In the discussion of molecular excitation, a single graph is used consistently because of its ability to summarize key information. This graphical representation is referred to as a Jablonski diagram.
The Jablonski diagram is capable of showing the transition between ground states and excited states by using quantized Morse potentials. As we will see later, it is also capable of showing other events, like fluorescence, which is an example of molecular emission which follows an excitation event.
A simple Jablonksi diagram is shown to the right involving a transition from a ground state to an excited state. Excitation events happen on extremely fast time scales, so excitation from a ground state to an excited state appears instantaneously on our diagram. When comparing time scales, we find that bond oscillation (10-12 to 10-14 s) is much slower than absorption and excitation (~10-15 s).
In the interactive module to the left you decide when to promote an electron. Based on the inter-nuclear distance and vibrational state the electron will get promoted to a certain excited vibrational state in accordance to the Franck-Condon principle. A diagram outlining the Franck-Condon principle is shown below.
Over time, the molecule will lose energy through vibrational relaxation where energy is lost to its surroundings (ie. transfer of energy through molecular collisions). Internal conversion is a process whereby the electron switches from a 'low vibrational excited state', to a 'high vibrational ground state'.
Try promoting the electron from different internuclear distances (click the pi orbital or the promote electron button). What happens to the excitation photon? Hover over the labels to highlight the transitions, why do they jump from state to state?
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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.