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The Walters Research Group - Spectroscopy
Our measurements center around observing electronic transitions within a molecule based on its interaction with light. We use three major techniques to monitor this interaction: Absorption, Emission, and Transient Absorption spectroscopic techniques. We also use Stark spectroscopy to measure the distance electrons actually move within a molecule! All this instrumentation is available in our laser lab (SC 467); email Dr. Walters if you'd like to take a tour! Basically, all spectroscopy involves the promotion of an electron from a normal (i.e., "ground") state to a higher-energy, "excited" state. For optical spectroscopy, these states are different electronic states in the molecule of interest. For the purposes of this discussion, let's assume a fictional molecule has a ground state (labeled S0), the lowest lying excited state (labeled S1), and some higher-energy excited state (labeled Sn).
In absorption spectroscopy, the absorption of light by the molecule of interest is measured. The diagram below to the right shows a schematic of an absorption spectrometer. Light is passed through a sample, separated to individual wavelengths by a monochromator, and measured with a detector. If less light gets to the detector when the sample is placed in the light beam, light is absorbed by the sample. The absorption process is shown in the Jablonski diagram shown below to the left. These diagrams illustrate the various electronic states of a molecule, with energy increasing as you go up the diagram. A photon of light (traditionally represented by the symbol hu) is absorbed (e.g., "added" to the system, which is why the plus is in the diagram) when it has the right amount of energy to promote an electron in the molecule from the ground to excited state (S0 to S1). A typical absorption spectrum is shown below in the middle, where the peak illustrates wavelengths of light where absorption occurs.
Once a molecule is excited by light, it can't stay excited forever. When the excited electron "decays" back to the ground state, energy is given off by the molecule, either by a photon or heat. Emission spectroscopy measures the emission of this light. The schematic of an emission spectrometer is very similar to an absorption instrument, with the exception that a second monochromator sets the wavelength of light excitation and light is detected at a right angle (why?). The Jablonski diagram shows the emission (i.e., "subtraction") of a photon after the absorption event. The emission spectrum corresponding to the typical absorption spectrum is shown below. The molecule was excited at 450 nm, the maximum of the absorption spectrum peak. Emission spectra peaks are always at longer wavelengths (lower energy) than absorption spectra peaks because of interactions of the molecule with the solvent following excitation, which dissipates energy.
Transient Absorption Spectroscopy
What if you really tried to push a molecule to the limit? Once you've excited a molecule with the first photon of light, there's nothing stopping you from hitting it with more photons of light and get further absorptions. This is the essence of Transient Absorption spectroscopy, which is essentially the "absorption spectrum of an absorption spectrum". The instrument schematic is exactly like an absorption experiment, with the exception that a laser is used to provide the initial excitation (S0 to S1) shown in the Jablonski diagram. Once this first excitation is achieved, the light provides additional photons that could lead to a second absorption (S0 to Sn). The presence of the "excited-state absorptions" are great signals to indicate that a certain process is occurring or if an excited-state reaction happens. We don't normally care what Sn is, but it will provide a way to measure the lifetime of the initial excited state, which is of great use in understanding the excited-state processes of a molecule. Since this technique has a time axis (it's a transient, after all), we can monitor the time that the second absorption exists, which in turn tells us how long the initial excited state lives before its decay to the ground state.
It is important to remember that the transient absorption "signal" is the difference in the amount of light absorbed by the sample at some point in time after the laser fires and the amount of light absorbed before the laser fires. Therefore, if an excited-state absorption occurs, the transient absorption signal should be positive. The diagram below helps us understand what happens in a transient absorption spectrum. When the laser fires, the majority of the molecules in our sample are promoted from the ground state to the lowest excited state. At this instant, there are no longer many ground state molecules, so the 450 nm absorption will not be as intense. This loss of intensity results in the sudden negative peak in the transient absorption signal at 450 nm (the bottom plot). At the same time, there are suddenly many molecules in the lowest excited state, so the 555 nm absorption will be much more intense. This increase of intensity results in the sudden positive peak in the 555 nm transient absorption signal (the top plot). However, the molecules will not stay at the lowest excited state for very long (a few microseconds in this example). As the molecules start to decay back to the ground state, the 450 nm and 555 nm absorptions begin to increase and decrease, respectively, until the point in time when all the molecules have returned to the ground state. At this point, the transient absorption signals for both absorptions are zero, since the absorptions have reached the same levels that were observed before the laser fired.
A typical transient absorption spectrum is a plot of delta absorbance versus wavelength for all the wavelengths observed. Multiple lines in the spectrum represent different times after the laser excites the sample. A sample spectrum is shown below, where both negative and positive signals like those described above are observed to decay back to zero with time.
Coming Soon!
Varian Cary 100 Scanning UV-Vis Spectrophotometer
Used to measure the absorption spectra of molecules.
JY-Horibe SPEX Emission Spectometer
Used to measure the emission spectra of molecules. Also used to measure Stark emission spectra.
Nanosecond-Regime Transient Absorption Spectrometer
Used to measure the transient absorption spectra of molecules. Also used to measure Stark absorption spectra.
Signals are measured with a Lecroy digitizing oscilloscope (shown above)
The spectrometer is controlled with electronics designed and built in-house (shown above)
Quantel Brilliant Nd:YAG Laser
Excitation source for the transient absorption spectrometer.
Emits laser light at 1054 nm (IR), 532 nm (green visible), and 355 nm (near UV).
Oxford Instruments Optistat Liquid Nitrogen Optical Cryostat
Used to measure the absorption, emission, or transient absorption spectra of molecules at various temperatures from 77 to 300 K.
Stark Spectroscopy Instrumentation (Under Construction)
A lock-in amplifier and high voltage frequency generator is needed for this technique (both shown above)
