Ba+2 ion trapping using organic submonolayer for ultra-low background neutrinoless double beta detector

Abstract

If neutrinos are their own antiparticles the otherwise-forbidden nuclear reaction known as neutrinoless double beta decay can occur. The very long lifetime expected for these exceptional events makes its detection a daunting task. In order to conduct an almost background-free experiment, the NEXT collaboration is investigating novel synthetic molecular sensors that may capture the Ba dication produced in the decay of certain Xe isotopes in a high-pressure gas experiment. The use of such molecular detectors immobilized on surfaces must be explored in the ultra-dry environment of a xenon gas chamber. Here, using a combination of highly sensitive surface science techniques in ultra-high vacuum, we demonstrate the possibility of employing the so-called Fluorescent Bicolor Indicator as the molecular component of the sensor. We unravel the ion capture process for these molecular indicators immobilized on a surface and explain the origin of the emission fluorescence shift associated to the ion trapping.

Fig. 1. Model of the FBI molecules before and after chelation.

Schematic representation of the experiment we have carried out. FBI molecules were sublimated on a Au(111) surface, chelated in situ and characterized inside the UHV chamber.

Fig. 2. XPS demonstration of the chemical changes induced by the molecular chelation.

a O 1s, N 1s, and C 1s core level spectra measured after sublimation of 0.6 ML of FBI deposition on Au(111); b FBI core level spectra measured on the previous sample (dashed spectra) after chelation with 0.80 Ba⁺² ions per FBI molecule (blue spectra); c O 1s, N 1s, and C 1s core level spectra measured after 0.6 ML of FBI deposited on Au(111) (green spectra) and after chelation with Na⁺ (2.60 Na⁺ ions per FBI molecule) (orange line). For the three panels, dots correspond to raw values and solid lines to fitted curves (the fitting procedure is discussed in the Methods section). d Curve-component deconvolution of O 1s spectra in a and b as well as after chelation with θ_Ba = 3. e Evolution of the O 1s core level as a function of the Ba⁺² deposition on 0.9 ML of FBI on Au(111). The spectra were manually shifted in the y-axis to better show the evolution. The O 1s spectra in d and e are displayed after subtraction of the contribution from Au 4p 3/2, in order to emphasize the spectral changes.

Fig. 3. STM images of FBI and FBI derivative molecules on Au(111).

a Large-scale STM image (50 × 50 nm²) of around 0.4 ML of FBI molecules deposited on Au(111) surface and measured at 4 K. Red squares show isolated molecules. Inset: constant current zoom STM image of a FBI molecule (I = 60 pA / U = 1.4 V) deposited on Au(111). b–d Bond-resolved STM images measured with a CO-functionalized probe (constant height, U = 5 mV) of individual FBI molecules: b benzo[a]imidazo[5,1,2-cd]indolizine fluorophore with phenyl ring and aza-crown ether (exactly the same molecule shown in the inset in a); c benzo[a]imidazo[5,1,2-cd]indolizine fluorophore; d benzo[a]imidazo[5,1,2-cd]indolizine fluorophore with phenyl ring but without the crown ether. For clarity, the same image is shown twice, top one with and bottom one without the molecular model superimposed to guide the eyes.

Fig. 4. Molecular chelation probed by STM/STS.

a Representation of the most energetically stable conformation calculated for the molecules in gas phase. b–d STM images of b Na⁺-complexed (U = 0.9 V, I = 60 pA), c native (U = 1.4 V, I = 60 pA), and d Ba⁺²-complexed (U = −0.5 V, I = 20 pA) FBI. (Scale bars = 0.3 nm). e STS spectra of pristine (green), Na⁺-complexed (red), and Ba⁺²-complexed FBI (blue)FBI. Extra spectra for unchelated and chelated molecules are shown in Supplementary Fig. 5.

Fig. 5. O 1s core level evolution upon chelation measured on two submonolayer FBI-functionalized surfaces.

a chelation on Cu(111) sublimating BaCl₂ and NaCl (inset); b chelation on ITO. In this case, the substrate component (O_ITO) was subtracted to emphasize the O 1s contribution coming from FBI (O_FBI). The entire spectra are shown inserted in b. Dot spectra correspond to raw values and solid lines to fitted curves.

Fig. 6. Curve deconvolution for O 1s in FBI 0.6 ML on Au(111) before and after chelation.

Detail of fitting components (dotted lines) for O 1s in FBI 0.6 ML (green) and FBI 0.6 ML + BaCl₂, θ = 0.8 (blue) from Fig. 2a and b. Circles and solid lines represent raw data and best fit respectively. The green component was fitted to the unchelated FBI data (green circles) at 532.95 ± 0.05 eV, with width 2.16 ± 0.04 eV at FWHM. This component was then fixed in position and width for the chelated FBI data (blue dots). An additional component was fitted to the chelated FBI CL, shown as blue filled area, at 533.91 ± 0.05 eV, with 1.41 ± 0.05. These values were obtained from the fit as free parameters. The background of each curve is shown as dashed lines in green and blue respectively. Inset: same blue curve and components with its background subtracted.