Quantitative tests of mixed crystal exciton theory. II. Energy denominator study of naphthalene 1B2u resonance pairs

A spectroscopic absorption study is presented of 1‐D1C10H7, 2‐D1C10H7, and 1,4‐D2C10H7 resonance pairs in C10D8, over a concentration range of 0.009% to 5% mole, at 2 °K, with a resolution of 0.5 cm−1. The resulting structure is interpreted in terms of the pairwise exciton exchange interactions, M1/2(a+b), Ma, Mb, Mc, Ma+c, and M1/2(a+b)+c. The energy denominator dependence is interpreted in terms of both exciton quasiresonance and exciton superexchange. The three parameter sets derived by Hong and Kopelman to fit earlier data on neat and isotopic mixed crystals are corroborated by this study, as they were in our previous study on monomers (I). However, it was not possible to narrow down completely the choice of parameters. A discussion is given on the feasibility of deriving a unique dispersion relation (and Green’s function) for the first naphthalene singlet exciton and on its likely precision.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70131/2/JCPSA6-66-4-1599-1.pd

Triplet exciton percolation and superexchange: Naphthalene C10H8–C10D8

The phosphorescence of betamethylnaphthalene doped into a naphthalene−h8/naphthalene−d8 mixed crystal has been measured. The results demonstrate that (1) dynamical exciton percolation does occur (i.e., a transition from an exciton insulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70872/2/JCPSA6-62-1-292-1.pd

Long range exciton percolation and superexchange: Energy denominator study on 3B1u naphthalene

The long-range exciton percolation model is found to describe the lowest triplet exciton superexchange ("tunneling") migration at low temperature (2 K), in our model alloy system: Binary isotopic mixed naphthalene crystals with dispersed exciton sensors (supertraps) consisting of small concentration of betamethylnaphthalene (-10-3 mole fraction) or isotopic substituted naphthalene molecules (with lower excitation energies than the partially deuterated naphthalene guest species). While the "host" is C10D8 throughout, the "guest" species in our five experimental systems are: C10H8, 2-DC10H7. 1-DC10H7, 1,2-D2C10H6 and 1,4,5,8-D4C10H4. The variation in guest--host (and supertrap--guest) energy denominator in the above systems enables a quantitative test of our physical exciton superechange (tunnelling) migration model. In conjunction with a mathematical long-range percolation model (J. Hoshen, E.M. Monberg and R. Kopelman, unpublished). The experimental monitoring of the exciton migration dynamics consists of refined phosphorescence measurements to our systems, under highly controlled conditions (crystal quality, purity, concentration, temperature and excitation). Using only the known nearest neighbor (interchange-equivalent) exciton exchange interaction, quantitative agreement with the experimental dynamic percolation concentration is achieved, without adjustable parameters, for four of the five investigated systems. The fifth one is known to involve a cooperative percolation--thermalization exciton migration, and is effective in qualitative agreement with the predicted upper limit for the exciton percolation concentration. The nearest-neighbor 3B1u excitation exchange interactions, and their square lattice topology, play the dominant role in determining the guest triplet exciton energy transfer and migration. This energy conduction involves an extremely narrow "impurity band", on the order of 10 to 103 Hz, formed by the superexchange (tunneling) exciton interactions resulting from the above mentioned exciton exchange interactions (integrals). The latter are thus confirmed as the major contributors to the 3B1u exciton transfer, migration and energy bond (3 x 1011 Hz) in the ordinary naphthalene crystal. Just below the percolation concentration the "impurity conduction band" further shrinks by one or two orders of magnitude, resulting in a bandwidth of about one hertz or less, and thus practically resulting in the "switching off" of the exciton transport. The tunneling radius is about 30 A or larger, depending on the system, but essentially in the ab plane.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22992/1/0000560.pd

Variable range cluster model of exciton migration: Dimensionality and critical exponents for naphthalene

Relative luminescence intensities for randomly substituted ternary systems with two major components and a minor one (sensor), for four triplet and one singlet exciton systems, identify the maximal effective exciton interaction distance for each system. The critical exponents [beta] and [gamma] show an effective 2-dimensional exciton topology and are consistent with dynamic exciton percolation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23661/1/0000629.pd

Expansion of immunoglobulin-secreting cells and defects in B cell tolerance in Rag-dependent immunodeficiency

The contribution of B cells to the pathology of Omenn syndrome and leaky severe combined immunodeficiency (SCID) has not been previously investigated. We have studied a mut/mut mouse model of leaky SCID with a homozygous Rag1 S723C mutation that impairs, but does not abrogate, V(D)J recombination activity. In spite of a severe block at the pro–B cell stage and profound B cell lymphopenia, significant serum levels of immunoglobulin (Ig) G, IgM, IgA, and IgE and a high proportion of Ig-secreting cells were detected in mut/mut mice. Antibody responses to trinitrophenyl (TNP)-Ficoll and production of high-affinity antibodies to TNP–keyhole limpet hemocyanin were severely impaired, even after adoptive transfer of wild-type CD4+ T cells. Mut/mut mice produced high amounts of low-affinity self-reactive antibodies and showed significant lymphocytic infiltrates in peripheral tissues. Autoantibody production was associated with impaired receptor editing and increased serum B cell–activating factor (BAFF) concentrations. Autoantibodies and elevated BAFF levels were also identified in patients with Omenn syndrome and leaky SCID as a result of hypomorphic RAG mutations. These data indicate that the stochastic generation of an autoreactive B cell repertoire, which is associated with defects in central and peripheral checkpoints of B cell tolerance, is an important, previously unrecognized, aspect of immunodeficiencies associated with hypomorphic RAG mutations

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Entire Phonon Spectrum of Molecular Crystals by the Localized Exciton Sideband Method: Naphthalene

The complete one‐phonon density‐of‐states of a molecular crystal can be mapped out by a carefully chosen exciton sideband involving a localized molecular internal excitation in a dilute isotopic mixed crystal. A theoretical derivation shows that the necessary criteria are: absence of localized in‐band phonons; applicability of a phonon amalgamation limit for the mixed crystal; weak exciton‐phonon, guest‐lattice coupling and weak guest‐host exciton interaction. Experimentally, at least two exciton‐phonon sidebands should be investigated, so as to exclude trap and defect transitions, and at least one of the guest transitions should belong to the deep‐trap limit and have sidebands weak enough to minimize multiphonon transitions. The above is demonstrated for the naphthalene crystal by high‐resolution fluorescence and phosphorescence spectra of 0.25% C10H8 in C10D8 and by phosphorescence of 0.14% 2‐DC10H7 in C10D8 at 2°K. The phosphorescence phonon sidebands correlate surprisingly well with Pawley's ``atom‐atom'' calculated phonon density‐of‐states, and seems to give a much better phonon density map than the very recent high‐resolution inelastic, incoherent neutron scattering data. New Raman data are also presented, showing defect (isotopic impurity) induced bands, which are interpreted in terms of the above phonon density‐of‐states peaks. Applications to reversible and irreversible thermodynamics are mentioned.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69440/2/JCPSA6-57-12-5409-1.pd

Comment: phonon spectrum of molecular crystals from exciton sidebands

Phonon sidebands of excitons in molecular crystals with negligibly small excitation exchange interactions, including isotopic guest systems, are not, in general, expected to be predominantly weighted by translational modes that are predictable from Brillouin zone centre (q = 0) ones. Some phonon sideband data on naphthalene, hexamethylbenzene, etc. are discussed and argued to map the extent of the phonon density-of-states as well as some of the q [not equal to] 0 singularities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22236/1/0000671.pd

Exciton percolation. II: Naphthalene 1B2u supertransfer

The exciton percolation theory has been tested for the migration of the lowest singlet exciton in our model organic alloy system at low temperature (2 K): binary isotopic mixed crystals of naphthalene (C10H8--C10D8), with an added exciton sensor (supertrap) of betamethylnaphthalene (about 10-3 mole fraction). At these relatively high sensor concentrations the system's migration dynamics are described quantitatively by the simple limiting case of supertransfer, i.e. by a dynamic percolation formulation of Hoshen and Kopelman that depends only on the concentrations and interaction topology of the ternary crystal. Experimentally, the exciton's migration dynamics is monitored by fluorescence spectra, taken under controlled conditions (crystal quality, purity, concentration, excitation and temperature). The effects of exciton tunneling (superexchange), exciton--phonon coupling, coherence, exciton delocalization and non-equilibrium chemical solubility are considered. We show that a complete concentration study of this simple energy transport model system reveals four kinds of exciton transfer regions, but only one (above the critical guest site percolation concentration) with long ranged, multistep, direct guest exciton transport. The same study, on the low concentration side, yields relative trapping efficiencies and thus eliminates the need for adjustable parameters. The substitutional randomness of the isotopic mixed crystal is confirmed, as well as the short range nature of the 1B2u exciton interactions and the dominance of the interchange equivalent, nearest neighbor, pairwise (molecular) excitation (1B2u) exchange integrals in the pure, perfect, low-temperature, naphthalene crystal. Our present exciton percolation study is also relevant to the primary energy transport process in heterogeneous photosynthetic units.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22931/1/0000497.pd