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An international team of scientists has developed a new nano-cell for laser spectroscopy. The atoms and molecules in a sample are in constant motion, temperature dictates just how much movement there is and unless chilled to incredibly low temperatures, there is inevitably line-broadening in the spectra of any sample. However, confining a sample in a tiny space could sharpen those spectral lines, in laser spectroscopy, at least. A new kind of cell developed by researchers, led by David Sarkisyan of the Armenian Academy of Sciences and Paris-Nord University traps a sample of an atomic vapour, commonly an alkali metal, within a small enough space (just 10 micrometres thick) but with a window on the thin layer of 10-20mm diameter. In such a cell, there is a strong spatial anisotropy, as opposed to the isotropy seen in standard cells, during the 50 time that the sample would be exposed to laser radiation during a study. For example, caesium atoms have an average velocity at room temperature of about 200 metres per second and will cross the cell gap distance of 10 micrometres in about 50 nanoseconds. In contrast, there is no limit on those atoms moving parallel to the windows of the cell, where they have 10-20 mm to traverse and so have much longer to interact with the laser radiation. A laser beam orthogonal to the window of the cell sees the atoms moving parallel to the window's surface as essentially fixed, which leads to a strong reduction of the Doppler effect and to the related Doppler broadening of the spectral lines. The laser thus sees two "kinds" of atoms, those moving parallel and those moving perpendicular, i.e. in the direction of the beam. The team's second-generation extremely thin cell can contain atomic nano layer (L=100-1500 nm) and laser spectroscopy with such a confined sample offer new hope of ultra-high resolution laser spectroscopy in which the sample actually has one dimension of the same order of magnitude as the wavelength of the laser light. Tests with the new cell reveal major differences between the fluorescence and transmission spectra because the fluorescence signal is due to the parallel moving atoms, whereas the fast atoms moving in the direction of the beam have insufficient time to undergo a complete absorption-emission cycle and so do not fluoresce. Therefore transitions are completely resolved in the fluorescence case. Only the fast atoms can contribute to the absorption spectrum in the nano layer because the absorption of photons is sufficiently rapid, but this leads to the opportunity for a much greater Doppler effect to affect the spectral lines. Tests with caesium vapour in the nano cell allow the different Cs-transitions to be resolved whereas in a conventional centimetre-scale cell they overlap completely and cannot be distinguished. The new nano cell will enable the study of absorption and fluorescence dynamics in more detail than previously possible with laser spectroscopy. The Institute of Electronics at the Bulgarian Academy of Sciences is now coordinating a European contract in the INTAS program with partners from Armenia, France, Italy, and Latvia. The program will study the properties of thin metal-vapour nano layers with a view to understanding gas-surface interactions. In addition, teams at the Institute of Electronics, Sofia University and the Paris-Nord University are collaborating under the Rila scientific program.
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