What fundamental process causes the transition of electrons from the ground state to the excited state?
Absorption of UV/visible radiation.
Define Emission Spectroscopy.
It is a spectroscopic technique that examines the wavelengths of photons emitted by atoms or molecules during their transition from an excited state to a lower energy state.
Define Luminescence.
The emission of light by a substance, occurring when an electron returns to the electronic ground state from an excited state and loses its excess energy as a photon.
Name the two categories into which luminescence is divided.
Fluorescence and phosphorescence.
Define Fluorescence.
Radiation emitted in the transition of a molecule from a singlet excited state to a singlet ground state.
What is the relationship between fluorescence emission and the incident light?
Fluorescence starts immediately after the absorption of light and stops as soon as the incident light is cut off.
What is a typical fluorescence lifetime?
Near 10 ns (emission rates are typically 10^8 s−1).
Define Phosphorescence.
Emission of radiation resulting from the transition of a molecule from a triplet excited state to a singlet ground state.
What happens to phosphorescence emission after the incident light is cut off?
The emission is continuous, even after the incident light is cut off.
Compare the typical lifetimes of fluorescence and phosphorescence.
The lifetime of phosphorescence (typically milliseconds to seconds) is much longer than fluorescence (near 10 ns).
Define Stokes’ Shift.
The difference between the positions of the band maxima of the absorption and emission spectra of the same electronic spectra.
According to the Pauli exclusion principle, what must be true about the spin states of the two electrons occupying an orbital?
The two electrons must have opposed spin states, meaning the spins are paired.
Define a diamagnetic molecule.
A molecule that exhibits no net magnetic field because of spin pairing, resulting in the molecule being neither attracted nor repelled by static magnetic fields.
Define a paramagnetic atom or molecule.
An atom or molecule with unpaired electrons, which has a magnetic moment and is consequently attracted by a magnetic field (e.g., free radicals).
What is a molecular electronic state in which all electron spins are paired called?
A singlet state.
Describe the spin arrangement in the excited singlet state.
The spin of the promoted electron is still paired with the ground-state electron.
Describe the spin arrangement in the triplet state.
The spins of the two electrons have become unpaired and are thus parallel.
Which is less energetic: the excited singlet state or the corresponding excited triplet state?
The excited triplet state.
Which magnetic property is associated with the triplet state?
Paramagnetic (The singlet state is diamagnetic).
What is the average lifetime range for an excited triplet state?
10−4 to several seconds.
How is the singlet state derived from the number of unpaired electron spins in the absence of a magnetic field?
If n=0 (no unpaired electron spins), then n+1=1. This is the associated electron spin for the singlet state.
How is the triplet excited state defined using the number of unpaired electron spins?
When one set of electron spins may undergo unpairing, it results in the formation of 2 unpaired electrons, so n+1=2+1=3, which is the triplet excited state.
When a molecule absorbs radiation of a specific frequency, it can end up in which vibrational levels of the first excited electronic state?
Any one of the vibrational levels (V0, V1, V2, V3, V4).
Name the four radiationless processes listed as potentially occurring from the excited singlet state.
Intersystem crossing, Vibration relaxation, Internal conversion, External conversion.
