Hitherto unrecognized intermolecular Coulombic decay mechanism in gases

Abstract

Excited atoms and molecules can utilize their excess energy to ionize a neighboring system by a process named interatomic and intermolecular Coulombic decay (ICD). ICD is ultrafast, in the femtosecond regime, and has many modes of appearance. Ample applications of ICD have been reported spanning a wide range of fields and it is expected to be ubiquitous in nature. Essentially all the investigations on ICD were for weakly bound systems, like clusters and fluids. We demonstrate that, unexpectedly, ICD can be efficiently active in atomic and molecular gases in spite of the very large distances between the units. We uncover the underlying mechanism, which differs from that prevailing in weakly bound systems. The dynamics of ICD in gases is analyzed. The results considerably broaden the impact of ICD and open the gateway to new kinds of applications.

Figures (6)

• Figure 1: Schematic picture of ICD in gases. Panel A) Neutral species in their electronic ground state (small cyan balls) and electronically excited species (larger magenta balls). Each excited species can potentially transfer its excess energy to ionize another excited species. In other words, any excited species can be an acceptor as well as a donor. An energy transfer is by a so called virtual photon Averbukh04 and indicated by a wavy line. Four possible transfers are explicitly shown which might take place at an arbitrary small time interval. Panel B) After the transfers indicated in panel A have taken place, the four donors have turned into ground state species and the four acceptors into ions (small orange balls). The respective four ICD electrons are also indicated.
• Figure 2: A Ne gas in a volume of 0.125 cm$^3$ with 10% of excited atoms evolves in time. Shown are the results for different numbers of atoms corresponding to a pressure of 0.032 to 0.288 Pa at room temperature. The excited atoms can decay by radiation and by ICD. In each ICD event one excited atom is transferred to its ground state and one is ionized. The left panel depicts the evolution of the number $N^*(t)$ of excited Ne atoms, the middle panel shows the number $N(t)$ of Ne atoms in their ground state created by radiative decay and ICD and the right panel reports the ion yield $N^+(t)$ produced by ICD.
• Figure 3: The ICD time and the average ICD rate for the Ne gases as in Figure \ref{['Number_Particles_vs_Time']}. The total ICD rate of a gas $\Gamma_{_{ICD}}$ is determined by summing up all possible ICD rates between the excited species of the gas taking into account attenuation due to the gas (see Methods for details). As usual, $\hbar/\Gamma_{_{ICD}}$ defines the time scale of the ICD, which is here named ICD time. The total ICD rate and the ICD time are temporal quantities because the number of excited atoms decreases in time. The left panel shows the ICD time on a logarithmic scale as function of time. It is seen that the ICD time increases exponentially as time proceeds. ICD is initially very fast (ps and sub ps) and dominates the decay by radiation (1.7 ns radiative lifetime of the excited Ne atom). The situation reverses at later times, but by then the ion yield has already saturated (see right panel of Figure \ref{['Number_Particles_vs_Time']}). The average ICD rate, i.e., $\Gamma_{_{ICD}}/N^*$ at $t=0$, is depicted in the right panel for the different Ne gases and broken into its three contributions. It is seen that all terms except of retardation can be neglected.
• Figure 4: Ion yield (left panel) and ICD time (right panel) for Ne, Ar, Kr, Xe and CO gases as a function of time. Each gas is in a volume of 0.125 cm$^3$ and initially contains $5 \times 10^{11}$ excited species which constitute 10% of its species. The ion yield of the Ne gas is seen to be the smallest and that of the molecular gas by about an order of magnitude larger than that of all the atomic gases. At short times, the ICD times of all the atomic gases are similar, while that of the CO gas is much shorter and stays so over time. For comparison, the radiative lifetimes are: 1.7 ns (Ne), 1.88 ns (Ar), 3.36 ns (Kr), 3.66 ns (Xe) and 9.71 ns (CO), see SI.
• Figure 5: Time evolution of the number of excited (left panel) and ground state (middle panel) atoms as well as of the ion yield (right panel) for different peak intensities ranging from $1\times 10^{5}$ to $9\times 10^{5}$ W/cm$^2$. A Ne gas of $5\times 10^{12}$ Ne atoms in the interaction volume of 0.125 cm$^3$ (0.162 Pa pressure at room temperature) is illuminated by a Gaussian pulse of 1 ns duration. To avoid negative times, the peak of the pulse arrives at the gas after 10 ns. The radiative lifetime of an excited Ne atom is 1.7 ns. The ion yield is essentially only due to ICD.