Heat Transfer at Solid–Gas Interfaces by Photoacoustics at Brillouin Frequencies

• Published in: Journal of physical chemistry. C 2012-05, Vol.116 (20), p.10896-10903
• Main Authors: Min, Chang-Ki, Chen, Kejia, Bae, Sung Chul, Cahill, David G, Granick, Steve
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Optical instruments, equipment and techniques; Physics; Polarimeters and ellipsometers
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp302726u

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 α 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 α values for bare and ODT-terminated Au are similar and α > 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 (CH3-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.