SCIENCE: RESEARCH

The research of this laboratory falls into the following three phases:

  1. GASEOUS REACTION DYNAMICS BY MEASUREMENT OF STATE-SPECIFIED RATES

    Past

  2. In 1956 Polanyi's first graduate student, Ken Cashion, looked for non-thermal infrared chemiluminescence from the exoergic reaction H + Cl2 HCl (v', J') + Cl. He observed this, and resolved the vibrational energy-distribution (references 9-17; 1958-60). It was not, however, until a decade later (references 47-53; 1967-72) that these results began to produce reliable data on the initial vibrational and hence translational energy distributions in the products of some simple exchange reactions that could be linked through among the earliest classical trajectory studies (references 26, 38, 51-56; 1962-69) to the forces operative in the reaction. This line of work, which led to proposals for a broad category of 'vibrational' lasers (then a new concept), through the idea of 'partial population inversion' (references 19, 35: 1960-65) employing both physical and chemical pumping, continued until approximately 1980. (This work was recognized by the joint award with D. R. Herschbach and Y. T. Lee of the Nobel Prize in 1986 with the citation: "…for the development of a new field of research in chemistry—reaction dynamics—[which] has provided a much more detailed understanding of how chemical reactions take place." The Polanyi laboratory was cited for developing "the method of infrared chemiluminescence, in which the extremely weak infrared emission from a newly-formed molecule is measured and analysed. He has used this method to elucidate the detailed energy disposal during chemical reactions."

  3. 'TRANSITION STATE SPECTROSCOPY'

    Past

  4. There had been theoretical discussion of the possibility of guiding chemical reactions through the interaction of intense laser field with the reacting particles. A proposal from Polanyi's laboratory shifted the discussion in 1979 (reference 124) to the observation of transition states by a phenomenon analogous to line-broadening; reacting molecules, it was pointed out, should emit or absorb radiation substantially altered in wavelength by the effect of the reactive collision.

    This phenomenon, termed 'Transition State Spectroscopy,' was explored in the Polanyi laboratory in emission, and elsewhere in absorption, in the early 1980s. Theoretical studies first performed in this laboratory, and extended elsewhere, showed how revealing such far-wing emission could be of the transition states' duration and geometry, especially for the simplest of chemical reactions (references 124, 129, 135, 141, 143, 146-147, 149, 151, 158, 160). The most important work in this field at the present time is to be found in California Institute of Technology in the laboratory of A. H. Zewail, awarded the Nobel Prize in 1999 for "studies of the transition states of chemical reactions using femtosecond spectroscopy," and at the University of California, Berkeley in the laboratory of D. Neumark. There is no doubt that this field, which aroused profound scepticism at first, is destined to be of great importance in supplementing measurements of the type described under 1 above.

    Present

    Polanyi's laboratory has renewed its work in the area of 'transition state spectroscopy' in recent times by forming (for the first time) gaseous complexes (Na)m…(XR)n and (Li)m..(XR)n in crossed beams of alkali metals plus alkyl halides, and then photoexciting the complex with broadly-tunable visible laser radiation so that a 'harpooning' reaction takes place on the lowest electronically-excited state starting in a designated restricted region of configurations intermediate between the reagents and the reaction product, i.e., in the transition state region. The probability of reaction can be measured as a function of the transition state configuration by tuning the visible laser. See references 193, 204, 208, 214, 218-220 and 222. At the present time these studies are being extended to include studies of photoinduced harpooning reaction in simple complexes of the type Li..FH that can be treated (and are being concurrently studied) by ab initio computation.

  5. SURFACE ALIGNED PHOTOCHEMISTRY (SAP)

    Present

  6. In the early 1980s the prevailing view was that photoprocesses at surfaces would be inefficient, and excessively complicated by energy transfer to the substrate. This laboratory pointed out that photoinduced processes (photodissociation and photoreaction) could be completed in picoseconds or femtoseconds, and consequently could compete on favourable terms with the transfer of energy into generalised heating of an underlying surface. Observation of surface aligned photodissociation (in UHV on a single-crystal substrate) was reported in 1984 (reference 152) and surface aligned photoreaction in 1986 (reference 157; see reference162 for theory and also references 164-167, 170, 172, 177-183, 185, 177-183, 185-187, 190-191, 194-195 (theory), 197-200, 202-203 (theory), and 205, 207, 209-213, 215-217, 221, 223 for further studies).

    The appeal of the field from a fundamental standpoint lies in the possibility of catalysing photoprocesses, and doing so in a comprehensible fashion. This would be effected primarily by aligning, orienting and positioning the adsorbate, and therefore restricting the angles of approach and ranges of impact parameter (the distance by which an attacking atom misses its target) in the ensuing photoreactions. Photoinduced reactions between co-adsorbed molecules have in fact been observed in this laboratory (e.g., references 157, 165, 172, 179, 183, 185, 198, 203, 216, 223). In addition more subtle types of catalysis have been identified in which light is channeled into the adsorbate by way of absorption of UV by the substrate (reference 166) or by electron-transfer from the substrate to the adsorbate (references 177, 181, 190-191, 195, 197-198) resulting, in either case, in orders-of-magnitude increases in the cross-section for photodissociation.

    The papers on surface aligned photochemistry from this laboratory are indicated as S-1 to S-31 on the publication list. Work in this laboratory has given direct evidence of electron-transfer by the observation and measurement of negative ions leaving an illuminated surface. It has shown that hot electrons photoformed within the metal (i.e., not only free electrons) can give this outcome. Studies have been made of photoreaction with the surface for a series of adsorbates, and also photoinduced ion-molecule reaction in the adsorbate. (See references 193 and 194 for these last two topics). The photodissociation of aligned and positioned adsorbates can lead to a new type of surface-scattering ('Localised Atomic Scattering') in which scattering is localised at a particular atomic site (references 195, 200, 202-203, 205, 211-212, 221).

    The most recent extension of these studies is from 'Localised Atomic Scattering,' LAS, to 'Localised Atomic Reaction,' LAR (reference 224, Journal of Chemical Physics, 111, 9905 (1999)). In this work the electron-induced reaction with a silicon substrate is shown to be localised to only one adatom site away from the adsorbate molecule being dissociated. In work being prepared for publication this process is further developed as a means to controlled atomic-patterning of the substrate (reference 226, Journal of Chemical Physics, submitted.).

For a summary of research, until 1986 only, see Some Concepts in Reaction Dynamics. Science, 236 (May 8, 1987).

For a summary of recent research, until 2000, see Localized Atomic Reactions Imprint Molecular Structures. Chemical & Engineering News, 78, (30) (July 24, 2000).

For a short biography and explanation of his science see Science.ca

FUNDING

For the year 2000, and beyond, the Polanyi group is funded by:

Natural Sciences and Engineering Council of Canada (NSERC)

Canadian Institute for Photonic Innovations (CIPI), a National Centre of Excellence

Photonics Research Ontario (PRO), an Ontario Centre of Excellence

Ontario Research and Development Challenge Fund (ORDCF)

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