Spatial Separation of Molecular Conformers and Clusters

Daniel Horke; Sebastian Trippel; Yuan-Pin Chang; Stephan Stern; Terry Mullins; Thomas Kierspel; Jochen Küpper

doi:10.3791/51137

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Method Article

Spatial Separation of Molecular Conformers and Clusters

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DOI:

10.3791/51137

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January 9th, 2014

Daniel Horke¹ , Sebastian Trippel¹ , Yuan-Pin Chang¹ , Stephan Stern¹^,² , Terry Mullins¹ , Thomas Kierspel¹^,³ , Jochen Küpper¹^,²^,³

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

Summary

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

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

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 syste....