Collective Energy Transfer to a Spectator Atom via Multi-Center Intermolecular Coulombic Decay

Abstract

Molecular mechanisms that enable collective and upconverted energy transfer from multiple photoacceptors to a non-absorbing spectator reaction center are highly desirable for efficient light-energy utilization. Here, we show that intermolecular Coulombic decay (ICD), a nonlocal energy relaxation channel in photoexcited molecules, offers an avenue for such a novel energy transfer mechanism. On irradiation of pyridine-argon gas mixture at 266 nm and at low laser intensities, we observed a surprisingly dominant formation of argon cations. Measurements of the laser power dependence, together with systematic studies of Ar$^+$ yield versus laser intensity and molecular density, reveal that ICD mediates the collective funneling of excitation energy from multiple photoexcited pyridine molecules to a non-photoabsorbing argon atom, leading to its ionization. The density of the reaction center offers an efficient handle to optimize this collective energy transfer. This mechanism opens new avenues in light harvesting design and may help explain the remarkable resistance of biomolecules to photodamage.

Figures (15)

• Figure 1: TOF mass spectrum of cations: A supersonic expansion of a pyridine-argon mixture, prepared by diluting pyridine vapor with 1 atm of Ar, was irradiated with 266 nm laser light at an intensity of 1 $\times$ 10$^6$ Wcm$^{-2}$. The resulting TOF mass spectrum shows dominant formation of Ar$^+$ and pyridine-derived cations. Raw data were smoothed using adjacent averaging in Origin 7.5.
• Figure 2: Cation TOF spectrum from skimmed supersonic expansion: A pyridine-argon mixture (2 atm) is expanded through a 1mm skimmer and irradiated with 266 nm photons. The spectrum shows cations of pyridine, its fragments, and clusters, with no formation of Ar$^+$. Heavier-than-pyridine cations reported by Barik et al. barik2022 are also absent due to the collision-free nature of the skimmed beam.
• Figure 3: Schematic and theoretical illustration of a collision-mediated ICD scenario involving argon and photoexcited pyridine dimers.a, Green: argon atom; red: pyridine molecules. Valence electron density is shown as a haze, with the outer blue haze representing photoexcited electron density. The figure illustrates a plausible configuration in which a diffusing argon atom enters the region between two electronically excited pyridine dimers, a scenario that can facilitate multi-center interactions leading to ICD. b-c, Comparison of total electron density surfaces (iso-density: $1\times 10^{-6}$) with and without Ar illustrates wavefunction delocalization induced by argon diffusion. d, Intermolecular orbital formed via covalent mixing of the pyridine 1$\pi$ orbitals and the argon $p_z$ orbital, which provides a plausible theoretical pathway for energy transfer associated with argon ionization via ICD.
• Figure 4: VMI of ICD electrons: VMI showing isotropic emission of ICD electrons from Ar atoms and pyridine molecules, where the imaging is not done in coincidence with any particular cation. The spectrum was recorded at a laser intensity of about 7 $\times$ 10$^5$ Wcm$^{-2}$. au: atomic units.
• Figure 5: Relative yield of Ar$^+$: The ratio of the Ar$^+$ yield to the total yield of all the other cations originating from photoexcited pyridine monomers is displayed. (a) The laser intensity is varied and the argon pressure is fixed at 0.5 atm. (b) The argon pressure is varied for a fixed laser intensity of $1.49 \times 10^6$ Wcm$^{-2}$. For the details on the partial pressure of pyridine monomers, see the Supplemental Material NOTES. The systematic increase in relative Ar$^+$ yield with decreasing laser intensity or increasing argon pressure constitutes unambiguous experimental evidence of concerted energy transfer from photoexcited pyridine molecules to non-photoabsorbing Ar atoms via the ICD mechanism.