Time-Resolved Measurement of Interatomic Coulombic Decay in Ne_2 (1308.0118v1)

Abstract: The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in Ne_2 is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne^{+}(2p^{-1}) - Ne^{+}(2p^{-1}) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne^{+} - Ne^{2+} pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the Ne_{2}^{+}(2s^{-1}) state to be (150 +/- 50) fs in agreement with quantum calculations.

• The paper reports a precise measurement of ICD lifetimes (~150 fs) in neon dimers using an ultrafast pump-probe technique.
• The methodology employs ion-ion coincidence spectroscopy to capture low-energy electron emissions and align the results with quantum mechanical models.
• The findings confirm theoretical predictions and open new avenues for exploring ICD processes in diverse molecular systems.

Time-Resolved Interatomic Coulombic Decay in Neon Dimers

The paper concerning the measurement of Interatomic Coulombic Decay (ICD) in neon dimers (Ne$_2$) represents a critical advancement in understanding radiationless relaxation processes in molecular systems. The research, conducted through an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg, elucidates both the experimental technique and the theoretical underpinning required to analyze ICD events, specifically in the neon dimer system. The robust data and subsequent analysis have yielded a measured ICD lifetime, aligned with theoretical predictions and quantum mechanical models.

Methodological Framework and Experimental Findings

The experimental method involved using an ultrafast pump-probe technique on the neon dimer. The pump pulse induced a $2s$ inner-shell vacancy, leading to a repulsive state in the ionized dimer. The ICD was then triggered as the vacancy was energetically filled, with energy emitted resulting in low-energy electron ejection—a crucial step for probing damage pathways such as in radiation therapy. The subsequent probe pulse facilitated the measurement by inducing a further ionization event which, when timed with the decay, allowed the deduction of the ICD lifetime from the recorded ion coincidences as a function of delay.

The experimental setup allowed for the precise control and detection of the resulting ion states using ion-ion coincidence spectroscopy, an innovative approach that obviates the need for prior detailed spectroscopic knowledge of potential energy curves. From the data collected, the ICD lifetime for this decay in Ne$_2$ was determined to be approximately 150 femtoseconds with a statistical uncertainty of ±50 femtoseconds.

Theoretical Considerations and Computational Insights

The experimental findings are corroborated by quantum calculations that model the nuclear dynamics and decay width of the $2^2 \Sigma^+_u$ and $2^2 \Sigma^+_g$ states of the dimer, factoring in the internuclear distance and the dipole-dipole character of the ICD process. This concurrence with theoretical estimates not only validates the measurement technique employed but also solidifies our understanding of the ICD process as a function of changing internuclear distances.

One of the intriguing aspects of the theory addresses the system's vibrational dynamics post ICD initiation, challenging initial assumptions of negligible nuclear motion for certain ICD variants and accentuating the need for distance-dependent decay lifetime calculations. This resonance with the theoretical models assures the data integrity, showcasing the necessity of factoring nuclear motion into the calculation of decay transitions.

Implications and Prospective Investigations

The measurement technique and findings from this research open prospective avenues for further investigations into ICD processes across various molecular clusters with potential applications spanning medical physics to material sciences. The method is poised to adapt and extend to different systems like endohedral fullerenes or water clusters, potentially revealing even swifter ICD phenomena. Noteworthy inquiries may involve experiments on mixed dimers or polyatomic systems, which could yield insights into the role of molecular environment and motion in dictating ICD rates.

Beyond immediate implications, the work suggests future investigations into ICD processes using other advanced ultrafast pump-probe methodologies, perhaps integrating real-time reaction-to-dynamics visualization or higher energy regime applications. Such progress will enrich our understanding of electron correlation effects across diverse molecular systems, underpinning potential technological innovations in X-ray free-electron laser facilities that continue defining the future landscape of molecular dynamics research.

In conclusion, the meticulous approach adopted in this paper provides a notably precise measurement framework for ICD lifetimes, demonstrating that definitive experimental validation of quantum predictions is attainable within the complex landscape of correlated electron dynamics.