Optical Pulse-Train Interference Spectroscopy

Let US consider atoms with optical transitions between a single level / and a state k) that is split into two sublevels k ) and 1 2 (Fig. 12.8). If the atoms are irradiated by a short laser pulse of duration r h/AE = ti/ Ek — Ek2) and mean optical frequency co = (Ei — Ek)/h, an induced dipole moment is produced, which oscillates at the frequency co. The envelope of the damped oscillation shows a modulation with the beat frequency Aco = AE/h (quantum beats. Sect. 12.2). 

If the sample is now exposed not to a single pulse but to a regular train of pulses with the repetition frequency / such that Ao) = q-lnf (q e N), the laser pulses always arrive in phase with the oscillating dipole moments. With this synchronization the contributions of subsequent pulses in the regular pulse train add up coherently in phase and a macroscopic oscillating dipole moment is produced in the sample, where the damping between successive pulses is always compensated by the next pulse [12.62]. 

The regular pulse train of a mode-locked laser with the pulse-repetition frequency / corresponds in the frequency domain to a spectrum consisting of the carrier frequency vq — col ll and sidebands at vo -/ ( A) (Sect. 11.2). If a molecular sample with a level scheme depicted in Fig. 12.12b and a sublevel splitting Av = Iqf is irradiated by such a pulse train, where the frequeny vq is chosen as vq = ( 1 + 2)/2, the two frequencies vo qf are absorbed on the two molecular transitions V], V2- This may be regarded as the superposition of two Raman processes ( 1) 2) 3) Stokes process) and 

The experimental arrangement which is similar to that of time-resolved polarization spectroscopy (Fig. 12.11) is depicted in Fig. 12.16. The pulse train is provided by a synchroneously pumped, mode-locked CW dye laser. A fraction of each pulse is split by the beam splitter BS and passes 

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