Absorbing particles transfer

Thus, the major conclusions of tiie early studies by Volkenshtein and his colleagues applicable to the theory of the method of semiconductor gas sensors are the following a) chemisorption of particles on a semiconductor surface can be accompanied by a charge transfer between adsorption-induced surface levels and volume bands of adsorbent and b) only a certain fraction of absorbed particles is charged, the fraction being dependent on adsorbate and adsorbent. 

The fact of a transfer of an electron from an absorbed particle to adsorbent [25] is widely considered as a criterion to differentiate between various forms of adsorption. Yet, as it has been already mentioned in previous section, there is a neutral form of chemisorption, i.e. weak binding formed without changing the surface charge state which only affects the dipole component of the work function. On the other hand, in several cases the physical adsorption can result in electron transitions in solids. Indeed, apart from formation of a double layer, changing the work function of adsorbent [26] the formation of surface dipoles accompanying physical adsorption can bring free charge carriers to substan-... 

In this paper we review the work performed on absorbing particles. Both Gaussian and Laguerre-Gauss modes are used. We outline a model for the trapping of absorbing particles for these beam types based on the effects of transfer of momentum from the beam to the particle. We also consider in some detail the heating effects. 

In our experiments using an optical tweezers set-up, only slightly modified compared to the conventional system described above, we have shown that using phase singular fields we can not only trap absorbing particles but also set them into rotation. This demonstrates the transfer of the angular momentum from the light beam to the particles. 

The transfer of momentum from the beam to an absorbing particle is therefore straightforward compared to other cases, such as transparent particles and atoms. For transparent particles, refraction and induced polarisation must be taken into account. For an atom, the frequency dependence of the absorption and spontaneous emission must be considered, while for an absorbing particle, the absorption can be assumed to be independent of frequency, and inter-atomic collision rates within the particle can be assumed to be high enough to cause deexcitation without re-emission. 

M.E.J. Friese, H.He, N.R. Heckenberg and H. Rubinsztein-Dunlop Transfer of angular momentum to absorbing particles from a laser beam with a phase singularity in N.B. Abraham and Y.I. Kanin (eds) Laser Optics 95 Nonlinear Dynamics in Lasers Proc SPIE 2792 190 -195(1996)... 

As noted above, the phenomenological two-fiux theories that have been developed on the basis of the radiation transfer equation can be considered continuum theories. Continuum theories consider the absorption and scattering coefficients as properties of an irradiated isotropic layer of infinitesimal thickness. On the other hand, discontinuum theories consider layers containing a collection of particles. Consequently, the thickness of a layer is dictated by the size of the scattering and absorbing particles. Optical constants can then be determined from the scattering and absorption properties of these particles. 

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