Characterization of second-order bands in Raman scattering spectra of lead phthalocyanine thin films

The structure, optical absorption (500…950 nm) and resonance Raman spectra (within the range 100…3000 cm–1) of lead phthalocyanine (PbPc) thin solid films with the thickness 190 nm were studied. The films were deposited using thermal evaporation in vacuum 6.5 mPa onto silica substrates held at room temperature. It was found that in the process of depositing the PbPc thin solid films monoclinic and triclinic PbPc crystallites were grown, and the amount of crystallites in the triclinic phase in the as-deposited PbPc films was approximately two times less than those in the monoclinic one. The resonance Raman spectroscopy, with application of the He-Ne laser line 632.8 nm as an excitation source, was used for studying the 190-nm thick PbPc films. Due to resonance enhancement, the second-order Raman spectrum of PbPc films within the region 1700…2950 cm–1 was successfully registered and analyzed for the first time. It has been shown that the second-order PbPc Raman spectrum is mainly formed by the overtones and combination modes of B1 symmetry fundamental vibrations. The second-order Raman region of 2550…2900 cm–1 appeared to be highly specific for PbPc and could be used for its identification along with the finger-print region of fundamental vibrational modes.