This paper deals with triplet-triplet annihilation in pure and mixed organic crystals. In crystals containing a small concentration of impurity traps, triplet excitation migration may proceed from trap to trap on a time scale which is short compared with the long triplet state lifetime but which is long compared with the normal fluorescence lifetime. Nearest-neighbor and long-range mutual annihilation of two triplets may then take place giving rise to delayed fluorescence. The rates of long-range triplet excitation migration and annihilation show a concentration dependence, a temperature dependence, and a solvent dependence. Providing the triplet-triplet annihilation rate is not too fast, the intensity of the delayed fluorescence can be shown to depend upon the square of the intensity of the exciting light. This expectation is borne out by experiments, briefly reported here, on delayed fluorescence in dilute isotopic mixed crystals. In crystals containing high concentrations of such impurity traps, or in pure crystals, the annihilation rate becomes extremely rapid and this mechanism effectively quenches phosphorescence in many, but not all, classes of pure organic crystals. The kinetics of the over-all process are discussed in both the limits of fast and slow annihilation rates. A theoretical investigation of the origin of the annihilation matrix element is carried out, and it is shown that exchange interactions play the largest role in determining annihilation rates.Past work [H. Sponer, Y. Kanda, and L. A. Blackwell, J. Chern. Phys. 29, 721 (1958); N. W. Blake and D. S. McClure, ibid., p. 722] on delayed fluorescence of assumed pure naphthalene crystals containing very small amounts of ~-methyl naphthalene (traps) can be understood within the framework of this paper. The need in organic crystals for the major delayed fluorescence mechanism to be based upon ionization and electron trapping seems now to be considerably lessened.

The two- and three-photon spectrum of expansion-cooled ammonia has been recorded in the 380–500 nm region using multiphoton ionization (MPI) detection. In addition, the vacuum ultraviolet spectrum of gas phase ammonia has been measured photoelectrically in the 125–160 nm region. Features due to transitions to eleven electronic states are assigned in the 5.7–9.3 eV energy range, where only five band systems were previously assigned. By utilizing the spectral simplification provided by expansion cooling, the different selection rules for multiphoton absorption, and the differences between the MPI and VUV spectra, assignments can be made with much more certainty. Three previously assigned band systems have been reassigned, and seven additional electronic states identified. It is found that states arising from nd orbitals are responsible for the most intense features in the VUV spectrum, in contrast to previous experimental and theoretical work where they were excluded from consideration. Evidence is found for direct competition between photodissociation and photoionization of NH3 excited states. Earlier photochemical studies of ammonia are reinterpreted in light of the new spectroscopic assignments.

Equilibrium constants for the chemisorption reactions of molecular nitrogen with nickel clusters Nin have been determined as a function of temperature for n=19 to 71. Van’t Hoff analysis of the data yields standard-state changes in reaction enthalpy and entropy. These changes are related to what is known about nickel cluster structure and the nature of the cluster–N2 interaction. In general, the adsorption energy is highest for the smallest clusters studied, reaching values twice those for N2 adsorption on bulk nickel surfaces. In many cases, there is a correlation between enthalpy and entropy: high adsorption energy is accompanied by a large change in entropy, and vice versa. These effects are discussed in terms of the configurational entropy of reaction and the frequencies of the frustrated translational and rotational motions of the adsorbed N2 molecules.

Articles you may be interested inEquilibrium structure and bonding of small iron-carbon clusters Magic numbers through chemistry: Evidence for icosahedral structure of hydrogenated cobalt clusters J. Chem. Phys. 92, 2110 (1990); 10.1063/1.458045 Tworeagent reactions of iron clusters with ammonia and deuterium: Saturated compositions and the kinetics of reactions of deuterium with ammoniated clusters J. Chem. Phys. 90, 1526 (1989); 10.1063/1.456095The uptake of ammonia by iron clusters: A new procedure for the study of metal cluster chemistry Evidence is presented for structural changes in iron clusters in the Fe J3 to Fe 23 size range. Abrupt changes with cluster size are found for several chemical properties, including reactivity with hydrogen and binding energies of ammonia and water. These changes often come at the same cluster sizes, pointing to a common origin-fundamental changes in the structure of the bare iron clusters. In addition, changes in structure as a consequence of adsorbate binding are suggested. The experimental observations leading to these conclusions are detailed, and possible structures for clusters in this size range are proposed.1622

The gas-phase reactions of nitrogen, hydrogen, and carbon monoxide with Ni38 are studied as a function of reagent pressure at several reaction temperatures. Saturation coverage of the cluster is found at Ni38(N2)24, Ni38H36, and Ni38(CO)36. These saturation levels are consistent with the metal core of the ligated cluster having the structure of a truncated octahedron in each case. An alternate fcc structure derived from a 40-atom truncated tetrahedron is consistent with the nitrogen data, but not with the hydrogen or carbon monoxide results. In addition, the nitrogen uptake data indicate that the bare Ni38 cluster also has the structure of a truncated octahedron or possibly a deformed truncated octahedron. There is no indication that Ni38 has an icosahedral or polyicosahedral structure. The nature of the binding of the three reagents to the cluster is discussed. Evidence is presented that CO initially binds to atop sites, but following saturation of these sites a local rearrangement to bridge sites occurs that allows an increase in coverage to the observed saturation at Ni38(CO)36. At high reagent pressures all three reagents cause adsorbate-induced structural changes to isomers that bind more ligands and whose structures have yet to be determined.