Observation of Nuclear Wave-Packet Interference in Ultrafast Interatomic Energy Transfer

We report the experimental observation of quantum interference in the nuclear wave-packet dynamics driving ultrafast excitation-energy transfer in argon dimers below the threshold of interatomic Coulombic decay (ICD). Using time-resolved photoion-photoion coincidence spectroscopy and quantum dynamics simulations, we reveal that the electronic relaxation dynamics of the inner-valence $3s$ hole on one atom leading to a $4s$ or $4p$ excitation on the other one is influenced by nuclear quantum dynamics in the initial state, giving rise to a deep, periodic modulation on the kinetic-energy-release (KER) spectra of the coincident ${\mathrm{Ar}}^{+}\text{−}{\mathrm{Ar}}^{+}$ ion pairs. Moreover, the time-resolved KER spectra show characteristic fingerprints of quantum interference effects during the energy-transfer process. Our findings pave the way to elucidating quantum-interference effects in ultrafast charge- and energy-transfer dynamics in more complex systems, such as molecular clusters and solvated molecules.

Figure 1

(a) Illustration of the pump-probe principle for measuring the frustrated-ICD process of the $3s$ hole in an argon dimer. The XUV-pump pulse removes a $3s$ electron from one of the atoms of the dimer (magenta arrow) initiating a frustrated-ICD process, in which a $3p$ electron fills the $3s$ vacancy transferring the released energy to the other argon atom where an electron is excited to a $4s$ or $4p$ orbital (green arrows). The excited electron is then ionized by a delayed IR pulse (red arrow) creating a dication that undergoes a Coulomb explosion. (b) Measured photoion-photoion coincidence (PIPICO) spectrum, where the XUV-IR delay was integrated from zero to 2 ps.

Figure 2

Measured (a) and calculated (b) time-resolved KER spectra of the Coulomb explosion channel of ${\mathrm{Ar}}_2$.

Figure 3

Potential-energy curves of argon dimers relevant for the process belonging to ungerade ${}^2{\mathrm{\Sigma }}_u^{+}$ (a) and gerade ${}^2{\mathrm{\Sigma }}_g^{+}$ (b) symmetry, taken from Ref. [20]. The magenta arrows indicate the equilibrium internuclear distance of the ground state of the neutral dimer, i.e., the center of the Franck-Condon region. (c),(d) Calculated time-resolved KER spectra from the ungerade and gerade states, respectively.

Figure 4

(i)–(iii) Schematic representation of nuclear dynamics in ${\mathrm{Ar}}_2$ during frustrated ICD in the XUV-IR-delay ranges (i), (ii), and (iii), as defined in Fig. 3. The red, blue, and black curves represent the $3s$-hole [${\mathrm{Ar}}^{+*}(3{\mathrm{s}}^{-1})\text{−}\mathrm{Ar}$], $4s$-, and $4p$-excited [${\mathrm{Ar}}^{+}(3{\mathrm{p}}^{-1})\text{−}{\mathrm{Ar}}^{*}(3{\mathrm{p}}^{-1}4\mathrm{s}/4\mathrm{p})$] states, respectively. The arrows mark the propagation direction.