The calculated two-photon absorption (TPA) cross-section amplitude depends on the spectral line width (Γ) of the transition and there is not a consensus on the most appropriate way to obtain the Γ value. Usually it is arbitrarily defined or ambiguously estimated based on experimental data or using a transition energy variation determined from a quantum mechanics/molecular mechanics approach. In this study we compare different procedures to estimate Γ values and different methods of spectral convolution to simulate TPA cross-section for para-Nitroaniline (pNA) in liquid environment. The effects of the liquid environment on the nonlinear absorption were approximated using Monte Carlo and Born-Oppenheimer Molecular Dynamics as well as the commonly used Polarizable Continuum Model. The different procedures adopted here provided large difference in the calculated TPA amplitudes, mainly because of the underestimate/overestimate of the homogeneous and/or inhomogeneous contributions to the spectral line width. Based on these difficulties, we suggest a new procedure in which the value assigned to Γ is the one used to induce the total line width of the simulated TPA spectrum to reproduce the experimental one-photon absorption line width. Within this procedure the Γ value, which becomes a fitting parameter (Γfit), represents part of the homogeneous contribution to the spectral line width. Finally, although time dependent density functional theory calculations reproduced only reasonably well the energy of the lowest-energy π → π* transition of pNA, the fitting procedure here proposed provided the calculated TPA cross-section in good agreement with the available experimental data.
- Broadening effects
- Two-photon absorption