Volume 175. number 3 CHEMICAL PHYSICS LETTERS 7 December 1990 Multiresonance CARS spectra and excitation profiles of dye molecules in liquid solutions: 1,4-dihydroxy-anthraquinone in chloroform A. Feis, C. Ferrante and R. Bozio Department ofphysical Chemistry, University of Padoa. 2, Via Loredan, 35131 Padoa. Italy Received 3 I August 1990 The resonance CARS spectra and the excitation profile of the ground state vibrational mode at 460 cm- ’ of I ,Cdihydroxy- anthraquinone in chloroform solution at room temperature are reported. Experimental evidence has been obtained for the build up of excited state population during the CARS process and for the participation of higher (two-photon allowed) excited states to the resonance enhancement of excited state Raman bands and of the background signal. The third-order multiresonance CARS susceptibility is analysed theoretically for a six-level system including a two-photon allowed state and incoherent initial popula- tions both in the ground and in the first excited state. The main features of the observed CARS spectra and excitation profile as well as the relation of the latter to the spontaneous Raman excitation profile are discussed on the basis of this model, 1. Introduction Coherent anti-Stokes Raman spectroscopy (CARS) is currently one of the most popular among the several coherent analogs of spontaneous Raman scattering spectroscopy that have been developed in the last two decades [ l-3 1. Resonance Raman scat- tering (RRS) owes most of its success in studies of biologically relevant systems and processes [ 41 and of electro- and photo-active [ $61 materials to its ability to provide structural and dynamical infor- mation on excited states. In particular, measure- ments and analyses of Raman excitation profiles (REP) are powerfool tools for investigating the structure of electronically excited states, their vi- bronic couplings and environmental effects [ 7-91. Although numerous experimental and theoretical studies have been dealing with CARS processes~in which one of the input laser beams (the pump beam of frequency IX,) is resonant with an allowed one- photon transition, few of them have directly ad- dressed the question of the relation existing between REP and CARS excitation profile (CEP). Basically two main views emerge from the literature. One em- phasizes the similarity existing between RRS and resonance CARS and the corresponding excitation profiles [ lo- 16 1. In fact it can be shown theoreti- cally that, when the resonance CARS process is de- scribed in terms of the corresponding third-order susceptibility, xi:) (We= 2w, -wZ), for the system with an initial population in the ground state only and no inhomogeneous broadening, the CEP is sim- ply given by the product of two REPS shifted in fre- quency, one in respect to the other, by an amount corresponding to the Raman transition [ 10.151. Ex- perimentally measured CEPs have been analysed us- ing either a sum-over-states approach [ 11,121 or transform relations [ 14 ] derived from those relating REPS and absorption spectra [ 171. In these ap- proaches inclusion of the inhomogeneous broaden- ing must account for the coherent nature of CARS and thus modifies the simple relation between CEPs and REPS [ 10,161. Experimentally, the second view is mainly based on results obtained for chromophoric molecules in low temperature crystalline matrices [ 18-221. It has been shown in fact that resonance CARS spectra contain a number of features that have no counter- part in ordinary RRS spectra obtained by cw exci- tation, namely (i) excited state Raman resonances with typical vibrational linewidths [ 19 1, (ii) broader resonances shifting with the detuning of wr from the 156 0009-2614/90/$ 03.50 0 1990 - Blsevier Science Publishers B.V. (North-Holland)