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Femtosecond transition-state spectroscopy of chemical reactions


Dantus, Marcos (1991) Femtosecond transition-state spectroscopy of chemical reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6aam-8f30.


The field of molecular dynamics emerged in the beginning of this century with the goal of understanding chemical reactions at the molecular level. The main interest was to study elementary processes such as bond rupture and bond formation as they occur on isolated molecules. Progress in that field has been directly related to technological advances. The development of the cross molecular beam techniques, for example, allowed the first observations of single molecule reactive collisions. Since the early experiments, it was recognized that the elementary steps in chemistry occurred on the time scale of a single vibration, many thought they might be unmeasurably fast. This thesis presents the first real-time observations of elementary chemical processes occurring on dissociative and bound potentials with a time resolution of 10(-14) s. The technique for achieving real-time monitoring of chemical reactions is Femtosecond Transition-State Spectroscopy (FTS). In essence, it involves two femtosecond (10(-15) s) laser pulses; one to initiate the reaction and the second to probe the progress at subsequent times. In addition to the time delay between the two pulses, the experiments include different wavelengths for the pump and probe pulses. The collection of data scans as a function of time delay for each combination of pump and probe wavelengths provides sufficient information about the dynamics to allow inversion to the potential energy surface of the reaction. The potential energy surface then contains all information about the reaction. The FTS technique has been applied with success to several systems. For repulsive states, such as the dissociation reaction of ICN, it has yielded a direct clocking of the bond rupture process and the real-time observation of transition states. For bound and quasi-bound states, the technique has allowed the real-time observation of wave packet oscillation in alkali-halides, halogens and inter-halogens. The technique has been applied to more than one dimensional systems such as HgI2. In addition to the scalar quantities, FTS has also been successful in the measurement of vectorial quantities such as the angular momentum and torque during dissociation reactions. Of all these, and many other measurements under different conditions, only a few representative examples are discussed here. The technique is now well established and it has become an important tool for the study of molecular dynamics on bound and repulsive potential energy surfaces.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Awards:The Herbert Newby McCoy Award, 1991
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Zewail, Ahmed H.
Thesis Committee:
  • Unknown, Unknown
Defense Date:7 December 1990
Record Number:CaltechETD:etd-06202007-112232
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2661
Deposited By: Imported from ETD-db
Deposited On:16 Jul 2007
Last Modified:16 Apr 2021 22:23

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