Real-time {\it ab initio} simulations of excited-state dynamics in nanostructures

Combining time-dependent {\em ab initio} density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the effect of excited-state dynamics in nanostructures. In carbon nanotubes, we find electronic excitations to last for a large fraction of a picosecond.\footnote{Yoshiyuki Miyamoto, Angel Rubio, and David Tomanek, Phys.\ Rev.\ Lett.\ {\bf 97}, 126104 (2006).} The de-excitation process is dominated by coupling to other electronic degrees of freedom during the first few hundred femtoseconds. Later, the de-excitation process becomes dominated by coupling to ionic motion. The onset point and damping rate in that regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling. Considering the fact that the force field in the electronically excited state differs significantly from the ground state, as reflected in the Franck-Condon effect, atomic bonds can easily be broken or restored during the relatively long lifetime of electronic excitations. This effect can be utilized in a ``photo-surgery" of nanotubes, causing structural self-healing at vacancy sites\footnote{Yoshiyuki Miyamoto, Savas Berber, Mina Yoon, Angel Rubio, and David Tomanek, Chem.\ Phys.\ Lett.\ {\bf 392}, 209 (2004).} or selective de-oxidation processes induced by photo-absorption.\footnote{Yoshiyuki Miyamoto, Noboru Jinbo, Hisashi Nakamura, Angel Rubio, and David Tomanek, Phys.\ Rev.\ B {\bf 70}, 233408 (2004).} Also, electronic excitations are a key ingredient for the understanding of sputtering processes in nanostructures, induced by energetic collisions with ions.\footnote{Yoshiyuki Miyamoto, Arkady Krasheninnikov, and David Tomanek (in preparation).}