The effect of internal excitation on the collision induced dissociation of I+2(2Πg,υ) ions

Cross sections for collision induced dissociation of 0.65 to 3.2 keV I⁺₂(²Π_g, υ) ions in I⁺₂(²Π_g, υ) + N₂(X ¹Σ⁺_g, υ = 0) interactions have been determined. Reaction cross sections for I⁺₂(²Π_{$\text{3}\text{2}$,g}, υ) ions in low vibrational levels vary smoothly from 6 to 10 A² with increasing kinetic energy. Dissociation cross sections for I⁺₂(²Π_{$\text{1}\text{2}$,g}, υ) ions are larger than those involving ground state ions. Processes involving highly excited metastable states of I⁺₂ are not observed in this investigation.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

Electronic-to-vibrational energy transfer reactions: Na(3 2P) + CO (X1Σ+, υ=0)

The vibrational distribution of CO produced from the electronic-to-vibrational energy transfer reaction: Na(3²P) + CO(X¹Σ⁺, υ=0)→Na(3²S) + CO(X¹Σ⁺, υ?8) has been determined by means of infrared resonance absorption measurements employing a cw CO laser. A flash-lamp-pumped dye laser is used to excite the ground state Na to the 3²P_{$\text{1}\text{2}$} and 3²P_{$\text{3}\text{2}$} states. The CO molecules formed in the reaction were found to be vibrationally excited up to the limits of available electronic energies carried by the excited Na atoms, and the vibrational population exhibits a maximum at υ=2. The efficiency of E→V energy transfer was determined to be 35%. Our present results were found to be consistent with the impulsive (half-collision) and curve-crossing models.     

Determinations of bond energies by time-of-flight single-collision chemiluminescence

A pulsed beam of metastable atoms traverses a scattering chamber filled with oxidant gas at low pressures (beam + gas arrangement); the resulting chemiluminescence is spectroscopically resolved as a function of time to yield a time-of-flight (TOF) spectrum for different internal states. From this data, the initial relative translational energy distribution is derived for the reactants that populate the excited internal state observed. Lower bounds are placed on the barium halide (BaX) dissociation energies, using the reactions Ba(³D) + X₂ → BaX* + X, where X = Br, I. Arguments are presented to show that these lower bounds represent measurements of the true bond energies: it is concluded that D₀⁰(BaBr) = 85.8 ± 2 kcal/mole and D₀⁰(BaI) = 72.9 ± 2 kcal/mole. The present work corrects previous determinations of bond energies from single-collision chemiluminescent studies which were in error because of unrecognized metastable contamination in the high-temperature atomic beam.     

Molecular beam chemiluminescence XI: kinetic and internal energy dependence of the NO + O3 → NO2*,→ NO23 reaction

Seeded supersonic NO beams were used to study the kinetic energy dependence of both the electronic (NO₂^*) and vibrational (NO₂³) chemiluminescence of the NO + O₃ reaction. In addition the electronic CL is found to be enhanced by raising the NO internal temperature. This is shown to be due to enhanced reactivity of the NO(²Π,_{$\text{3}\text{2}$}) fine structure component. By difference NO(²Π_{$\text{1}\text{2}$}) is concluded to yield predominantly groundstate NO₂³. The excitation function for NO₂^* formation from NO(²Π_{$\text{3}\text{2}$}) is of the form σ_{$\text{3}\text{2}$}(E) = C(E/E₀ - 1)ⁿ over the 3–6 kcal energy range where n = 2.4 ± 0.15, C = 0.163 Ų and E₀ = 3.2 ± 0.3 kcal/mole. Vibrational IR emission from NO₂³ has an energy dependence different from electronic NO₂^* emission, confirming that emitters are formed predominantly in distinct reaction channels rather than via a common precursor (either NO₂^* or NO₂³). The short wavelength cutoff of the CL spectra recorded at elevated collision energies E ? 15 kcal/mole corresponds to the total available energy. These and literature results are discussed in the light of general properties of the (generally unknown) ONO₃ potential energy surfaces. The formation of electronically excited NO₂^* rather than energetically preferred O₂ (¹ Δg) (Gauthier and Snelling) can be rationalized in terms of surface hopping near a known intersection of potential energy surfaces more easily than by vibronic interaction in the asymptotic NO₂ product.     

Molecular predissociation dynamics revealed through multiphoton ionisation spectroscopy. III. New 1A2 and 1B1 rydberg states in H2S D2S

Two new Rydberg series in H₂S and D₂S habe been characterized as three-photon resonances in four-photon ionisation spectrometry. Members of the two series exhibit sufficient rotational structure to permit characterisation of their electronic symmetries as, respectively. A₂ and B₁. The first Rydberg series is identified with the (one-photon forbidden) excitations npb₂ ← 2b₁ (¹A₂ ← $\text{X}$¹A₁) on the basis of the observed quantum defects. Geometry considerations indicate that second series, of ¹B₁ states, also arises as a result of electronic promotion from the highest occupied 2b₁ orbital. The acceptor (a₁) Rydberg orbitals possess substanial s character, but the polarisation dependence of the various ¹B₁-$\text{X}$¹A₁ three-photon transition probabilities their hybrid I character, d (and quite possibly p) functions contribute also. The results provide further clear demonstration of the way in which multiphoton excitations, and MPI techniques in particular, can complement conventional one-photon absorption techniques. Members of both series are predissociated. Vibronic predissociation rates are found generally to decline with increasing n and to be slower in D₂S than in H₂S. The lowest (n = 4) member of the ¹A₂ series in both isotopic species appears immune from rovibronic predissociation but higher members show evidence of a ($\text{J}$_a²)-dependent rotationally-induced predissociation, the severity of which increases dramatically with n. This observation is explained in terms of electronic-rotational Coriolis coupling to a dissociated ¹B₂ state is presumed to be responsible for the observed ($\text{J}$_b²)-dependent heterogeneous predissociation of the ¹B₁ (n = 6) member in H₂S. However, the dominant rotationally-induced predissociation mechanism that affects the counterpart in D₂S scales with ($\text{J}$_a²). Wherever possible comparisons are drawn with the known spectroscopy and photophysics of the isovalent molecules H₂O and D₂O.     

Vibrational state analysis of unrelaxed BaI from the reactions Ba + CH3I and Ba + CH2I2

The reactions Ba + CH₃I → BaI + CH₃ and Ba + CH₂I₂ → BaI + CH₂I have been investigated by the method of laser-induced fluorescence. Excitation spectra are reported for BaI products formed under single-collision conditions in a “beam-gas” arrangement. The production of BaI for Ba + CH₂I₂ is found to be a major reaction pathway with a cross section about twice that for Ba + CH₃I. The relative vibrational populations show for both reactions bell-shaped distributions peaking close to υ = 21 for Ba + CH₃I and υ = 39 for Ba + CH₂I₂. The corresponding average fraction of the total reaction exoergicity that appears as BaI vibration is $\text{f}$_υ = 0.18 for Ba + CH₃I and $\text{f}$_υ = 0.29 for Ba + CH₂I₂. In the case of Ba + CH₃I, an estimate for the average relative translational energy of the products, obtained from the primitive angular distribution measurements of Lin, Mims and Herm, can be combined with the average vibrational excitation of BaI to provide evidence that the internal excitation of the methyl radical exceeds that of BaI. A model is discussed which postulates an electron jump in the exit valley of the Ba + CH₃I reaction to account for this feature of the reaction dynamics.     

Molecular predissociation dynamics revealed through multiphoton ionisation spectroscopy. I. The C1B1 states of H2O and D2O

The $\text{C}$¹B₁ states of H₂O and D₂O have been observed by means of three photon absorption (four photon ionisation) spectroscopy. Differences between the experimentally observed 3 + 1 multiphoton ionisation spectrum and that predicted by the appropriate asymmetric-top three-photon line-strength theory are attributed to $\text{C}$ state predissociation. Two separate predissociation mechanisms have been identified, one (heterogeneous) relying on a-axis parent molecular rotation to couple the bound B₁ state to an unbound state of A₁ electronic symmetry, the other (homogeneous) involving a second, dissociative excited electronic state of B₁ symmetry. Having established the detailed $\text{C}$ state predissociation dynamics, two photon absorption spectra of H₂O and D₂O ($\text{C}$ ← $\text{X}$) can be predicted accurately: studies of individual quantum-state-selected photofragmentation processes from H₂O($\text{C}$) are proposed.     

Laser-induced fluorescence in supersonic nozzle beams predissociation in the Rb2 C1πu and D 1πu states

By exciting Rb₂ in a supersonic nozzle beam with a pulsed dye laser in the C ¹Π_u-X ¹Σ⁺_g and the D ¹Π_u-X ¹Σ⁺_g band system, we find evidence tor different predissociation processes The products appear as follows from the C state, Rb^* (5 ²P_{$\text{3}\text{2}$}) exclusively, and from the D state Rb*(4²D_{$\text{3}\text{2}$}) predominantly, followcd by Rb^*(5 ²P_i-5²S) cascade radiation In addition, a lower bound of D_e(Rb₂X¹Σ⁺_g)? 3939± 10 cm^?1 is obtained.     

Microwave—optical double resonance in the A1A2 excited state of thioformaldehyde

Microwave—optical double resonance experiments have been carried out on the 4_o¹ band of the $\text{A}$¹A₂$\text{X}$¹A₁ system of thioformaldehyde (H₂CS). More than 100 microwave and radiofrequency transitions have been observed in the $\text{A}$¹A₂ excited state. Many of these transitions are magnetically sensitive. Some of the excited state levels are perturbed by triplet levels and others by high vibrational levels of the ground state.     

Theoretical study of the 12A″ (X2II) State of N2 O+: implications for isotopic scrambling

Ab initio calculations suggest that the potential energy surface for the 1²$\text{A}$″ state of N₂O⁺ has a secondary minimum corresponding to a strongly bent structure. This structure is computed to lie below the $\text{A}$²Σ ⁺ state energetically and therefore may be responsible for the isotopic scrambling observed in this energy region.     

Observation of X1A1 vinylidene by photoelectron spectroscopy of the C2H2− ion

Photoelectron spectra of the vinylidene anion (C₂H₂^?) show vibrational structure in X¹A₁ vinylidene up 12 kcal/ mol above the vibrational ground state. Analysis yields an EA(C₂H₂$\text{X}$¹ A₁) of 0.47 ± 0.02 eV, and frequencies for the CC stretch and HCH bend. Absence of the ³B₂ state in the photoelectron spectra indicates the ¹A₁-³B₂ splitting in vinylidene is ? 1.7 eV.     

Production of electronically excited iodine atoms I(5p52P12) by collisional release in molecular I2

The appearance rate of 1^* (5p⁵²P_{$\text{1}\text{2}$}) following laser photolysis of molecular I₂ 1.2 kT below the dissociation limit o the I₂ (B³ Π_ou+) state has been monitored by time-resolved atomic absorption as a function of I₂ pressure. Data were also taken with N₂ as an added gas. The results confirm the production of I^* from the B state by a collisional process and reveal an additional process by which I^* continues to appear for several hundred nanoseconds after the laser pulse even at N₂ pressures as high as 750 torr.     

Branching ratio for the production of I2Psol12) in the photodissociation of I2

The production of atomic iodine in the ground (²Pfrsol|3/2) and electronically excited (²P$\text{1}\text{3}$) states following laser-induced photodissociation of I₂ the region 425–498 nm was monitored directly by resonance spectroscopy. The branching ratio for iodine atom formation. R = [I(²P$\text{1}\text{2}$)]/[I(²P$\text{3}\text{2}$)], is above 0.5 in the region 495–498 nm in agreement with the recent observation of laser action on the atomic transition at 1315 nm following photolysis of I₂ using a dye laser. The present experiments permitted deconvolution of the I₂ continuous absorption spectrum below 498 into contributions from the B³ Π_o,u → X ¹Σ_g⁺ and ¹Π_tu → X¹σ_g^? transitions.     

Diffusion of point defects participating in solid-phase chemical reactions (trap diffusion): Demonstration for the Ti3+ → Ti4+ transition in corundum

A problem of trap diffusion, that is diffusion of point defects in crystals participating in a solid-phase chemical reaction with motionless impurity ions, is solved. Time dependences of the reaction-front displacement, X_f, and its steepness, $\text{(}\text{?C}\text{?X}\text{)}{}_{\mathrm{f}}$ are determined analytically for N₀ ? C₀ and numerically for all relations of N₀ and C₀$\text{x}{}_{\mathrm{f}}{}^2\text{=2}\text{N}{}_0\text{C}{}_0\text{Dt; (}\text{ac}\text{ax}\text{)}{}_{\mathrm{f}}\text{=0.3C}{}_0{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{(}\text{g}\text{D}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2></mtext>}}$where C₀ and N₀ are the initial concentration of impurity and the eqilibrium defect concentration, respectively, D is a diffusion coefficient, and g is a chemical reaction constant. Dependence of X_f vs C₀ and t is confirmed for oxygen annealing of corundum crystals doped with titanium which, reacting with the point defects, changes its valency. The data are obtained for dependence of displacement X_f upon partial oxygen pressure and thermotreatment temperature as well as upon the sign of the constant electric field applied to the sample. From these data we conclude that the reaction of titanium impurity, changing from the three-valent to the tetravalent state at the activation energy of 80 ± 8.5 kcal/mole is due to anisotropic diffusion of charged aluminum vacancy and holes in the valence band. The diffusion coefficient for that process at 1500°C is estimated to be larger than 10^?5 cm²/sec. Using the trap-diffusion features, the concentration of optical centers of the 0.315-μm absorption band in ruby is also estimated.     

Fluorescence spectra of laser-excited YO molecules in a H2O2Ar flame

CW dye laser induced fluorescence emission and thermal emission spectra of YO-molecules in a 1 atm H₂O₂Ar flame of 2430 K were recorded simultaneously. Narrow band laser excitation was applied to four rotational lines in the (1, 1) Q-branch of the A²Π_{$\text{3}\text{2}$}→X²Σ⁺ transition and broadband excitation was applied to several separate Q-branches of the A²Π_{$\text{1}\text{2}$,$\text{3}\text{2}$}→X²Σ⁺ transitions. From the differences between the fluorescence emission spectra and thermal emission spectra, we conclude that collisional de-excitation of an excited vibronic level takes place by vibrational relaxation, decay to the electronic ground state and by intermultiplet transfer in order of increasing probability.     

Energy levels of iodine monochloride near the first dissociation limit

Absorption transitions to vibrational levels close to the A state dissociation limit of ICI have been examined using a two-photon sequential absorption technique. The discrete rotational structures of I³⁷ Cl bands to within 0.7 cm^?1 of the limit have been selectively excited and analysed. A value of 17557.514 ± 0.030 cm^?1 has been obtained for the I(²P^o_{$\text{3}\text{2}$}) + Cl(²P^o_{$\text{3}\text{2}$}) dissociation energy D_e, relative to the minimum of the ICI ground state potential well. The two-photon technique can be used to excite and display separately the high resolution absorption spectra of different isotopic species of a molecule which are contained in a mixture.     

Beam-gas chemiluminescent reactions of Eu and Sm with O3, N2O,NO2 and F2

Studies are made of the visible chemiluminescence resulting from the reaction of an atomic beam of samarium or europium with O₃, N₂O, NO₂ and F₂ under single-collision conditions (～10^?4 torr). The spectra obtained for SmO, EuO, SmF, and EuF are considerably more extensive than previously observed. The variation of the chemiluminescent intensity with metal flux and with oxidant flux is investigated, and it's concluded that the reactions are bimolecular. From the short wavelength curoff of the chemiluminescent spectra, the following lower bounds to the ground state dissociation energies are obtained: D⁰₀(SmO) > 135.5 +- 0.7 kcal/mole, D⁰₀(EuO) > 131.4 ± 0.7 kcal/mole, D⁰₀(SmF) > 123.6 ± 2.1 kcal/mole, and D⁰₀(EuF) > 129.6 ± 2.1 kcal/mole. Using the Clausius-Clapeyron equation, the latent heats of sublimation are found to be ΔH₁₀₅₂ (Eu) = 42.3 ± 0.7 kcal/mole for europium and ΔH₁₀₈₄(Sm) = 47.9 ± 0.7 kcal/mole for samarium. Total phenomena- logical cross sections are determined for metal atom removal. Relative photon yields per product molecule are calculated from the integrated chemiluminescent spectra and it is found that Sm + F₂ → SmF^* + F is the brightest reaction. The comparison of the photon yields under single-collision conditions with those at several torr shows that energy transfer collisons play an important role in the mechanism for chemiluminescence at the higher pressures. A simple model is presented which explains the larger photon yields of the Sm reactions compared to the Eu reactions in terms of the greater number of electronic states correlating with the reactants in the case of samarium.     

C2O(Ã 3Πi↔-~X3Σ−: laser induced excitation and fluorescence spectra

Laser induced fluorescence of C₂O is observed following the 266 nm laser photodissociation Of C₃O₂. Excitation spectra of C₂O(óΠ_i?-~X³Σ^? are consistent with previous absorption studies of C₂O. A number of new transitions are identified and assigned. Fluorescence spectra have been recorded following single vibrational level laser excitation. Bands are assigned to ground state vibrational progressions. Values of 1967 and 1063 cm^?1 are found for υ₁″ and υ₃″ stretching vibrations in the X?³Σ ^? state. A subband structure in the fluorescence spectrum is observed and discussed.     

Structural studies in the Li2MoO4MoO3 system: Part 1 the low temperature form of lithium tetramolybdate,LLi2Mo4O13

LLi₂Mo₄o₁₃ crystallizes in the triclinic system with unit-cell dimensions a = 8.578 Å, b = 11.450 Å, c = 8.225 Å, α = 109.24°, β = 96.04°, γ = 95.95° and space group $\text{P}\text{1}\text{, Z = 3}$. The calculated and measured densities are 4.02 g/cm³ and 4.1 g/cm³ respectively. The structure was solved using three-dimensional Patterson and Fourier techniques. Of the 2468 unique reflections collected by counter methods, 1813 with I ? 3σ(I) were used in the least-squares refinement of the model to a conventional R of 0.031 (ωR = 0.038). LLi₂Mo₄O₁₃ is a derivative of the V₆O₁₃ structure with oxygen ions arranged in a face-centred cubic type array with octahedrally coordinated molybdenum and lithium ions ordered into layers.