Multi-photon absorption effect and intra-molecular charge transfer of donor-π-acceptor chromophore ethyl p-amino benzoate.

Fourier transform (FT)-Raman and infrared (IR) spectra of the nonlinear optical (NLO) material ethyl p-amino benzoate (EPAB) have been recorded and analyzed. The geometry and harmonic vibrational wavenumbers are calculated with the help of B3LYP density functional theory method. The detailed interpretation of the vibrational spectra has been carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. Stability of the molecule arising from hyperconjugative interactions leading to its NLO activity and charge delocalization have been analyzed using natural bond orbital (NBO) analysis. Employing the open-aperture z-scan technique, NLO absorption of the sample has been studied in two excitation regimes, using 100 fs and 5 ns laser pulses respectively. It is found that EPAB is a three-photon absorber for 100 fs pulses at the excitation wavelength of 800 nm. For ns pulses at 532 nm it exhibits strong optical limiting, indicating possible photonics applications.