absorbance
- (A) common logarithm of the reciprocal of the transmittance of a pure solvent
- also called optical density
- directly proportional to the concentration
absorption spectrum
- graph showing how (A) varies with wavelength
- absorption has higher energy than fluorescence and phosphorescence
Beer's Law
- the heart of spectrophotometry of applied analytical chemistry
- The mathematical relationship between concentration and absorbance
- fails in concentrated solutions
chemiluminescence
- emission of light from a chemical reaction
- ex light from a firefly or light stick
chromophore
the part of the molecule responsible for light absorption
cuvet
- a cell that contains the liquid sample for visable end ultraviolet spectroscopy
- has flat, fused-silica (SiO?) faces or glass
- 1.000 cm path length
derivatization
the chemical alteration of an analyte so that it can be detected or separated easily from other species
electromagnetic spectrum
All of the frequencies or wavelengths of electromagnetic radiation
electronic transition
- an electron from one molecular orbital moves to another orbital, with a constant increase or decrease in the energy of the molecule
- usually involve vibrational and rotational transitions
** electronic absorption bands are usually broad because many di
emission spectrum
- graph of emission intensity vs emission wavelength
- constant ?ex and variable ?em
- measure emitted radiation rather than fraction of incident radiation striking the detector
excitation spectrum
- measure by varying the excitation wavelength and measuring emitted light at one particular wavelength (?cm)
- graph of emission intensity vs excitation wavelength; look slike absorbtion spectrum
- constant ?em and variable ?ex
excited state
when a molecule absorbs a photon and the energy of the molecules increases
fluorescence
- emission of a photon during a transition between states with the same spin quantum numbers
- radiational transition S?? S?
- relatively rare
- lifetime: 10?? to 10??s
frequency
(1/second) number of complete oscillations that a wave makes each second
ground state
the lowest energy state of a molecule
hertz
one oscillation per second
irradiance
- (P) the energy/time unit area in the light beam
- watts per square meter (W/m�)
- also called intensity or radiant power
luminescence
- emission of light from an excited molecule
- generally more sensitive than absorption - sensitive enough to observe single molecules
masking
- when reagens like neocuprine or thiourea form strong complexes that prevent Cu�? from reacting with ferrozine
molar absorptivity
- characteristic of a substance that tells how much light is absorbed in a particular wavelength
- also called extinction coefficient
- units: M?� cm?�
molecular orbital
describe the distribution of electrons in a molecule
monochromatic light
- consists of 1 color (one wavelength)
- the better the ______________, the narrower the range of wavelengths in the emerging beam
monochromator
- what light is passed through to select one wavelength
- ex. prism, grating or filter
phosphorescence
- emission of a photon during transition between states with different spin quantum numbers
- radiational transition T?? S?
- has lower energy than fluorescence; energy comes at longer wavelengths
- relatively rare
- lifetime: 10?? to 10�s; involves chang
photon
a particle of electromagnetic radiation having zero mass and carrying a quantum of energy
reagent blank
- contains all reagents but with analyte replaced by distilled water
- important in most spectrophotometric analyses
- must be subtracted from the absorbance of samples and standards before doing any calculations
refractive index
(n), a measure of the light-bending ability of a medium
- for visible wavelengths, n>1
- when light moves between media with different refractive indexes, frequency remains constant but wavelength changes
rotational transition
changes in the rotational states
self-absorbtion
- absorption of excitation or emission energy by analyte molecules in the solution
- also called the inner filter effect
singlet state
- the state in which the spins are opposed
- S?= lowest energy state
- not split by a magnetic field
spectrophotometry
- any technique that uses light to measure chemical concentrations
transmittance
- (T) the fraction of the original light that passes through the sample
- ranges from 0-1
triplet state
- if the spins are parallel
- T?= lowest energy state; T?< S?
- split into 3 slightly different energy levels in a magnetic field
vibrational transition
- change in the vibrational state
- usually involve simultaneous rotational transistions
wavelength
(?) crest to crest distance between waves
wavenumber
(v=1/?)
The reciprocal of wavelength; the number of waves in a unit length or distance per cycle
nonbonding orbital
(n) highest energy occupied orbital
- ex. formaldehyde: mostly O
antibonding orbital
(pi*) lowest energy unoccupied orbital
- produce repulsion, rather than attraction for the C & O atoms
vibrational relaxation
- usually the first process after absorbtion
- relaxation to the lowest vibrational level of S1
radiationless transition
- vibrational energy is transferred to other molecules through collisions, not by emission of a photon
- net effect: convert part of energy of absorbed photon into heat throughout medium
internal conversion
- when a molecule enters a highly excited vibrational level of So, having the same energy as S1
- radiationless transition between states with the same spin quantum numbers (S?? S?)
intersystem crossing
- radiationless transition between states with different spin quantum numbers (T?? S?)
vibrational energy
- observed in non-polar or less polar solvents
- broadened beyond recognition in polar solvents
solution phase spectra
- broadened absorption spectrum
- absorbing molecules are surrounded by solvent molecules with a variety of orientations that create slightly different energy levels for different absorbing molecules
simple gas phase molucules
not in close contact with neightbos, limited # of energy levels
- have extrememly sharp absorptions
essentials of a luminescence experiment
- sample is irradiated at 1 wavelength
- emission is observed over a range of wavelengths
- excitation monochromator selects teh excitation wavelength
- emission monochromator selects one wavelength at a time to obserce