The picture shows an introductory experiment on vibrational relaxation by Erik van Dijk.
(A) The ground state |1, v=0> is coupled to an electronically and vibrationally excited state (|2,v>) by saturation with a broad band laser pulse.
In the absence of other states the balance between stimulated absorption and emission gives equal state probability for both states. However, the
excited state |2,v> couples to other vibrational states |2,v'> with sub-picosecond redistribution time and the |2,v'> states will ultimately decay
by spontaneous emission with nanosecond decay time. The states |2,v'> have reduced coupling constants for stimulated emission by the laser pulse to
the ground state, and therefore a new equilibrium between level |1, v=0> and |2,v > will be reached. Employing two equal pulses with a controllable
delay the stimulated photo cycle can be manipulated resulting in enhanced spontaneous decay. (B) A train of short pulses (280 fs, circularly polarized)
is split by a 50/50 beam splitter (BS), and recombined after reflection on two corner mirrors (CM). One mirror is placed on a translation stage to
provide a variable delay (Δt) between the two beams. The combined "double pulse" beam is coupled into a confocal microscope to excite single
molecules (SM). The single molecular fluorescence is collected by a high NA objective (Obj) and passed via a dichroic beam splitter (DBS) onto a
photon counting detector (Det). (C) The fluorescence probability for a single saturating pulse is 0.5 (solid blue line). In the delay interval from
-2 to 2 ps the calculated response for two femtosecond pulses is plotted where the relaxation time from level |2,v> to |2,v'> is set to 90 fs
(solid red line). The fluorescence probability increases from 0.5 at zero delay to 0.75 for longer delay, resulting in a dip in the fluorescence
intensity for zero delay. For a realistic longer pulse duration (280 fs, green dashed line,) the dip becomes convoluted with the pulse width, less
pronounced and the femtosecond decay time is retrieved by deconvolution.