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. The Laboratory is directed by Professor Michael S. Feld. 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 of the Laboratory.
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 thirteen MIT Chemistry and Physics faculty members. Information about the equipment and facilities of the LRF and the LBRC can be found in the Spectroscopy Laboratory Researcher's Guide.
Professors Field and Robert J. Silbey have developed new methods for extracting information about intramolecular vibrational redistribution (IVR) from frequency-domain dispersed fluorescence spectra of acetylene. A technique called extended cross correlation enables overlapping polyads to be "unzipped". The unzipping procedure tests the goodness of generalized vibrational quantum numbers. The progressive failure of unzipping as vibrational energy and evolution time (spectral resolution) are increased may provide a general way of detecting the change in resonance structure that occurs near an isomerization barrier. Professors Field and Silbey are also examining intersystem crossing and internal conversion processes, the electronic counterparts of IVR. Experiments on acetylene with Dr. Stephen Drucker are developing new methods to populate triplet states of small polyatomic molecules and monitor the ensuing unimolecular dynamics.
Professor Field is also pursuing time domain (~1 ps) studies of intramolecular dynamics. Initial experiments on acetylene test a hypothesis concerning the profound effect of a DC electric field on the intensity, but not the apparent decay rate, of fluorescence from the S1 state.
Professors Field and Steinfeld continue development of a free radical facility based on pulse-amplified FM laser spectroscopy. FM spectroscopy is a zero-background, linear technique capable of unprecedented sensitivity, especially in the ultraviolet region. Initial results have been obtained using a cw single mode ring dye laser, pulse amplified by a long-pulse (20us) injection-seeded Nd:YAG laser.
Professor Steinfeld and Dr. Brian Gilbert have characterized explosive compounds, the objective being detection at ultratrace levels. TNT has been detected at picogram levels using near-IR surface-enhanced Raman spectroscopy, and enhancement factors up to 104 for mono- and trinitrotoluene Raman signals have been achieved using UV resonance-enhanced Raman spectroscopy.
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. The data on dilute samples of dots are being used to develop models of relaxation mechanisms and the fine structure in the electronic transitions. Time-resolved studies of the heterostructures (close packed arrays of dots) show that energy transfer between nearest neighbor dots is important. This last result is particularly relevant to potential device applications. Professor Bawendi has also developed a new apparatus to study the spectrocopy of individual quantum dots. Initial fluorescence results show that transition linewidths are at least 50 times narrower than expected. This result is especially relevent to quantum dot laser applications.
Professors Klavs F. Jensen and Bawendi continue their work on spectroscopic characterization of CdSe-ZnSe quantum dot composite films deposited by electrospray organomettalic chemical vapor deposition. Photoluminescence spectroscopy was employed in combination with other characterization techniques to optimize the new processing technology and assess the materials performance for optoelectronic applications.
Professor Lippard and his associates have used Raman spectroscopy to characterize iron (III) peroxo compounds as potential models for metalloenzyme systems. The (u -1,2-peroxo)diiron (III) complex, [Fe2(u -O2)( -O2CCH2Ph2) 2{HB(pz')3}2], was prepared by Dr. Kimoon Kim and investigated by X-ray crystallographic and electronic, resonance Raman and Mössbauer spectroscopic methods, which revealed it to be a valuable model for the peroxo intermediate, Hperoxo, in the methane monooxygenase reaction cycle. A novel ferric [eta]2- u 4-peroxo complex, [Fe6O2(O2)3(OAc)9]-, was prepared and characterized by X-ray crystallography. Raman spectroscopy was used to characterized the O-O stretching frequency at 844 cm-1.
Professor Nelson has used a streak camera to record acoustic oscillations following excitation with crossed picosecond pulses. This detection method increases the acoustic frequency range accessible for optical study. Current applications include characterization of the mechanical properties of thin films, bulk polymers and other viscoelastic liquids, and biological samples whose acoustic properties are important in medical imaging.
Professor George Benedek and Drs. Jayanti Pande and Ramasamy Manoharan have studied oxidation-related structural changes in the protein, [gamma]II crystallin from the bovine ocular lens, using Raman spectroscopy. The vibrational spectra in the 2500-2700 and 500-550 cm-1 regions of the Raman spectra of native and oxidized [gamma]II show clear evidence for the loss of cysteine thiol groups to form disulfide bridges in the protein. No other changes in secondary structure or protein functional groups are seen. Preliminary Raman spectra of a young intact, human lens in vitro have also been obtained.
Professor Christopher Cummins used resonance Raman equipment to study a molybdenum-mediated dinitrogen cleavage reaction. Data were collected for characterizing of (u-N2){Mo[N(R)Ar] 3}2 and (u-15N2){Mo[N(R) Ar] 3}2, clarifying the degree of N-N bond activation in these complexes. The information obtained was collated with information from a plethora of techniques pertaining to the mechanism of dinitrogen cleavage.
Professor Tannenbaum and Drs. Paul L. Skipper, Dasari and V. Bhaskaran Kartha have set up an ultrasensitive high performance liquid chromatography system with laser-induced fluorescence detection to detect and quantify benzopyrene and cyclopentapyrene adducts at the attomole levels in DNA, and histone adducts of several brush biopsy and synthetic samples.
Professor Alexander Rich and Drs. Yang Wang and Dasari have continued investigations using Raman microspectroscopy to study the sequence-dependent DNA conformations in crystalline and solution states. Their aim is to further probe the detailed conformation of the DNA tetra-helix, d(CCCC)4. Experiments that probe the binding of proteins to Z-DNA inside intact cells utilize a YAG Laser and are being conducted in collaboration with Drs. Alan Herbert (Biology/MIT) and Stefan Wöfl (Hans-Knoll-Inst., Germany).
Professor Ali Javan continues to explore metal-oxide-superconductor junctions for optical frequency-mixing. Demonstration of direct transfer from a near-IR frequency to a near-UV frequency is under way.
Professor Kleppner has employed recurrence spectroscopy of the Rydberg states of lithium in parallel electric and magnetic fields to study quantum chaos. The system under study is a Rydberg atom in a static electric field with an additional RF electric field whose frequency is resonant with some classical orbits.
Professor Feld and Drs. Dasari and Kyungwon An have studied the single-atom microlaser, in which laser oscillation is achieved via coherent interaction between a two level atom and a single mode optical resonator. Recent progress includes realization of a traveling-wave microlaser and extensive numerical simulations based on quantum stochastic wave-function methods.
Professors Tanaka and Feld and Drs. Kartha and Dasari have studied the FT-IR and Raman spectra of N-isopropyl acrylamide (NIPA) polymer gels in the temperature range 20-500 C across the volume phase transition. The spectra showed changes in the C-C and C-H stretching mode frequencies of the isopropyl group, indicating that the phase transition is accompanied by a reorientation of the isopropyl group.
Professor Feld and Drs. Dasari, Geurt Deinum, Irving Itzkan, Manoharan, and Lev Perelman are pursuing basic and applied applications of lasers biology and medicine. Fluorescence and near-IR Raman spectroscopy are being used for biochemical analysis of tissues and blood, and diagnosis of dysplasia, cancer, atherosclerosis and other diseases. A novel system for acquiring real-time excitation-emission matrices with 10 nm excitation intervals and 5 nm spectral resolution is identifying optimal wavelengths for in vivo detection of dysplasia in many organs. Clinical studies are being pursued with researchers from the Cleveland Clinic Foundation, Brigham and Women's Hospital, Metrowest Hospital and New England Medical Center in colon, bladder, breast, and coronary and peripheral arteries. Quantitative analysis of blood analytes using Raman spectroscopy is under development. UV resonance Raman spectroscopy is being explored to characterize dysplasia. Photon migration using a newly developed time-resolved optical tomographic system is being used to image small fluorescent objects (lesions) imbedded in turbid biological tissue, and to study paths of early arriving photons. Finally, the mechanism of pulsed laser ablation of soft and hard biological tissues has been 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.
Michael S. Feld