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

Abstract

AbstractWe 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.