1. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Dipole Aligned Solids A Metastable Phase of Molecular Materials Martin McCoustra

2. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Field and co-workers measurements of slow (meV) electron interactions at surfaces  Slow electrons (meV) produced by synchrotron radiation induced photoemission from rare gases  Vapour deposition of molecular films on a variety of substrates  Simple current measurement Observations on Dipole Alignment

3. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Field and co-workers measurements of slow (meV) electron interactions at surfaces  Slow electrons (meV) produced by synchrotron radiation induced photoemission from rare gases  Vapour deposition of molecular films on a variety of substrates  Simple current measurement Observations on Dipole Alignment

4. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Field and co-workers measurements of slow (meV) electron interactions at surfaces Observations on Dipole Alignment  Range of molecular species investigated but let’s look at N 2 O as an example

5. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Observations on Dipole Alignment  Field and co-workers measurements of slow (meV) electron interactions at surfaces  Range of molecular species investigated but let’s look at N 2 O as an example  Surface potential increases linear with film thickness  Surface potential decreases with increasing deposition and annealing temperature  Some systems exhibit a Curie Temperature

6. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Dipole alignment is assumed to be the source of the potential  Non-linear and non-local in that an initial alignment of dipoles creates the electric field which itself creates further dipole alignment through the film  Creates a film with some degree of order while thermal motion creates disorder Observations on Dipole Alignment

7. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Measurements on films diluted with a rare gas demonstrate the long range nature of this effect  Increasing dilution of N 2 O in Xe reduces the dipole aligned field strength  No field is formed when the average distance between N 2 O is greater than 1.25 nm and less than 1.75 nm Observations on Dipole Alignment

8. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  This is the point at which we entered the picture as Field and co-workers asked us to take a look at the IR spectra of the N 2 O – Xe films using reflection-absorption IR (RAIR) spectroscopy  No evidence for segregation of N 2 O as bulk solid  Only monomer, dimer, trimer and other small clusters An Aside… A. Cassidy, O. Plekan, J. Dunger, R. Balog, N. C. Jones, J. Lasne, A. Rosu-Finsen,c M. R. S. McCoustra and D. Field, Phys. Chem. Chem. Phys., 2014, 16, 23843

9. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  A simple mean-field model has been constructed to describe the local electric field in the thin film generated by the spontaneous alignment of dipoles (i.e. the spontelectric field) A Simple Model… Symmetric local average intermolecular dipole-dipole interaction Asymmetric field created by the averaged dipoles and experienced by the average dipole

10. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University A Simple Model…  The simple model works very well for N 2 O and other systems  Allows estimates to be made of the spontelectric field strength associated with the degree of alignment expressed in the film,   Deviation at high temperature is probably due to N 2 O sublimation

11. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Seeing in a New Light…  The simple model works very well for N 2 O and other systems  Allows estimates to be made of the spontelectric field strength associated with the degree of alignment expressed in the film,   Deviation at high temperature is probably due to N 2 O sublimation

12. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Seeing in a New Light…  At this point, we decided to look at the IR spectroscopy of N 2 O more closely!  N 2 O has 3 vibrational modes (actually 4 but 2 are degenerate)  In the solid state, the vibrations are split due to coupling with the lattice phonons into LO-TO pairs  Our experimental arrangement allows us to see them both! Bend 569 cm -1 NO 1298 cm -1 NN 2282 cm -1

13. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Seeing in a New Light…  At this point, we decided to look at the IR spectroscopy of N 2 O more closely!  N 2 O has 3 vibrational modes (actually 4 but 2 are degenerate)  In the solid state, the vibrations are split due to coupling with the lattice phonons into LO-TO pairs  Our experimental arrangement allows us to see them both! J. Lasne, A. Rosu-Finsen, A. Cassidy, M. R. S. McCoustra and D. Field, Phys. Chem. Chem. Phys., 2015, 17, 20971

14. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Seeing in a New Light…  At this point, we decided to look at the IR spectroscopy of N 2 O more closely!  N 2 O has 3 vibrational modes (actually 4 but 2 are degenerate)  In the solid state, the vibrations are split due to coupling with the lattice phonons into LO-TO pairs  Our experimental arrangement allows us to see them both!

15. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Seeing in a New Light…  We observe a contraction in the LO-TO splitting of the IR modes of N 2 O  Consistent with a Vibrational Stark Effect arising from the electric field associated with dipole alignment  Extension of the simple model reproduces the IR observations

16. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Seeing in a New Light…  We observe a contraction in the LO-TO splitting of the IR modes of N 2 O  Consistent with a Vibrational Stark Effect arising from the electric field associated with dipole alignment  Extension of the simple model reproduces the IR observations Deposition Temperature / K LO-TO Splitting / cm -1

17. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University So what now?  Demonstrate use of RAIRS in other systems that might exhibit this behaviour  CO – Online in Phys. Chem. Chem. Phys.  CO on Water Ice Surface – Relevant to Astrophysical Ices – In preparation for Phys. Chem. Chem. Phys. and Mon. Not. Roy. Astron. Soc.  Recent work by Baragiola and co-workers (J. Chem. Phys., 2015, 142, 134702) on H 2 O ices demonstrates the potential of Kelvin Probe measurements in detecting the spontelectric state  Implement a Kelvin Probe and investigate N 2 O and CO – Should be in place by Christmas!  Look for other probes (e.g. THz Spectroscopy; EFISHG; AFM) and develop deposition simulations with realistic potentials

18. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Professor David Field and Dr. Andrew Cassidy (ASTRID and Aarhus University) Dr. Jérôme Lasne Dr. Mark Collings Alexander Rosu-Finsen ££ European Community FP7-ITN Marie-Curie Programme (LASSIE project, grant agreement #238258) ££ Acknowledgements