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 Feld, Steinfeld, Moungi G. Bawendi, Robert W. Field, Daniel Kleppner, Keith A. Nelson, Stephen J. Lippard, Jeffrey I. 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.
RESEARCH HIGHLIGHTS
Professor Field and Dr. Steven Coy have developed a powerful pattern recognition technique, extended spectral cross-correlation, to extract patterns (relative intensities, energy splittings) repeated in two or more spectra without any prior knowledge of the nature of the patterns or even the number of repeated patterns present. Using this technique, complete information has been extracted from the dispersed fluorescence (DF) spectrum of acetylene about the early time (t < 1ps) dynamics of a "Franck-Condon bright state" on the electronic ground state potential surface, at all energies up to 16,000 cm-1 above the zero-point level.
Professors Field and Steinfeld have initiated a spectroscopic study of the isocynanogen NCNC molecule to observe the NCNC->NCCN isomerization process in the DF and/or stimulated-emission-pumping spectrum. Our initial spectroscopic goal is to record the electronic spectrum of NCNC in the ultraviolet region (250nm), first by direct absorption and then by cavity ringdown spectroscopy.
Professor Field and Dr. Steven Drucker are developing methods for studying triplet electronic states of small, unsaturated hydrocarbon molecules (e.g. acetylene). Triplets are long-lived energetic species that often play an unsuspected role in photochemical processes. Our first complete prepare-probe-detect apparatus for triplet spectroscopy has been designed, and the experiments are in progress.
Professors Steinfeld and Field and their students are investigating the use of advanced optical techniques for atmospheric monitoring. They have recently demonstrated that backscattered light preserves the phase information necessary for FM detection. Strong backscattered FM signals were observed from molecular iodine at a vapor density of 109-1010 cm-1, suggesting that FM-based remote sensing could be a sensitive and versatile technique for measuring atmospheric trace gases. The addition of FM capability to pulsed lasers used in atmospheric remote sensing promises a considerable enhancement of sensitivity in comparison to the traditional differential absorption lidar. By using a frequency modulated pulse, the absorber distribution could be obtained directly in the FM-detected response from a single pulse. Moreover, since the amplitude of the FM signals is approximately proportional to the ratio of rf to linewidth, relatively sharp molecular absorption lines (pressure broadened at 1 atm to 5 GHz, or 0.1 cm-1), may be readily distinguished from the near-continuous background attenuation due to particulate scattering or molecular aggregates. This technique can be extended into the ultraviolet and mid-infrared regions, where key atmospheric trace gases such as methane, non-methane hydrocarbons, and nitrogen oxides can be detected.
Professor Bawendi is using a picosecond laser and time-correlated photon counting to study the electronic properties of semiconductor quantum dots and heterostructures containing those dots. Data on dilute samples of dots have been used to develop models of relaxation mechanisms and fine structure in the electronic transitions. Time-resolved studies of the heterostructures (close packed arrays of dots) have been used to study and understand energy transfer between dots. Professor Bawendi has also developed a new apparatus to study the spectroscopy of individual quantum dots. His group has found that the linewidth are ultranarrow (<0.120meV), a result which has important implications for the physics and applications of the materials. They have also begun to study Stark effects of individual dots, which is important for any device application that uses dots and electric fields to modulate light.
Professors Marc Kastner and Bawendi studied charge transport in close-packed dot heterostructures . They used the picosecond apparatus to study the temperature dependence of charge separation dynamics following photoexcitation.
Professor Lippard and his associates have used Raman spectroscopy to investigate the reactions of dioxygen with diiron (II) and dicopper (I) complexes as models for metalloenzyme active sites. The O-O stretching frequencies of the resulting peroxo-bridged dimetallic complexes were measured and used to characterize the species present. Fluorescence resonance energy transfer studies were also carried out to investigate 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 have investigated molecular changes in the protein crystallin and eye lens using Raman spectroscopy. Oxidative stress, which leads to a variety of non-enzymatic modifications in crystallins, is the major cause of cataract formation. The intensity of S-S and S-H stretching modes in Raman spectra has shown that sulfur centered oxidative dimerization occurs in crystallins
Professor Tannenbaum and Drs. Paul L. Skipper, Dasari and V. Bhaskaran Kartha 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 100 volunteers have been analyzed, with 17 samples showing BP adducts ranging from 0.05 to 4.8 fmol adduct per mg of albumin. Another 145 albumin samples and 35 lung histone samples remain to be analyzed to complete a comprehensive epidemiological study.
Professor Alexander Rich and Drs. Imre Berger, Dasari and Manoharan have demonstrated the specificity of human double-stranded RNA deaminase enzyme for left-handed Z-DNA using Raman spectroscopy. Raman spectra of B-DNA/Za-peptide complex exhibit characteristic Z-DNA peaks at 627 and 1318 cm-1, which are not observed in either B-DNA or Za-peptide. The occurrence of these bands in the B-DNA/Za-Peptide complex shows that poly d(GC) DNA adopts a Z-DNA conformation and binds to the Za-peptide. This study provides the first direct evidence for the actual existence of left-handed DNA in the protein-DNA complex.
Professor Ali Javan's research has focused on the resistance vs. voltage characteristics (RVC) and optical response of superconductor-normal metal Point Contacts (SNPCs). SNPCs are the simplest means of constructing nanoscale conductance paths between a bulk super conductor and a metal. Novel RVC features measured on these SNPCs have been shown to result from flow of critical current in the SNPC. The optical response has been used in a new technique to measure superconductor relaxation rates in real time.
Professor Kleppner and his students have extended our understanding of the connections between quantum mechanics and classical motion by their new technique of recurrence spectroscopy in a microwave field. Periodic orbits in their system, a Rydberg atom in an electric field, can be identified from the Fourier transform of the spectrum. They found that by applying microwave fields near resonance with the periodic orbits, the intensity of the recurrences was systematically modified in a fashion that could be related to the detailed motion of the corresponding classical system. These results illustrate one case in which quantum mechanics can describe detailed classical motion.
Professor Feld and Drs. Dasari and Kyungwon An have studied the single-atom microlaser, a fundamental laser device with a single atom as the gain medium. Recent progresses include demonstration of a traveling-wave atom-cavity interaction in the microlaser and development of a precision spectroscopic technique to measure mirror absorption at sub-ppm levels using thermally induced optical bistability.
Professors Tanaka and Feld and Drs. Kartha and Dasari studied spectroscopies of gels near phase transitions. Random heteropolymers are known to be in three phases: swollen, collapsed but fluctuating, and collapsed and frozen in a conformation. The third phase, which is considered to be responsible for the stability and memory of conformation of proteins, was found in copolymer gels, where major interactions are hydrogen bonding. The degree of hydrogen bonding is being studied using FT-IR and Raman spectroscopy.
Professor Feld and Drs. Dasari, Geurt Deinum, Irving Itzkan, Manoharan, and Lev Perelman pursued basic and applied applications of lasers in biology and medicine. Reflectance, fluorescence and near-IR Raman spectroscopy were used for biochemical analysis of tissues and blood, and diagnosis of dysplasia, cancer, atherosclerosis and other diseases. Clinical studies were pursued with researchers from the Cleveland Clinic Foundation, Brigham and Women's Hospital, Metrowest Hospital and New England Medical Center in colon, Barretts' esophagus, bladder, breast and coronary and peripheral arteries. Quantitative analysis of blood analytes using Raman spectroscopy is under development. Observation of the nuclear signatures in reflectance spectra lead to a new technique for measuring nuclear size distribution in biological tissues. UV resonance Raman spectroscopy was used to characterize dysplasia. Photon migration using a newly developed time-resolved optical tomographic system was used to image small fluorescent objects (lesions) imbedded in turbid biological tissue in the presence of background fluorescence, and to study paths of early arriving photons. Finally, the mechanism of pulsed laser ablation of soft and hard biological tissues was shown to be thermoelastic in origin. 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/www
Michael S. Feld
MIT Reports to the President 1996-97