| Autor: |
Sylvia H. Stevenson, Gerald J. Small, Maureen A. Connolly |
| Rok vydání: |
1988 |
| Předmět: |
|
| Zdroj: |
Chemical Physics. 128:157-184 |
| ISSN: |
0301-0104 |
| DOI: |
10.1016/0301-0104(88)85068-7 |
| Popis: |
Phase-matched two-photon excited (TPE) fluorescence and second-harmonic generation (SHG) are used to determine the transvere polariton dispersion curves and longitudinal exciton associated with the 31475 cm − a -exciton of naphthalene. Crystal angle tuning is shown to provide polariton group velocities in the range 10 6 gVp ≲ν p ≲5×10 8 cm s − . Scattering of these polaritons with phonons is completely characterized and compared with the same for the longitudinal exciton and the relatively slow polaritons (ν p ≈ 2×10 5 cm s − ) produced by one-photon excitation. Narrowing of the exciton-phonon interaction results from fast polariton motion. As a result, the semi-classical theory of nonlinear susceptibilities is shown to break down. The temperature-dependent branching ratio for SHG and two-photon absorption yield a low T residual scattering frequency of γ 0 ≈4×l0 9 s − for the polaritons in pure strain-free crystals. Macroscopic polariton coherence lengths ( l p ) are achieved at low T . A novel methodology for the study of energy transfer in doped molecular crystals is described which employs wavevector- and velocity-selected polaritons. For naphthalene doped with anthracene (trap) the donor-trap “reaction complex” radius ( a ) is ≈ 50 A with an associated scattering frequency of p ≈ 10 12 s − . It is necessary to consider scattering of the polariton (exciton) off of the trap ( p sc ) as well as actual trapping or capture ( p tr ). The ratio p sc / p tr ≈7 and p = p sc + p tr . Analysis of the data with a simple theoretical model shows that energy transfer via the phase-matched polaritons utilized is trap limited (i.e. not motion limited) with concomitant (and “tunable”) high probabilities for transmission of the polariton through the trap region. On the other hand, the energy transfer via the relatively slow polaritons (ν p ≈10 5 cm s − ) or even slower unclothed excitons is shown to be motionlimited. In this limit, the transmission probability is zero and the quasiparticle dephasing frequency (sum of the rate constants for capture and scattering) is given by the trap encounter frequency, R enc . For the intermediate regime it is necessary to write R enc p /( p + t enc −1 ) for the dephasing frequency, where t enc −1 ) is the quasiparticle escape frequency (from the trap region). |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
|
