Laser induced photodissociation spectroscopy: Br2

The continuous absorption spectrum of molecular bromine has been examined using laser induced photodissociation spectroscopy. In this technique, Br₂ molecules are photolyzed using a flashlamp-pumped dye laser; the atomic products of the dissociation are then monitored by time-resolved resonance absorption spectroscopy in the vacuum ultraviolet. The relative absorptivities for the transitions B³Π_o⁺u ← X¹Σ⁺_g and ¹Π_1u ← X¹Σ⁺_g have been obtained at 18350, 21010 and 22125 cm⁻¹.     

Quenching of the dissociative electron attachment resonances of O2 physisorbed on amorphous ice

Measurements of electron stimulated desorption (ESD) yields of O⁻, at incident electron energies below 20 eV, from 0.15 monolayers (ML) of O₂ physisorbed at 20 K on a variety of molecular solids have been performed. It is observed that for O₂ condensed on 4 ML of H₂O, the O⁻ signal from dissociative electron attachment (DEA) to O₂ is entirely absent. We attribute this to a complete quenching of the dissociative ²Π_u, ²Σ⁺_g, and ²Σ⁺_u, resonances of O⁻₂ by the adjacent water molecules.     

Calculation of CO(XΣ) + 0(P) recombination chemiluminescence spectrum

Relative emission spectra for the bent to linear, CO₂(¹B₂)---CO₂(X¹Σ⁺_g) transitions have been calculated using the model: harmonic oscillator, symmetric-top wavefunctions and energy levels for CO₂(¹B₂); first-orer Fermi resonance vibrational wavefunctions and energy levels for CO₂(X¹Σ⁺_g); a Boltzmann distribution of vibrational and rotational states in CO₂(¹B₂); and a constant electronic transition moment. With the literature CO₂(¹B₂) molecular structure, spectra calculated using this model show characteristics similar to the low-temperature chemiluminescence from the combination of atomic oxygen and carbon monoxide. The calculated spectra account for experimental band positions to wavelengths of 570 nm and the weak dependence of the spectra on temperature over the range 206–353 K. The latter result was obtained from a CO₂(¹B₂) bending fundamental of 600 cm⁻¹. The calculated spectra also show a violet-shift in intensity and an attenuated band structure at higher temperatures. The magnitude of these effects depends on the CO₂(¹B₂) force constants and not on the CO₂(¹B₂) molecular structure.     

Mechanism for resonant O electron stimulated desorption from condensed O2. Non-adiabatic transitions and dynamics

Absolute kinetic energy distributions and yields associated with ground state ³P and excited state ¹D oxygen atoms have been obtained for O⁻ anion electron stimulated desorption from condensed O₂ in the electron energy range 6–15 eV. The observed yields are understood as resulting essentially from dissociative electron attachment reactions via the two lowest ²Σ⁺_g O⁻₂ resonance states through adiabatic and non-adiabatic transitions to the limits O⁻(²P) + O(³P) and O⁻(²P) + O(¹D). The kinetic energy distributions show the prominent role of electron multiple collision processes and post-dissociation interactions of the O⁻ anions in the condensed phase.     

Study of the N2 bΠu state via 1 + 1 multiphoton ionization

Medium-resolution spectra of the N₂ b¹Π_u-X¹Σ_g⁺ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b ¹Π_u state: Δν₀ = 0.50 ± 0.05 cm⁻¹, Δν₁ = 0.28 ± 0.02 cm⁻¹, Δν₂ = 0.65 ± 0.06 cm⁻¹, Δν₃ = 3.2 ± 0.5 cm⁻¹, Δν₄ = 0.60 ± 0.07 cm⁻¹, and Δν₅ = 0.28 ± 0.02 cm⁻¹. From these linewidths, predissociation lifetimes τ_ν were obtained: τ₀ = 16 ± 3 ps, τ₁ > 150 ps, τ₂ = 10 ± 2 ps, τ₃ = 1.6 ± 0.3 ps, τ₄ = 9 ± 2 ps, and τ₅ > 150 ps. Band origins and rotational constants for the b ¹Π_uν = 0 and 1 levels were determined for the ¹⁴N₂ and ¹⁴N¹⁵N molecules.     

Autoionization spectra of Li2 and the XΣg ground state of Li2: Experimental and theoretical investigations

Autoionizing Rydberg levels of Li₂ molecules in a supersonic molecular beam are populated by stepwise excitation with two tunable pulsed dye lasers. The observed autoionization spectra show severe perturbations. Based on calculations of quantum defects and a perturbation treatment of l-uncoupling a tentative assignment of Rydberg series up to n = 32 is proposed. The convergence limits of these series yield a value of IP = 41475 cm⁻¹ for the adiabatic ionization potential and a vibrational constant ω_e = 263 cm⁻¹ for the X²Σ⁺_g ground state of Li⁺₂. The experimental results are compared with ab initio calculations combined with a core polarization potential, which yield the potential curve. the dissociation energy, the quadrupole moment and the vibrational frequency for the X²Σ⁺_g ground state of Li⁺₂, in the excellent agreement with experimental findings.     

All electron ab initio investigations of the electronic states of the MoN molecule

The low lying electronic states of the molecule MoN were investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction were determined in perturbation calculations. The electronic ground state is confirmed as being ⁴∑⁻. The chemical bond of MoN has a triple bond character because of the approximately fully occupied delocalized bonding π and σ orbitals. The spectroscopic constants for the ground state and ten excited states were derived. The excited doublet states ²∑⁻, ²Γ, ²Δ, and ²∑⁺ are found to be lower lying than the ⁴Π state that was investigated experimentally. Elaborate multi-configuration configuration-interaction (MRCI) calculations were carried out for the states ⁴∑⁻ and ⁴∏ using various basis sets. The spectroscopic constants for the ⁴∑⁻ ground state were determined as r_e=1.636 Å and ω_e=1109 cm⁻¹, and for the ⁴∏ state as r_e=1.662 Å and ω_e=941 cm⁻¹. The values for the ground state are in excellent agreement with available experimental data. The MoN molecule is polar with a charge transfer from Mo to N. The dipole moment was determined as 2.11 D in the ⁴∑⁻ state and as 4.60 D in the ⁴∏ state. These values agree well with the revised experimental values determined from molecular Stark spectroscopic measurements. The dissociation energy, D_e, is determined as 5.17 eV, and D₀ as 5.10 eV.     

Spectroscopic investigation of ground state pyrrole (C4H5N): the NH stretch

We have recorded the infrared absorption spectrum of pyrrole at 0.005 cm⁻¹ spectral resolution using a Fourier transform interferometer. The rotational analysis of the fundamental N---H stretch (1¹₀) at 3530.811343(82) cm⁻¹ was performed. A set of 13 upper state rovibrational parameters was determined, allowing the 2715 assigned rovibrational lines to be reproduced with a standard deviation of 1.3 10⁻³ cm⁻¹. An attempt to record the fundamental band under slit-jet conditions is reported. The role of hot bands accompanying the series of the N---H stretch excitation is investigated. Effective vibrational parameters — ω⁰₁, X⁰₁₁, Y₁₁₁, X_1,24 — are obtained. The lower level in the hot band series is unambiguously identified as the V₂₄ = 1 level, by retrieving X_1,24 independently, from other spectral data. The observation of the complex band pattern accompanying the N---H series in the higher overtone range is discussed with the help of new data, recorded around the 1⁵₀ band at different temperatures using intracavity laser optoacoustic spectroscopy.     

Photodissociation spectroscopy of Mg—Ar

Mg⁺—Ar ion—molecule complexes are produced in a pulsed supersonic nozzle cluster source. The complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as X ²Σ⁺ → ²Π is observed with an origin at 31387 cm⁻¹ (vac) for ²⁴Mg⁺—Ar. The ²⁴Mg⁺—Ar spectrum is characterized by a 15 member progression with a frequency (ω′_e) of 272 cm⁻¹. An extrapolation of this progression fixes the excited state dissociation energy (D′_o) at 5552 cm⁻¹. The corresponding ground-state value (D″_o) is 1270 cm⁻¹ (3.6 kcal/mol). The ²Π _, spin—orbit splitting is 76 cm⁻.     

The vibrational state distributions in both products of the reaction: O(P) + NO2 → O2 + NO

A laser pulse-and-probe method has been used to determine the nascent vibrational populations in NO(v=0–4) and O₂(v=6–11) formed in the thermal reaction: O(³P) + NO₂ → O₂(v) + NO(v). A frequency-tripled Nd: YAG laser is used to photolyse NO₂, diluted tenfold in Ar, and laser-induced fluorescence spectroscopy in the NO A ²Σ⁺-X ²Π and O₂ B ³Σ⁻_u -X ³Σ⁻_g electronic band system is used both to follow the kinetics of individual vibrational states and to determine the nascent vibrational distributions. The majority of the NO product is formed in v = 0 and the average vibrational yield is ≈ 4.6%. The O₂ populations fall monotonically from v = 6 to 11 in a distribution close to what is expected on prior grounds. Based on a surprisal analysis, the average vibrational energy yield in O₂ is ≈ 26%. The nature of the reaction dynamics is discussed.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

The dehydroxylation of basic aluminum sulfate: An infrared emission spectroscopic study

The tridecameric aluminum polymer [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ was prepared by forced hydrolysis of Al³⁺ up to an OH/Al molar ratio of 2.2. Upon addition of sulfate, the tridecamer crystallized as the monoclinic basic aluminum sulfate Na_0.1[AlO₄Al₁₂(OH)₂₄(H₂O)₁₂](SO₄)_3.55. The dehydroxylation of the basic aluminum sulfate has been studied by Fourier transform in-situ infrared emission spectroscopy over a temperature range of 200° to 750°C at 50°C intervals. The spectrum is characterized by the sulfate ν₁ (1024 cm⁻¹), ν₃ doublet (1117 and 1168 cm⁻¹) and the ν₄ doublet (568 and 611 cm⁻¹) modes. Furthermore, minor bands assigned to nitrate are observed. Upon heating from ≈350° to 400°C major changes are observed, especially in the bandwidth and band intensities. The bands in the hydroxyl stretching region due to the Al₁₃ group disappear, whereas the bands around 1050 cm⁻¹ display various changes in bandwidths, intensities and positions associated with the dehydration and dehydroxylation of the basic sulfate and the changing of the structure into an aluminum oxosulfate. The nitrate bands diminish upon heating.     

Characterization and decomposition mechanism of an unusual nickel, thiocyanate and 4-methylpyridine polymeric complex

The synthesis, characterization, and thermal decomposition of the [Ni(SCN)₂(H⁺SCN)₂(4-mepy)₂] compound with an octahedral structure in polymeric chain were reported, in which SCN groups form bridges among Ni(II) ions. The compound decomposes in water resulting in a pH<4 solution. The FT-IR spectrum presented doublet bands at 2117; 2128 cm⁻¹, 788; 773 cm⁻¹ assigned to ν(C---N) and ν(C---S) stretching modes, respectively, and δ(SCN) deformation modes at 468; 476 cm⁻¹. The Raman spectrum of the compound presented the ν(C---N) stretching as a strong doublet at 2122; 2128 cm⁻¹, ν(C---S) at 783; 770 cm⁻¹, and δ(SCN) at 468; 477 cm⁻¹. No significant changes were observed in the 4-mepy ligand bands compared with the vibrational frequencies of the pure compound or the compound in aqueous solution 0.2 mol l⁻¹. The crystal UV–vis reflectance spectrum presented two bands centered in 626 and 424 nm tentatively assigned to the d→d type transitions, ³A_2g→³T_1g and ³A_2g→³T_1g, for a symmetry close to O_h. The TG curve showed a mass loss between 120 and 200 °C assigned to the loss of the two 4-mepy molecules; from 200 to 265 °C, the loss of the two H⁺SCN groups; and from 265 to 450 °C, the loss of the two SCN groups that formed the bridges among the nickel atoms. Based on these mass loss data, a mechanism of thermal decomposition for the compound was proposed.     

Lifetime measurements of electronically excited C3(Πu) radicals in different vibrational states

The radiative lifetimes of nine vibrational levels of the C₃(¹Π_u) radical were obtained from decay time studies of the C₃(¹Π_u → ¹Σ⁺_g) fluorescence induced by a tunable dye laser. The lifetimes of the different vibronic levels were found to be constant within the experimental error limits, namely, τ_o = (200 ± 10) ns. The collisional deactivation of the C₃(¹Π_u) states by helium gives rate constants between 2.5 and 4 in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ units.     

Vibrational spectrum of [(CD3)3S]I and normal coordinate calculations for [(CH3)3S] and [(CD3)3S] cations

IR (4000-30 cm⁻¹) and Raman (4000-0 cm⁻) spectra of [(CD₃)₃S]I have been observed, together with those of [(CH₃)₃S]I. By assuming a C_3v molecular symmetry for the cations [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺, all the active fundamentals of [(CD₃)₃s]⁺ have been assigned and normal coordinate calculations have been carried out by a symmetry force field for [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺. The strength of the S---C and C---H bonds in the compound has been compared with that in dimethyl sulfide by using their valence stretching force constants.     

Proton transfer between hydrogen halides and dimethylether in nitrogen matrices. An example of infrared-induced transfer

The infrared absorption of mixtures of dimethyl ether and hydrogen halide (HX) in nitrogen at 13 K display relatively narrow bands in the range 650–800 cm⁻¹ with an isotopic ratio ν_H/ν_D larger than 1.4 and weakly halogen dependent; these features are assigned to the antisymmetric O…H…O stretching within the [(CH₃)₂ O…H…O(CH₃)₂]⁺X⁻ ion pair. With HI—ether mixtures, the intensity of the 660 cm⁻¹ band decreases under infrared irradiation of the matrix, which might be due to the transfer of the proton back to the I⁻ anion.     

The A Πu-X Πg emission spectrum of I2

The A ²Π_u-X ²Π_g electronic emission spectrum of I₂⁺ has been recorded at a low rotational temperature in a crossed molecular beam/electron beam apparatus. Six vibrational sequences with five or more members have been assigned to progressions in ν′, giving ω′_e = 122±8 cm⁻¹, but a full vibrational analysis has not been possible. It is not known whether this is due to the relatively poor resolution (≈5 cm⁻¹) at which the spectrum has been recorded or because the A ²Π_u state is perturbed in one or both spin-orbit components. Existing data on the A state of I₂⁺ are reviewed.     

A theoretical investigation of C5

Large-scale CEPA-1 calculations have been carried out for linear C₅, a molecule of substantial interest to combustion processes and astrochemistry. The equilibrium bond lengths are predicted to be 1.289 Å (outer CC bond) and 1.283 Å (inner CC bond), with an accuracy of 0.002 Å. The calculated ν₃ band origins of 2161 cm⁻¹ (105 CGTO basis) and 2137 cm⁻¹ (150 CGTO basis) are in good agreement with the experimental value of 2169 cm⁻¹. This band has an extremely large transition moment of 0.74 D. The less intense stretching fundamental ν₄ (μ=0.18 D) is predicted to occur at 1478 ± 10 cm⁻¹. Predictions for the totally symmetric stretching and the bending vibrational frequencies (in cm⁻¹) are 2008 (1σ_g⁺), 792 (2σ_g⁺), 570 (1π_u), 209 (1π_g) and 119 (2π_u).     

A monomeric bacteriochlorophyll triplet state (B) in reaction centres of Rhodobacter sphaeroides R26, studied by absorption-detected magnetic resonance

Using absorption-detected magnetic resonance, a new triplet state was observed in reaction centres of Rhodobacter sphaeroides R26 at 7 K. Its zero-field splitting parameters &|;D&|; = 220.5 × 10⁻⁴ cm⁻¹ and &;|;E&|; = 65.4 × 10⁻⁴ cm⁻¹, its spin polarization and its microwave-induced absorption spectrum indicate that it belongs to one of the accessory monomeric bacteriochlorophyll molecules and that it is populated through the same excitation path as the primary donor triplet ³P₈₇₀, via the radical pair ³[P₈₇₀⁺H⁻].     

Vibration-rotation spectra of C containing acetylene: anharmonic resonances

High resolution vibration-rotation spectra of ¹³C₂H₂ were recorded in a number of regions from 2000 to 5200 cm⁻¹ at Doppler or pressure limited resolution. In these spectral ranges cold and hot bands involving the bending-stretching combination levels have been analyzed up to high J values. Anharmonic quartic resonances for the combination levels ν₁ + mν₄ + nν₅, ν₂ + mν₄ + (n + 2) ν₅ and ν₃ + (m − 1) ν₄ + (n + 1) ν₅ have been studied, and the l-type resonances within each polyad have been explicitly taken into account in the analysis of the data. The least-squares refinement provides deperturbed values for band origins and rotational constants, obtained by fitting rotation lines only up to J ≈ 20 with root mean square errors of ≈ 0.0003 cm⁻¹. The band origins allowed us to determine a number of the anharmonicity constants x_ij⁰.