Free Induction Decay of a Large Molecule

In what follows, we consider the time evolution within a subset of discrete levels js) of an assembly of collision-free polyatomic molecules, which interact on the timescale 0 t At with a short laser pulse and where the background manifolds 11) do not carry oscillator strength to g).These time evolutions during the pulseO f At are given by U(t, 0) = exp(-iHgfft), while after the pulse t = At + r are determined by the time evolution operator in the diagonalized form as 

Considering the radiative coupling with a jt/2 pulse, that is, coRAf = jt/2, Equation 6.78 together with Equations 6.75 and 6.77 results in 

The polarization per molecule after the pulse is obtained from Equation 6.69 is 

For some application to molecular physics, we may wish to generalize (6.85) to include the reaction of the detector to determine the OFID signal. This may be obtained by squaring of Equation 6.85 and performing an optical cycle averaging (sin (A x) = 1/2. ..) to find 

Equation 6.86 provides us with an expression for the macroscopic polarization and the following remarks are worthy of note  

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