Name: spatial separation of molecular conformers and clusters
Uploaded: 2014-01-09T14:00:00
Description: Watch this Scientific Journal Video about Spatial Separation of Molecular Conformers and Clusters at JoVE.com

This work has been supported by the excellence cluster &#34;The Hamburg Center for Ultrafast Imaging &#8211; Structure, Dynamics and Control of Matter at the Atomic Scale&#34; of the Deutsche Forschungsgemeinschaft and by the Helmholtz Virtual Institute &#34;Dynamic Pathways in Multidimensional Landscapes&#34;.

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
<ol>
	<li>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 ...

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...

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 &#34;cluster soup&#34;, 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 force<img fo:content-width="0.25in" src="/files/ftp_upload/51137/force.jpg" width="30px" /> is dependent on the effective dipole moment, &#956;eff, of the molecule and can be evaluated as
<img src="/files/ftp_upload/51137/51137eq1.jpg" /> (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 &#956;eff, 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/&#956;eff) 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.

<table border="0" cellpadding="0" cellspacing="0" width="687">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Vacuum system</td>
			<td style="width:175px;">various, e.g. Pfeiffer Vacuum, Varian, Edwards, Leybold</td>
			<td style="width:119px;"></td>
			<td style="width:177px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Dye laser system</td>
			<td style="width:175px;">various, e.g. Coherent, Spectra Physics, Syrah, LIOP-TEC, Radiant Dyes&#8230;</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Pulsed valve</td>
			<td style="width:175px;">Even-Lavie</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">High voltage power supply</td>
			<td style="width:175px;">eg. FUG</td>
			<td style="width:119px;">HCP 14-20000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Deflector</td>
			<td style="width:175px;"></td>
			<td style="width:119px;"></td>
			<td>Custom made</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Time-of-flight spectrometer</td>
			<td style="width:175px;">Jordan TOF</td>
			<td style="width:119px;">C-677</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">TOF power supply</td>
			<td style="width:175px;">Jordan TOF</td>
			<td style="width:119px;">D-603</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Focusing lens</td>
			<td style="width:175px;">e.g. Thorlabs</td>
			<td>LA4745</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Translation stage</td>
			<td style="width:175px;">e.g. Vision Lasertechnik</td>
			<td style="width:119px;">8MT167-25</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Digitizer</td>
			<td style="width:175px;">e.g. Agilent</td>
			<td style="width:119px;">Acquiris DC440</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Digital delay generator</td>
			<td style="width:175px;">e.g. Stanford Systems</td>
			<td style="width:119px;">SRS DG645</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Molecular beam skimmer</td>
			<td style="width:175px;">Beam Dynamics Inc.</td>
			<td style="width:119px;"></td>
			<td>http://www.beamdynamicsinc.com/</td>
		</tr>
	</tbody>
</table>

spatial separation of molecular conformers and clusters

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.

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...

Watch this Scientific Journal Video about Spatial Separation of Molecular Conformers and Clusters at JoVE.com

Spatial Separation of Molecular Conformers and Clusters

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.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

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