Recently an acoustic destabilizing pressure was predicted, which could be shown experimentally via a dewetting pattern in thin polymer films. The wavelength λ of the fastest growing mode is a signature of the acting forces. Even in cases with stabilizing van der Waals forces, films became unstable. The present paper also considers thermally excited acoustic waves confined in a thin liquid film of thickness d. A new concept is developed to calculate the acoustic pressure for different boundary conditions: the free-standing film, the film rigid at one surface and the film deposited on a substrate, liquid or solid. For characteristic examples the calculation is carried out numerically. The results for the limiting cases are simple. The acoustic pressure of the free-standing film grows monotonically with d up to a level strongly depending on the temperature, it cannot destabilize the film. The acoustic pressure of the film rigid at one surface rapidly grows with d to a maximum and then decreases monotonically to the same level as for the free-standing film. On the right side of the maximum the film is unstable and λ grows quadratically with d, similar to the case of a destabilizing van der Waals pressure. For a film deposited on a substrate the acoustic pressure comes to a smaller level directly, depending on the excess sound velocity in the substrate: generally it yields a rather linear dependence of λ on d.It was derived by Schäffer et al [6] using the same energy k B T for all states and the Debye approximation for a free-standing
Isotope shifts and hyperfine structures of four transitions in W I have been measured as well as isotope shifts in Lu I by means of atomic-beam laser spectroscopy. Magnetic-dipole hyperfine constants 3 of '"W are determined for the atomic states 'D&, 'F&, and 'F3 of the 5d 6s6p configuration. The nuclear parameters A, and 5(r') are derived for W stable isotopes. J dependences of the isotope shifts are observed for the D term of the 5d6s configuration in LuI and the 'D term of the 5d 6s configuration in WI. Parameters z, d of the crossed-second-order effects are derived for the LuI Sd6s and WI 5d 6s configurations, and zsd /A, is found to be 411(26) MHz/fm for W. A systematic behavior of the normalized parameter z"t /A, g", is discussed. PACS number(s): 32.30.Jc, 32.70.Jz, 35.10,Fk, 21.90.+ f
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