  We present a new method that combines fluorescence correlation spectroscopy
(FCS) on the microsecond time scale with fluorescence antibunching measurements
on the nanosecond time scale for measuring photophysical rate constants of
fluorescent molecules. The antibunching measurements allow us to quantify the
average excitation rate of fluorescent molecules within the confocal detection
volume of the FCS measurement setup. Knowledge of this value allows us then to
quantify, in an absolute manner, the intersystem crossing rate and triplet
state lifetime from the microsecond temporal decay of the FCS curves. We
present a theoretical analysis of the method and estimate the maximum bias
caused by the averaging of all quantities (excitation rate, photophysical
rates) over the confocal detection volume, and we show that this bias is
smaller than 5\% in most cases. We apply the method for measuring the
photophysical rate constants of the widely used dyes Rhodamine~110 and
ATTO~655.
