Thermal accommodation

Thermal accommodation 1st stage of chemisorption molecular transient Dissociative chemisorption with formation of hot transient O 5 (s)... 

Support for the special reactivity of hot oxygen adatoms also came from Matsushima s temperature-programmed desorption study4 of CO oxidation at Pt(lll), when CO and 02 were coadsorbed at low temperature with C02 desorbed at 150 K, the temperature at which 02 dissociates. This temperature is some 150 K lower than that for C02 formation when oxygen is preadsorbed (thermally accommodated) at the Pt(lll) surface. 

The diode laser is scanned up and down in frequency by a triangle wave, so that the scan should be linear in time and have the same rate in both directions. In the thermal accommodation coefficient experiments, the external beam heats the microsphere to a few K above room temperature and is then turned off. The diode laser is kept at fairly low power ( 7 pW) so that it does not appreciably heat the microsphere. Displacement of a WGM s throughput dip from one scan trace to the next is analyzed to find the relaxation time constant as the microsphere returns to room temperature. Results from the two scan directions are averaged to reduce error due to residual scan nonlinearity. This is done over a wide range of pressures (about four orders of magnitude). The time constant provides the measured thermal conductivity of the surrounding air, and fitting the thermal conductivity vs. pressure curve determines the thermal accommodation coefficient, as described in Sect. 5.5.2. 

Fig. 5.6 Pressure dependence of thermal conductivity of air, measured using a PDDA coated microsphere of effective radius 298 pm. The fit to (5.11), shown as the curve, gives a thermal accommodation coefficient of 0.92 for air on PDDA. Reprinted from Ref. 5 with permission. 2008 International Society for Optical Engineering...

The sensing methods summarized thus far are intended for absorption detection of molecules in the ambient, but molecules (or indeed thin films) on the microresonator surface can also be detected. In particular, if the surface is covered to such an extent that the optical energy absorbed heats the microresonator, the resulting thermal bistability in the frequency-scan response can be used to determine the absorption and/or thickness of the thin-film coating. This and surface characterization by measurement of the thermal accommodation coefficient were described in Sect. 5.5. These methods offer quite precise measurement, provided that certain reasonable and easily implemented assumptions are satisfied. 

Saxena, S. C. Joshi, R. K., Thermal accommodation and adsorption coefficients of gases, In Vol. II 1 of McGraw Hill/CEMDAS Data Series on Material Properties Touloukian, Y. S. Ho, C. Y Eds. McGraw Hill, New York, NY, 1981... 

Analogous to the slip velocity between gas and particle at Kn above the continuum flow range discussed in Section A above, a temperature discontinuity exists close to the surface at high Kn. Such a discontinuity represents an additional resistance to transfer. Hence, transfer rates are generally lowered by compressibility and noncontinuum effects. The temperature jump occurs over a distance 1.996kl 2 — a )/Fva k + 1) (K2, Sll) where is the thermal accommodation coefficient, interpreted as the extent to which the thermal energy of reflected molecules has adjusted to the surface temperature. 

According to the generally accepted mechanism for growth from the vapor phase, a fraction of molecules or atoms incident on a substrate are adsorbed. Those molecules or atoms that are not adsorbed are reflected back into the gas phase. The fraction reflected depends on the energy of the incident species and on the thermal-accommodation characteristics of the substrate. 

Mass Accommodation Coefficient. For a given molecule the mass accommodation coefficient is a physical constant which depends only on the temperature and on the nature of the liquid surface. The process of the molecule entering the liquid phase might proceed as follows. Since the surface of water is non-rigid it is likely that a molecule which strikes the surface achieves thermal accommodation with near-unit probability. The molecule is bound to the surface in a potential well of depth aU, where aUs is the binding energy of the molecule to the liquid surface. 

Accurate modeling is only possible by the consideration of wavelength-dependent optical and temperature-dependent thermodynamic parameters and the correct application of the thermal accommodation coefficient which is dependent on the ambient particle conditions and is described in detail elsewhere (Schulz et al., 2006 Daun et al., 2007). Moreover, Michelsen (2003) suggested the inclusion of a nonthermal photodesorption mechanism for heat and mass loss, the sublimation of multiple cluster species from the surface, and the influence of annealing on absorption, emission, and sublimation. A more general form of the energy equation including in more detail mass transfer processes has been derived recently by Hiers (2008). For practical use, Equation (1) turns out to be of sufficient physical detail. 

The rate coefficient km would contain factors for the thermal accommodation of a molecule in collision with the filament, the geometry of the filament and the trap, and the efficiency of capture of thermally excited molecules by the trap. The observed fall in pressure in a static atomisation reaction, occurring at the rate ua = kaP21/2 for a given filament temperature, would then be... 

The observed RID s can be explained if we assume that just after switching off the discharge, the gas is at a slightly higher temperature (AT< 10mK) than the walls and cools down to T in 10-15 s. Even a small AT should show up as a substantial RID due to the strong temperature dependence of Vy. However, on the basis of a computer simulation of the decay we expect an exponential temperature dependence of the thermal accommodation time. This is not in line with the measurements, moreover the observed RID s are larger than we expect. This point deserves further study. 

Hurst et al. attributed the second of these spectral peaks (the peak occurring at later times) to Xe atoms which had accommodated to the surface (i.e. come into equilibrium with the surface at a surface temperature of Ts) and subsequently desorbed from the surface. The other peak present in the spectra was attributed to Xe atoms that had directly scattered, without thermal accommodation, from the surface. 

Figure 3 Data adapted from Hurst et al. [24] for Xe on Pt(l 11). Time of flight (TOF) spectra obtained at a Pt(l 11) surface temperature of Ts = 185 K. Curve (a) Incident Xe beam with, Emg = 0.14eV Curve (b) Xe beam scattered at 0° Curve (c) Xe beam scattered at 45° Curve (d) Xe beam scattered at 75°. First peak attributed to scattered Xe atoms and second peak attributed to thermally accommodated, desorbing Xe atoms.

The parameter Tr is the temperature corresponding to the half-range Maxwellian distribution of the particles reflected diffusively. The temperature 7 r is related to the temperature of the reflecting surface through the thermal accommodation coefficient E, defined as... 

In his original derivation Brock used a value of % for Cs, the thermal slip coefficient. More recent data by Ivchenko and Yalamov (1971) have shown that Cs = 1.147 for complete thermal accommodation. In Brock s equation... 

As mentioned earlier, the factor a is the thermal accommodation coefficient and am the momentum accommodation or reflection coefficient. From the data of Rosenblatt and LaMer (1946), Schmitt (1959), and Keng and Orr (1966), as a first approximation a value of 1.25 seems reasonable for Cm, whereas for Ct a value of 2 is a good approximation (Brock, 1962b). These numbers then imply values of 0.89 for am and 0.97 for at. 

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