Contains a report on one research project.Joint Services Electronics Program (Contract DAAG29-80-C-0104)National Science Foundation (Grant DMR78-23555

Larson, Brent D.

Litster, James David

Solomon, Lorraine E.

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42 Time-Resolved Spectroscopy of CondensedMatter8. Time-Resolved Spectroscopy of CondensedMatterAcademic Research StaffProf. J.D. Litster, Dr. L.C. Kupferberg 2Graduate StudentsD.E. Cooper, B.D. Larson, L.E. SolomonJoint Services Electronics Program (Contract DAAG29-80-C-0104)National Science Foundation (Grant DMR78-23555)James D. Litster, Brent D. Larson, Lorraine E. SolomonWe are using time-resolved spectroscopy in conjunction with light scattering and x-ray scatteringto study the properties of condensed phases (solids, liquids, liquid crystals, and other phases) ofmatter. Our goal is to understand why large numbers of interacting particles form the phases whichthey do and why they have the properties which they have; this problem of phase transitions is one ofthe most important problems of current interest in physics.The tools we use provide different kinds of information. From x-ray scattering we learn about themass density fluctuations and structure on length scales as small as I A to as large as I pm using highresolution, but we learn nothing about dynamical behavior since it is not possible to measure energychanges in the scattered radiation with sufficient precision. Quasi-elastic light scattering enables usto study the dynamical behavior of fluctuations which alter the refractive index of the material studied;by digital autocorrelation we can measure energy changes in the scattered light of I part in 1014. Lightscattering, however, restricts us to studying properties on a length scale of half the wavelength oflight or longer. The use of time-resolved spectroscopy enables us to study the dynamical behavior ofprocesses as fast as 10-12 second and, in addition, it is frequently a local probe of behavior, thusproviding information on the same length scale as x rays combined with dynamical information thatcan be obtained optically.In the past, we have applied this singular technique to studying phase separation in binary criticalmixtures and to studying the microscopic fluid properties of polymer solutions and gel phases.During the past year we have been concentrating on the behavior of surfactant molecules in watersolutions with salt. These molecules are amphiphilic, that is, one end prefers to interact with polarmaterials like water, and the other end prefers an oily environment. In a sufficiently concentrated2 Assistant Professor, Department of Physics, Worcester Polytechnic Institute, Worcester, MassachusettsRLE P.R. No. 12443 Time-Resolved Spectroscopy of CondensedMattersolution (about 10 " molar) they orygmize themiselv e into miceles, rougl ly spherical structures withthe oil-loving ends of the molecule in the middle and the polar moiety on the outside. As moresurfactant is added to the solution more micelles form, and when about 50% of the volume isoccupied by micelles, the micelle-micelle interaction has become so strong that they form an orderedphase. The micelles we have been studying are decylammonium chloride (DACI) and have beenfound by x-ray scattering to be diskshaped at high concentrations, with a diameter of 60 A and athickness of 20 A. They order first into a phase in which the symmetry axes of the disk align parallel,but without positional ordering. On lowering the temperature, the disks fuse themselves into bilayerswith the oil-loving moiety in the interior of the layer. This occurs as a second-order (continuous)transition. New hydrodynamic normal modes appear as a result of this ordering, and we have beenstudying these by light scattering.We have been using pulses from a mode-locked dye laser to study the orientational dynamics of themicelles as a function of temperature and surfactant concentrations. This is done in a way quiteanalogous to using a strobe light to study rotating machinery. By adding a small amount of dye(about one molecule to each micelle) we can monitor the orientation of the micelles by measuring thedecay of dichroism following an excitation pulse of light. The reorientation of a single micelle insolution would lead to a dichroism decaying as e-60D where the rotary diffusion constant D is given byD = kT/6V, where 71 is the viscosity of the water, and V is the volume of the micelle. At low micelleconcentrations, the micelles behave as independent particles with a volume that agrees with the x-raymeasurements; we find 1/6D - 400 psec at 250C and scales with the viscosity of water as thetemperature is changed. At higher concentrations, when the micelles interact, the reorientation timeappears to be temperature-independent, which is a quite unexpected result. We believe the statisticalmechanics of how these micelles interact is of fundamental interest and will pursue it vigorously.Future directions will also include a study of nonradiative energy transfer in micelles; this will yieldinformation on the structure of the micelle, as well as being of some practical importance for buildingmore efficiently pumped dye lasers for some spectral regions. We expect that the elucidation of thebehavior these materials, which are lyotropic liquid crystals, will contribute to progress in the field ofstatistical mechanics. Less broadly, we expect to gain greater insight into the physics of surfactants,and the formation and stability of microemulsions and bilayers, which are the basic structuralcomponent of cell walls in biology.Publications1. J.D. Litster, C.W. Garland, K.J. Lushington, and R. Schaetzing, "Experimental Studies of LiquidCrystal Phase Transitions," Mol. Cryst. and Liq. Cryst. 63, 145 (1981).2. P.S. Pershan, G. Aeppli, J.D. Litster, and R.J. Birgeneau, "High Resolution X-ray Study of theSmectic A - Smectic B Phase Transition in Butyloxybenzylidine Octylaniline," Mol. Cryst. andLiq. Cryst. 67, 205 (1981).3. H. von Kanel, J.D. Litster, "Light Scattering Studies on the Single-Layer Smectic p-Butoxy-benzylidene p-Octylaniline," Phys. Rev. A 23, 3251 (1981).4. H.V. Kanel, J.D. Litster, J. Melngailis, and H.I. Smith, Phys. Rev. A 24, 2713 (1981).RLE P.R. No. 12444 Time-Resolved Spectroscopy of CondensedMatter5. C.R. Safinya. R.J. Birgeneau, J.D. Litster. and M.E. Neubert, "Critical Fluctuations Near a Nematic-Smectic A-Smectic C Multicritical Point." Phys. Rev. Lett. 47, 668 (1981).6. R. Kortan, H.V. Kanel, R.J. Birgeneau, and J.D. Litster, "High Resolution X-ray Scattering Study ofthe Nematic-Smectic A-Re-entrant Nematic Transitions in 80CB/60CB Mixtures," Phys. Rev.Lett. 47, 1206 (1981).RLE P.R. No. 124

Time-Resolved Spectroscopy of Condensed Matter

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Time-Resolved Spectroscopy of Condensed Matter

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