Giant dipole plasmon resonance in fluorene $\left({\mathrm{C}}_{13}{\mathrm{H}}_{10}\right)$ molecules

We report the observation of the giant dipole plasmon resonance in the fluorene molecule $\left({\mathrm{C}}_{13}{\mathrm{H}}_{10}\right)$, a polycyclic aromatic hydrocarbon, upon collisions with H-like silicon ions. An idea of using highly charged ions to create a large perturbation strength is exploited to observe the plasmon state effectively in the electron double-differential distribution. The signature of plasmon excitation is observed directly as a characteristic broad peak in the electron double-differential spectrum. Such an excited state could not be observed for the same molecule when the low charged ions having lower perturbation were used. This is explained by a model based on dipole approximation and linear response theory for the giant plasmon resonance. The angular distribution of these electrons is modeled using the photoelectron angular distribution for an oscillating dipole superimposed with the postcollision interaction due to the long-range Coulomb interaction between the plasmon electrons and the receding projectile ions.

Figure 1

(a) Experimental scheme in which the projectile ions perturb the system and the electron cloud (red and blue circles depict above and below the ionic cage) collectively oscillates, which then predominantly decays via the electron emission (not to scale). (b) The GDPR plasmon excitation probability $P\left(b\right)$ as a function of impact parameter $b$ for 94-MeV Si ions ($v_p=11.63$ a.u.) having $8+$ and $13+$ charge states. The dotted line shows impact parameter corresponding to the radius of the fluorene molecule ($\sim 6.5$ a.u.).

Figure 2

Energy distributions of the DDCS at particular angles for 94-MeV ${\mathrm{Si}}^{13+}$: (a) $\theta ={30}^{\circ }$, (b) $\theta ={45}^{\circ }$, (c) $\theta ={60}^{\circ }$, and (d) $\theta ={160}^{\circ }$. Closed (red) circles and open circles represent the ${\mathrm{C}}_{13}{\mathrm{H}}_{10}$ and $\mathrm{C}{\mathrm{H}}_4$ data, respectively. The solid lines represent the CDW-EIS calculations for fluorene. The insets display the DDCS in linear scale and dashed lines are a guide to the eye. The black square in the inset of (d) shows the data for the 98-MeV ${\mathrm{Si}}^{8+}$ ion on fluorene [32]. The $\mathrm{C}{\mathrm{H}}_4$ data are multiplied by suitable factors for comparison only, as indicated in each panel.

Figure 3

Angular distributions of the DDCS of the ejected electrons for the fluorene (closed circles), $\mathrm{C}{\mathrm{H}}_4$ (open circles with dots), and CDW-EIS calculations (solid lines) in collisions with 94-MeV ${\mathrm{Si}}^{13+}$ ions at (a) 10 eV, (b) 13 eV, (c) 40 eV, and (d) 60 eV. The dashed lines are a guide to the eye.

Figure 4

Angular distribution of the $d{\sigma }_{pl}^{exp}/d\mathrm{\Omega }$ for plasmon electrons, denoted as relative GDPR cross section [experiment (closed circles)], with the scale on the left axis. The scaled PCI correction factor $\left[C_{PCI}\left(\theta \right)\right]$, with the numbers on the left axis, is shown as dashed green line. The oscillating dipole distribution including the PCI correction factor (i.e. Eq. (2)) is indicated as solid black line, with the scale on the left axis. The plasmon electron angular distribution corrected for the PCI effect [i.e. $(d{\sigma }_{pl}^{exp}/d\mathrm{\Omega })/C_{PCI}\left(\theta \right)$] is shown as closed square symbols with the scale on the right axis. Angular distribution for the oscillating dipole i.e. Eq. (1) is indicated as dotted blue line, with the scale on the right axis.

Figure 5

Energy distribution of the SDCS of the electron emission for the fluorene (closed circles), $\mathrm{C}{\mathrm{H}}_4$ (open circles with dots), and CDW-EIS calculations (solid line). The inset shows a close-up in the linear scale spectrum and the dashed line is a guide to the eye.