Electric quadrupole second-harmonic generation spectra in epitaxial vanadyl and titanyl phthalocyanine films grown by molecular-beam epitaxy

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Abstract
For epitaxial vanadyl and titanyl phthalocyanine films with Laue symmetry 4/mmm, second-harmonic generation (SHG) spectra due to only electric quadrupole and electric quadrupole coupling mechanisms were obtained under a suitable geometry, while the contributions of electric dipole, magnetic dipole, and magnetic dipole coupling mechanisms were eliminated. In addition to a resonant peak previously reported, two peaks were observed at higher energies. An oriented gas model analysis showed that in-plane components of molecular hyperquadrupole polarizabilities cause the resonances. On the basis of the molecular electronic structure calculated by the density functional theory, the resonant processes are attributed to electric quadrupole transitions at SH frequencies.