Fundamental laser spectroscopy of molecules in solids: statistical fine structure

Abstract

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.