Electronic Vibrational Spectroscopy of Molecules Flashcards
Energy levels quantised?
Energy levels quantised at a range of energy scales, Electronic, Vibrational, Rotational
How can electronic spectroscopy probe these processes?
Electronic absorption spectroscopy, Fluorescence spectroscopy
What do transitions between electronic energy levels correspond to?
UV/visible wavelengths, Absorption promotes an electron from an occupied molecular orbital to a higher unoccupied molecular orbital, But also get changes to vibrational energy levels (and rotational), changes in vibrational levels add “structure” to the spectrum
How to electronic absorption spectra arise?
Transitions can occur into the vibrational states of the electronically excited state, Not just adjacent levels, delta v in this case is not simply ±1, This leads to a number of absorption peaks in the spectrum – often as a broad band in solution spectra (but can be resolved in high res), Differences between peaks give info on vibrational levels in the excited state
Franck Condon Principle?
Electronic transitions are generally very fast, Nuclei do not move, Franck and
Condon showed what this means for absorption, Transition could be from bottom of lower electronic state to bottom of the upper, BUT this would mean r changes (i.e. the nuclei would have to move), Instead the transition happens vertically (to the “turning point”
Where is the largest wave function overlap?
Largest wavefunction overlap is where the vertical meets the potential in the excited state, the “turning point”, This will usually mean that the most favoured transition is to one of the higher level vibrational states in the excited electronic state, This will have the highest intensity absorption peak, Others will be observed (to other vibrational levels in the electronic excited state), These will have lower intensity
Most intense peak?
Most intense is most
vertical, this is the 0 (v, lower) –> 2 (v’, upper) vibrational transition, The 0 –> 0, 0 –> 1,
0 –>3, etc, v–>v’ will also happen, No delta v selection rule between different electronic states, But will be of lower intensity, as indicated, Excitation may also be to a dissociated state
UV absorption for liquid benzene?
Vibrational fine structure is seen in the pi-pi* electronic transition, Transitions can be labelled according to the relevant vibrational states), Here the v = 0 to v’ = 2 transition is strongest, We can calculate vibrational energy level differences in the excited electronic state, e.g. gap between v’=0 and v’=1 is difference in energy of lowest two transitions, This is the energy difference between v’=0 and v’=1 in the excited electronic state
Fluorescence spectroscopy?
Relaxation” to the lowest vibrational state (of Relaxation” to the lowest vibrational state (of the excited electronic state)
Return to the ground state – Emission of a photon at lower energy, this is fluorescence, Fluorescence lasts around 10^-10 – 10^-8 s and stops when the exciting light is taken away, Phosphorescence is a related phenomenon
Fluorescence spectroscopy?
When in the vibrationally excited state (of the excited electronic state), the molecule loses vibrational energy to the surroundings – radiationless decay, When at the ground vibrational state of the excited electronic state, cannot lose further thermal energy, The molecule now decays to the ground electronic state, emitting a photon (of lower energy than that originally absorbed)
Phosphorescence?
Another route by which the excited molecule can lose energy is by phosphorescence, The absorbed photon undergoes what is called intersystem crossing from a singlet to a triplet state (forbidden in theory!), It then decays by emitting a photon in another “spin- forbidden” process, The forbidden processes become allowed in molecules where there is spin-orbit coupling (which happens often)
Fluorescence spectra important points>
Emission occurs at lower energy (higher wavelength) than absorption, Pattern of lines is the same, but (almost) a mirror image
What does structure in the emissions spectrum give information on?
Structure in the absorption spectrum gives information on the vibrational energy levels in the excited electronic state – info on delta E for the vibrational levels, Structure in the emission (Fluorescence) spectrum gives information on the vibrational energy levels in the ground electronic state, Analysed in the same way as for the Absorption spectrum of liquid benzene, Differences between peak positions gives info on vibrational states in upper (Absorption) and lower (Fluorescence) electronic states
