The importance of spin relaxation in the delayed fluorescence of molecular crystals

Deactivation rate constants of spin-orbital excited Br atoms in the reactions Br(²P_{$\text{1}\text{2}$}) + O₂ → Br(²P_{$\text{3}\text{2}$}) + O₂ (k₁), and Br(²P_{$\text{1}\text{2}$}) + NO → Br(²P_{$\text{3}\text{2}$}) + NO (k₄) have been measured with a photodissociative IBr laser on the electronic transition ²P_{$\text{1}\text{2}$}?²P_{$\text{3}\text{2}$} in the Br atom (λ = 2.7 μm). The values obtained are (6.4 ± 1.8) × 10^?14 cm³ s^?1 and (1.9 ± 0.6) × 10^?12 cm³ s^?1, respectively. Comparison with published data leads to the conclusion that, contrary to a widely accepted point of view, the high rate constants for the quenching of excited halogen atoms are due to resonant energy transfer processes and not to the paramagnetic nature of the quencher.     

The quenching of excited iodine atoms by oxygen molecules as studied by opto-acoustic spectroscopy

The opto-acoustic spectrum of I₂ in the presence of various quenching gases — NO, O₂, CH₃I, SO₂, C₃H_S, N₂, and He — has been studied. Of these, the I₂/O₂ spectrum is quite different due to the near-resonant energy transfer I(²P$\text{1}\text{2}$) + O₂(³Σ) → I(²P$\text{3}\text{2}$) + O₂(^IΔ), wherein the resistance of the O₂((^IΔ) species to collisional relaxation severely distorts the acoustic signal. The photochemical production of excited ²P$\text{1}\text{2}$ iodine atoms commences at wavelengths considerably longer than the dissociation limit of the I₂B? state.     

Surface-hopping model for near-resonant electronic energy transfer

A surface-hopping model is applied to near-resonant electronic energy transfer in the NFBi and O₂I systems. Multiple surface crossings occur in NFBi at ca. 8 A, corresponding well with measured transfer cross section of 200 A². A Landau-Zener model yields the temperature dependence of the thermally averaged cross section for the laser pumping reaction, O^*₂(a¹Δ) + I(²P_{$\text{3}\text{2}$}) → O₂(X³Σ^?_g) + 1^*(²P_{$\text{1}\text{2}$}).     

Laser induced photodissociation of C2F5I. Radiative lifetime of the metastable iodine atom (2P12)

Excited iodine atoms I(²P_{$\text{1}\text{2}$}) are formed by laser irradiation of C₂F₅I at 2950 Å. The mean radiative lifetime τ of these metastable atoms and their bimolecular rate constant k₂ for deactivation in collissions with C₂F₅I were measured to be: τ = 108 ± 10 ms; k₂ = (1.8 ± 0.1) × 10^?17 cm³/molec s.     

Yield of I(2Psol:12) in the photodissociation of CH2I2

The production of I(²P_{$\text{1}\text{2}$}) in the photolysis of CH₂I₂ has been studied optoacoustically at excitation wavelengths between 365.5 and 247.5 nm. Bands found at 32200 and 47000 cm^?1 correlate with I(²P_{$\text{3}\text{2}$}) whilst those at 34700 and 40100 cm^?1, which correlate with I(²P_{$\text{1}\text{2}$}), give final ²P_{$\text{3}\text{2}$}/²P_{$\text{1}\text{2}$} ratios of 1.75 and 1.1, respectively, after curve crossing.     

A photoionization study of the charge transfer reactions: Xe+ + O2 → O+2 + Xe and O+2 + Xe → Xe+ + O2

Photoionization mass spectrometer techniques have been employed to study the charge transfer reactions: Xe⁺ + O₂ → O⁺₂ + Xe and O⁺₂ + Xe → Xe⁺ + O₂. The results show the reaction of Xe⁺(²P_{$\text{3}\text{2}$}) ions with O₂ molecules is much more efficient than the reaction of Xe⁺(²P_{$\text{1}\text{2}$}) ions with O₂ molecules. The charge transfer reaction of O⁺₂ ions with Xe atoms was detected for O⁺₂ ions in the a ⁴Π_u state.     

Comparison of reactivity of Ar(3Po) and Ar(3P2) metastable states. Products of quenching by some halogen-containing compounds

Vacuum UV emission from the products of quenching ofAr(³P_o) and Ar(³P₂) by several reagents has been compared. The large differences suggest that Ar(³P_o) and Ar(³P₂) preferentially yield respectively the D($\text{1}\text{2}$) and B($\text{1}\text{2}$) excited states of ArBr^★ or ArCl^★, which show specific, but very different, predissociation patterns.     

Diffusion of oxide ions in LaFeO3 single crystal

The tracer diffusion coefficient, $\text{D1}{}_{\mathrm{<mtext>O</mtext>}}$, of oxide ions in LaFeO₃ single crystal was determined over the temperature range of 900–1100°C by the gas-solid isotopic exchange technique using ¹⁸O as a tracer. For the determination of $\text{D1}{}_{\mathrm{<mtext>O</mtext>}}$, the depth profile of ¹⁸O was measured by means of a secondary ion mass spectrometer (SIMS). The surface exchange reaction was found to be slow and the surface exchange rate constant, k, was determined together with $\text{D1}{}_{\mathrm{<mtext>O</mtext>}}$. It was found that $\text{D1}{}_{\mathrm{<mtext>O</mtext>}}$ at 950°C is proportional to P^?0.58_O2, where P_O2 is an oxygen pressure. The vacancy mechanism was determined for the diffusion of oxide ions from the P_O2 dependence. The vacancy diffusion coefficient, D_V, for LaFeO₃ was nearly the same as that for LaCoO₃ at the same temperature. The activation energy for migration of oxide ion vacancies was 74 kJ · mole^?1 for both oxides.     

Proton and carbon nuclear magnetic resonance study on some n- and o-acyl derivatives of monohydroxypyridines

Comparative study on the proton and carbon NMR spectra for a series of $\text{N}$- and $\text{O}$-acyl substituted monohydroxypyridines (C₅H₄NO$\text{R}$: $\text{R}$=-H, -CHO, -COCH₃, -COC(CH₃)₃, -COCF₃, -COC₆H₅, -SO₂CH₃, -SO₂C₆H₄CH₃ is reported. p]Characteristic ¹H, ¹³ NMR and IR spectral features allow simple and unambiguous distinction between the isomeric $\text{N}$- and/or $\text{O}$-acyl-derivatives of 2-, 3- and 4-hydroxypyridines, so that both forms can clearly be identified when tautomeric equilibria occur, since the tautomerism rate is slow on the NMR time scale     

The crystal structure of hexathallium(I) hexatellurodigermanate(III), Tl6[Ge2Te6]

The new compound Tl₆[Ge₂Te₆] was prepared by thermal synthesis from the elements. The material is triclinic, space group $\text{P}\text{1}$, with a = 9.471(2), b = 9.714(2), c = 10.389(2) Å, α = 89.39(1), β = 97.27(1), γ = 100.79(1)°, and Z = 2. The crystal structure was determined from single-crystal intensity data measured by means of an automated four-circle diffractometer and refined to an R value of 0.053 for 1831 observed reflections. Tl₆[Ge₂Te₆] is characterized by Ge₂Te₆ units with GeGe bonds which are linked into a three-dimensional structure by Tl atoms coordinated to essentially six Te atoms. The most important mean distances are $\text{d}\text{GeGe}\text{) = 2.456}$ Å, $\text{d}\text{(}\text{GeTe}\text{) = 2.573}$ Å, and $\text{d}\text{(}\text{TlTe}\text{) = 3.515}$Å. The lone 6s electron pairs of the thallium(I) atoms exhibit significant stereochemical activity. Tl₆[Ge₂Te₆] represents a new structure type.     

The crystal structure of Na7Mg4.5(P2O7)4

The crystal structure of Na₇Mg_4.5(P₂O₇)₄ has been solved by direct methods from the three-dimensional X-ray data. The space group is $\text{P}\text{1}$. The crystal structure consists of Mg²⁺, Na⁺, and P₂O^4?₇ ions. One magnesium atom at symmetry center (0,0,0) and two sodium atoms at ±(?0.0421, ?0.0596, 0.2230) display occupation factors 0.5 each. A short interatomic distance between these Na⁺ and Mg²⁺ ions (1.80 ± 0.01 Å) excludes the occupation of both sites in the same unit cell. The crystal structure of Na₇Mg_4.5(P₂O₇)₄ consists of unit cells containing Na₈Mg₄(P₂O₇)₄ or Na₆Mg₅(P₂O₇)₄ with a statistical occurrence 1:1.Each Mg²⁺ ion is octahedrally coordinated by six O^2? ions at distances 1.979 – 2.270 Å. The coordination polyhedra around the Na⁺ ions are ill-defined. The bond angles POP in the P₂O^4?₇ groups are 126.6 and 133.6° (±0.3°). The final reliability factor R is 7.1%.     

Die molekül- und kristallstruktur von thallium-tris(bistrimethylsilylamin), Tl[N(SiMe3)2]3

The molecular and crystal structure of tris(bistrimethylsilylamin)thallium was determined by means of single-crystal X-ray spectroscopy: in the space group P$\text{3}$1c with a = 16.447(7), c = 8.456(7) Å; and D_c = 1.149 g cm^?3 two molecules are located in the unit cell. The compound is isomorphous to the analogues Fe[N(SiMe₃)₂]₃ or Al[N(SiMe₃)₂]₃, respectively, which show a propellar-twist of the Si₂N-groups versus the plane of the metal atom and the three nitrogen-atoms: Tl(N)₃/Si₂N 49.1°; SiNSi 122.6°; NSiC 111.8°; CSiC 107.1°; TlN 2.089 Å;; SiN 1.738 Å;; $\text{SiC}$ 1.889 Å;.     

The optical absorption and emission spectra of Cs2NaSmCl6

Optical absorption and emission spectra are reported for single crystals of the cubic elpasolite Cs₂NaSmCl₆. The variable temperature spectra obtained at high resolution are assigned using energies and relative intensities. Transitions from the ground level, ⁶H_{$\text{5}\text{2}$} to cystal fi levels of ⁶H_{$\text{7}\text{2}$-$\text{15}\text{2}$}, ⁶F_{$\text{1}\text{2}$-$\text{11}\text{2}$}, ⁴G_{$\text{5}\text{2}$-$\text{9}\text{2}$}, ⁴F_{$\text{3}\text{2}$,$\text{5}\text{2}$}, ⁴I_{$\text{9}\text{2}$}, and ⁶P_{$\text{3}\text{2}$, $\text{5}\text{2}$} are located and characterized. Intensity calculations are reported for magnetic dipole allowed transitions. The dominance of vibronic intensity in ⁶H_{$\text{5}\text{2}$}→⁶F _{$\text{1}\text{2}$-$\text{9}\text{2}$} and ⁶P_{$\text{3}\text{2}$, $\text{5}\text{2}$} transitions is accounted for qualitatively through the ligand polarization model involving quadrupole metal (Sm³⁺)-ligand (Cl^?) interaction mechanisms. The E_u″(⁶H_{$\text{5}\text{2}$})→E′(⁶H_{$\text{1}\text{2}$}) E_u′(⁶F_{$\text{1}\text{2}$}) no-phonon transition is postulated to be pure electric quadrupole allowed. The ground state magnetic moment is determined to be very small from magnetic circular dichroism (MCD) spectra.This study has led to the assignment of nearly all of the crystal field levels in the visible and IR region for Cs₂NaSmCl₆. A total of 27 such levels were identified, 17 from no-phonon transitions and the rest from vibronic transitions. The magnetic dipole intensity calculated using intermediate coupling O_h wavefunctions along with a crystal field analysis of the splitting pattern was used in the assignment of the levels. Vibronic bands were observed for all transitions and their vibrational symmetries were tentatively assigned. MCD data were used to determine the magnet moment of the ground state.     

C2O(X3Σ−): absolute reaction rates measured by laser induced fluoresence

Absolute rate constants are reported for reactions of C₂O($\text{X}\text{?}$³Σ^?) under pseudo-first-order decay conditions. C₂O is generated by laser photodissociation of C₃O₂ at 266 nm, and detected by dye-laser induced fluorescence on the $\text{A}\text{?}$³Π_i-$\text{X}\text{?}$³Σ^? transition. Rate constants of (433 ± 12), (3.30 ± 0.12) and (1.12 ± 0.05) × 10^?13 cm³ molecule^?1 s^?1 are reported for reactions with NO, O₂ and isobutene. The NO value is approximate due to an apparent dark reaction between NO and C₃O₂. Upper limits of 1 × 10^?14 cm³ molecule^?1 s^?1 are reported for reactions with H₂, CO₂, C₃H₂ and C₂H₄. The C₂O + C₃O₂ reaction does not follow pseudo-first-order decay kinetics. Two explanations are proposed to explain this observation. Results are compared with previous relative rate measurements and are discussed in terms of their relevance to combustion chemistry.     

Trimethymetall-urotropinaddukte des aluminiums,galliums, indiums und thalliums

Reactions of urotropine (C₆H₁₂N₄) with trimethylmetal derivatives of Al, Ga, In und Tl in various ($\text{1}\text{1}$$\text{1}\text{4}$) molar ratios lead to stable and monomeric $\text{1}\text{1}$, $\text{1}\text{2}$ or $\text{1}\text{3}$ adducts in good yields, but no $\text{1}\text{4}$ addition product could be isolated. The vibrational spectra (IR and Raman) of all compounds are recorded and partly assigned. Some characteristic frequencies of the C₆N₄-skeletons clearly show the symmetry changes in the series of the $\text{1}\text{1}$ → $\text{1}\text{2}$ → $\text{1}\text{3}$ adducts (C_3ν → C_2ν → C_3ν). The X-ray structure determinations of C₆H₁₂N₄·.GaMe₃ (MeCH₃) and C₆H₁₂N₄·.2GaMe₃, are in good agreement with the vibrational spectra. Both compounds crystallize in monoclinic space groups (P2₁ and C2/c). The GaN distances are around 214 pm, and the values for the C₆N₄-skeletons are not significantly different from those for free urotropine.     

Structure cristalline du tétramétaphosphate de nickel-ammonium heptahydraté: Ni(NH4)2P4O12 · 7H2O

Nickel-ammonium tetrametaphosphate, Ni(NH₄)₂P₄O₁₂ · 7H₂O is triclinic with a = 13.841(3); b = 9.621(5); c = 7.482(2)Å; α = 98.05(4); β = 97.25(4); γ = 103.01(4)°; M = 536.59; V = 947.9ų; Z = 2; D_x = 1.879 g cm^?3; μ = 14.524 cm^?1, and space group $\text{P}\text{1}$. The crystal structure was solved using 1661 independent reflections measured on a single-crystal diffractometer (MoKα). The final R value is 0.056. The two crystallographic independent nickel atoms Ni(1) and Ni(2) are octahedrally coordinated: Ni(1) by four oxygen atoms and two water molecules, Ni(2) by six water molecules. Ni(1), closely connected to two P₄O₁₂ rings, forms a complex anion [Ni(P₄O₁₂)₂(H₂O)₂]^6? which is associated to ammonium polyhedra and [Ni(H₂O)₆]²⁺ octahedra. Another interesting feature of this atomic arrangement is the presence of a large channel (10 × 4) Ų parallel to the $\text{c}$ axis. The internal surface of this channel is covered by six zeolitic water molecules.     

Direct absorption measurement of the spin—orbit splitting of the ground state in atomic fluorine

We have measured the three hyperfine components of the spin—orbit split (²P_{$\text{3}\text{2}$} → ²P_{$\text{1}\text{2}$}) ground state in atomic fluorine by diode laser absorption spectroscopy. The measurement improves the accuracy of the two lines previously reported (404.175 and 403.969 cm^?1), with the third line at 404.210 cm^?1. This confirms the spacing of the hyperfine components measured by EPR, and establishes diode laser absorption as a viable technique for determining F-atom concentrations.     

Excited state dynamics of C2O(A3Πi)

The fluorescence decay and bimolecular electronic quenching behavior of C₂O ($\text{A}$³Π_i) is reported. C₂O($\text{X}$³Σ^?) is produced by laser photolysis of C₃O₂ at 266 nm and is subsequently excited by a tunable flashlamp pumped dye laser. The fluorescence decay is highly nonexponential and dominated by both short (≈ 15 μs) and long (50–250 μs) decay components. The long-lived emission, itself, is nonexponential. The fluorescence decay is modeled as the sum of three exponential components. The short-lived emission is quenched by C₃O₂ at higher than the gas kinetic rate while the long-lived fluorescence is quenched much less efficiently. Fluorescence quenching measurements are also reported for collisions with Ar, N₂ and O₂.     

Raman spectra of lanthanide sesquioxide single crystals: Correlation between A and B-type structures

Structures and Raman spectra of lanthanide sesquioxide single crystals with A-type trigonal structure (La₂O₃, Pr₂O₃, Nd₂O₃, Sm₂O₃) and B-type monoclinic structure (Sm₂O₃, Eu₂O₃, Gd₂O₃) are compared. The B form (C³_2h or $\text{C2}\text{m}\text{, Z = 6}$) derives from the A form (D³_3d or $\text{P}\text{3}\text{m1, Z = 1}$) by a slight lattice deformation, implying a splitting of D_3d and C_3v atomic positions into less symmetrical C_2h and C_s sites. This close structural relationship allows one to relate the Raman active modes of the B-type crystals to vibrations of the A-type crystals and to deduce an interpretation of the complex B-type spectra from those of the simpler A-type spectra. Furthermore, it is shown that the frequency of the modes which mainly involve metal-oxygen stretching motion increases with the lanthanide atomic number in the A and B series. This evolution is interpreted in terms of increasing compactness of the structure.     

State determination of atoms formed by dye-laser induced photodissociation of KI in the near UV

By means of a time of flight technique, using high temperature surface ionization, we have been able to measure the internal energy of atoms formed in the photodissociation of KI at wavelengths between 265 and 335 nm. At all wavelengths only ground state K(²S_{$\text{1}\text{2}$}) is formed. Between 265 and 295 nm iodine is solely formed in the excited I(²P_{$\text{1}\text{2}$}) state, above 305 nm only in the ground state I(²P_{$\text{3}\text{2}$}).