Dissipative Equation of Motion for Electromagnetic Radiation in Quantum Dynamics

The dynamical description of the radiative decay of an electronically excited state in realistic many-particle systems is an unresolved challenge. In the present investigation electromagnetic radiation of the charge density is approximated as the power dissipated by a classical dipole, to cast the emission in closed form as a unitary single-electron theory. This results in a formalism of unprecedented efficiency, critical for ab initio modeling, which exhibits at the same time remarkable properties: it quantitatively predicts decay rates, natural broadening, and absorption intensities. Exquisitely accurate excitation lifetimes are obtained from time-dependent DFT simulations for ${\mathrm{C}}^{2+}$, ${\mathrm{B}}^{+}$, and Be, of 0.565, 0.831, and 1.97 ns, respectively, in accord with experimental values of $0.57\ifmmode\pm\else\textpm\fi{}0.02$, $0.86\ifmmode\pm\else\textpm\fi{}0.07$, and 1.77--2.5 ns. Hence, the present development expands the frontiers of quantum dynamics, bringing within reach first-principles simulations of a wealth of photophysical phenomena, from fluorescence to time-resolved spectroscopies.