Learn: Optical Spectroscopy Lexicon

Absorbance ($A$)

A logarithmic measure of light absorbed: A = -\n\log(T) = \log\left(\frac{P_0}{P}\right).

Absorption Spectrum

A plot of molar absorptivity or absorbance versus wavelength or wavenumber.

Beer's Law

Absorbance is directly proportional to concentration ($c$) and pathlength ($b$): $A = \epsilon \cdot b \cdot c$.

Electromagnetic Spectrum

The entire range of electromagnetic radiation classified by wavelength or frequency.

Frequency ($\nu$)

The number of wave oscillations per second, linked to speed of light by $c = \nu \cdot \lambda$.

Hertz ($Hz$)

The SI unit of frequency, equal to one cycle or oscillation per second ($s^{-1}$).

Irradiance / Radiant Power

The energy per unit area per second delivered by an electromagnetic beam.

Monochromatic Light

Radiation consisting of a single, isolated wavelength or extremely narrow band.

Polychromatic Light

Radiation consisting of multiple different wavelengths.

Photon

A discrete packet of electromagnetic energy governed by $E = h \cdot \nu$.

Refractive Index ($n$)

Ratio of the speed of light in a vacuum to its speed in a medium: $n = \frac{c}{v}$.

Transmittance ($T$)

The fraction of incident light power passing through a sample: $T = \frac{P}{P_0}$.

Wavelength ($\lambda$)

The physical distance between consecutive peaks or troughs of a wave.

Wavenumber ($\bar{\nu}$)

The reciprocal of wavelength ($\bar{\nu} = \frac{1}{\lambda}$), typically in $cm^{-1}$ for infrared or Raman.

Chromophore

The specific structural part of a molecule responsible for absorbing light.

Electronic Transition

Promotion of an electron from an occupied orbital to an unoccupied anti-bonding orbital.

Excited State

A high-energy state achieved when a molecule or atom absorbs energy.

Fluorescence

Radiative relaxation where an excited singlet state ($S_1$) emits a photon to return to $S_0$.

Ground State

The lowest, most stable energy state where electrons occupy lowest orbitals.

Luminescence

Emission of light from electronically excited states, including fluorescence and phosphorescence.

Molecular Orbital

Orbital formed by hybridizing atomic orbitals describing electron probabilities.

Phosphorescence

Radiative relaxation via intersystem crossing from $S_1$ to triplet state $T_1$, with longer lifetimes.

Quantum Yield ($\phi$)

Ratio of emitted photons (via luminescence) to absorbed photons.

Quenching

Process or chemical interaction decreasing fluorescence intensity by competing with relaxation.

Rotational Transition

Low-energy transitions induced by microwaves that alter rotational motion.

Self-Absorption

Attenuation of emitted luminescence when high analyte concentrations reabsorb energy.

Singlet State ($S$)

An electronic state where all electron spins are paired (net spin = $0$).

Triplet State ($T$)

An electronic state where two electron spins are unpaired and parallel.

Vibrational Transition

Mid-energy transitions induced by infrared radiation altering bond stretching and bending.

Bioluminescence

Chemiluminescence occurring within living organisms via enzyme-catalyzed reactions.

Chemiluminescence

Light emission produced directly by a chemical reaction without external optical excitation.

FRET

Non-radiative dipole-dipole energy transfer from an excited donor to a nearby acceptor.

Photobleaching

Permanent chemical destruction of a fluorophore's ability to luminesce from light exposure.

Raman Scattering

Inelastic light scattering interacting with molecular vibrational modes with altered wavelengths.

Rayleigh Scattering

Elastic light scattering where photons bounce off molecules without changing wavelength.

ATR

Infrared sampling technique using total internal reflection inside a high-index crystal.

Bandwidth

The range of wavelengths passed through a monochromator's exit slit.

Beam Chopping

Mechanical modulation alternating light between sample and blank to eliminate flicker noise.

Blackbody Radiation

Continuous thermal radiation emitted by a heated solid object based solely on temperature.

Charge Coupled Device (CCD)

Solid-state semiconductor array detector storing charges in 2D pixels for simultaneous collection.

Colorimeter

Simple optical instrument using broad filters to measure absorbance at specific visible wavelengths.

Cuvette

Specialized sample cell made of transparent glass, plastic, or quartz.

Diffraction

Angular bending and spreading of light waves when encountering apertures or lines.

Digital Smoothing

Mathematical algorithms applied to digitized spectra to reduce baseline noise.

Dispersion

Angular separation of light into constituent wavelengths by a prism or grating.

Ferroelectric Material

Temperature-sensitive pyroelectric material used as an infrared detector.

Fourier Analysis

Mathematical transformation converting an interferogram into a frequency-domain spectrum.

Diffraction Grating

Optical mirror ruled with parallel lines that disperses light via diffraction and interference.

Greenhouse Gas

Atmospheric gases like $CO_2$ that absorb infrared radiation via vibrational modes.

Interferogram

Plot of detected light intensity versus optical path difference from an interferometer.

Interferometer

Assembly of beamsplitter and mirrors splitting and recombining light to produce interference.

Laser

Source of intense, coherent, monochromatic radiation produced by stimulated emission.

Monochromator

Optical system with slits and a dispersing element to isolate a narrow band of wavelengths.

Optical Fiber

Transparent glass or plastic strand transmitting light via total internal reflection.

Optode

Chemical sensor on an optical fiber tip whose properties change in an analyte's presence.

Photoconductive Detector

Solid-state IR detector whose electrical conductivity changes upon absorbing IR photons.

Photodiode

Semiconductor junction detector producing electrical current proportional to light intensity.

Photodiode Array (PDA)

Linear array of photodiodes recording an entire spectrum simultaneously.

Photomultiplier Tube (PMT)

Ultra-sensitive detector where photoelectrons are amplified through a dynode cascade.

Phototube

Evacuated tube detector emitting photoelectrons via the photoelectric effect.

Photovoltaic Detector

Semiconductor device generating a voltage across a p-n junction when exposed to radiation.

Polychromator

Optical instrument dispersing light across a focal plane for multi-wavelength array detection.

Refraction

Bending of light as it passes between media of differing refractive indices.

Resolution

Minimum wavelength separation required to distinguish adjacent spectral lines.

Resolving Power ($R$)

Measure of ability to separate close wavelengths: $R = \frac{\lambda}{\Delta\lambda}$.

Root-Mean-Square Noise

Statistical metric quantifying the magnitude of random signal noise.

Signal Averaging

Noise reduction technique summing $n$ independent scans to improve SNR by $\sqrt{n}$.

Snell's Law

Equation relating incidence and refraction angles: $n_1 \cdot \sin(\theta_1) = n_2 \cdot \sin(\theta_2)$

Stray Light

Unwanted radiation at wavelengths outside the isolated band, causing negative absorbance errors.

Surface Plasmon Resonance

Optical sensing technique detecting binding events via shifts in minimum reflectance angle.

Thermocouple

Thermal detector in IR spectrophotometry converting temperature changes to electric potential.

Waveguide

Structure confining and transmitting light along a path by total internal reflection.

Derivatization

Chemically modifying an analyte to improve chromatographic behavior or detection.

Discrete Analyzer

Automated analytical instrument performing assays in isolated reaction vessels.

Flow Injection Analysis

Automated technique injecting sample plugs into a continuous unsegmented carrier stream.

Immunoassay

Analytical method relying on high-affinity antibody-antigen binding to quantify molecules.

Isosbestic Point

Wavelength where two chemical species share identical molar absorptivity.

Microplate

Flat multi-well plastic plate used for high-throughput automated assays.

Molar Absorptivity ($\epsilon$)

Constant describing how strongly a solute absorbs light, in $M^{-1}cm^{-1}$.

Multivariate Calibration

Algorithms using absorbance data across multiple wavelengths to determine mixture concentrations.

Reagent Blank

Reference solution containing all reagents except analyte, used to set zero absorbance.

Sequential Injection

Automated flow analysis where sample and reagents are sequentially aspirated before mixing.

Spectrophotometric Titration

Titration where the endpoint is determined by monitoring light absorbance changes.

Spectrophotometry

Quantitative measurement of light reflection, transmission, or absorption as a function of wavelength.

Laser Ablation

Using a pulsed laser beam to vaporize material directly from a sample surface into an aerosol.

Aerosol

Fine suspension of liquid droplets or solid particles in a carrier gas stream.

Atomic Absorption (AA)

Elemental analysis measuring absorption of resonance lines by ground-state atoms in a flame/furnace.

Atomic Emission (AE)

Elemental analysis measuring light emitted by thermally excited atoms or ions.

Atomic Fluorescence (AF)

Elemental analysis where gaseous atoms absorb radiation and re-emit fluorescent light.

Atomization

Mechanical and thermal process of breaking a sample down into individual gaseous atoms.

Background Correction

Instrumental techniques to subtract matrix absorption or light scattering from atomic signals.

Boltzmann Distribution

Equation predicting population ratio of excited to ground-state atoms as a function of temperature.

Chemical Interference

Interference caused by chemical reactions preventing analyte atomization.

Detection Limit

Lowest concentration yielding a signal equal to twice peak-to-peak baseline noise.

Doppler Effect

Spectral line broadening caused by thermal motion of absorbing/emitting atoms.

Graphite Furnace

Electrically heated graphite tube atomizing micro-volume samples with ultra-high sensitivity.

Heisenberg Uncertainty Principle

Physical principle contributing to natural atomic linewidths due to finite excited state lifetimes.

Hollow-Cathode Lamp

Element-specific radiation source emitting sharp resonance lines upon inert gas bombardment.

Inductively Coupled Plasma (ICP)

High-temperature ($\sim 10,000\,K$) argon gas plasma generated by radiofrequency coils.

Ionization Interference

Interference when thermal energy ionizes alkali metals, reducing neutral ground-state population.