Detailed studies of the low-temperature inhomogeneously broadened 0-0 S(1)-S(0) electronic transition of pentacene dopant molecules in p-terphenyl crystals have produced two novel observations which open up a new regime for ultra-sensitive laser spectroscopy in solids. The first result, direct detection of intrinsic static absorption fine structure on a megaHertzs scale, called statistical fine structure, arises from number fluctuations in the spectral density of absorbers which scale as the square root of the number of absorbers per homogeneous width. Surprisingly, this effect provides a new method for determining the homogeneous width that does not rely on hole-burning or coherent transients. The second recent observation, detection of the optical absorption of a single pentacene molecule in a p-terphenyl crystal, opens the door to new studies of single local environments in solids as well as to studies of the interactions of a single absorber with external perturbations in which no averaging is performed over large numbers of nominally equivalent local configurations. Both achievements utilized high sensitivity variations of laser frequency modulation spectroscopy to perform the measurement. Keywords: Statistical fine structure, Atomic properties, Single molecule detection, Molecule properties, Laser spectroscopy of solids, Instrumentation, Pentacene in p-terphenyl, Organic compounds, Near field.

In spite of detection intensity constraints necessary to avoid power broadening, the optical absorption spectrum of single molecules of pentacene in p-terphenyl crystals can be measured by (1) using laser FM spectroscopy combined with Stark and/or ultrasonic double modulation (to remove residual amplitude modulation) and (2) recording spectra far out in the wings of the inhomogeneous line to reduce the number of molecules in resonance to one.

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Ultrasensitive Laser Spectroscopy covers the experimental methods involved in various sensitive techniques to which lasers have been applied for the study of weak transitions. This book is organized into seven chapters. Each chapter discusses the theories, experiments, and application of the specific technique. A discussion on the advantages, disadvantages, and modifications made in each technique is also provided. Ultrasensitive techniques considered in this text include photoacoustic, one- and two-photon excitation, absorption, mass, and laser ionization spectroscopies. Other chapters examine the techniques of laser intracavity-enhanced, laser absorption, and emission spectroscopy. This book will be of value to spectroscopists, analytical chemists, and researchers in the field of ultrasensitive analysis.

Recent progress in the optical detection and spectroscopy of single ions in vacuum confined in electromagnetic traps has led to novel measurements that further our understanding of quantum physics. For example, various workers have achieved direct measurement of quantum jumps, Doppler sidebands, and other fundamental phenomena such as ion crystallization of ions in vacuum trapped by electromagnetic fields. By using laser induced fluorescence and a novel hydrodynamically-focused flow to confine the molecules and reduce the scattering volume, single molecules of the protein B-phycoerythrin with the equivalent of 25 rhodamine 6G chromophores have also been detected. In contrast to the far- field optical approach, recent advances with various near-field spectroscopies such as scanning tunnelling microscopy (STM) have provided images of single molecules of benzene and CO on Rh surfaces and images of liquid crystal molecules on graphite to name a few examples.