18 research outputs found
Sub-harmonic resonant excitation of confined acoustic modes at GHz frequencies with a high-repetition-rate femtosecond laser
We propose sub-harmonic resonant optical excitation with femtosecond lasers
as a new method for the characterization of phononic and nanomechanical systems
in the gigahertz to terahertz frequency range. This method is applied for the
investigation of confined acoustic modes in a free-standing semiconductor
membrane. By tuning the repetition rate of a femtosecond laser through a
sub-harmonic of a mechanical resonance we amplify the mechanical amplitude,
directly measure the linewidth with megahertz resolution, infer the lifetime of
the coherently excited vibrational states, accurately determine the system's
quality factor, and determine the amplitude of the mechanical motion with
femtometer resolution
Equilibrium speciation in moderately concentrated formaldehydeâmethanolâwater solutions investigated using 13C and 1H nuclear magnetic resonance spectroscopy
We used 13C and 1H NMR spectroscopy to examine the equilibrium speciation in formaldehydeâmethanolâwater solutions at moderate formaldehyde concentrations such as those used in the synthesis of formaldehyde-based organic gels. Concentrations of small methylene glycol oligomers and their methoxylated forms found in these solutions were quantitatively determined over a range of formaldehyde concentrations and methanolâwater ratios, and at temperatures between 10 and 55 °C. Using the measured concentrations, equilibrium constants for methylene glycol dimer and trimer formation as well as methoxylation of these oligomers were calculated. Based on this, we developed a quantitative equilibrium model for calculation of formaldehyde-related species concentrations over a range compositions relevant for formaldehyde based solâgel processes allowing for more rational design of formaldehyde polymerization systems