Time Resolved Direct Probing of the Change in the Local Solvent Response Following Excitation of a Solute

Abstract. The change in the low frequency (0.1- 500 cm−1) non-resonant Raman response following excitation of a dye molecule in solution has been measured as a function of time after resonant excitation of a solute using a two-dimensional mixed resonant, non-resonant time domain spectroscopy. This method provides a direct measurement of the change in the spectrum of the local environment as a dynamic event proceeds in a condensed phase. Recently we reported the first results from a new experimental technique involv-ing a resonant excitation of a chromophore followed by a third-order Raman probe of the solvent (RaPTORS, ResonAnt-Pump Third Order Raman Spectroscopy) [1, 2]. This two-dimensional method allows direct observation of the solvent partici-pation in response to a dynamic event, such as the creation of an excited state in a chromophore or a charge transfer reaction. Here, our efforts are focused on directly measuring the change in the complete spectrum of the low-frequency (0.1- 500 cm−1) molecular solvent response as a function of time from photo-excitation of a solute. The experiment involves an initial pulse that is resonant with an electronic tran-sition in the solute, Er. A time t later, a set of three electronically non-resonant pulses, Enr1-Enr3, follow to probe the third order Raman response of the solvent as a function of the intrinsic time delay τ between time coincident pair Enr1/Enr2 and Enr3. The two dimensional signal is detected along the same direction, ks ′ = kr−kr+knr1−knr2+knr3 as the non-resonant third order signal ks = knr1−knr2+knr3. The result is heterodyne detection of the two dimensional response with the non-resonant bulk solvent signal providing the local oscillator. The two dimensional response can be interpreted as a small change in the TOR probe measured as the cross term [2], IRaPTORS(t, τ) ≈ (n