Heat Transfer at Solid-Gas Interfaces by Photoacoustics at Brillouin Frequencies
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- Heat Transfer at Solid-Gas Interfaces by Photoacoustics at Brillouin Frequencies
- Min, Chang-Ki; Chen, Kejia; Bae, Sung Chul; Cahill, David G.; Granick, Steve
- Issue Date
- AMER CHEMICAL SOC
- JOURNAL OF PHYSICAL CHEMISTRY C, v.116, no.20, pp.10896 - 10903
- We use time-resolved ellipsometry to investigate the rate of heat transfer at solid-gas interfaces through measurements of the amplitude and phase of acoustic waves at Brillouin frequencies, 100-400 MHz, at pressures 2 orders of magnitude higher than earlier comparable studies. An ultrafast optical pulse heats a thin metal film deposited on a sapphire substrate. Heat flow from the substrate into the gas causes expansion of the gas and generates an acoustic wave that is probed by off-null ellipsometry with subpicosecond time resolution. We compare the amplitudes and phases of photoacoustic signals generated in inert gases Ne, Ar, Kr, and Xe to a continuum theoretical model that includes the thermal accommodation coefficient ± at the gas-solid interface. For the surfaces we have studied, bare Au and Au coated by a self-assembled monolayer of 1-octadecanethiol (ODT), this comparison between experiment and theory for the amplitude of the photoacoustic waves suggests that ± values for bare and ODT-terminated Au are similar and ± > 0.3. This conclusion is tentative, however, because the phases of the photoacoustic waves show systematic differences that are not predicted by the model. For tetrafluoroethane vapor (R-134a refrigerant), the photoacoustic signal generated by a Au surface coated with a hydrophilic (COOH-terminated) self-assembled monolayer is a factor of 2 larger than the photoacoustic signal generated by a hydrophobic (CH 3-terminated) monolayer. We also report measurements of the ultrafast ellipsometry signals generated by the sudden desorption of physisorbed methanol and water on hydrophobic and hydrophilic self-assembled monolayers.
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