Imaging the Rovibrational Ground State of the Helium-Neon Dimers 4He20Ne and 4He22Ne

Abstract

The helium-neon dimer has been subject to many theoretical studies, in which the interaction potential of the helium-neon system has been calculated with ever increasing accuracy. Calculations predict that the helium-neon system supports only a few bound states, which makes the system inaccessible to standard spectroscopic techniques. Previous experiments have probed the helium-neon potential by comparing measured and predicted scattering cross sections. However, the spatial structure and energetics of the bound states of the helium-neon system have not been studied experimentally in great detail. We employ Coulomb explosion imaging (CEI) to measure the pair distance distributions of the helium-neon dimers ⁴He²⁰Ne and ⁴He²²Ne in their rovibrational ground state. For each dimer, the binding energy is extracted from the measured pair distance distribution. Additionally, the pair distance distribution provides access to the helium-neon potential.

Figure 1

(a) Sketch of the experimental setup (see the text for details). (b) Measured diffraction patterns of single helium atoms (black curve), single neon atoms (gray curve), and helium–neon dimers (green curve). The diffraction patterns of the individual species are post-selected from the data by gating on the time-of-flight peaks of the ions He⁺, Ne⁺, and HeNe⁺, respectively. Because all species travel in the molecular beam with roughly the same mean velocity v_jet, the spacing between the diffraction peaks is solely determined by the cluster mass m due to the de Broglie wavelength λ_dB = 2π/mv_jet (in the present experiment, v_jet = 600 ms^–1 and λ_dB,⁴He = 0.2 nm). In order to reduce the background from the ionization of single helium and neon atoms, the laser is focused on the first-order diffraction peak of the helium–neon dimer (magenta dashed vertical line).

Figure 2

(a) Green dots with error bars: measured pair distance distribution of ⁴He²⁰Ne. Error bars correspond to the statistical error √N, where N is the number of counts per bin. Solid, dashed, and dash-dotted lines: computed pair distance distributions |u_J(R)|² of the three helium–neon dimer bound states J = 0, 1, and 2 based on the interaction potentials (b). The distributions |u_J(R)|² are scaled to the experimental distribution. (b) Sum of the helium–neon potential and repulsive centrifugal barrier (eq 4).

Figure 3

(a) Measured pair distance distribution of ⁴He²⁰Ne. (b) Measured pair distance distribution of ⁴He²²Ne. Error bars correspond to the statistical error √N, where N is the number of counts per bin. Note the different ranges of the y axis in panels a and b. The binding energy of each dimer is obtained from a fit of the exponential tail of the pair distance distribution. The binding energies that we computed from the potential of Cacheiro et al. are −3.71 K for ⁴He²⁰Ne and −3.79 K for ⁴He²²Ne.

Figure 4

Purple dots represent the helium–neon potential computed from the measured pair distance distribution of ⁴He²⁰Ne via eq 3. Horizontal error bars correspond to the bin width ΔR. For each data point of the measured potential curve, the vertical error bar is derived from eq 3 by computing a Gaussian error propagation for the statistical error √N of the pair distance distribution (green dots). The black solid line corresponds to the theoretical helium–neon potential of Cacheiro et al.