Dynamics of Energy Transport in Molecular Crystals: The Picosecond Transient-Grating Method

Abstract

PHYSICAL REVIEW LETTERS 10JULY1978 tion 14 of P(cr, t) reduce after a single RG step either to the form assumed in this work or to terms of second order in the intercell interaction. Of course, if the RG is performed to higher orders in the cumulant expansion, more terms must be used in <p(o, t) and the effective time-dependent Hamiltonian and transition probability become more complicated than the simple nearestneighbor form.We have used this method in the second-order cumulant approximation and confirmed dynamic scaling to this order. The calculations are lengthy and memory effects unavoidable. Details will be published elsewhere. ^On leave from University of Birmingham, Birmingham B15 2TT, England. 1 B. I. Halperin, P. C. Hohenberg, and S.-k. Ma, Phys. Rev. Lett. 29, 1548 (1972), and Phys. Rev. B 10, 139 (1974); P. C. Hohenberg and B. I. Halperin, Rev. Mod. Phys. 49, 435 (1977). 2 C. de Dominicis, E. Brezin, and J. Zinn-Justin, Phys. Rev. B 12, 4945 (1975). 3 C. de Dominicis, S.-k. Ma, and L. Peliti, Phys. Rev. B 15, 4313 (1977). 4 S.-k. Ma, Modern Theory of Critical PhenomenaInter molecular interactions in molecular crystals give rise to electronic excited-state energy migration. In pure crystals energy transport may be excitonic 1 in nature, and in disorded or impure crystals it may be due to long-range resonant transfer. 2 In this paper we present the first results from a new method which utilizes a picosecond transient diffraction grating to directly probe the time-dependent position of excited . There appears to be an error in this because our calculation of Au = 4.43 disagrees with the value quoted of 2.37. * 6 M. Suzuki, Prog. Theor. Phys. 43, 882 (1970). 17 L. van Hove, Phys. Rev. 93, 1374 (1954.states in the bulk of a crystal. As such, this method allows determination of the rate of energy transfer along any crystallographic direction, and thus the influence of intermolecular interactions, 1 molecular orientation, 1 impurity effects, 3 and exciton-phonon coupling 4 on the transport process can be directly studied. In this experiment, two time-coincident, Gaussian picosecond excitation pulses of wavelength Energy transport in molecular solids is investigated using a picosecond transient-grating technique, A model is presented which directly relates the rate of energy transport to the experimental observable. The diffusion coefficient for singlet electronic excitation transport is reported for pentacene in£-terphenyl. Acoustically induced optical absorption effects are also observed.

Visualization of exciton transport in ordered and disordered molecular solids

“…Müller and Bardeen 20 used time-resolved microscopy to image transport, but did not detect energy migration in a molecular solid. Diffraction from exciton density gratings has been used to study dynamics of exciton transport 21 , but this approach requires high exciton densities to be created, which introduces challenges such as exciton-exciton interactions and material damage. Due to the lack of a general technique, the temporal dynamics of exciton transport in molecular solids and how it relates to the nanoscale disorder, the defining characteristic of molecular and nanostructured materials, remains elusive.…”

High sensitivity transient infrared spectroscopy: a UV/Visible transient grating spectrometer with a heterodyne detected infrared probe

“…Using appropriate sample concentrations, pathlengths and pulse energies, the experimental arrangement in Fig. 1 allows conventional pump-probe signals from Re(CO) 3 (dmbpy)Br to be measured. In our setup these were comparable in size to previously published spectra [16] and could therefore be used to determine the level of signal enhancement obtained when detecting the heterodyned transient grating signal.…”

“…Transient grating spectroscopy has a long history of development in the visible part of the spectrum, both decades ago [2][3][4][5][6] and also more recently [7,8]. The use of diffractive optics has simplified considerably the design of simple and robust spectrometers [5,6,9].…”

Исследование скорости релаксации фотовозбужденных молекул индола интерферометрическим методом pump and probe с пикосекундным разрешением

“…Необходимость учета изменения показателя преломления в фото-акустических методах была высказана впервые в работе [11] и затем развита в [10] при исследовании молекул пентацена в молекулярных кристаллах п-терфенила. В этой работе регистрировался фотоаку-стический сигнал при оптическом возбуждении молекул пентацена пикосекундным лазером с плотностью мощности 500 MW/cm 2 .…”