Femtosecond Resolved Fluorescence 509
three values of the two-photon phase relaxation time T
2(2ph)
= 20 fs (dotted line),
T
2(2ph)
= 50 fs (solid line), and T
2(2ph)
= 75 fs (dashed line).
The experiment thus indicates a two-photon resonant DFWM and a phase
relaxation time of 50 fs (decay of the DFWM signal versus delay) for the
two-photon transition. There is no resolvable effect of a single-photon res-
onant DFWM (rise time of DFWM signal versus delay). The dominance of
the two-photon enhancement of DFWM in DODCI at 620 nm is confirmed by
theory. Simple numerical estimates indicate that indeed, the contribution of the
two-photon resonance dominates the DFWM signal [9].
10.7. FEMTOSECOND RESOLVED FLUORESCENCE
If an excitation is followed by fluorescence (luminescence), ...