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Japanese Journal of Applied Physics

Fine Resolution of Energy Structures of Large Molecules in Solution by Coherent Population-Grating Spectroscopy

Hirotoshi Souma, C. Horie, Masayuki Hoshi, Taeko Daiguji Tashiro

Published Sep 1, 1994• 5 citations
Publisher

Abstract

We have succeeded in observing finely resolved spectra in the spectral regions of absorption and emission bands of cresyl violet in ethanol, by means of population-grating spectroscopy based on four-photon parametric mixing. The inhomogeneous width is investigated in terms of the excitation frequency dependence. The energy separation and the homogeneous width are also analyzed by assuming that the observed fine bands consist of several Lorentzian lines. It is revealed that the observed spectra reflect the detailed vibrational and vibronic structures of molecules. This high-resolution spectroscopy can be readily applied to more general disordered systems in the condensed phase.

DOI
10.1143/jjap.33.l1218

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We have succeeded in observing finely resolved spectra in the spectral regions of absorption and emission bands of cresyl violet in ethanol, by means of population-grating spectroscopy based on four-photon parametric mixing. The inhomogeneous width is investigated in terms of the excitation frequency dependence. The energy separation and the homogeneous width are also analyzed by assuming that the observed fine bands consist of several Lorentzian lines. It is revealed that the observed spectra reflect the detailed vibrational and vibronic structures of molecules. This high-resolution spectroscopy can be readily applied to more general disordered systems in the condensed phase.
Recently, we reported the first observation of the statistical fine structure (SFS) on an inhomogeneously broadened optical absorption line, the 0-0 origin transition of pentacene molecules in p-terphenyl crystals at liquid helium temperatures.1 SFS is a time-independent spectral structure resulting from number fluctuations in the spectral density of absorbers that we have detected directly using laser frequency modulation (FM) techniques. In our samples where the number of molecules per homogeneous linewidth N¯ H is large, a zero-background technique such as FM spectroscopy is optimal for detection of SFS because only the deviations of the absorption coefficient from the background value produce FM signals. In addition, for extremely low signal levels, we use double modulation with a Stark field to remove the effects of residual amplitude modulation in the modulator. Our goal is to describe the systematics and analysis of SFS spectra at a fixed low temperature, 1.6 K. For example, we show that the rms value of the SFS grows as the square root of N¯ H . More important, by computing the autocorrelation of the measured frozen noise spectra, the homogeneous linewidth can be determined to good accuracy without requiring spectral hole-burning or coherent transients. Since the source for SFS is fundamental statistical fluctuations in the absorber spectral density, SFS should occur for all inhomogeneously broadened lines; furthermore, the effect imposes a fundamental limit on the detectability of shallow spectral features in solids.
By using laser FM spectroscopy, we have observed statistical fine structure (SFS) in the inhomogeneously broadened zero‐phonon S1←S0 (0–0) absorption of pentacene molecules in crystals of p‐terphenyl at liquid helium temperatures. SFS results from variations in the spectral density of absorbers with optical wavelength due to statistics, illustrating that the inhomogeneous line profile is not a simple, smooth Gaussian function as has been previously assumed but rather contains significant fine structure. Theoretical analysis of the SFS spectra using autocorrelation techniques can be used to estimate the underlying value of the homogeneous linewidth. This new observation not only provides a novel way to study the statistics of inhomogeneous broadening, but it also allows estimation of the homogeneous width without requiring spectral hole‐burning or coherent transient techniques.
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We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.
Using laser frequency-modulation spectroscopy, we have observed statistical fine structure (SFS) in the inhomogeneously broadened optical absorption of pentacene in ppp-terphenyl at liquid-helium temperatures. SFS is time-independent structure on the inhomogeneous line profile caused by statistical variations in the spectral density of absorbers in each frequency interval. The size of the SFS varies as the square root of the number of centers, and the autocorrelation of the SFS yields an estimate of the homogeneous linewidth without the requirement of spectral hole burning or coherent transients.
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