Light-particle interaction

The previous sections covered the light-molecule interactions. However, there is a lot more to near-infrared spectroscopy than molecular scale phenomena. In dilute, nonscattering liquid systems, the Beer-Lambert law describes the absorption of the light  

When measurements are performed in transmittance (the light is sent through the sample and the light not absorbed is measured). Beer s law is 

When the medium is not a dilute solution, but a concentrated solution, a colloid, a semisolid, or a solid made of compacted particles, the absorption of the light deviates from Beer s law. Beer s law assumes that when a photon encounters an absorbing particle, it will be either absorbed or transmitted. However, light-particle interaction can also generate a scattering event (forward or backward scatter) when not reflected. In a particulate sample, multiple scattering events can occur, thus generating a 

A lot of care is given in designing the detection geometry to avoid specular reflection (the reflection of light that has not interacted with the sample). 

Dahm and Dahm [19] provide other deviations from Beer s law (no change in the absorbing power of any species due to interactions, using mass to prepare sample over volume fraction, and the use of nonmono-chromatic light) and ask the following question Is there a Beer s law for scattering samples Authors describe experimental setup (very thin samples and very small detectors) for which Beer s law applied. Mark et al. published a very interesting paper on the deviation of Beer s law when mass is used to prepare samples over volume fraction [20]. 

---

Harmonium represents the two-electron Hooke atom. A Hodce diaromic molecule means two heavy particles (nuclei) interacting by Coulomb forces. The same is true with electrons, but the heavy particle-light particle interactions are harmonic. 

The origins and principles of NIR spectroscopy, including early instrumentation, spectroscopic theory, and light-particle interaction... 

Free-space optical manipulation techniques in microfluidic systems have recently generated a significant amount of interest. Classically the advantage of these optical approaches lies in their ability to provide remote operation and handle individual particles directly as opposed to indirect manipulation of the surrounding flow field. The precision with which particles can be transported and separated with these optical techniques makes them particularly useful for bio-medical analysis devices. While extremely flexible and relatively easy to implement (see the section on optofluidics - applications and optofluidics - techniques for fabrication and integration), these systems are ultimately limited by the fundamentals of the free-space optics. Specifically, the light-particle interaction length is... 

P. N. Pusey and R. J. A. Tough, Particle interactions, in Dynamic Light Scattering Applications cfPhoton Correlation Spectroscopy (R. Pecora, ed.), pp. 85-179, Plenum Press, New York (1985). 

When the imposed light beam interacts with the sample, its response must then be detected by the instrument. Generally, for a given detector, when the incident light source is shorter in wavelength, the instrument is more sensitive to smaller particles. A combination of transmitted, forward-scatter and back-scatter detectors and black mirrors increase the accuracy and stability of the instrument and decrease the stray light (for instance in Instruments A and F). The source/de-tector combination defines the effective spectral characteristics of the instrument and the manner in which it responds to a sample. 

---