Vocab 4 Flashcards
The ability to separate adjacent absorption bands or spectral lines; equivalent to the spectral bandpass or amount of energy exiting a monochromator at any one time.
Resolution
The amount of radiation exiting a monochromator relative to the amount that enters the monochromator.
throughput
a general optical, acoustic, or radio frequency instrument that uses interference phenomena between a reference wave and an experimental wave, or between two parts of an experimental wave, to determine wavelengths and wave velocities, measure very small distances and thicknesses, or calculate indices of refraction.
interfoerometer
an optical instrument that produces interference fringes by dividing the source radiation with a beamsplitter such that one beam is directed into an arm of the interferometer and strikes a fixed mirror, while a second beam is directed into a different arm of the interferometer and strikes a movable mirror. When the reflected beams are brought back together, an interference pattern (the interferogram) results.
Michelson Interferometer
An optical material that is partially transmitting and partially reflecting
beamsplitter
The optical path difference between radiation in the two arms of a Michelson interferometer
optical retardation
Two electromagnetic waves having a phase difference of 180° or 180° plus an integer multiple of 360°
destructive interference
The encoding of a carrier wave by varying its frequency in accordance with an input signal.
modulation
A recording of the interference pattern generated by an interferometer.
interferogram
When radiation of multiple wavelengths is emitted by a source, the measure is the summation of the individual interferograms corresponding to each wavelength in the spectrum.
polychromatic interferogram
In a Michelson interferometer, the term that contains information about the intensities of all the frequencies that have been modulated by the interferometer.
spectrum
The point of zero path difference in an interferometer, i.e. OM = OF; in a polychromatic interferogram it is the one point at which all wavelengths constructively interfere.
centerburst
The reduction in measurement time that results from measuring all the source radiation wavelengths simultaneously in a Fourier transform instrument. demonstrated by the signal-to-noise improvement of an FT instrument over a dispersive instrument
Fellgett’s advantage / multiplex advantage
The number of measurement subdivisions, n, contained in a spectral range between wavenumber limits
resolution elements
increased signal-to-noise ratio that results from the increases signal strength observed by the detector; related to the optical throughput of an interferometric IR spectrometer θI relative to the optical throughput of a grating IR spectrometer
jacquinot’s advantage / throughput advantage
The inherent precision in the wavenumber scale of Fourier transform IR spectrometers compared to dispersive IR instruments; this inherent precision is due to the referencing of the mirror position of the moving mirror in the interferometer to an internal laser source, thereby allowing very precise measurement of d, the optical retardation
cone’s advantage / precision advantage
The process of finding a weighting function that minimizes the distortions produced by truncation of the interferogram in experimental Fourier transform infrared spectroscopy.
apodization
default truncation function in experimental interferograms that induces distortions in the resultant FT-IR spectra.
boxcar apodization function
One of the most common of the interferogram apodization functions used to minimize distortions in FT-IR spectra
triangular apodization function
Any waveform that is a sinusoidal function of time or distance can be digitized unambiguously using a sampling frequency equal to twice the bandwidth of the system. Required digitization frequency and time can be expressed as
nyquist sampling theorem
The point where the sinusoidal output signal of the reference He-Ne laser crosses the 0 voltage level in the detector electronics of an FT-IR instrument. For a 632.8 nm He-Ne laser, each zero- crossing occurs at 316.4 nm, which corresponds to the retardation distance of the moving mirror between zero-crossings.
zero-crossing
A spectroscopic instrument in which the background reference or blank is obtained separately and independently from the sample. The background is usually stored in a computer file for subsequent calculation of absorbance, transmittance, etc
single-beam spectrophotometer
Interferogram obtained of the reference, which is then Fourier transformed to obtain the single beam background spectrum.
background interferogram
interferogram obtained of the sample which s then fourier transformed to obtain the single beam sample spectrum
sample interferogram
Law of elastic behavior, which states that a mass will experience a restoring force that is proportional to its displacement from equilibrium
Hooke’s Law
system that, when displaced from its equilibrium position, experiences a restoring force, f, proportional to the displacement from equilibrium
Harmonic Oscillator
Measure of the strength of a covalent molecular bond
force constant
The natural frequency of oscillation of an harmonic oscillator
harmonic oscillator vibrational frequency
The equilibrium vibrational energy associated with the frequency of the harmonic oscillator
harmonic oscillator wavenumber
As used in the harmonic oscillator frequency and wavenumber equations; for two
atoms with masses m and m
reduced mass
Quantized energy levels for the simple harmonic oscillator, obtained by substituting the harmonic oscillator potential energy function into the Schrödinger
equation
Harmonic Oscillator Vibrational Energy Levels
scalar quantum number that defines the energy state of an harmonic
or anharmonic vibrating molecular system; can take on values
vibrational quantum number
Lowest possible vibrational energy
Zero-Point Energy
Any potential energy function that exhibits non-harmonic oscillator, non-parabolic behavior for a diatomic molecular system; approximates a more realistic potential energy function;
anharmonic oscillator
Empirical mathematical function that satisfies the conditions of anharmonic oscillator behavior
morse potential
Quantized energy levels for the anharmonic oscillator, obtained by substituting the Morse potential energy function into the Schrödinger equation
anharmonic oscillator vibrational energy levels
Theoretical vibrational energy for the anharmonic oscillator at the equilibrium internuclear position
equilibrium vibrational wavenumber
measure of the deviation of anharmonic energy levels from harmonic energies
anharmonicity constant
An independent mode of motion of one of N atoms in a polyatomic molecule
degrees of freedom
Non-linear molecules lose 3 degrees of freedom for 3 all-atom, whole-molecule translations along the 3 Cartesian laboratory axes, as well as 3 degrees of freedom for all-atom, whole-molecule rotations about these axes, resulting in 3N – 6 possible fundamental molecular vibrations for non-linear polyatomic molecules.
Vibrational Modes in Non-Linear Polyatomic Molecules.
Linear molecules lose 3 degrees of freedom for all- atom, whole-molecule translations along the 3 Cartesian laboratory axes, but lose only 2 degrees of freedom for all-atom, whole-molecule rotations, since there is no discernable rotation along the major long axis of a linear molecule, resulting in 3N – 5 possible fundamental molecular vibrations for a linear polyatomic molecule.
Vibrational Modes in Linear Polyatomic Molecules
A vibrational motion in which all the atoms in a group move in-phase and with the same frequency; can be defined as the synchronous motion of atoms, or groups of atoms, that may be excited without leading to the excitation of any other normal mode.
normal mode of vibration
The product of the distance, d, separating two charges of equal magnitude and the
magnitude of the charge,
dipole moment
describes the symmetry present in molecules and the classification of molecules according to their symmetry. Symmetry also refers to the types of symmetry operations, e.g. rotations, reflections, etc., that can be applied to a molecule.
molecular symmetry
The case in which two or more normal modes have identical vibrational energies
degenerate vibrations
The empirical observation that vibrational bands are consistently produced by sub- molecular groups of atoms in characteristic wavenumber regions of the IR spectrum; arise due to the coupling of oscillating dipole moments and can be very sensitive to the configuration and conformation of the local molecular environment.
group frequencies
Common method of IR analysis analogous to standard UV/Vis absorption spectroscopy; Beer’s Law is applicable in IR spectral regions; however, when performing IR measurements there is a need to consider the unique IR transmission requirements for optical materials and solvents, as well as the use of short pathlength (10 – 100 μm) cells.
transmission IR spectroscopy
Type of reflectance spectroscopy in which IR radiation approaches the interface between two media of different refractive indices from the high refractive index medium side (i.e., the ATR crystal); if the angle of incidence of the incoming radiation is above a critical angle, total internal reflectance occurs; samples placed at the interface of the ATR crystal absorb IR wavelengths depending on their absorption spectra leading to attenuation of those wavelengths in the reflected radiation, hence the name attenuated total reflectance spectroscopy.
Internal Reflectance / Attenuated Total Reflectance IR Spectroscopy
The minimum angle of incidence that the incoming IR radiation must achieve relative to the interface between two media of different refractive indices to lead to total internal reflection,
critical angle
The standing wave electric field intensity that penetrates into the second phase of lower refractive index at the interface of the ATR crystal under conditions of total internal reflectance; the strength
of the electric field is a decreasing exponential function of the distance z from the surface
evanescent wave
The distance from the surface of the ATR crystal that the evanescent wave penetrates into the second phase of lower refractive index; defined as the distance from the sample-crystal interface where the intensity of the evanescent wave falls to 1/e of its original value
depth of penetration
IR specular reflection from reflective surfaces;Snell’s Law applies; can be used as a non-destructive method for analysis of the optical properties of surfaces and interfaces; can also be used to measure IR spectra of thin films on a reflective substrate, e.g. Ag or Au; highly sensitive, can be used to analyze thin films from monomolecular layers (10’s of Å’s) to several μm thick; known as IR reflection-absorption spectroscopy (IRRAS) if used for thin films; polarized IRRAS used for orientation analysis.
External Reflectance IR Spectroscopy
Effective method of analyzing powdered samples with little or no sample preparation; diffuse scattering of radiation occurs when radiation strikes a sample of a finely divided powder and penetrates slightly into the sample before re-emerging as reflected radiation; intensity of diffusely reflected radiation is isotropic; quantitative analysis is more difficult than standard IR sampling methods, as diffuse reflectance does not follow either normal absorbance (i.e. Beer’s Law) or reflectance (i.e. Snell’s Law) models.
diffuse reflectance IR spectroscopy
Widely used model that relates the intensity of the measured diffuse reflectance IR spectrum to sample concentration
Kubelka-Monk Equation
predicts a linear relationship between spectral
intensity and sample concentration when the scattering coefficient is a constant the sample is diluted in a non-absorbing scattering matrix such as KBr.
Kubelka-Monk model
first investigated by Alexander Graham Bell in 1880’s; photoacoustic effect based on light absorption by a sample in an enclosed cell with a gas; absorption of radiation leads to sample heating which is reflected in pressure fluctuations in the gas; frequency-dependent pressure fluctuations detected using sensitive microphone detection; not a widely used sampling method – primarily used to samples that are difficult to analyze by other methods.
Photoacoustic IR Spectroscopy
analog of atomic emission spectroscopy; increased fraction of samples at elevated temperatures exist in the excited state due to Boltzmann equilibrium, emit IR radiation upon relaxation back to the ground state; sample itself is the source; specialized sampling method not widely used in laboratory settings, but commonly used for environmental monitoring, e.g. CO2 or SO2 from smokestacks; sometimes need specialized IR telescope to increase collection efficiency.
Emission IR Spectroscopy
