Search for: All records

This thesis presents a comprehensive investigation into the low-temperature luminescence of nanoclusters doped with nitrogen and oxygen atoms. The nanoclusters were formed by condensing the products of a radio-frequency discharge in various impurity-helium gas mixtures into the cold helium gas and bulk superfluid helium. The research focused on two primary objectives: first, the discovery and mechanism of the phenomenon of enhanced oxygen β-group emission in nitrogen and nitrogen-rare gas nanoclusters, and second, the identification of the direct spectroscopic evidence of solidified helium layers on neon nanocluster surfaces. Optical spectroscopy was used as the primary analytical method. The mechanism of the enhanced oxygen β-group (O(1S→1D)) emission at temperatures between 16-36 K was established. This enhancement is driven by the recombination of nitrogen atoms from the gas jet on solid nanocluster surfaces, resulting in the formation of metastable N2(A3Σ+u ) molecules. The energy transfer from the excited molecule to the stabilized inside nanoclusters oxygen atoms via the nitrogen matrix was found to depend on the nanocluster’s internal structure. The enhancement effect was strong in pure molecular nitrogen, N2-Kr and N2-Ar nanoclusters, supporting a core-shell structure of nitrogen-rare gas nanoclusters with an outer N2 layer that provides an efficient energy transfer pathway. In contrast, the effect was suppressed in N2-Ne nanoclusters, indicating the core-shell structure where an insulating neon shell impedes energy transfer. This work also provides the first spectroscopic evidence for solidified helium layers on nanocluster surfaces. In neon nanoclusters with low nitrogen concentrations, a prominent and narrow emission line at λ=519.9 nm was observed in the nitrogen atom α-group (N(2D→4S)) spectrum. The line’s near-gas-phase position, long decay lifetime ( 280 s), and disappearance upon warming up collectively indicate that it originates from nitrogen atoms on the neon nanocluster surface, whose properties are predominantly influenced by a surrounding layer of solidified helium. 

We studied luminescence accompanied by an injection of nitrogen–krypton–helium gas mixtures after passing radiofrequency discharge into dense cold helium gas. In the cold helium gas N2–Kr nanoclusters were formed, with a core of Kr atoms and N2 molecules on the surface. Atomic nitrogen and oxygen resided in the N2 surface layers. When the temperature in the observation zone was in the range of 20–36 K, we observed enhanced emission of oxygen atom β-group and molecular nitrogen Vegard–Kaplan bands from N2–Kr nanoclusters. At these temperatures, nitrogen atoms efficiently recombine on the surface of nanoclusters with the formation of exited nitrogen molecules, leading to enhanced emission of Vegard–Kaplan bands. Simultaneously, the energy transfer from exited nitrogen molecules to the oxygen atoms enhanced O atom β-group emission. 

We studied luminescence accompanied an injection of the nitrogen-helium gas mixture after passing discharge into dense cold helium gas. Initially, when the experimental beaker was filled with superfluid helium and the nitrogen-helium gas was injected into bulk superfluid helium at T ≈ 1.5 K, the dominant band in the emission spectra was the α-group of nitrogen atoms. At these conditions, the nanoclusters of molecular nitrogen with high concentrations of stabilized nitrogen atoms were formed. When superfluid helium was evaporated from the beaker and the temperature at the bottom of the beaker was increased to T ≈ 20 K, we observed a drastic change in the luminescence spectra. The β-group of oxygen atoms was dominated in the luminescence spectra, and the emission of the α-group became small. At high temperatures (T ≈ 20 K), most of the nitrogen atoms recombine on the surface of N2 nanoclusters with the formation of excited nitrogen molecules. We explained the effect of the enhancement of β-group emission by effective energy transfer from excited nitrogen molecules to the stabilized impurity oxygen atom inside N2 nanoclusters.