Plasmonic Sensing of Ultrafast Evaporation and Condensation

We use nanoscale plasmonic structures and ultrafast pump-probe spectroscopy to study the evaporation and condensation of an adsorbed layer of 2-chloro-1,1,1,2-tetrafluoroethane (R124) on Au surfaces functionallized by self-assembled monolayers. silica substrate is patterned with Au nanodisks with a diameter of 120 nm and thickness of 20 nm that have plasmonic resonances near the Ti: sapphire laser wavelength of 800 nm. The sensor is placed in contact with a vapor of R124 at room temperature and pressures between 0 and approximately half of the saturation pressure, 25 psi. The pump optical pulse heats the nanodisk and the changes in the thickness of the adsorbed layer are probed by transient absorption on timescales of picoseconds to nanoseconds. Due to the small thickness of the adsorbed layer, evaporation and condensation is limited by mass exchanged at the vapor-liquid interface and not by heat transport. We describe the relationships between the amplitude A and timescale tau of the response, thermodynamic parameters, thickness of the adsorbed layer, and effective evaporation coefficient eta. In particular, we show that the ratio A/tau is proportional to the product of the isosteric heat of adsorption and eta. The data are consistent with eta similar to 1.