DOUBLE EXCITATION SPECTROSCOPYBY ELECTRON AND LASER-LIGHT PULSES
Katsumi Tan.imura, Kazuo Soda and Noriaki Itoh
Department of Crystalline Materials Science, Faculty of Engineering,Nagoya University, Nagoya, JapanThe studies of the photochemical processes in alkali halides
by means of the double excitation spectroscopy, using an electron
pulse from an accelerater and a light pulse from a tunable dyelaser, are reported. The advantages of the double excitation spectroscopy over the single excitation spectroscopy and the implication of the obtained results are discussed.
In order to understand the photochemical processes in solids
and molecules it is important to make clear the dissociative states
at which the reaction products are evolved. For this purpose thestudies of the wavelength dependent reaction using intense uv andvisible light have been made extensively. We have undertaken
studies of the wavelength dependent reaction induced by the double
excitation: by an electron pulse and by a light pulse from a tunable
In this method, the lowest t;riplet excited states are
dye-laser.
populated by the excitation with an·electron pulse §1-nd·are~subse­
quently excited into higher states with a light pulse, which isdelayed by lo-6-lo-3 s from the electron pulse.
Advantages of the double excitation spectroscopy over thesingle excitation spectroscopy are as follows: (1) The triplet excited states can be surveyed directly; and (2) in the system inwhich some of the degeneracy in ground state is removed by the JahnTeller distortion at the lowest excited states, the double excitatipn can resolve the chemical reactions induced at the sublevel
split by the Jahn-Teller distortion of the lowest excited states.
In this paper we report the studies of the double excitation spectroscopy in alkali halide crystals. In alkali halide crystals withNaCl structure the self-trapping of the excitons is known to reduce
the symmetry from Oh to D2h·
The experimental apparatus 1 l for the double excitation spectroscopy is shown in Fig. 1. The amount of the chemical:reactio nproducts at the incidence of the electron and light pulses wasdetermined by means of optical absorption spectroscopy.
One-electron orbitals of the self-trapped excitons in alkali
halides is shown in Fig. 2. The lowest four orbitals are originated from the halogen p-orbitals, and the others are for a trappedelectron in a D2h potential. The electron occupation shown in thefigure is for the lowest self-trapped excitons.
The optical absorption from the lowest triplet excitons to thehigher state (T-T absorption) has been measured by Williams andThe direction of the transition dipole for each transiKabler. 2 ltion is determined using the double-excitatio n technique: The change
in the T-T absorption induced by polarized laser light is measured
using polarized light either parallel or perpendicular to the laser
The order of the excited states are shown in Fig. 2 inlight. 3 laccordance with the results of the polarization measurements.
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The F center creation yield in KCl by the excitation of thelowest self-trapped exci tons was obtained as a function of the exciThe result is shown in Fig. 3, in which the T-Ttation energy. 1 )absorption curve is also shown. Evidently the excitation within
the lowest optical absorption band which has been ascribed to thealg -+ b2u electron transition produces the F-centers. Moreover theexcitation into the· n.ext higher state is seen to produce the F centers at the same yi.eld. This result means that .no additional creation of the F centers takes place at the next excited state, namely
at the state in which the electron is excited into the b3u orbital.
The result has been interpreted on the basis of the configurational mixing between the electron-excite d states and the hole-excitedstates.·
We have already used. the double excitation spectroscopy forseveral other studies, such as the orientational correlation between the self-trapped excitons and the evolved interstitial centers3l and the. intersystem crossing from the electron-excite d andThese results appear
hole-excited states in KI 4 l and in NaCl!)
to yield very detailed information not only on the photochemical
processes but also on the non-radiative processes in the selftrapped excitons.
References.
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K. Soda and N. Itoh, J. Phys. Soc. Japan 46, 1618 (1980).
R. T. Williams and M. N. Kabler, Phys. ReV. B9, 1897 (1974).
K. Tanimura, K. Soda and N. Itoh, to be published.
Y.:Ban, K. Tanimura and N. Itoh, to be published.
K. Soda and N. Itoh, Physics Letters 73A, 45 (1979).

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