Caution: Make sure to be familiar with all possible hazards connected with the experiment and in the laboratory. The procedure below includes class-IV lasers, high voltage and vacuum. Consult the material safety data sheet (MSDS) for the species to be investigated.
1. Preparation
2. Turning on Spectrometer and Detectors
NOTE: This part of the protocol slightly depends on the actual implementation of the spectrometer and detector system. The description here is valid for a standard COLTRIMS setup with a hexagonal delay line detector (HEX75).10 In this implementation, a detector has 7 output channels: one for the microchannels plates (MCPs) and two for each of the three layers of the anode.
3. Sample Delivery
4. Measurement
NOTE: The following steps are performed in the data acquisition software.
5. Data Analysis
NOTE: Data analysis in a Coulomb Explosion Imaging experiment is a complex, yet rewarding task because many parameters can be fine-tuned after the experiment and a multitude of correlations between the measured momenta can be explored. All following steps are usually performed after the experiment in the data analysis software.
In this part, we show results obtained for halomethanes. These species are ideal for proof-of-principle experiments due to their simplicity and high vapor pressure. In the meantime, the more complex species halothane has been investigated using single soft-x-ray photons from a synchrotron source to induce multiple ionization.14
CHBrClF
Bromochlorofluoromethane (CHBrClF) is a textbook example for chiral molecules with a stereogenic carbon atom. It is also the ideal candidate for Coulomb Explosion Imaging due to its simple structure and the high vapor pressure (around 600 hPa at room temperature). Unfortunately, the species is not available commercially; for the experiment presented here, a racemic mixture was synthesized by reacting CHBr2Cl with HgF2 according to reference15. Enantio-enriched samples are difficult to obtain in the quantities needed so that only results for racemates have been obtained so far.
For the results presented here the sample was cooled to around 240 K to obtain an appropriate target density with the given nozzle (10% of ionization probability per pulse). The peak intensity of the laser was estimated to be 6 x 1014 W/cm2. The measurement at 100 kHz laser repetition rate took 11 h.
In order to distinguish R and S-enantiomers, a normalized triple product is calculated from the momentum vectors of the three halogens fluorine, chlorine, and bromine. Geometrically, this quantity can be interpreted as the cosine of the angle between the fluorine momentum and the plane of the chlorine and bromine momenta.
Figure 3 shows cosθ for the isotope CH79Br35ClF, together with the geometric definition. Two clear peaks are visible, indicating the enantiomers. The position of the peaks is consistent with a classical molecular dynamics simulation. As almost no background is present, the assignment of handedness works on a single molecule level.
CHBrCl2
The chirality of CHBrCl2 occurs only if the both isotopes 35Cl and 37Cl are present in the same molecule. A sample with natural abundance of isotopes thus contains chiral and achiral molecules. Two additional complications arise here: Firstly, the time-of-flight distributions of the chlorine and bromine isotopes overlap respectively due to the small mass difference. This is particularly relevant for chlorine as the determination of handedness depends on the correct assignment of the isotopes. Secondly, the chiral species CH79Br35Cl37Cl has (within the setup's accuracy) the same total mass as the achiral species CH81Br35Cl2. The investigation of this species can thus be seen as a benchmark test for the method.
With the spectrometer used (spectrometer length s = 60.5 mm, electric field strength E = 57.1 V/cm), the data for the chiral isotope CH79Br35Cl37Cl could be selected via the total momentum, using an algorithm suggested by reference16 to assign which of the hits belongs to which isotope.
Geometrical considerations lead to the conclusion that there can be orientations of the molecule in space where the two chlorine isotopes have the same time-of-flight; in this case, they cannot be distinguished as a matter of principle. A procedure to sort out these events has been described in the supplementary materials reference4. As a result, the configuration even of isotopically chiral molecules can be determined with high reliability.
Figure 1: View into a COLTRIMS Setup. Molecules enter the setup through the nozzle and pass through a pair of skimmers. In the interaction chamber, the laser pulses cross with the molecular jet under 90°. Ions are guided by the electric field of the spectrometer to the detector (top). For better visibility, not all spectrometer plates are shown. The remaining molecules are dumped in a differentially pumped section (jet dump) to keep the background pressure in the interaction region as low as possible. Figure modified from reference17 with permission by G. Kastirke. Please click here to view a larger version of this figure.
Figure 2: Four-Particle Coincidence Spectrum. This histogram is an extension of a time-of-flight mass spectrum to four particles: The sum of the time-of-flights for the first and second hit on the detector are plotted on the x-axis, the sum for the third and fourth hit on the y-axis. The center of the peaks allows to identify the masses of the four detected fragments. The shape of the structures contains additional information: If the momenta of the fragments add up to zero, the events are contained in a narrow line (H, CF, Cl, Br). If an undetected fragment carries momentum, the non-zero total momentum of the measured particle leads to a broadening of the features. For illustration purposes, data from synchrotron, not laser, measurements are used here due to higher statistics. Figure reproduced from reference5 with permission by Wiley-VCH. Please click here to view a larger version of this figure.
Figure 3: Distinction of enantiomers in the five-particle break-up of CHBrClF via the chirality parameter cos θ as defined in the text. The peak at positive values corresponds to the R-enantiomer, the peak at negative values to the S-enantiomer. The inset illustrates cos θ geometrically. The low background allows for an assignment of handedness for individual molecules. Figure reproduced from reference4 with permission by AAAS. Please click here to view a larger version of this figure.
CHBrCl2 | SigmaAldrich | 139181-10G | or other suitable sample |
femtosecond laser system | KMLabs | Wyvern500 | |
High-reflective mirrors | EKSMA | 042-0800 | |
mirror mounts | Newport | U100-A-LH-2K | |
focusing mirror (protected silver, f = 75 mm) | Thorlabs | CM254-075-P01 | (if available: f = 60 mm) |
COLTRIMS spectrometer, including electronics and data acquisition system | RoentDek | custom | contrary to the standard COLTRIMS, only one detector is needed |
This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the “reaction microscope” technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.
Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.
This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the “reaction microscope” technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.
Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.
This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the “reaction microscope” technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.
Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.