Method Article

Spatial Separation of Molecular Conformers and Clusters

DOI:

10.3791/51137

⸱

January 9th, 2014

 ,  ,  ,  ,  ,  , 
1Center for Free-Electron Laser Science, CFEL, DESY, 2Department of Physics, University of Hamburg, 3The Hamburg Center for Ultrafast Imaging, University of Hamburg

In This Article

Summary

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We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Abstract

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Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

Introduction

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Modern gas-phase molecular physics and physical chemistry experiments often use supersonic expansions of target molecules to produce rotationally cold molecular samples within a molecular beam. However, even at low rotational temperatures of 1 K, which can routinely be achieved using supersonic expansions, large molecules can still remain in multiple conformations within the beam1. Similarly the production of molecular clusters in a beam source does not result in a single species, but rather in the formation of a "cluster soup", containing many different cluster stoichiometries, as well as remaining pure parent molecules. This makes the study of these systems with novel techniques such as imaging of molecular orbitals2, molecular-frame photoelectron angular distributions3-5 or electron6-10 and X-ray diffraction11-13 difficult, as these require pure, consistent, and homogenous samples in the gas-phase.

While several methodologies are now available to separate different conformers of charged species in the gas-phase (e.g. ion mobility drift tubes14,15) and charged clusters are easily separated by their mass-to-charge ratio, these techniques are not applicable to neutral species. We have recently demonstrated that these issues can be overcome with the use of an electrostatic deflection device16,17, allowing the separation of molecular conformers as well as clusters and the production of rotationally cold molecular beams.

The use of electrostatic deflection is a classic molecular beam technique, the origins of which go a long way back18,19. First ideas of utilizing electrostatic deflection for the separation of quantum states were introduced by Stern in 192620. While early experiments were conducted on small molecules at high temperatures, we demonstrate the application of this technique to large polar molecules and clusters at low temperatures16,21.

Polar molecules experience a force inside an inhomogenous electric field (E) due to the spatial differences in potential energy. This forceVector notation symbol (F) with arrow, representing force in physics equations and diagrams. is dependent on the effective dipole moment, μeff, of the molecule and can be evaluated as

Vector force equation with electric field gradient, symbol, scientific analysis. (1)

As different molecular conformers typically posses different dipole moments and differing numbers of solvent molecules within a cluster lead to different cluster masses and dipole moments, these species will experience a different acceleration in the presence of a strong inhomogeneous electric field. The resulting Stark effect force from an inhomogeneous electric field can therefore be used for the separation of conformers and quantum states22. This is indicated in Figure 1, showing the calculated Stark curves for the J = 0,1,2 rotational states of the cis and trans conformers of 3-fluorophenol, respectively. This leads to large differences in μeff, as shown in Figures 1c and 1d, and hence a different acceleration is experienced by the two conformers in inhomogeneous electric fields. Therefore, an electrostatic deflection device can be used as a mass-to-dipole moment ratio (m/μeff) separator, in analogy to a mass spectrometer acting as a mass-to-charge ratio (m/z) filter23.

Furthermore, these techniques allow the separation of rotational quantum states24,25. As the ground rotational states (blue curves in Figures 1a and 1b) exhibit the largest Stark shift, these will be deflected most and can be spatially separated from molecules in higher J states17. The coldest part of a molecular beam can therefore be selected, significantly aiding in many applications, such as alignment and orientation of target molecules17, 26-28.

In this contribution we show how an electrostatic deflection device can be used to spatially separate different species of large polar molecules and clusters. Example data is presented for the production of a pure beam of an individual conformer and of a solute-solvent cluster of well-defined size and ratio. Specifically we present data on 3-fluorophenol, where a pure beam containing only the trans conformer is produced, and on indole-water clusters, where the indole(H2O)1 cluster can be spatially separated from water, indole, indole(H2O)2 , etc.

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Protocol

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1. Description of the Experimental Setup

A schematic of the gas-phase molecular beam setup and deflector is shown in Figure 221. It consists of

  1. A pulsed Even-Lavie valve29 containing the molecular sample. Other pulsed molecular beam valves can be used equally well as long as a cold molecular beam (O(1 K)) is formed. The following parameters are specific for the employed Even-Lavie valve. In the experiments presented here the valve is operated at 20 Hz repetition rate with high backing pressures (helium at ~50 bar) and expanded into a vacuum chamber evacuated to <10-6 mbar.
  2. A molecular beam skimmer (2 mm diameter) is placed 22 cm downstream from the valve, collimating the molecular beam and leading to differential pumping conditions between the pulsed valve and the rest of the vacuum system.
  3. Immediately after the skimmer the molecules enter the electrostatic deflection device. This consists of a rod (radius 3.0 mm) and a trough (radius of curvature 3.2 mm), each 24 cm long. The vertical gap between the electrodes in the center of the device is 2.3 mm. A potential difference between 0-26 kV is applied between the rod and trough, producing a strong inhomogeneous electric field with a nearly constant field gradient30, as indicated in the inset of Figure 2.
  4. Directly after the deflector molecules enter the interaction region through a second skimmer, providing a further differential pumping stage.
  5. The interaction region (evacuated to pressures <10-9 mbar) contains a standard Wiley-McLaren time-of-flight (TOF) setup. Molecules are ionized by focused laser pulses in the center of the extraction region, between the repeller and extractor electrodes. Produced ions are accelerated towards a multichannel plate (MCP) detector, where a mass spectrum is recorded.
  6. Laser pulses are derived from an Nd:YAG pumped dye laser, providing typical output wavelengths around 283 nm (indole experiments) or 272 nm (3-fluorophenol experiments) and pulse energies of a few mJ. Pulse durations are on the order of 10 nsec and pulses are focused with a f = 750 mm lens to a spot size of ~100 μm in the interaction region.
  7. The timing sequence is controlled by a digital delay generator providing the master clock. This triggers the Nd:YAG laser (flash lamps and Q-switch), the pulsed valve, and the digitizer card used to record mass spectra.
  8. Mass spectra are recorded on a digitizer card, triggered at the same time as the laser Q-switch. Molecular beam densities are extracted from appropriate mass gates in the recorded time-of-flight spectra.

2. Production and Characterization of a Conformer Selected Molecular Beam

  1. A cold molecular beam of the target molecules is created via supersonic expansion and characterized using spatial (x, y directions) and temporal (z direction) profiling. 
  2. Load the sample reservoir of the pulsed valve with the chemical sample. Dissolve solid samples in an appropriate solvent and place a few drops on a small piece of filter paper which is inserted into the sample cartridge. Place liquid samples directly on the filter paper.
  3. Produce the supersonic expansion, using a high-purity high-pressure backing gas. Adjust the temperature of the sample reservoir within the valve such that the partial pressure of the sample is approximately 10 mbar.
    Note: For liquid samples typically no heating is necessary. The valve opening time depends on the exact model of pulsed valve used, for the experiments presented here the Even-Lavie valve is operated with an electric pulse duration of 10 μsec.
  4. Characterize the produced molecular beam with the electrostatic deflector turned off. Set the ionization laser to a known wavelength for resonance-enhanced multiphoton ionization (REMPI) of a particular conformer of the sample. Record a temporal profile of the molecular beam pulse by monitoring the total parent ion yield on the MCP detector as a function of valve-laser delay.
  5. Fix the valve-laser delay at the position of maximum intensity for all subsequent measurements.
  6. Record a transverse spatial profile of the molecular beam by monitoring the total parent ion yield as a function of the y position of the laser focus. Do this by moving the focusing lens perpendicular to the laser propagation direction, such that the focus moves in the y direction relative to the molecular beam.
  7. Repeat the temporal and spatial profiling for all conformers of interest in the beam.
    Note: These typically have distinct REMPI resonances, such that each conformer can be probed separately. In the absence of a deflection field however, the temporal and spatial profiles are identical for all conformers.
  8. Characterization of the deflected beam. Turn on the high voltage supply to the deflector and record spatial profiles for all isomers. These should now be deflected according to mass-to-dipole moment ratio.
    Note: For species undergoing large deflections it may be necessary to move the skimmer immediately following the deflector to ensure good transmission of the deflected beam into the detection region.
  9. Conduct experiments on the conformer or size-selected part of the molecular beam by ensuring the interaction (e.g. a crossing laser beam) takes place within the part of the molecular beam containing only the species of interest.

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Results

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The electrostatic deflection technique has been successfully applied to the separation of structural isomers16 and neutral clusters21, as well as the production of rotational quantum state selected molecular samples31. We demonstrate this with representative results for the separation of cis and trans conformers of 3-fluorophenol, and size selected indole(H2O)n clusters.

3-Fluorophenol conformers were separated in a molec...

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Discussion

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Throughout this manuscript, familiarity with ultra-high vacuum components, pulsed molecular beam valves and laser sources is assumed and the associated safety procedures should always be adhered to. Special care needs to be taken when handling the high voltage electrodes for the deflector. Their surfaces need to be polished to a high standard and must be absolutely clean to avoid arcing inside the vacuum chamber. Before first use the electrodes should be conditioned under vacuum. The voltage applied is slowly increased and the current throug...

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Disclosures

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The authors have nothing to disclose.

Acknowledgements

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This work has been supported by the excellence cluster "The Hamburg Center for Ultrafast Imaging – Structure, Dynamics and Control of Matter at the Atomic Scale" of the Deutsche Forschungsgemeinschaft and by the Helmholtz Virtual Institute "Dynamic Pathways in Multidimensional Landscapes".

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Vacuum systemvarious, e.g. Pfeiffer Vacuum, Varian, Edwards, Leybold
Dye laser systemvarious, e.g. Coherent, Spectra Physics, Syrah, LIOP-TEC, Radiant Dyes…
Pulsed valveEven-Lavie
High voltage power supplyeg. FUGHCP 14-20000
DeflectorCustom made
Time-of-flight spectrometerJordan TOFC-677
TOF power supplyJordan TOFD-603
Focusing lensThorlabsLA4745
Translation stagee.g. Vision Lasertechnik8MT167-25
Digitizere.g. AgilentAcquiris DC440
Digital delay generatorStanford SystemsSRS DG645
Molecular beam skimmerBeam Dynamics Inc.http://www.beamdynamicsinc.com/

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Molecular Beam SeparationElectrostatic DeflectorStark EffectConformer IsolationCluster SelectionResonance Enhanced Multiphoton IonizationTime of Flight Mass SpectrometryPulsed Valve ExpansionSpatial ProfilingDipole Moment Deflection

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