Quantitative polarization-sensitive resonance CARS and resonance Raman spectroscopy on octaethylporphine

Abstract

We present polarization-sensitive (PS) multiplex coherent anti-Stokes Raman scattering (CARS) spectra of octaethylporphine (OEP) in solution employing Q band resonance conditions. Several advantages of this four-wave mixing technique compared with spontaneous resonance Raman (RR) spectroscopy are demonstrated. PS CARS spectroscopy allows the acquisition of high-quality polarized vibrational spectra of this luminescent D2h symmetric free-base porphyrin. From a quantitative CARS lineshape analysis, a set of accurate band parameters for the normal modes in the range 1510-1670 cm-1 is determined; in particular, individual spectral contributions from the nearly degenerate modes ν19 and ν2 can be differentiated and unambiguously assigned. The observed difference of π between the coherent phases of totally symmetric ag modes (ν11, ν2, ν10) and non totally symmetric b1g modes (ν19) confirms a correlation with normal mode symmetry. Using the CARS lineshape parameters, hypothetical polarized RR spectra for Q band excitation are generated: this enables a direct comparison with experimentally detected polarized RR spectra obtained with preresonant B band excitation. Major changes in spectral parameters such as intensity enhancements and depolarization ratios of prominent macrocycle modes due to the different electronic resonance conditions are discussed. Copyright © 2006 John Wiley & Sons, Ltd.