Theory of rotational coherence spectroscopy as implemented by picosecond fluorescence depletion schemes

Abstract

We present a perturbation theory analysis of four time-resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherence spectroscopy. The analysis shows that ground-state rotational constants determine the rotational coherence effects in fully resonant, time-resolved stimulated Raman-induced fluorescence depletion (TRSRFD), excited-state rotational constants determine such effects in time-resolved stimulated emission spectroscopy (TRSES), and both ground- and excited-state constants do so in time-resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance-enhanced shows that this method gives rise to qualitatively different rotational coherence effects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground-state rotational coherence spectroscopy. Expressions for the calculation of rotational coherence effects in TRFD, TRSRFD, and TRSES traces are also presented. Such expressions are used to show that the magnitudes of rotational coherence transients are similar in all three schemes. Finally, experimental results on molecular iodine are presented to show that, indeed, both ground- and excited-state rotational coherence effects are manifest in TRFD traces.