Emergence of Excited-State Plasmon Modes in Linear Hydrogen Chains from Time-Dependent Quantum Mechanical Methods

Explicitly time-dependent configuration-interaction theory is used to predict a new type of plasmonic behavior in linear hydrogen chains. After an intense ultrashort laser pulse brings the system into a broad superposition of excited states, the electronic dipole of the entire chain oscillates coherently, and the system is predicted to emit radiation at energies significantly lower than the first absorption band. A simple classical model accurately predicts the energy of this plasmon resonance for different hydrogen chain lengths and electron densities, demonstrating that collective, free-electron-like behavior can arise in chains of as few as 20 hydrogen atoms. The excitation mechanism for this plasmonic resonance is a highly nonlinear, multiphoton process, different from the linear excitation of ordinary surface plasmons.

Figure 1

Lower panel: Induced charge density in ${\mathrm{H}}_{50}$ as a function of time, as computed by the TDCIS method. The induced density is defined as the difference between the density before the pulse arrives and at time $t$, integrated over all $x$ and $y$. Upper panel: dipole moment, $\mu$, (black), and the shape of the incident optical pulse (red, arbitrary units).

Figure 2

Emission spectra computed by the TDCIS and TDCISD methods for hydrogen chains of varying length (a) and total electron density (b). Also shown are the positions of the first optical absorption peak (green) and the plasmon resonance as predicted by a classical model (black).

Figure 3

Dipole moment as a function of time for various interatomic separations in ${\mathrm{H}}_{50}$, as computed by the TDCISD method. The vertical dashed line at 1 fs denotes the end of the laser pulse.

Figure 4

Emission spectra for ${\mathrm{H}}_{50}$ after interacting with a laser field of varying strength (a) and energy content (b) as computed by the TDCIS method. The field strength, $E_0$, and frequency, $\omega$, are given on the right.