Laser dependent shifting of Raman bands with phthalocyanine pigments

Phthalocyanine pigments are commonly used in artist’s paints [PG7 (1936), PG36 (1957), PB15 (1928), PB16 (1939)], can easily be detected by Raman spectroscopy and are thus of interest for authentication purposes. Apart from PG36 and PB16, they all give a good response with the 785nm laser, which seems the most versatile wavelength for pigment analysis. Pigment blue 15 exists in different crystal modifications (α, β, γ, δ, ε), whereby the distinction of the alpha (1928) versus the beta (1953) modification is of particular interest. Phthalocyanine pigments exhibit a peculiar behaviour when exposed to variable laser power of standard Raman excitation sources. Several observations were made: a) spectra of the phthalocyanine PG7 acquired with 514, 633 and 785nm excitation wavelengths differ such that they will not be matched in a reference database unless acquired by the same laser; b) increasing laser intensity produces a significant shift (up to 10cm-1) of the main bands of the macrocycle (C-N bond lengths); c) band shifting is reversible, despite visible alteration at the spot of analysis; d) sharp and distinct multiple bands are reduced to broad bands with excessive laser power. Due to the thermal and chemical stability of phthalocyanines, excessive laser power may not be noticed, as a spectrum will be delivered in any case. Explanations for the peak shifting in relation to excessive excitation energy is likely related to the unique physical properties such as reflectivity and dielectric constant as functions of the photon energy.