Infrared Radiation Spectrum
In the infrared (IR) spectroscopic range (200–4000 cm−1), radiation generally characterized by its wavenumber ν (cm−1), related to the wavelength λ (µm), frequency, ν (s−1), and angular frequency ω (s−1) as
ν = 1 /λ = ω/2πc
where c =2.99793×108 m·s−1 is the velocity of the electromagnetic spectrum in a vacuum.
Photons Frequencies
When IR radiation containing a broad range of frequencies passes through a sample, which represented as a system of oscillators with resonance frequencies ν0, then according to the Plank equation,
E = hν
where ΔE = difference between the energy of the oscillator in the excited and ground states, ν = the frequency of photons and h = 6.626069×10−34 J s, Planck’s constant, photons with frequencies ν = ν0, absorbed.
These photons will be eliminated from the initial composition of the radiation. Since all the elementary excitations have unique energy levels.
Measurements of the disappearing energy as a function of ν (atomic spectrum of the sample) enable these excitations to be identified, and microscopic information about the sample (e.g., molecular identity and conformation, intra- and intermolecular interactions, or crystal field effects, etc.) may be obtained.
Infrared (IR) Spectra of Ultrathin Films
The primary characteristics that one identifies from infrared (IR) spectra of ultrathin films for further analysis are the resonance frequencies, oscillator strengths (extinction coefficients), and damping (bandwidths), related to different kinds of vibrational, translational, and frustrated rotational motion inside the thin-film material.
However, the microscopic processes inside or at the surface of a film (motion of atoms and electron particles) give rise to the frequency dependence (the dispersion) not only of the extinction coefficient but also of the refractive index of the film.
As a result, a real IR spectrum of an ultrathin film is, as a rule, distorted by so-called optical effects. Specifically, the spectrum strongly depends upon the conditions of the measurement, the film thickness, and the optical parameters of the surroundings and substrate impeding the extraction of physically meaningful information from the spectrum.
Thus after the introduction of the nomenclature accepted in optical spectroscopy and a brief discussion of the physical mechanisms responsible for absorption by crystalline solid on a qualitative level, this introductory chapter will concentrate on the basic macroscopic or phenomenological theory of the optical response of an ultrathin film immobilized on a surface or at an interface.
