Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

The overall goal of this experiment is to use Coulomb explosion to image the handedness of individual molecules in the gas phase. This method is a new approach to address important questions in stereochemistry. In particular, it allows to determine the absolute configuration of small chiral species and their photoinduced dynamics.

The main advantage of this technique is that energy and emission direction of all charge fragments from a molecular breakup can be recorded in coincidence. We have developed this multicoincidence imaging technique over a period for more than 25 years to learn more about and to measure the secrets of stereochemistry on the basis of single molecules. The heart of the experiment is a setup for coincident imaging of ions and a femtosecond laser such as this COLTRIMS apparatus.

The femtosecond laser beam enters from an adjacent room. The basics of the COLTRIMS technique are depicted in this animation. A molecular beam enters the vacuum chamber through a nozzle and a skimmer.

A laser beam crosses the molecular beam at right angles and leads to ionization and subsequent fragmentation. The electric field in the spectrometer guides the ions to the detector where the times and the positions of their impacts are recorded. Prepare the setup for operation and check the vacuum in the interaction chamber.

The pressure should be less than 10 to the negative nine hectopascal. With the laser on and at low intensity perform the beam alignment. Be sure the beam enters the experiment chamber and is reflected from the chamber’s focusing mirror.

Use a beam card to check that the incoming and outgoing beams coincide spatially. Block the laser beam before continuing. Prepare to turn on the spectrometer and detector power supplies.

First, turn off the vacuum gauges in the interaction chamber. Next, connect the amplified signal output of the microchannel plates to a fast oscilloscope. Now turn on the high voltage power supplies for the detector and the electric field in the spectrometer.

Return to work with the laser beam by first unblocking it. Have a rotatable polarization filter ready to adjust the beam intensity. Place a power meter in the beam just before the chamber and use the filter to adjust the beam intensity to below 10 to the 14th watts per square centimeter.

For this setup, this corresponds to 100 milliwatts on the power meter. Remove the power meter and observe the oscilloscope trace. It is crucial to check the signal quality and the detector settings.

If too much noise is recorded the true coincidences are very difficult to find. The signal frequency should be about 5%of the laser repetition rate. The signal should have one peak of several hundred millivolts without any ringing.

The width of the signals from the microchannel plate should not exceed 10 nanoseconds. Now turn to the the data acquisition software. There, display an image of the detector, which should be a circle with a diffused spot at its center due to the laser.

At this point, provide one bar of argon from a gas cylinder as the source for the gas jet. Observe the detector image live in the data acquisition software while adjusting the focusing mirror. If a narrow spot appears overlap between the molecular jet and the laser beam has been located.

The goal is to maximize the jet beam overlap and increase the counts in the jet spot. After optimizing the jet beam overlap, prepare the sample for use in the experiment. This sample is already in a cylinder that is compatible with the setup.

First, cool the cylinder in liquid nitrogen for one to two minutes to avoid sample losses. Connect the cooled cylinder to the jet system and tighten the connection to make it vacuum-proof. Next, open the valve to pump the system for a few seconds to remove air.

Wait for the sample to return to room temperature before opening the valve to the jet nozzle. The pressure in the source chamber should increase. Return to the data acquisition software.

At the computer, verify that a jet spot is still visible, and identify the most prominent peaks in the time-of-flight spectrum. Continue to work in the data acquisition software. For analysis, make a plot using the time-of-flight of the first detected ion along the x-axis and the time-of-flight of the second detected ion along the y-axis.

When two fragments have been detected in coincidence, they appear as regions with many counts. If two fragments add up to the parent mass, they fulfill momentum conservation and show up as sharp diagonal lines. Perform a time-of-flight analysis of four particles with a plot created by summing the times-of-flight for the first and second hits to use as the x-axis value and summing the times-of-flight for the third and fourth hits to use for the y-axis value.

The positions and shapes of the resulting structures reveal information of the masses and momenta. Adjust the laser power so that the number of counts in the spectrum is optimized. When optimizing the rate for the relevant breakup we have to kep in mind that the overall rate should not exceed 10%of the laser repetition rate to avoid false coincidences.

These data are from a synthetic racemic mixture of bromochlorofluoromethane. Only events where a fragmentation into five singly charged ions were recorded are shown. The horizontal axis is calculated using a triple product of the momentum vectors of the halogen ions.

The S enantiomer is at the left of the histogram. The diagram illustrates its associated angle data. The R enantiomer is at the right of the histogram along with a diagram to illustrate its angle.

The laser repetition rate was 100 kilohertz and the measurement took about 11 hours. This animation demonstrates how it is possible to use the recorded momentum vector data of the fragments to transform their random orientation in the jets into a molecular coordinate system. After watching this video, you should have an idea how Coulomb explosion imaging is performed using the COLTRIMS technique.

Recording the momentum vectors of four or five fragments in coincidence allows a detailed view into the molecule. Extracting this information from the raw data however, requires a lot of effort in the analysis step. The secrets of life science in the world of molecules occurs on a time scale of a billionth of a billionth of a second and our method can reveal and visualize these fast dynamics.