MIT Reports to the President 1997-98
The George Russell Harrison Spectroscopy Laboratory is engaged in research in the field of modern optics and spectroscopy for the purpose of furthering fundamental knowledge of atoms and molecules and pursuing advanced engineering and biomedical applications. Professor Michael S. Feld is Director; Professor Jeffrey I. Steinfeld and Dr. Ramachandra R. Dasari are Associate Directors. An Interdepartmental Laboratory, the Spectroscopy Laboratory encourages participation and collaboration among researchers in various disciplines of science and engineering. Professors Moungi G. Bawendi, Feld, Robert W. Field, Daniel Kleppner, Stephen J. Lippard, Keith A. Nelson, Steinfeld, Toyoichi Tanaka, Steven R. Tannenbaum and Dr. Dasari are core investigators.
The Laboratory operates two laser resource facilities. The MIT Laser Biomedical Research Center (LBRC), a Biotechnology Resource Center of the National Institutes of Health, develops basic scientific understanding, new techniques and technology for advanced biomedical applications of lasers; core, collaborative and outside research are conducted. The National Science Foundation-supported MIT Laser Research Facility (LRF) provides resources for core research programs in the physical sciences for 13 MIT Chemistry and Physics faculty. Information about the equipment and facilities of the LRF and the LBRC can be found in the Spectroscopy Laboratory Researcher's Guide.
Powerful pattern recognition techniques have been developed in Professor Field's research group: extended cross correlation(XCC), extended autocorrelation and baseline stripping. These techniques have been adopted by scientists at many laboratories. In the Spectroscopy Laboratory, XCC has been used to discover local bend and counter-rotator vibrations in highly excited HCCH, as well as other unexpected dynamical features.
Professors Field and Steinfeld, in collaboration with Drs. Alexander Kachanov and Sergei Panov have studied cavity ringdown, an untrasensitive absorption technique. They have developed a new variant which employs relatively low-reflectance mirrors (99.5% rather than the usual 99.9995%), yet still provides quantum limited sensitivity. This scheme is ideal for UV and IR applications, where high reflectance mirrors are often not available, and for double resonance studies, where the cavity decay time and the ~1 us lifetime of the labeled level should be comparable.
Professor Field and Dr. Steven Drucker developed a sensitive new spectral technique for studying triplet states of small unsaturated hydrocarbon molecules. This scheme is based on electron ejection from a metal surface when impacted by an electronically excited molecule, with electronic (not vibrational) excitation energy greater than the metal's work function. A clear example of "gateway mediated intersystem crossing" was observed. Statistical methods are being developed for extracting information about the bright and gateway states from complex spectra.
Professor Bawendi's group has developed an apparatus to study the optical spectroscopy of individual quantum dots. Ultranarrow linewidths, < 0.120 meV, have been observed. Individual dots have been found to be highly polarizable. Large spectral diffusion effects have also been observed; these are attributed to changes in local electric fields caused by the thermal motion of trapped charges. These findings are important for quantum dot applications to electro-optic systems. An apparatus is now being constructed to study the time resolved spectroscopy of single dots.
Prof. Steinfeld and Dr. Arturo Gonzalez-Casielles, a postdoctoral fellow supported by the Fundacion Repsol (Spain), are investigating the use of self-assembled monolayers incorporating colloidal metallic particles as substrates for surface-enhanced Raman spectroscopy. With these substrates, molecular explosives such as TNT have been detected with good reproducibility in 104 molar solutions.
Professor Mildred Dresselhaus and Drs. Gene Dresselhaus, Marcos Pimenta, Alessandra Marucci and Huiming Cheng studied resonant Raman scattering of single wall carbon nanotubes, and discovered a new way to observe differences in behavior between metallic and semiconducting nanotubes. The diameter dependence of the Raman spectra of tubes less than 2 nm in diameter was studied. Other carbon-based systems that have been investigated include polyparaphenylene to study conformational changes associated with heat treatment.
Professor Feld and Drs. Dasari, Kyungwon An and Chung-chieh Yu continued their work on the single atom laser. They have observed nonlinear dependence of laser output on atom density, and also in the large atom-cavity detuning range, and have developed a two-laser optical pumping scheme for improved atomic velocity selection.
Professors Tanaka, Kevin Otto, Jane Morningstar and Feld and Drs. Taro Oya, Yasar Yilmaz, Takashi Enoki and Dasari are developing polymer gels that reversibly change their affinity to target molecules by orders of magnitude. The gels are made of copolymers of backbone monomers and monomers that attract the target through electrostatic interactions. Fluorescence spectra revealed the target being in two states, free and attached to absorption sites. Phase transitions in single network gels that undergo phase transitions in response to pH and temperature changes are also being studied. Raman and IR spectroscopy were used to study the level of hydrogen bonding.
Professor Lippard and his collaborators characterized synthetic models of metalloenzyme active sites using Raman spectroscopy. They measured the O-O stretch frequencies of peroxo-bridged species resulting from the interaction of diiron (II) complexes with dioxygen. Azido- and peroxo-bound models of the diiron active site in hemerythrin, a dioxygen carrier in marine invertebrates, were synthesized by Dr. Tadashi J. Mizoguchi and characterized by resonance Raman spectroscopy. Resonance energy transfer was used to study the interactions of high mobility group domain proteins with cisplatin-modified DNA containing pendant fluorescent donor and acceptor molecules.
Professor George Benedek and Drs. Jayanti Pande and Manoharan continue investigating molecular changes in the protein crystallin and eye lens using Raman spectroscopy. Oxidative stress in crystallins is the major cause of cataract formation. The oxidation of crystallin protein was studied by monitoring the intensity changes of the S-S and S-H stretching modes. The results show that sulfur centered oxidative dimerization occurs in crystallins, and suggest that disulfide formation or oxidation does not cause significant changes in secondary protein conformation.
Professor Tannenbaum and Drs. Paul L. Skipper and Dasari have analyzed and quantified levels of benzo[a]pyrene (BP) adducts in samples of human serum albumin and human lung histone proteins using the ultrasensitive HPLC with laser-induced fluorescence detection system. Human albumin samples from volunteers have been analyzed, with some showing BP adducts ranging from 0.05 to 4.8 fmol adduct per mg of albumin. Additional results from albumin and lung histone samples, currently under analysis, will provide a comprehensive epidemiological study.
Professor Kleppner and his students have completed their study of Rydberg atoms in an electric field to explore the connections between quantum mechanics and classical motion. Using the technique of recurrence spectroscopy in a microwave field, periodic Rydberg orbits were identified from the Fourier transform of the spectrum. By applying microwave fields near resonance with the periodic orbits, the recurrence intensities were shown to be systematically modified in a fashion that could be related to the detailed motion of the corresponding classical system.
Professor Alexander Rich and Drs. Imre Berger and Ramasamy Manoharan established that a human protein, ds RNA deminase (ds RAD) has specificity for binding Z-DNA. Raman spectra of poly (GC) in low and high salt solutions, control protein, protein/poly (GC) complex were obtained, and spectral features of left and right-handed DNA conformers were characterized. By comparing these with the spectra of the protein/poly (GC) complex, it was concluded that DNA exists in the left-handed conformer when it binds to ds RAD.
Professor Feld and Drs. Dasari, Rajan Gurjar, Eugen Hanlon, Irving Itzkan, Lev Perelman and Qingguo Zhang are pursuing basic and applied applications of lasers and spectroscopy in biology and medicine. Reflectance, fluorescence, coherent and acousto-optical techniques and near-IR Raman spectroscopy are being used for biochemical analysis of tissues and diagnosis of disease. Clinical studies are being conducted with researchers from the Cleveland Clinic Foundation, Brigham and Women's Hospital, Metrowest Hospital, Beth Israel Hospital and New England Medical Center. Highlights include: (1) Demonstration that Raman spectroscopy can accurately measure concentrations of glucose and other blood analytes in serum and blood at physiological levels. (2) Clinical demonstration of a light scattering technique to detect the precancerous condition known as dysplasia, accomplished by determining the size distribution of epithelial cell nuclei. (3) The use of fluorescence spectroscopy of brain tissue to identify Alzheimer's disease, opening the prospect of non-invasive diagnosis and perhaps also determining the severity of this disease in vivo. (4) The use of Michelson interferometry to study the interplay of stress waves and cavitation in soft biological tissues, induced by short pulses of laser light. (5) The use of heterodyne detection to study forward-scattered light propagating in a turbid medium (such as biological tissue); the degree of spatial coherence was found to be significantly higher for photons scattered just once, and to approach a small constant value for a large number of scattering events. The experimental and theoretical work being conducted in this program is advancing new laser diagnostic technologies in the field of medicine.
More information about the Laboratory can be found on the World Wide Web at http://web.mit.edu/spectroscopy
Michael S. Feld
MIT Reports to the President 1997-98