研究・研究者
研究会・セミナー
| 演 題 | 「The Quest for the Perfect Pluck」 |
|---|---|
| 日 時 | 2008年05月13日(火) 15:00 |
| 講演者 | Prof. R W. Field (MIT) |
| 場 所 | 分子研 研究棟 201号室 |
| 概 要 |
A molecule is not a "bag of atoms." Even when a molecule is highly excited, its network of bonds and the existence of approximate integer ratios between some of its vibrational frequencies exert a profound influence on the dominant dynamical pathways for the intramolecular flow of energy. Bond-breaking unimolecular isomerization is the simplest chemical process: one bonded network is exchanged for another. Spectroscopic and theoretical studies of the acetylene S_1-S_0 spectrum have demonstrated the existence of a small number of eigenstates that express large- amplitude (LAM) nuclear displacements, principally localized in a chemically interesting region of the S_0 potential energy surface, the minimum energy acetylene/vinylidene isomerization path. Two difficulties must be overcome in order to exploit the information encoded in LAM eigenstates. These difficulties are "access" and "recognition". LAM eigenstates are difficult to access spectroscopically because they are localized in regions of coordinate space displaced (along Franck-Condon-dark normal modes) far from the near equilibrium regions of the S_1 and S_0 potential energy surfaces. Access, via Stimulated Emission Pumping, to a LAM state on S_0 is gained through uniquely schizophrenic vibrational eigenstates on S_1. These eigenstates must contain significant admixtures of a Franck-Condon-bright basis state that provides intensity to the S_1(bright)-S_0(0^0_0) PUMP step and a Franck-Condon-dark basis state that provides intensity to the S_1(special dark)-S_0(LAM) DUMP step. In acetylene, the S_1(bright) basis state is an overtone of the trans-bend (nu3') and the S_1(special dark) basis state is an overtone of the antisymmetric in-plane bend (cis-bend, nu6'). The spectroscopically useful mixture of these bright and special- dark basis states is a consequence of the relatively low barrier on the S_1 potential energy surface between the trans and cis conformers. This is one of the key ideas of this project: it takes a low-barrier process on one potential energy surface to view a high-barrier process on another potential surface via access to LAM and barrier-proximal eigenstates. The second key idea is the use of embedded reporters to distinguish the rare LAM eigenstates from the vastly more abundant small amplitude or ergodic eigenstates. Electronic properties, such as the electric dipole moment or a hyperfine coupling constant, permit both recognition of LAM eigenstates and a measure of progress along the LAM displacement. The ability to read these electronic properties requires two normally antithetical spectroscopic capabilities: exceptionally high resolution and capability to survey broad spectral regions. The ability of the Chirped Pulse millimeter-wave technique, developed by Brooks Pate (University of Virginia), to survey multi-GHz regions at sub-MHz resolution is the third major idea of this project. |
| その他 |
講師のR.W. Field教授（マサチューセッツ工科大学化学科）は、気相分子の可視・紫外分光研究の大家でSEP法の創始者の1人であり、また、2原子分子のスペクトルを詳細に論じた教科書の著者としても高名です。 |
