ and D for _∞²[(UO₂)₂(VO₄)₃]⁵⁻ built from UO₆ square bipyramids and connected through VO₄ tetrahedra to _∞¹[U(3)O₅-U(4)O₅]⁸⁻ infinite chains of edge-shared U(3)O₇ and U(4)O₇ pentagonal bipyramids alternatively parallel to a- and b-axis to construct a three-dimensional uranyl vanadate arrangement. It is noticeable that similar _∞[UO₅]⁴⁻ chains are connected only by S-type sheets in A₂(UO₂)₃(VO₄)₂O and by D-type sheets in A(UO₂)₄(VO₄)₃, thus A₃(UO₂)₇(VO₄)₅O appears as an intergrowth structure between the two previously reported series. The mobility of the monovalent ion in the mutually perpendicular channels created in the three-dimensional arrangement is correlated to the occupation rate of the sites and by the geometry of the different sites occupied by either Na, Ag or Li. Crystallographic data: 293 K, Bruker X8-APEX2 X-ray diffractometer equipped with a 4 K CCD detector, MoKα, λ=0.71073 Å, tetragonal symmetry, space group P4¯m2, Z=1, full-matrix least-squares refinement on the basis of F²; 1,a=7.2794(9) Å, c=14.514(4) Å, R1=0.021 and wR2=0.048 for 62 parameters with 782 independent reflections with I?2σ(I); 3, a=7.2373(3) Å, c=14.7973(15) Å, R1=0.041 and wR2=0.085 for 60 parameters with 1066 independent reflections with I?2σ(I).     

Threshold energies for production of CH2(3B1) and CH2(1A1) from ketene photolysis. The CH2(3B1) ↔ CH2(1A1) energy splitting

By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH₂CO(¹A₁) → CH₂(³B₁) + CO(¹Σ⁺) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔH_f298⁰[CH₂(³B₁)]. By measuring the relative ratio of CH₂(¹A₁)/CH₂(³B₁) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH₂CO(¹A₁) → CH₂(¹A₁) + CO(¹Σ⁺) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔH_f298⁰[CH₂(¹A₁)]. Thus a value for the CH₂(³B₁) ? CH₂(¹A₁) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations.     

Reactions of C2(a 3πu) with CH4, C2H2, C2H4, C2H6, and O2 using multiphoton UV excimer laser photolysis

C₂(a ³π_u) disappearance rate constants of 1.44, 0.96, 0.0296, 0.0130 and < 10^?6(x10^?10cm³s^?1) are reported for reactions with C₂H₄, C₂H₂, O₂, C₂H₆, and CH₄, respectively at 298 K. C₂(a ³π_u) fragments are generated by multiphoton ArF excimer laser photodissociation at C₂H₂, and monitored by dye laser induced fluorescence. Arguments are presented which favor chemical reactions over the C₂(a ³π_u) → (X ¹σ⁺_g) quenching channel. C₂ + C₂H₂ represents the one possible exception to the reactive channel.     

Kinetics of the reaction of O(3P) with CF3NO

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of O(³P) with CF₃NO (k₂) as a function of temperature. Our results are described by the Arrhenius expression k₂(T) = (4.54 ± 0.70) × 10^?12 exp[(?560± 46)/T] cm³molecule^?1 s^?1 (243 K ? T ? 424 K); errors are 2σ and represent precision only. The O(³P) + CF₃NO reaction is sufficiently rapid that CF₃NO cannot be employed as a selective quencher for O₂(a¹Δ_g) in laboratory systems where O(³P) and O₂(a¹Δ_g) coexist, and where O(³P) kinetics are being investigated. © 1995 John Wiley & Sons, Inc.     

Photodissociation reaction CO−3

Photodissociation reaction CO^?₃ + hv → CO₂ + O^? has been observed at seven photon energies between 2.35 and 2.71 eV using a drift tube mass spectrometer and an argon ion laser. The total cross sections for the destruction of CO^?₃ due to photons of these energies have been measured, and it is concluded that essentially all of this destruction is due to photodissociation. The photodestruction of CO^?₃.H₂O has also been observed at four photon energies.