New light on hidden surfaces New light on hidden surfaces PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR AAN DE UNIVERSITEIT LEIDEN, OP GEZAG VAN DE RECTOR MAGNIFICUS DR.D.D.BREIMER, HOOGLERAAR IN DE FACULTEIT DER WISKUNDE EN NATUURWETENSCHAPPEN EN DIE DER GENEESKUNDE, VOLGENS BESLUIT VAN HET COLLEGE VOOR PROMOTIES TE VERDEDIGEN OP WOENSDAG 15 SEPTEMBER 2004 KLOKKE 15.15 UUR DOOR SYLVIE ROKE GEBOREN TE DE BILT IN 1977 Promotiecommissie Promotor: Prof. Dr. A. W. Kleyn Co-promotor Dr. M. Bonn Referent Prof. Dr. H. J. Bakker Overige leden: Prof. Dr. J. Reedijk Prof. Dr. J. W. M. Frenken Prof. Dr. G. J. Kroes Prof. Dr. A. van Blaaderen The work described in this thesis was made possible by financial support from the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organization for Scientific Research (NWO). ”...Thereisnoproblemknowntosciencethatcannotbecured by the liberal application of chocolate.” RICHARD BUTTERWORTH Contents 1Introduction 1 1.1Surfaces................................. 1 1.2Second-ordersumfrequencygeneration............... 3 1.3Femtosecondsumfrequencygeneration............... 4 1.4Thisthesis................................ 6 2 Experimental 9 2.1Introduction............................... 9 2.2Thelasersystem............................ 9 2.3Second-orderprocesses........................ 10 2.4Generatinginfraredpulses....................... 12 2.5Thesumfrequencyexperiment.................... 12 3 Time vs. frequency domain sum frequency generation 15 3.1Introduction............................... 16 3.2Theoreticalbackground........................ 17 3.3Experimental.............................. 19 3.4Homogeneousbroadening....................... 21 3.5Inhomogeneousbroadening...................... 23 3.6Theinfluenceofthesurface...................... 27 3.7Conclusions............................... 28 4 Time resolved sum frequency generation 29 4.1Introduction............................... 30 4.2 Modeling the CO/Ru(001) system . 32 4.3EffectofFIDontransientspectralfeatures.............. 35 4.4EffectofFIDontimeresolution.................... 36 4.5Observingtransitionstates....................... 40 4.6Conclusions............................... 43 vii 5 The phase behavior of phospholipids 45 5.1Introduction............................... 46 5.2Experimental.............................. 47 5.3 Sum frequency generation and fluorescence microscopy . 48 5.4Anewphasetransition......................... 51 5.4.1Heatingeffect.......................... 54 5.5Order-disordertransition........................ 54 5.6Conclusions............................... 58 6 Sum frequency generation scattering 61 6.1Introduction............................... 62 6.2Experimental.............................. 62 6.3Results.................................. 64 6.4Conclusions............................... 70 7 Nonlinear optical scattering: the concept of an effective susceptibility 71 7.1Introduction............................... 72 7.2Theory.................................. 72 7.2.1Reciprocity........................... 73 7.2.2Symmetry............................ 75 7.3Sumfrequencygenerationscattering................. 76 7.3.1Smallparticles......................... 76 7.3.2Indexmatchedparticles.................... 78 7.3.3Smallindexdifference..................... 81 7.3.4Correlatedscattering...................... 83 7.3.5ComparisonofRGDandWKBapproximations........ 85 7.4Conclusions............................... 87 8 Molecular origin of a phase transition of colloids 89 8.1Introduction............................... 90 8.2Experimental.............................. 91 8.3Molecularsurfacestructure...................... 92 8.3.1Theoreticalconsiderations................... 92 8.3.2Gelaging............................ 97 8.4Temperaturedependenceandtheroleofthesolvent........ 98 8.4.1Calorimetricmeasurements.................. 98 8.4.2Phasetransitioninn-hexadecane............... 99 8.4.3Phasetransitioninbenzene..................100 8.4.4Discussionofsolventeffect..................101 8.5Conclusions...............................102 A Molecular orientation from SFG spectra 103 Bibliography 107 ix Summary and outlook 115 Samenvatting 119 List of publications 123 Curriculum Vitae 125 Nawoord 127 x Chapter 1 Introduction 1.1 Surfaces Asurfaceistheendofabulk.Itisthemembraneinacell,theactiveareaina catalyst, the exterior of an ice particle in the stratosphere, the place where oxygen becomes part of the human body. It is at these places that proteins, enzymes and other active molecules are selectively permitted into the cell, reactions are accel- erated, the place where ozone is destroyed and the blood cell takes up oxygen from the blood. Surfaces or interfaces are also the place to find interesting and complex physics, because the endless repetition of atoms and molecules of the bulk comes to a halt and in some way makes the transition to another bulk medium. This asymmetry offers a great challenge to physicists in describing surface phenomena [1]. It is illustrative to quote the famous physicist Enrico Fermi (Nobel Prize 1938) who said: ”God made the solid state. He left the surface to the Devil.” Important physical, chemical and biological processes occur at surfaces. Often these surfaces are hidden, because they are masked by the two bulk media that they separate [2]. The number of molecules residing on the surface of a (spher- ical) 1 ml drop of water is in the order of 1015,whereasthebulkiscomposed of 1022 molecules. Since the importance of surface processes became clear in the early years of the 20th century, many techniques have been developed success- fully to characterize and understand surface processes. The exploration of solid-gas interfaces matured with the introduction of ultra high vacuum (UHV) in the 1960’s, which made it possible to investigate atom- ically clean surfaces. The techniques used employ the interaction of ions, elec- trons and x-rays with surfaces [3, 4]. This research is mainly motivated by the development of heterogeneous catalysts, the understanding of surface processes in material sciences like corrosion and the construction of semiconductor devices. To study buried interfaces one needs to make use of interactions that are not 1 2CHAPTER ONE hindered by the presence of intervening solids or liquids. One means is to distort the surface region and measure its reaction using either the restoring thermo- dynamic forces or the restoring proximal forces. Restoring thermodynamic forces generate a picture that reflects changes on a macroscopic scale and can be measu- red by, e.g., recording the surface tension. Proximal forces can be used to display the surface on a microscopic scale. This method is applied in, e.g., the atomic force microscope. A different, non-invasive, approach is to probe the surface with photons. If these photons have the same energy as the energy difference between energy lev- els of the surface molecules one can obtain information on the molecular level. The vibrational modes of molecules are sensitive to the local environment. Prob- ing these modes with infrared photons thus generates a molecular picture of the molecule and its local surroundings. This type of spectroscopy is called vibrational w (1) w I(0 ) c I( ) I/I0 Pot. Energy w RES w w RES Bond Distance Figure 1.1: Schematic illustration of the optical processes involved in vibrational spectroscopy. It shows the potential energy surface of a vibrational mode with a number of possible energy states. The infrared light interacts resonantly with a vibrational mode. At parts of the spectrum where the frequency is resonant the light is absorbed. In the spectrum this appears as a dip. spectroscopy and is illustrated in Fig. 1.1. The electromagnetic infrared field (E) drives the oscillations in the molecules and creates a polarization (P)thatisgiven by: P(r, t)=χ(1)(r, t)E(r, t). (1.1) The amount of polarization that is built up is determined by the first order sus- ceptibility (χ(1)(r, t)). If the infrared field becomes resonant with the vibrational mode, the first order susceptibility becomes large, resulting in a large polariza- tion. The polarization radiates an electric field that interferes destructively with INTRODUCTION 3 the incoming field. These destructive interferences show up as dips in the result- ing intensity spectrum (see Fig. 1.1). As there is no specific surface sensitivity in the light-matter interactions other than the one imposed by the experimental geometry, both the bulk and the surface are measured simultaneously. 1.2 Second-order sum frequency generation This lack of surface sensitivity can to a large extent [5] be overcome by the appli- cation of second-order vibrational sum frequency generation (SFG). In a second- order vibrational sum frequency experiment an infrared photon (with a wave- length of (e.g.) ∼ 3333 nm=3000 cm−1) and a visible photon (with a wavelength of (e.g.) ∼ 800 nm=12500 cm−1) interact with the molecules on the surface (see Fig. 1.2). The sum frequency polarization that is created due to interaction with c(2) (1) w w c 1 w 1 w 1 0 c(1) w w Pot. Energy 2 2 w w 2 0 Bond Distance Figure 1.2: Schematic illustration of the optical processes involved in a sum fre- quency generation experiment on a flat surface. It shows the infrared (with fre- quency ω2) and visible laser beams (with frequency ω1) reflecting from the sur- face. This reflection is governed by the first order susceptibility. At the surface, a photon with the sum of the frequencies (ω0=ω1+ω2) is formed through