Femtosecond activation of reactions and the concept of nonergodic molecules

Science

Volumn 279, Issue 5352, 1998, Pages 847-851

  Diau, Eric W G ^a   Herek, Jennifer L ^a,b   Kim, Zee Hwan ^a   Zewail, Ahmed H ^a  

^a CALIFORNIA INSTITUTE OF TECHNOLOGY   (United States)

^b LUND UNIVERSITY   (Sweden)

Author keywords

[No Author keywords available]

Indexed keywords

HYDROCARBON;

ARTICLE; CHEMICAL REACTION; CHEMICAL STRUCTURE; ENERGY; MOLECULAR DYNAMICS; PRIORITY JOURNAL; STATISTICAL ANALYSIS; VIBRATION;

EID: 0032488870     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.279.5352.847     Document Type: Article

References (62)

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35. R. Bersohn, unpublished results.
36. The experimental setup for the present study has already been described in detail elsewhere (26). Briefly, the femtosecond pulses were generated from a colliding-pulse, mode-locked oscillator and then amplified by a four-stage dye amplifier. The laser pulse had a typical output of 500 μJ with a duration of ∼60 fs at 615 nm. The laser beam was split into two parts to form the time delay in an interferometer arrangement. One part of the beam was frequency doubled to give the initiation pulse of the reaction. The other part was used for probing. The pump and probe pulses were recombined, focused onto the molecular beam, and appropriately attenuated. The reaction chamber consisted of a differentially pumped molecular beam system with a pulsed nozzle and a TOF mass spectrometer. A series of experiments for measuring the power dependence of the pump beam were carried out; as in our previous work on Norrish reactions (22), we found that the initial excitation is two photons in the region of Rydberg state absorption (27). The probe pulse ionized the reaction components, and the resulting mass spectra were recorded as a function of delay times. We also obtained the temporal evolution of a certain mass (parent or fragments) by gating while varying the delay time from -500 fs to a few picoseconds. The theoretical fits of the temporal data were made, with the proper convolution, with a least squares procedure.
37. - were added to those of n = 5 with the use of Benson's rule (31, 32), that is, with the assumption that these active vibrational and rotational modes were decoupled from the reaction coordinate. In other words, because they are not close to the reaction coordinates, they are considered to be conserved and additive in nature.
38. O is the dissociation energy required to break cyclic ketone's primary C-C bond: 61.1, 78.5, 81.5, 82.2, 80.1, and 80.7 kcal/mol for n = 4, 5, 6, 8, 10, and 12, respectively (37, 33).
39. J(E). The same is true for the effect of anharmonicity (6). Thus, the RRKM calculations were carried out for J = 0 and harmonic modes. We present details of these calculations as a function of energy E and for different values of n in Fig. 4.
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47. The rise of the diradicals and decay of the parent must be related (Fig. 3), and we have detailed the kinetics of the processes involved (E. W.-G. Diau and A. H. Zewail, in preparation). For example, for cyclopentanone and its tetramethyl derivative, the decay of the parent mass is faster than the rise of the diradical mass, indicating a nonconcerted process. By self-consistent analysis of the decay of the parent and the rise of the intermediate, we can obtain the time constants of the elementary steps. Generally, the decay of the parent (or the rise of the intermediate) is the one of relevance here and both the decay and rise are on the femtosecond time scale, independent of n (Fig. 3).
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62. Supported by a grant from the Office of Naval Research and by the NSF. We thank R. Bersohn for communicating unpublished results to us and S. Baskin for helpful discussions.