IR-photon quenching of delayed electron detachment from hot pentacene anions

Spontaneous and photoinduced delayed electron detachment from pentacene anions was measured in the electrostatic ion storage ring TMU E-ring. The dissipation mechanisms of the internal energy of the stored hot ions were inferred both from the temporal profile of the spontaneous detachment and from the laser-firing time dependence of the photoinduced temporal profile. Simulations based on statistical emission processes reproduced the observed behaviors, providing the value for the radiative cooling rates in the time range of a few tens of milliseconds. Using the obtained information on the competition between electron detachment and radiative cooling, we discuss the anion survival probabilities as a function of the initial internal energies and the nature of the observed quasiexponential decrease in the temporal profile of the spontaneous detachment at long times.

Figure 1

(a) Structural representation of the pentacene anion placed in the $XY$ plane. Carbon and hydrogen atoms are shown in gray and white, respectively. (b) Energy level diagram of the pentacene anion showing the electron detachment threshold $E_{\mathrm{th}}$ and the electronic excited state below $E_{\mathrm{th}}$.

Figure 2

Schematic of the experimental setup. Stored pentacene anions were photoexcited by the OPO laser, which intersected the anion beam at a small angle to avoid hitting $\mathrm{M}\mathrm{C}{\mathrm{P}}_1$ (dumped-ion-beam detector). The neutral species generated in the upper straight section were detected with $\mathrm{M}\mathrm{C}{\mathrm{P}}_2$.

Figure 3

(a) Detachment rate constant $k_{\mathrm{d}}$ (${\mathrm{s}}^{-1}$) as a function of internal energy $E$. $E_{\mathrm{th}}$ represents the electron detachment threshold of 1.43 eV. $E_0^{1/2}$ of 3.5 eV refers to the center of the energy window with FWHM of 0.55 eV for neutral detected after one half revolution. (b) Calculated vibrational radiative cooling rates $k_{\mathrm{v}}^{\left(i\right)}$ as a function of internal energy $E$, for eight major vibrational modes in the rate constants $i=84$ (1503.4 ${\mathrm{cm}}^{-1}$), 80 (1470.3 ${\mathrm{cm}}^{-1}$), 74 (1355.6 ${\mathrm{cm}}^{-1}$), 73 (1347.1 ${\mathrm{cm}}^{-1}$), 71 (1328.6 ${\mathrm{cm}}^{-1}$), 64 (1202.6 ${\mathrm{cm}}^{-1}$), 60 (1145.4 ${\mathrm{cm}}^{-1}$), and 44 (845.9 ${\mathrm{cm}}^{-1}$). The ${\mathrm{k}}_{\mathrm{d}}$ and the sum of the rate constants of these modes $k_{\mathrm{v}}^{8-\mathrm{modes}}$ at around 2 eV, shown by the dashed black line, are discussed in Sec. 5b. The structures of $k_{\mathrm{v}}^{\left(i\right)}$ at internal energies $E$ less than 0.3 eV are attributed to the finite small internal energy to be distributed to each vibrational mode. (c) Calculated total vibrational cooling power, $P_{\mathrm{v}}$ (black), and the eight main contributions, $h{\nu }_i{k}_{\mathrm{v}}^{\left(i\right)}$ (other colors), as a function of internal energy $E$.

Figure 4

(a) The internal energy $E$ distribution for storage times of 0, 0.1, 1, 20, 40, 60, and 80 ms (from right to left) without laser excitation. (b) The short time evolution of the energy distributions, which is primarily governed by depletion via electron detachment after one-photon excitation at 20 ms storage time. The red curve is made by shifting the red curve in (a) by the photon energy (1.39 eV); for visibility, the vertical axis is scaled to reflect photoexcitation of half of the stored ions. Black colored areas show the depleted components at the revolution of $①$ turn 0.5, $②$ turn 2.5, $③$ turn 6.5, $④$ turn 14.5, and $⑤$ turn 22.5, to be compared with the red circles in Fig. 7. (c) The internal energy distributions for storage times of 60 and 80 ms [the same as in (a)], and those of the photoexcited component shifted by the photon energy (1.39 eV); for visibility, the vertical axis is again scaled. They are compared with the $E$ distribution at 0.1 ms [the same as in (a)].

Figure 5

A typical example of the raw detected neutral yield as a function of storage time. Here, an 890-nm laser pulse was introduced at 40 ms.

Figure 6

The temporal profile of the neutral yields originating from spontaneous detachment. The raw temporal profile (upper solid black line) and the profile (lower solid orange line) after subtracting the component due to the residual-gas collisions (dash-dotted black line) are shown along with the background level of MCP noises (dashed black line). The simulated profile is shown as a solid blue curve.

Figure 7

The temporal profiles of the laser-induced neutral yields for ions stored for 20 ms (red circles), 40 ms (orange triangles), 60 ms (green squares), and 80 ms (blue diamonds) before laser firing. The spectra are normalized to the number of stored ions at the time the laser is fired and to the laser intensity. The contributions from spontaneous detachment and residual-gas collisions have been subtracted. The error bars are calculated by adding statistical and systematic errors in quadrature. The latter is larger and dominated by fluctuations of the stored ion intensities for normalization from betatron oscillations. The solid curves represent simulated decay profiles. The solid black ticks numbered from $①$ to $⑤$ refer to the time regions for which the energy distributions of the ions stored for 20 ms are shown on Fig. 4. See the details in the text.

Figure 8

A contour plot of ${\chi }^2$/dof as a function of the initial effective temperature $T_{\mathrm{eff}}^{\mathrm{ini}}$ and the correction factor $S_{\mathrm{v}}$ for the $A$ coefficients of IR emission.

Figure 9

The temporal profile of the neutral yields originating from spontaneous detachment and those for laser-induced profiles at 60 and 80 ms storage times, zoomed in the time region up to 3 ms. Note that for the spontaneous decays shown by the red curve, the region prior to the time of the kicking out of the unwanted ions at 707.8 $\mu \mathrm{s}$ is not shown.

Figure 10

Survival probabilities $f\left(E_{\mathrm{ini}}\right)$ as a function of the initial internal energy $E_{\mathrm{ini}}$. The solid black curve shows a Monte Carlo simulation. The dashed red curve is $f\left(E_{\mathrm{ini}}\right)=k_{\mathrm{v}}^{8-\mathrm{modes}}/(k_{\mathrm{d}}+k_{\mathrm{v}}^{8-\mathrm{modes}})$, where the eight major vibration modes are taken into account. The inset shows a zoomed view for the region where the difference in the two curves starts to become visible.