ABSTRACT
Until the late 1980s, virtually all molecular spectroscopic measurements involved observing a signal having contributions from a large number of different molecules, ‘large’ meaning much greater than one. While spectroscopists have long had the ability to detect and count individual photons or ions each originating from a single molecule, the statistical averaging required to make a meaningful measurement generally required observing many such events from many different molecules. Only recently have experimental techniques been developed that allow interrogation of fundamentally quantum mechanical entities on a one-by-one basis. These developments are driving revolutionary changes in the way molecular scientists make and interpret physical measurements. For example, simple organic molecules that are chemically identical, distinguished only by slightly different local environments within a solid, have been shown to have distinctly different electronic and vibrational spectra, linewidths, electric field and pressure-induced spectral shifts, and fluorescence lifetimes and quantum yields. The ability to correlate these various spectroscopic properties on a molecule-by-molecule basis is providing powerful insight into the details of intermolecular interactions. Single-molecule techniques have also shown that apparently pure, homogeneous enzyme preparations contain molecules having a wide range of catalytic activities and that an individual enzyme’s catalytic activity retains a ‘memory’ of its past history. This new information is stimulating a re-evaluation of established models for the chemical kinetics of biological systems. Single-molecule experiments involve sequential measurements of a given observable on the same molecule at different times and, if a time average is equivalent to an ensemble average (the ergodic hypothesis), no additional information is gained by probing individual members. The value of single-molecule measurements lies precisely in the fact that in many systems of interest, different members of the ensemble remain distinct on time scales much longer than that required to perform an experiment.
