High coherence and intensity at FERMI enables new X-ray interfacial probe

Interfaces are involved in a wide range of systems and have significant implications in many fields of scientific and technological advancement, often determining device performance or chemical reactivity. Vital examples include solar cells, protein folding, and computer chips.A class of commonly used surface science techniques are comprised of even-ordered nonlinear spectroscopies (i.e., second harmonic and sum frequency generation) which exhibit no response in centrosymmetric media due to symmetry constraints.As a result, they have been widely used at optical wavelengths to explore physical and chemical properties of interfaces, where centrosymmetry is broken. Extending this to x-ray wavelengths would effectively combine the element specificity and spectral sensitivity of x-ray spectroscopy with the rigorous interfacial/surface specificity of optical even-ordered nonlinear spectroscopies. Unfortunately, at hard x-ray energies (x-ray wavelength order of the spacing between atoms) these even-ordered nonlinear spectroscopies are effectively bulk probes, as each individual atom breaks inversion symmetry. As soft x-ray wavelengths fall in between the UV and hard x-ray regimes, there has been uncertainty regarding the interface specificity of soft x-ray second harmonic generation.
Utilizing the FERMI FEL-2 free electron laser, a light source with the requisite high coherence and intensity, at the EIS-TIMEX beamline and measuring the light transmitted following the excitation of graphite films with soft x-ray radiation, the generation of second harmonic light from soft x-rays was observed for the first time (Figure 1). Moreover, the second harmonic signal did not show a dependence with the film thickness, implying that the signal is not bulk-volume dependent. Finally, by varying the x-ray energy, the effect of resonance enhancement was observed, consistent with the energy sensitivity seen in traditional x-ray spectroscopy. Accompanying calculations indicate that this technique is indeed highly surface specific, with the signal originating primarily from the top-most molecular layer. This result stands in stark contrast to traditional ‘surface specific’ x-ray spectroscopies, which effectively probe the first few nanometers of a sample as they are restricted by the inelastic mean free paths of the photoionized electrons/ions or by the penetration depth of the incident x-rays. As modern free electron lasers are inherently ultrafast probes, it should be possible to combine the technique with tools to study dynamics at a surface, such as watching a catalyst work or a protein unfold.

Figure 1 Experimental Design. X-ray pulses are passed through a 2 mm iris and focused onto the graphite sample at normal incidence. The transmitted beam is then passed through a 600 nm aluminum filter and onto a spectrometer grating, spatially resolving the second harmonic signal from the fundamental. Inset: A schematic energy level diagram of the second harmonic generation process.



This research was conducted by the following research team:

Royce K. Lam,1,2 Sumana L. Raj,1,2 Tod A. Pascal,3 C. D. Pemmaraju,4 Laura Foglia,5 Alberto Simoncig,5 Nicola Fabris,6,7 Paolo Miotti,6,7 Christopher J. Hull,1,2 Anthony M. Rizzuto,1,2 Jacob W. Smith,1,2 Riccardo Mincigrucci,5 Claudio Masciovecchio,5 Alessandro Gessini,5 Enrico Allaria,5 Giovanni De Ninno,5,8 Bruno Diviacco,5 Eleonore Roussel,5 Simone Spampinati,5 Giuseppe Penco,5 Simone Di Mitri,5 Mauro Trovò,5 Miltcho Danailov,5 Steven T. Christensen,9 Dimosthenis Sokaras,10 Tsu-Chien Weng,11 Marcello Coreno,5,12 Luca Poletto,6 Walter S. Drisdell,2 David Prendergast,3 Luca Giannessi,5,13 Emiliano Principi,5 Dennis Nordlund,10 Richard J. Saykally,1,2 Craig P. Schwartz3,10

1 Department of Chemistry, University of California, Berkeley, California, USA
2 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
3 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, USA
4 Theory Institute for Materials and Energy Spectroscopies, SLAC National Accelerator Laboratory, Menlo Park, California, USA
5 Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
6 Institute of Photonics and Nanotechnologies, National Research Council of Italy,  Padova, Italy
7 Department of Information Engineering, University of Padova, via Gradenigo 6/B, I-35131 Padova, Italy
8 Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia
9 National Renewable Energy Laboratory, Golden, Colorado, USA
10 SLAC National Accelerator Laboratory, Menlo Park, California, USA
11Center for High Pressure Science & Technology Advanced Research, Pudong, Shanghai, China
12 ISM-CNR, Elettra Laboratory, Trieste, Italy
13 ENEA, C.R. Frascati, Frascati (Rome), Italy

Contact persons:

Emiliano Principi, email:


R.K. Lam, S.L. Raj, T.A. Pascal, C.D. Pemmaraju, L. Foglia, A. Simoncig, N. Fabris, P. Miotti, C.J. Hull, A.M. Rizzuto, J.W. Smith, R. Mincigrucci, C. Masciovecchio, A. Gessini, E. Allaria, G. De Ninno, B. Diviacco, E. Roussel, S. Spampinati, G. Penco, S. Di Mitri, M. Trovò, M. Danailov, S.T. Christensen, D. Sokaras, T.-C. Weng, M. Coreno, L. Poletto, W.S. Drisdell, D. Prendergast, L. Giannessi, E. Principi, D. Nordlund, R.J. Saykally, C.P. Schwartz, "Soft X-Ray Second Harmonic Generation as an Interfacial Probe", Phys. Rev. Lett. 120, 023901 (2018), DOI: 10.1103/PhysRevLett.120.023901. 

Last Updated on Friday, 25 June 2021 11:22