Spectroscopy: Chapter 15 Flashcards
What is Luminescence?
Re-emission of absorbed energy
At what wavelength does fluorescence cease to occur because it breaks bonds?
<250 nm
What are the three spectroscopic methods used for quanatative elemental anaylsis?
- Mass Spectrometry
- Optical Spectrometry
- X-Ray Spectrometry
What is resonance radiation or resonance fluorescence ?
a type of fluorescence where absorbed radiation is reemitted without a change in frequency.
what is Pauli exclusion principle?
No more than two electrons can occupy an orbital and they must have opposed spin states.
what is a singlet state?
a molecular electronic state in which all electron spins are paired.
what is the average lifetime range of an excited triplet state?
from 10-4to several seconds
what is the average lifetime of an excited of an excited singlet state?
~10-8 seconds
describe internal conversion.
when a molecule passes to a lower-energy electronic state without emission of radiation.
what is internal conversion?
a crossover between two states of the same multiplicity (singlet to singlet of triplet to triplet)
Define Intersystem Crossing.
when there is a crossover between electronic states of different multiplicty.
List the different deactivation processes
- Fluorescence/Phosphorescence
- Vibrational, rotational
- Internal Conversion
- External Conversion
- Intersystem Crossing
How do electon donating substitutents affect fluorescence?
it increases fluorescence
how do electron withdrawing substituents affect fluorescence?
It decreases fluorescence.
Why doesn’t fluorescence occur at wavelengths LESS THAN 250nm?
because it breaks bonds
What is an example of the heavy-atom effect?
the decrease of fluorescence with increasing molar mass of halogens.
When is intersystem crossing most common?
in molecules that contain heavy atoms and in paramagnetic species such as molecular oxygens
What is external conversion?
A process in which molecules in electronically excited states pass to a lower electronic state by colliding with other molecules.
**In this process the electronic energy is eventually converted into heat**
How does tempurature effect fluorescence?
increasing temperature causes a decrease in fluorescence which increases the chance of external conversion
how does viscosity effect fluorescence?
decrease in viscosity or zero viscosity results in a decrease in fluorescence
**increases external conversion**
excessive absorption larger than ~0.05 is referred to as ________ ________.
primary absorption
Emission wavelengths overlap with an absorption band is referred to as _______ ________.
Secondary Absorption
Primary and secondary absorption are reffered to as ______ ______ effects.
inner filter
Define *Dynamic (collisional) Quenching. *
nonradiative energy transfer between an excited species and the queching agent
Define Static Quenching
when the quencher and ground state fluorophore form a dark complex
Define Long Range (Förster) quenching
when an energy trasfer occurs without collisons between molecules.
What sources can be used for Fluorescence Spectroscopy
Xe lamps or lasers
What are advantages of using a laser as a source
- can used very little sample (microliter or less)
- can used for highly monochromatic excitation in order to minimize fluorescing interferences
What is the advantage of using a lamp as a source in fluorescence spectrometers
- inexpensive
- determines multiple anayltes with different excitation wavelengths.
What is the difference between an fluorescence emission spectrum and a fluorescence excitation spectrum
in the emission spectrum, the excitation wavelength is held constant and emission instensity is measured as a funcion of emission wavelength
while in the excitation spectrum, the emission is measured at one wavelength while excitation wavelengths are scanned.
Define Fluorescence.
when an molecule excited by absorption emits a photon from an excited singlet electonic state to a lower state with the same spin multiplicity.
(singlet to singlet)
Define Phosphorescence.
when an molecule excited by absorption emits a photon from an excited triplet electonic state to a lower state with a different spin multiplicity.
(triplet to singlet)
Define reasonance fluorescence.
when excitation and emission frequencies are the same
What is a triplet state?
paramagnetic electronic states in which all eletron spins are unpaired
define vibrational relaxation.
when a molecule losses vibrational energy without emitting radiation
when does predissociation occur?
when a molecule changes from a higher electronic state to a upper level vibrational level of a lower electronic state that is strong enough to break bonds
when does dissociation occur?
when radiation promotes a molecule directly to a state with enough vibrational energy to break bonds
why do some compounds fluoresce while others don’t?
if a compound relaxes too fast it will not fluoresce but if it relaxes slowly there is time for flourescence to occur.
Fluroescence involves what type of multiplicity electron transitions?
singlet to singlet
Phosphorescence involves what type of multiplicity electron transitions?
Triplet to singlet
why do more compounds show fluorescence rather than phosphorescence?
because with fluorescence you don’t have to flip spin states.
