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Radiationless Relaxation in "Large" Molecules: Experimental Evidence for Preparation of True Molecular Eigenstates and Born-Oppenheimer States by a Coherent Light Source

Zewail, A. H. and Orlowski, T. E. and Jones, K. E. (1977) Radiationless Relaxation in "Large" Molecules: Experimental Evidence for Preparation of True Molecular Eigenstates and Born-Oppenheimer States by a Coherent Light Source. Proceedings of the National Academy of Sciences of the United States of America, 74 (4). pp. 1310-1314. ISSN 0027-8424. http://resolver.caltech.edu/CaltechAUTHORS:ZEWpnas77

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Abstract

Photon absorption and emission by molecules that undergo radiationless transitions are examined using the single modes of lasers having well-defined coherence properties. Contrary to the usual beliefs, where it is assumed that the molecule is prepared in a Born-Oppenheimer singlet state and then "crosses-over" to other states (vibrationally "hot" singlets and/or triplets), it is shown experimentally that the true eigenstates of the molecule can be prepared, even in "large" molecules, if the laser correlation time is relatively long and the molecular relaxation is made slow. On the other hand, lasers with short (psec) correlation time have yielded effectively the singlet Born-Oppenheimer state, which has a much shorter lifetime than the true eigenstates. Effects of magnetic fields and temperature are also reported. The former changes the amount of mixing amongst the Born-Oppenheimer states. The latter, on the other hand, swings the molecule from being "small" (i.e., sparse vibronic structure with long lifetimes) to being "large" (i.e., dense statistical distribution of levels) since the relaxation between levels is very effective at high temperatures. Finally, the results of this work show that the words fluorescence and phosphorescence in their strict meaning are misleading if the true eigenstates, which may contain both singlet and triplet character, are prepared.


Item Type:Article
Additional Information:Copyright © 1977 by the National Academy of Sciences Communicated by John D. Baldeschwieler, January 27, 1977 We thank Profs. J. D. Baldeschwieler, G. W. Robinson, D. McClure, W. Goddard III, V. McKoy, and W. Gelbart for critical reading of the manuscript and valuable suggestions. We also thank Dr. A. Marchetti for providing us with the samples used in this work. The help of R. H. Reiner in the pulsed dye laser experiment performed in Prof. A. Kuppermann's laboratory is greatly appreciated. This work is contribution number 5465 and was supported in part by the California Institute of Technology President's Venture Fund and in part by the Sloan Foundation. Note Added in Proof. Recently, with a resolution of 10^-4 cm^-1 we have observed the optical free induction decay in molecular beams and the photon echo on the spontaneous emission of gases near zero pressure. These observations (Chem. Phys. Lett., in press) which give optical T1 and T2 in gases and beams will now allow us to separate the mechanisms for the spontaneous and dephasing processes at zero pressure (intramolecular) and in condensed phases. "Large" usually refers to molecules with more than two or three atoms. More precisely, it means that hp/T >> 1, where r is the decay time of the zeroth order levels (density p) that are located at the energy of øp (see Fig. 1).
Subject Keywords:radiationless transitions; optical coherence; resonance decay of excited states; laser spectroscopy
Record Number:CaltechAUTHORS:ZEWpnas77
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:ZEWpnas77
Alternative URL:http://www.pnas.org/cgi/content/abstract/74/4/1310
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4763
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:06 Sep 2006
Last Modified:26 Dec 2012 09:01

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