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Flashcards in Test 2 Deck (21):


is the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited singlet electronic state to a lower state of the same spin multiplicity (e.g., a singlet → singlet transition).



is the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited triplet state to a lower state of a different spin multiplicity (e.g., a triplet → singlet transition).


Resonance fluorescence

is observed when an excited species emits radiation of the same frequency at used to cause the excitation.


A singlet state

is one in which the spins of the electrons of an atom or molecule are all paired so there is no net spin angular momentum


A triplet state

is one in which the spins of the electrons of an atom or molecule are unpaired so that their spin angular moments add to give a net non-zero moment.


Vibrational relaxation

is the process by which a molecule loses its excess vibrational energy without emitting radiation.


Internal conversion

is the intermolecular process in which a molecule crosses to a lower electronic state without emitting radiation.


External conversion

is a radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute.


Intersystem crossing

is the process in which a molecule in one spin state changes to another spin state with nearly the same total energy (e.g., singlet → triplet).



occurs when a molecule changes from a higher electronic state to an upper vibrational level of a lower electronic state in which the vibrational energy is great enough to rupture the bond.



occurs when radiation promotes a molecule directly to a state with sufficient vibrational energy for a bond to break.


Quantum yield

is the fraction of excited molecules undergoing the process of interest. For example, the quantum yield of fluorescence is the fraction of molecules which have absorbed radiation that fluoresce.



is a process by which radiation is produced as a result of a chemical reaction.


Hydrogen vs deuterium lamps

Hydrogen and deuterium lamps differ only in the gases that are used in the discharge. Deuterium lamps generally produce higher intensity radiation.


Filters vs monochromators

Filters provide low resolution wavelength selection often suitable for quantitative work, but not for qualitative analysis or structural studies. Monochromators produce high resolution (narrow bandwidths) for both qualitative and quantitative work.


phototube vs photocell

Detectors. A phototube is a vacuum tube equipped with a photoemissive cathode and a collection anode. The photo electrons emitted as a result of photon bombardment are attracted to the positively charged anode to produce a small photocurrent proportional to the photon flux. A photovoltaic cell consists of a photosensitive semiconductor sandwiched between two electrodes. An incident beam of photons causes production of electron-hole pairs which when separated produce a voltage related to the photon flux. Phototubes are generally more sensitive and have a greater wavelength range. Photocells are in general simpler, cheaper and more rugged. Photocells do not require external power supplies.


photodiode vs pmt

Amplifiers. A photodiode consists of a photo-sensitive pn-junction diode that is normally reverse-biased. An incident beam of photons causes a photocurrent proportional to the photon flux. A photomultiplier tube is a vacuum tube consisting of a photoemissive cathode, a series of intermediate electrodes called dynodes, and a collection anode. Each photoelectron emitted by the photocathode is accelerated in the electric field to the first positively charged dynode where it can produce several secondary electrons. These are, in turn, attracted to the next positively charge dynode to give rise to multiple electrons.


double-beam-in-space vs in-time

Both types of spectrophotometers split the beam into two portions. One travels through the reference cell and one through the sample cell. With the double-beam-in-space arrangement, both beams travel at the same time through the two cells. They then strike two separate photodetectors where the signals are processed to produce the absorbance. With the double-beam-in-time arrangement, the two beams travel at different times through the cells. They are later recombined to strike one photodetector at different times.


spectrophotometers vs photometers

Spectrophotometers have monochromators or spectrographs for wavelength selection.
Photometers generally have filters and use an LED source for wavelength selection. The spectrophotometer can be used for wavelength scanning or for multiple wavelength selection. The photometer is restricted to one or a few wavelengths.


single-beam vs double-beam spectophotometer

A single-beam spectrophotometer employs one beam of radiation that irradiates one cell. To obtain the absorbance, the reference cell is replaced with the sample cell containing the analyte. With a double-beam instrument, the reference cell and sample cell are irradiated simultaneously or nearly so.


multichannel vs conventional spectrophotometers

Multichannel spectrophotometers detect the entire spectral range essentially simultaneously and can produce an entire spectrum in one second or less. They do not use mechanical means to obtain a spectrum. Conventional spectrophotometers use mechanical methods (rotation of a grating) to scan the spectrum. An entire spectrum requires several minutes to procure.