Once the system absorbs a photon of light and successfully excites an electron to an excited state, what happens to this excited electron? As with any system, molecules will slowly change from a high energy state to a lower stable state. The extra energy in this excited state can be lost in several ways, which include: a chemical reaction, non-radiative relaxation, and radiative relaxation.
In the case of a chemical reaction, the molecule is prone to undergoing a chemical reaction since it is already in an excited state. For non-radiative relaxation, the molecule collides with other molecules in its surroundings and energy is slowly transferred away. In the previous module we saw how the molecule converts from an excited state to a ground state in a process called internal conversion.
In cases of radiative relaxation, the energy between an excited state and the ground state is lost as a photon. This photon is always of lower energy than the photon that was absorbed. One example of radiative relaxation is the process of fluorescence.
Fluorescence begins with absorption of a photon resulting in an excited state. The electron will then fall to the lowest vibrational energy within that excited state. This happens relatively quickly (10-11 s). The transition from the excited state to the ground state through the loss of a photon happens on the order of nanoseconds (10-9 s), which is much slower than other processes. An example of an absorption spectrum from a fluorescent event is given below.
When moving from the excited state to the ground state, the electron follows the Franck-Condon principle to determine which vibrational state it enters. Essentially, the emission of the photon is nearly instantaneous compared to molecular vibration.
Click the system to the left to start, you can choose when to promote the electron and relax the electron. How does the photon change with respect to the vibrational state it falls too? How does this compare to its excitation photon?
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Physical chemistry attempts to understand chemistry through the physical world and using instrumentation.
Molecular excitation refers to the promotion of an electron to an excited state. This particular pheomenon is extremely important for current scientific discovery, particularly in the biological sciences.
A simple harmonic oscillator displays a very particular type of periodic motion called simple harmonic motion. A common example of a simple harmonic oscillator is a spring that is compressed or stretched.
Morse potentials are used to model the interaction between two atoms in a diatomic molecule.
A diatomic molecule has only two atoms which are connected through a chemical bond. This particular diatomic molecule is double bonded.
The energy of a diatomic molecule can be approximated using a Morse Potential. Quantum effects are not discussed.
The vibrational state of the diatomic molecule refers to the frequency at which the atoms oscillate (ie. the bond stretches and compresses).
A single rotational mode is available to the diatomic molecule and involves rotation around an axis that is perpendicular to the bond axis. The energy of the rotational mode is directly related to its angular momentum.
Electromagnetic radiation is a form of that travels in waves. Specifically, electromagnetic energy travels in a transverse wave that oscillates at a certain frequency.
Like other dipoles, the transition dipole refers to a difference in charge from one location of a molecule to another. The transition dipole occurs when an electron is excited from the ground state to an excited state.
The Jablonski diagram is capable of showing the transition between ground states and excited states by using quantized Morse potentials.
Fluorescence begins with absorption and molecular excitation into an excited state. Once promoted, the electron will fall to the lowest vibrational energy within that excited state.