A variable-path ultrasonic interferometer has been modified to permit automatic data collection and analysis utilizing an Apple IIe microcomputer and an ADC interface board possessing a crystal-controlled clock for precision data sampling. The micrometer screw is driven by a synchronous motor also controlled by the interface board. The system allows rapid sampling and recording of the crystal driving voltage for each resonant node in the gas column, thereby reducing running time and improving overall accuracy and sensitivity. For SiF4, the (single) vibrational relaxation time was found to be (2.20 _ 0.14) X 10 -s s at a temperature of 304 K and 1 atm of pressure.
A computer-controlled ultrasonic interferometer has been utilized for exploring room-temperature relaxation of gaseous SiF4–Ar and SiF4–Ne mixtures. A single relaxation process was observed for all mixtures, and the relaxation rates, 1/pτ, were found to vary linearly with the mole fraction of added argon or neon. Analysis of the mixture relaxation data shows that collisions between SiF4 molecules and either Ar or Ne atoms are less efficient for V−T transfer than are SiF4 self-collisions. This result can be explained within the Schwartz–Slawsky–Herzfeld formulation if it is assumed that a steep repulsive potential (i.e., the 7:28 function) is operative for SiF4–SiF4 collisions, while the repulsive interaction for SiF4–noble gas collisions is treated as being between a molecule obeying a 7:28 law and an atom characterized by a 6:12 function.
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