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.     

Photodissociation of alkyl bromides

The branching ratios for the production of Br(4²P_{$\text{1}\text{2}$}) following the broadband flash photolysis of the alkyl bromides, CH₃Br and C₂H₅Br, and the perfluorinated molecules, CF₃Br, C₂F₅ Br and n-C₃F₇ Br, have been determined using time-resolved atomic absorption spectroscopy. The production of electronically excited bromine atoms is shown to be inefficient in the case of the alkyl bromides while the perfluorinated molecules yield decreasing amounts of Br(4²P_{$\text{1}\text{2}$}) as the molecular complexity increases, i.e., CF₃Br > C₂F₅Br > C₃F₅Br > C₃F₇. It is also shown that the hydrogenated bromides deactive electronically excited atoms almost two orders of magnitude faster than do the perfluorinated bromides.     

Photofragmentation of CH3Br in the A band

CH₃Br is photodissociated in the first continuum. Dissociation takes place into ground state CH₃ and Br [ = Br(²P_{$\text{3}\text{2}$}] or Br* [ = Br(*P_{$\text{1}\text{2}$})]. Time of flight and angular distributions of the CH₃ fragments are measured. The Br*/Br ratios upon excitation at 222 and 193 nm are found to be 1.00 and 0.20 respectively. The anisotropy parameters at these wavelengths are β = 0.28±0.04 and β = ?0.23±0.02, respectively. The total absorption cross section is decomposed into partial absorption cross sections of the ¹Q, ³Q₀ and ³Q₁ states. It appears that excitation at 222 nm takes place to the ³Q₀ and ³Q ₁ states whereas at 193 nm the ¹Q and ³Q₀ states are excited. Contrary to CH₃I, the adiabatic curve crossing between the ³Q₀ and the ¹Q states in Ch₃Br is not important. The dissociation energy of the CBr bond is determined to be D₀(CH₃Br) = 2.87±0.02 eV.     

Collisional deactivation of laser-excited Br*(2P12) atoms with halogen and interhalogen molecules

Deactivation rates of spin-orbit excited Br*(²P_{$\text{1}\text{2}$}) atoms by halogens I₂, Br₂, and Cl₂ and interhalogens IBr, ICl and BrCl have been measured by laser-excited, time-resolved infrared emission techniques. The results are effectively explained in terms of a collision complex formation model.     

Preparation and synthetic utility of fluorinated phosphonium salts,bisphosphonium salts and phosphoranium salts

The synthesis and mechanism of formation of phosphonium salts of the type [R₃$\text{P}\text{+}$CFXY]Z^? (where X = F, Cl, Br; Y = Br, Cl; Z = Br, Cl), $\text{bis}$-phosphonium salts of the type [R₃$\text{P}\text{+}$CF₂$\text{P}\text{+}$R₃]2Br^?, and phosphoranium salts of the type [R₃$\text{P}\text{+}$$\text{C}\text{?}$F$\text{P}\text{+}$R₃]X^? (X = Br, Cl) will be presented. The applicability of these substrates in the generation of useful nucleophilic or electrophilic synthetic intermediates will be discussed.     

Classical models for electronic degrees of freedom: Quenching of Br*(2P1/2) by collision with H2 in three dimensions

Three-dimensional classical trajectory calculations have been carried out for the quenching of Br*(²P_1/2) by collision with ground-state H₂, using an approach that describes the electronic as well as heavy-particle (i.e. transition, rotation, vibration) degrees of freedom by classical mechanics. We find a sizeable quenching cross section (≈$\text{1}\text{2}$ Ų), with essentially all of the collision products having H₂ vibrationally excited to υ = 1, i.e. near-resonant EV transfer dominates the non-resonant ET,R processes. This is in agreement with experimental results.     

Electronic-to-vibrational energy transfer from Br (42P12 to HF

Room temperature experiments have measured the rate of electronic-to-vibrational energy transfer between spin—orbit excited Br(4²P_{$\text{1}\text{2}$}) and HF. The Br^* + HF quenching rate is very fast, (1.1 ± 0.2) × 10⁶ s^?1 torr^?1, due to a near resonance between the spin—orbit splitting and the vibrational spacing. The majority of the Br^* spin—orbit energy goes directly into HF vibration.     

Temperature dependence of collisional deactivation of Tl(62P32)

The rate coefficients for the collisional deactivation of Tl(6²P_{$\text{3}\text{2}$}) by several gases has been determined at 300°K. This data is compared with that previously obtained at higher temperatures and the Arrhenius parameters calculated. Both the overall rate coefficients and temperature effects display trends similar to those observed for 1(5²P_{$\text{1}\text{2}$}) relaxation. The deactivation of Tl(6²P_{$\text{3}\text{2}$}) by O₂ is shown to proceed by a process involving an equilibrium with Tl(6²P_{$\text{1}\text{2}$}) and electronically excited oxygen, probably O₂(¹Δ_g).     

Rare gas molecular ions: Formation of XeF+ by ion-molecule reactions in Xe and NF3 and evidence for radiative deactivation of Xe+ (2P12)

The formation of the XeF⁺ ion by ion-molecule reaction has been observed in an ionized mixture of Xe and NF₃ by ion cyclotron resonance mass spectrometry. The excited ²P_{$\text{1}\text{2}$} state of the xenon ion has unambiguously been identified as the major precursor by photoionization mass spectrometry. The NF⁺₃ ion makes an additional minor contribution. Evidence suggests that the excited ²P_{$\text{1}\text{2}$} xenon ion radiatively decays to the ²P_{$\text{3}\text{2}$} ground state on the time scale of the experiment. The transition probability deduced for this dipole forbidden emission, 18 ± 4 s^?1, is in good agreement with the theoretical value of 21 s^?1 for the sum of the magnetic dipole and electric quadrupole transition rates.     

Photoacoustic measurements of photofragmentation in CH3I

Photoacoustic measurements are described giving branching ratios for the I(²P_{$\text{1}\text{2}$}) and I(²P_{$\text{3}\text{2}$}) atom production following vapour-phase photolysis of CH₃I. A range of excitation wavelengths are used from the long wavelength tail up to 248 nm. The presence of three bands is shown within the σ^* ← n continuum; in the strong-coupling model these are E ← A₁(⊥), A^*₁ ← A₁(||) and E ← A₁(⊥) with only the A^*₁ ← A₁ transition giving excited iodine atoms.     

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.     

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.     

Electronic-to-vibrational energy transfer reactions.X* + CO (X = O,I and Br)

The vibrational distribution of CO produced from the following two electronic-to-vibrational energy transfer reactions:

have been determined by means of infrared resonance absorption measurements employing a cw CO laser. The CO molecules formed in both reactions were found to be vibrationally excited up to the limits of available electronic energies carried by the excited atoms. A similar result was also observed in the Br(4²P_{$\text{1}\text{2}$}) + CO reaction, in which absorption occurred only in the 1 → 2 band. For the O^* + CO reaction the efficiency of E → V energy transfer was determined to be 16%. Our present results were found to be inconsistent with the impulsive (half-collision) model.     

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.     

Kinetic spectroscopy in the far vacuum ultraviolet. Part I: Detection of (2p5) 2P32,12 fluorine atoms using atomic resonance spectrometry in the wavelength range 95–98 nm

A new experimental system for atomic resonance spectrometry at λ < 105 nm in a discharge-flow system is described. The spectrum of a fluorine resonance lamp has been studied, and possible precursors for the 2p⁴ 3s excited F atoms formed are suggested. Ground state (2p⁵²P_{$\text{3}\text{2}$}) and J-excited ²P_{$\text{1}\text{2}$} F atoms have been detected for the first time in resonance absorption and fluorescence using the first resonance transitions with wavelengths between 95.2 and 97.8 nm. Preliminary measurements (using both ⁴P-²P and ²P-²P lines) of the variations with concentration of absorption intensity by ground state F ²P_{$\text{3}\text{2}$} and by J-excited F ²P_{$\text{1}\text{2}$} atoms are reported; F atom concentrations were measured using a titration method based on the rapid reaction, F + Cl₂ → FCl + Cl.     

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.     

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_{$\text{3}\text{2}$}).     

Measurements of the electronic excitation in atomic collisions of potassium with the rare gases

Excitation cross sections for the potassium 4²P_{$\text{1}\text{2}$} and 4²P_{$\text{3}\text{3}$} states produced in collisions with rare gases are reported. They have been measured by the lightly yield as a function of the energy from 100 to 2500 eV. In addition to this, a spectral analysis from 3300 to 8600 Å of the light is given for some energies. This spectral analysis gives experimental evidence that the excited and ionized states of both potassium and the rare gases are involved in the collision process.     

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.     

Fine structure transition cross sections for several alkali + rare gas systems

The energy dependence, E_c.m. </ 0.2 eV, of the inelastic total cross sections for the ²P_{$\text{1}\text{2}$} → ²P_{$\text{3}\text{2}$} fine structure transition of the lowest excited states of the alkali atoms are calculated for the following systems: Na, K, Rb + He, Ne, Ar and Cs + He. Encouraging agreement between theory and experiment is obtained.