Two-Photon Absorption Spectroscopy

Beyond the electric-dipole approximation: A formulation and implementation of molecular response theory for the description of absorption of electromagnetic field radiation

“…Note that terms of odd order in the perturbation have only periodic contributions, which vanish upon time-averaging, and thus do not contribute to light absorption. 36 As will be shown below in the case of linear absorption, the rate of absorbed energy in orders of the intensity can be related to the imaginary parts of the even-order molecular response functions,…”

“…We follow the energy-loss approach, 36 which provides a natural link to the complexvalued molecular properties underlying the absorption process. One may note that going beyond the ED approximation introduces an explicit dependence in the interaction operator [see Eq.…”

“…By comparison with the first term in Eq. (17), the linear absorption cross section, defined as σ(ω) = α(ω)/N , 36 becomes equal to…”

“…It is a common practice 5 to choose a common damping term for all excited states, i.e., γ n0 = γ. Upon introducing this approximation, the equation of motion for the damped linear response function reduces to (36) which takes a form similar to the undamped counterpart.…”

“…We will base our derivation of a response theory description of linear absorption on the energy-loss approach, which makes it directly linked to the formulation of resonance-convergent complex molecular properties. 36 In the ED approximation, we recall that the linear absorption cross section is proportional to the imaginary part of the electric-dipole polarizability, a property which we have earlier addressed by means of the development of the complex polarization propagator (CPP) approach. [37][38][39] In Sec.…”