Detection of Recurrent Fluorescence Photons

We have detected visible photons emitted from the thermally populated electronic excited state, namely recurrent fluorescence (RF), of ${\mathrm{C}}_6^{-}$ stored in an electrostatic ion storage ring. Clear evidence is provided to distinguish RF from normal fluorescence, based on the temporal profile of detected photons synchronized with the revolution of ${\mathrm{C}}_6^{-}$ in the ring, for which the time scale is far longer than the lifetime of the intact photoexcited state. The relaxation (cooling) process via RF is likely to be commonplace for isolated molecular systems and crucial to the stabilization of molecules in interstellar environments.

Figure 1

Schematic energy diagram. (a) Photoexcitation followed by normal fluorescence and (b) the RF process at stationary state, respectively. The gray scale represents the vibrational level densities. GS: electronic ground state, ES: electronic excited state, Ex: photoexcitation, F: fluorescence, IC: internal conversion, IIC: inverse internal conversion, and RF: recurrent fluorescence.

Figure 2

Schematic layout of the experimental setup. Hot ${\mathrm{C}}_6^{-}$ ions were injected and stored in the TMU $\mathrm{E}$-ring. The time delay ($\mathrm{\Delta }t$) between the photon signals and the corresponding neutral product signals detected with a microchannel plate (MCP) is the time it takes for the neutral products of the velocity $v$ to travel a distance $L$ from the photon observation region to the position of the MCP, i.e., $\mathrm{\Delta }t=L/v$. The $\mathrm{\Delta }t$ is constant for each ion bunch revolution. The second harmonic of a pulsed Nd:YAG laser (10 Hz, 532 nm) is merged to excite the stored ions in the opposite straight section of the ring.

Figure 3

RF photons from the nascent hot ions. (a) Temporal profile of the photon yield after ion production in the source. The bin width is $0.1\mu \mathrm{s}$. The inset shows an expanded view of the peaks. The peak at $\sim 0.02\mathrm{ms}$, indicated by the asterisk, is unassigned. The first peak appearing just after ion injection originates from the switching noise from the injection deflector. (b) Reference temporal profile of the neutral products. The bin width is $0.1\mu \mathrm{s}$. The peaks assigned to ${\mathrm{C}}_6^{-}$ are depicted in red. Other peaks are also assigned to ${\mathrm{C}}_n^{-}$ ($n\ne 6$) (see the details in the Supplemental Material [26]). Note that each peak height does not necessarily reflect the neutral intensity, because the MCP became saturated for the intense irradiation at the time before the 1st revolution of ${\mathrm{C}}_6^{-}$. The time delay ($\mathrm{\Delta }t$) between the photon signals and the corresponding neutral product signals is $9.9\mu \mathrm{s}$ for ${\mathrm{C}}_6^{-}$ (see Fig. 2). (c) Plot of time-integrated yields at the region of the ion bunch ($1.9\mu \mathrm{s}$ in width) versus time after ion production. The error bars indicate a statistical uncertainty of $\pm 1\sigma$. The solid curves represent the simulated RF intensity assuming an initial temperature of 5000 K at the ion source. (see the Supplemental Material [26] and Ref. [19] for the details.)

Figure 4

RF from the photoexcited ions. (a) Temporal profile of the observed photon yield after photoexcitation (synchronized). The bin width is $1\mu \mathrm{s}$. The intense peak at the time of laser irradiation (approximately 0.475 ms after ion production) is due to stray laser light. The photon emission time expected from the ion bunch timing and width ($1.9\mu \mathrm{s}$) are shown by the yellow regions. (b) Observed temporal photon yield after laser irradiation half a turn out of phase with the ion motion (countersynchronized). The peak position of the stray laser light shifts correspondingly by the time the ions undergo a half of a revolution. (c) Integrated yields at the ion bunch regions for both synchronized and countersynchronized cases are represented by red filled circles and blue filled circles, respectively.