An Introduction to Processing, Fitting, and Interpreting Transient Absorption Data

We are a group of experimental photo chemists exploring how to use light to drive important chemical reactions and energy harvesting devices such as solar cells. We use a range of spectroscopy such as transient absorption spectroscopy, the data of which we use in this tutorial to monitor and quantify these dynamics. One of the current challenges is to monitor and quantify chemical dynamics and devices under more realistic conditions or what we would call operational conditions that truly represent how they would be implemented in real life.

The advantage of this protocol is to provide a relatively easy entry point for beginners. We wanted to outline the basics of fitting and some of the common challenges in an approachable way for those new to this area. We will always keep using light as our inspiration.

We hope to apply our expertise to understand how photo activated devices function under real life operating conditions to help address environmental pollution challenges and to harness energy from the sun to benefit our society. To begin, open the fitting software and load the sample dataset. Click on the surface menu and select subtract scattered light.

In the new window, use the arrow buttons and set the number of background spectra to average to 10 for performing background correction. Click on accept to perform the subtraction. Next, remove the unhelpful portions of the spectrum and focus on the spectrum displayed in the lower left.

Click on end wavelengths, input new values, and press enter. Adjust the spectral window to the desired wavelength range. Go to the surface menu, select crop, and confirm by clicking okay.

To save the changes, click on the file menu, select save file as, click okay, and then close the dataset. Open the solvent blank dataset and begin the trip correction process by adjusting the spectral display. In the heat map tile on the top left, click the cross hair tool and drag the vertical line to the blue end of the spectrum, starting near the blue wavelength range at the beginning of the spectral window.

Then navigate to the kinetics menu and select fit solvent response. In the new window titled Fit Solvent Response"click on the fit button. Perform this at least five different wavelengths, spaced out across the entire spectrum.

After the last point, click save, and then close the window. Next, open the previously cropped and background subtracted dataset. In the surface menu, select the chirp correction option.

Add the recently created chirp correction by choosing add from file, selecting the file ending in fit coefficients, and confirming with okay. Click on the preview chirp correction button and observe the correction in the top left window, ensuring that the data appears temporarily flattened without any curvature. If the correction is satisfactory, finalize it by clicking the apply and exit button.

Navigate to the file menu. Select save file as. Enter a suitable name to reflect the chirp correction application, and then click okay.

To locate features in the data that should be removed, navigate to the upper left heat map tile. Use the cross hair within the negative time region to identify the start and end wavelengths of the scattering feature. Begin with the lower or blue wavelength of 654 nanometers in the spectrum lower left tile.

Select the right end wavelength and input the value for the lower feature extent. Crop the data by selecting surface from the menu, choosing crop, and then clicking okay in the pop-up menu. Save the crop data with a unique file name indicating which side of the data it is, and then close the file.

Next, open the previously saved chirp correction file and proceed to the features higher or red wavelength extent. Select the left end wavelength on the spectrum in the lower left tile and input the value for the higher extent of the feature. To crop the data, navigate to the surface menu, select crop, and then click okay in the popup menu.

Save the crop data with a distinctive file name to indicate the side of the data it represents. Combine the two files by going to file and selecting combine multiple surfaces. Select both sides of the data in the new window using control plus click.

Ensure both files are chosen in the file name box, and then click okay to confirm. Click on file and choose save file as to save the combined file. Assign a unique file name to indicate its combined status, and click okay to save.

A corrected heat map and representative spectra data are shown here. The results demonstrate how the data should look after applying the corrections. The data set is now ready for fitting.

To initiate a single kinetic fit, navigate to the desired wavelength using the cursor in the top or bottom left tiles. Access the kinetics menu and select fit kinetic. Adjust the number of lifetimes in the finite lifetimes box using the arrow buttons.

Select the use infinite lifetime checkbox to include an infinite lifetime component. Input initial guess values for lifetimes, amplitudes, instrument response time, and time zero to facilitate the fitting process. Click on each parameter, enter a guess value in the value window and press the initial guess button to set it.

After entering all guess parameters, complete the fitting by clicking the fit button. Save the results by selecting the save button. For global analysis fitting, access the surface menu, and choose principal components via SVD.

Set the number of principal components using the arrow buttons. Save these settings by clicking the save button. Return to the surface menu and select global fit.

Use arrow buttons next to the number of the XP to set the number of exponential functions to include in the fit. To include an infinite lifetime component, check the use offset AINF checkbox. Initiate the fit by clicking the fit button and save the configuration with the save button.

To extract the raw data for plotting, create a heat map of the dataset. First, navigate to the file menu and select export to CSV. This action will open a new window.

Click okay to save the CSV file in the same directory as the data file and name it identically to the data file. To view an export representative spectra for plotting in another program, locate the horizontal cursor in the upper left of the heat map and drag it to the desired time point. Press CTRL plus S to capture the spectrum at this point and save it in the spectral window.

To export the data, right click on the window that displays the spectrum. Choose export data to clipboard. Finally, paste this data into a software document of choice and save.

The single wavelength fitting shown here displays the lifetimes obtained in table format, the data, and the residual plot. The global analysis fitting shows the resulting parameters and the decay associated difference spectra.