Chapter 3
Multiphoton Dissociation Dynamics of Organoiron and Organoselenium Molecules Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 2, 2017 | http://pubs.acs.org Publication Date: June 8, 1993 | doi: 10.1021/bk-1993-0530.ch003
Joseph J. BelBruno Department of Chemistry, Dartmouth College, Hanover, N H 03755
The non-linear photochemistry of selenium and iron containing organometallic molecules, both experimental and computational, is reviewed. It is shown that the mechanisms for the two different metals are not related. Organoiron complexes are typical of organometallics while the organselenium molecules exhibit a dissociation mechanism related to that typical of organic molecules.
An important step in gas phase organometallic chemistry is the production of free metal atoms or ions. Nonlinear photochemistry or multiphoton dissociation (MPD) provides a convenient method of production using visible or near uv radiation. If coupled to a sensitive detection technique such as multiphoton ionization or fluorescence, the products may be analyzed in the same laser pulse. In these Laser Chemistry of Organometallics experiments, the following questions are posed: (1) What are the reactive species? (2) What is the mechanism that results in the observed products? (3) How do laser properties control the reaction? (4) Can any novel and/or useful chemistry be derived from MPD? In this review, we primarily discuss the first three questions. However, a significant body of research describing applications of this chemistry is available. The subject matter is restricted to the title molecules, but again the reader is advised that the literature contains numerous references to other organometallic molecules. At the focus of an intense laser field, an organometallic molecule typically dissociates prior to ionization (whereas most organic molecules preferentially ionize prior to dissociation). Such photophysical/photochemical behavior implies that organometallic complexes are convenient sources of gas phase metal atoms or ions. This inviting prospect has led to a broad range of fundamental studies into the photodecomposition of this class of molecules. Prominent among the many organometallic molecules used in this research have been the carbonyls and, in particular, Fe(CO). Alkylmetals of the semiconductor groups have played a much smaller role as have substituted carbonyls such as Fe(CO)L. The effort in our laboratory has been concentrated on substituted carbonyls and the semiconductor groups. 5
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0097-6156/93/0530-0049$06.00/0 © 1993 American Chemical Society Chaiken; Laser Chemistry of Organometallics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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LASER CHEMISTRY OF ORGANOMETALLICS
Historical Review Early reports involving MPD of iron carbonyls were from Zare and coworkers.f I) A catalog of observed emission lines from iron carbonyl was reported. A sequential absorption scheme was postulated for excitation at both 193nm and 249nm. The efficiency of metal atom production from Fe(CO)5 was confirmed by Smalley and co-workers(2), who reported Fe+ at laser intensities as low as 10 W cnr . Engelking subsequently reported that the MPI spectrum of ferrocene consisted entirely of atomic iron lines.(j) All three of these studies used multiphoton excitation into the charge-transfer (CT) band of the iron complex. Grant and co-workers excited Fe(CO)5 into a mixed ligand field(LF)/charge transfer state.(4) They proposed a dissociation model involving excited states with lifetimes between 0.6 and 1 ps. A follow -up study by Zare and co-workers(5) cataloged numerous atomic iron transitions via MPI through the CT states. These authors also presented the first experimental evidence of the presence of unsaturated fragments such as Fe(CO) and Fe(CO)2 , but only at levels of - 1 % that of Fe+ Engelking and co-workers later proposed a the following mechanism for the MPD dynamics of ferrocene.(
