PHOTOSELECTION AND MAGNETIC FIELD LEVEL CROSSING IN $CS_{2}$ VAPOR.
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Date
1969
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Ohio State University
Abstract
Photoselection of molecules by excitation with polarized light is well known in condensed $media.^{1}$ but heretofore unobserved for polyatomic molecules in the gas phase. We report here the photoselection of gaseous $CS_{2}$ as demonstrated by the degree of polarization of $CS_{2}$ fluorescence. Using a 600 W high pressure xenon lamp in conjunction with a fast monochromator, we have excited the $3000-3300{\AA}$ absorption bands with plane-polarized light having a $40{\AA}$ bandwidth. The $4000-6500{\AA}$ $fluorescence,^{2}$ observed at right angles to the incident beam, shows a polarization of $+5.8 \pm 0.2$ percent extrapolated to zero $CS_{2}$ pressure. We have observed magnetic depolarization of the $CS_{2}$ fluorescence (molecular Hanle $effect^{3}$). This represents the first zero field level crossing performed on a polyatomic molecule. At 25 microns $CS_{2}$ pressure, a field of $18 \pm 8$ Gauss causes a 50 percent reduction in the degree of polarization. Observation of this molecular Hanle effect provides clear evidence for the existence of a magnetic moment in the excited state(s) of the $3000-3300{\AA}$ absorption system, in agreement with previous magnetic rotation $work.^{4}$ Further interpretation of this magnetic depolarization data must await the spectroscopic identification of the excited state(s) involved. A preliminary search has been made for the corresponding electric field depolarization of the $CS_{2}$ fluorescence, but we have been unable to observe this effect with our present experimental setup using field intensities up to 4.5 KV/cm.
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This work was supported by the National Science Foundation and the Alfred P. Sloan Foundation. Present address of J. W. Mills: Department of Chemistry, Drew University, Madison, N. J. 07940. Present address of R. N. Zare: Department of Chemistry, Columbia University, New York, N. Y. 10027. $^{1}$See A. C. Albrecht, J. Mol. Spec. 6, 84 (1961). $^{2}$J. Heicklen, J. Am. Chem. Soc. 85, 3562 (1963). $^{3}$R. N. Zare, J. Chem. Phys. 45, 4510 (1968). $^{4}$P. Kusch and F. W. Loomis, Phys. Rev. 55, 850 (1939); P. Kusch, J. Mol. Spec. 11, 385 (1963).
Author Institution: Department of Chemistry and Joint Institute for Laboratory Astrophysics, University of Colorado
Author Institution: Department of Chemistry and Joint Institute for Laboratory Astrophysics, University of Colorado