W6 Molecular Fluorescence (Emission) Spectroscopy Flashcards
properties of fluorescent instrument
emission spectrum is produced by keeping the excitation wavelength fixed and scanning the emitted radiation
has zero background > extremely sensitive
what are fluorescence sensors used for
used for determination of anions and dissolved oxygen
what is vibrational relaxation
involves the transfer of excess energy of a vibrationally excited species to the molecules of the solvent > leaves molecule in lowest vibrational state of an electronic excited state
what is internal conversion
a type of relaxation that involves relaxation between the lowest vibrational level of an excited electronic state and the upper vibrational level of another electronic state
definition of intersystem crossing
an isoenergetic radiationless transition between two electronic states having different multiplicities
fluorescence definition
the emission of a photon during a transition between states with the same spin quantum numbers
lifetime of fluorescence is very short
phosphorescence definition
the emission of a photon during the transition between the states with different quantum numbers
lifetime of phosphorescence is much longer
properties of stokes shift
stokes-shifted fluorescence is longer in wavelength than the radiation that causes the excitation
when spacings between vibrational levels are roughly equal and transition probabilities are similar > absorption and emission spectra will have approximate mirror image
definition of stokes shift
molecule absorbs light > excited to higher energy level > undergo non radiative relaxation before emitting light > lose some energy was heat while in excited state
due to this energy loss > light emitted as fluorescence has less energy than absorbed light > results in longer wavelength
main causes of stokes shift
reoptimisation of molecule in S1 excited state: electronic distribution changes upon excitation > shift in bond lengths, angles or overall molecular shape > molecule relaxes to lowest vibrational level of S1 state > adjusting its geometry to become more stable in this new electronic configuration
environmental interactions
definition of fluorescence quantum yield
the ratio of the number of molecules that fluoresce and the total number of excited molecules, or the ratio of photons emitted and the photons absorbed
properties of fluorescence quantum yield
highly fluorescent molecules have high quantum yields ~1
many substitutes aromatic compounds tend to fluoresce
the rigid molecules also tend to fluoresce
increasing the temperature tends to reduce the quantum efficiency of fluorescence
what is fluorescence quenching
process where energy of an exited molecule can be transferred to another molecule > undergoes deactivation and gives out heat
