A general conceptual problem of time-dependent single molecule spectra is discussed theoretically in the framework of recently developed intensity-time-frequency correlation spectroscopy. It is shown that the new method is closely related to a ''gedanken'' three-pulse photon echo experiment done on an ensemble of identical molecules interacting with statistically identical microscopic environments. The correlation function is an integral transform ͑under certain conditions a Fourier transform͒ of the echo amplitude as a function of the delay between the first and the second pulses.
͓S0163-1829͑99͒10907-X͔A common characteristic feature of single molecules, single quantum dots, or any other single quantum systems ͑later on all are referred to as SMs͒ is that each successive spectral measurement performed on the same SM can reveal a new ''spectrum'' even when the macroscopic conditions do not change. 1,2 In mathematics, such time-dependent spectra are called joint time-frequency distributions.3 The spectral dynamics result from fluctuations of the microscopic surroundings of each SM. These surroundings are sometimes simulated by a set of two-level systems ͑TLSs͒ interacting with a phonon bath.4,5 Each TLS is characterized by two parameters, the energy splitting E and the sum of downward and upward TLS flip rates K. The interaction between a SM and a TLS leads to a SM resonance frequency shift 2 when the TLS flips. These flips are random and are not correlated with flips of other TLSs. Thus the resonance frequency becomes a stochastic function of time.6 This effect is called spectral diffusion ͑SD͒. For a bulk sample SD also leads to time-dependent line shapes, and time-frequency distributions in bulk materials have been studied intensively for more than 20 years. 7,8 Different from nonreproducible SM spectra, a time-frequency distribution for an ensemble of molecules is a reproducible macroscopic characteristic.It is important to emphasize that time and frequency obey the uncertainty principle and one can speak about timedependent ''spectra'' only when the measuring procedure is exactly described. There are few methods for measuring time-dependent spectra of ensembles. These are two-and three-pulse photon echo 9-11 and ''hole burning. '' 12 Hole burning makes little sense for SMs. Hahn-echo-type experiments, which are limited by the lifetime of the exited state, can be done even on a SM, 13 but classical two-and three-pulse echoes require an ensemble. Strictly speaking, neither experimental methods nor even a consistent theoretical description for time-dependent reproducible spectral distributions for single molecules have existed. Only recently, a new approach called intensity-time-frequency correlation ͑ITFC͒ was suggested and demonstrated experimentally. 14 ITFC spectroscopy works in three steps: ͑a͒ N laser scans over the same spectral region are acquired and the SM luminescence intensity as a function of the laser frequency is measured, ͑b͒ a correlation function is calculated for each scan, and ͑c͒ thes...