Experimental and theoretical studies are reported of the short-lived and delayed fluorescence of anthracene single crystals, excited by single- and double-photon absorption. A giant-pulse ruby laser provides the primary source of radiation of 14 400 cm−1 (up to 1027 photons/cm2·sec) and is also used to generate second-harmonic radiation from ADP, as well as stimulated Raman radiation of 12 800 and 17 500 cm−1 from liquid oxygen. The time dependence of the fluorescence intensity is studied as a function of laser intensity, crystal temperature and excitation wavelength. The very intense fast fluorescence with a half-life of 30 nsec at 300°K, characteristic of singlet exciton decay, and the relatively weak delayed fluorescence which involves intermediate triplet states, are separated using sectored disks. It is concluded that the triplet state at 14 750 cm−1 can be populated (i) by direct absorption of laser photons involving an activation energy of 350 cm−1; (ii) via two-photon absorption, presumably leading to a vibrationally excited state of the 1B2u exciton, followed by intersystem crossing; (iii) via one-photon (second-harmonic) excitation from levels≥700 cm−1 into the singlet absorption band, followed by conversion of the singlet exciton into a triplet pair. The latter process is suggested by the observed activation energy of 700 cm−1. In agreement with these interpretations, the delayed fluorescence intensity is found to vary with the second to fourth power of the laser intensity depending on the experimental conditions. Also, light of 17 500 cm−1 leads exclusively to Process (i), light of 12 800 cm−1 exclusively to (ii). Triplet lifetimes from 2–17 msec are obtained, depending on crystal purity, which indicates that unimolecular triplet decay is an extrinsic, radiationless process. A singlet—triplet intersystem crossing rate constant of about 3×10−5 sec−1 is estimated. The triplet—triplet annihilation rate constant is found to be about 5×10−11 cm3 sec−1. This value considered together with the triplet-pair creation process suggests a triplet exchange rate ≳ 1013 sec−1 and a triplet diffusion constant ≳o5×10−4cm2/sec.
The absorption and fluorescence spectra of Tb3+ in LaCl3 have been used to find the levels of Tb3+. All Stark components of 7F and 5D4 were unambiguously established. The classification of the higher levels is still doubtful. The analysis was helped by Zeeman effects and particularly crystal field calculations. The latter gave as crystal field parameters: B20=92,B40=−40,B60=−30,B66=290 cm−1. Tb3+ in YCl3 has a similar spectrum but, because of different crystal symmetry, with different fine structure. A third type of spectrum is obtained from pure TbCl3.
profile of the directional proton flux as measured by the two satellites. The detector A fluxes have been converted from omnidirectional to directional for this comparison. The energy resolution of detector C makes it clear that the secondary peak is a phenomenon of the high-energy protons and disappears at lower energies.The first glance shows that the high-energy spectrum does not get noticeably softer as one goes to higher altitudes, in contradiction to the proposed loss mechanisms. Detailed comparison of the counting rates proves that the spectrum is softest at L = 1. 9, where the high-energy protons go through a minimum, , and actually gets harder toward higher I. values. Using all eight channels listed above, oiie can make a broad spatial survey of the proton spectrum. The spectrum of 1to 60-MeV protons is shown in Fig. 3 using data taken on or extrapolated to the magnetic equator. The error brackets allow for time variations, counter statistics, calibration uncertainties, and reasonable extrapolation errors. As observed by Naugle and Kniffen, a soft component appears at I values greater than 1.6 earth radii. However, the same explanation cannot be offered since the new measurements are in the polar shadow. A power-law fit of the form N(&E} = KE " at the high-energy end of the spectrum yields an exponent of 3 or 4. It is clear, though, that no single power 1am will fit the entire spectrum, and indeed, neither will any obvious relationship such as an exponential, Gaussian, etc.It must be concluded that the theory of geomagnetically trapped protons is incomplete. One rather old idea which appears to be worth developing is that some of the low-energy solar cosmic rays are perturbed into trapped orbits by the time-varying magnetic fields near the outer limits of the magnetosphere and that the subsequent diffusion (and acceleration} caused by magnetic field fluctuations brings some of these protons into the inner part of the earth' s magnetic field. Observations of particle motions during magnetic storms may be of great help in identifying the important mechanisms, but much more work appears necessary before the complete problem will be solved.
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