Spectroscopic Study of Photoexcited /%Naphtholate
Ion
The Journal of Physical Chemistty, Vol. 82, No. 10, 1978 1201
Picosecond Time-Resolved Spectroscopic Study of Solvated Electron Formation from the Photoexcited (?-Naphtholate Ion A. Matsuzakl, T. Kobayashl, and S. Nagakura" Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato, Tokyo, Japan and Institute of Physlcal and Chemical Research, Wako, Saitama, Japan (Received September 27, 1977) Publication costs assisted by the Institute for Solid State Physics, The University of Tokyo
The rate constant of solvated electron formation from the photoexcited P-naphtholate ion in aqueous alkaline solution was determined as (1.8 f 0.5) x 1O1O s-' at 23 "C using the picosecond laser photolysis method. From this value it was shown that the rate-determining step for solvated electron formation is the reorientation of surrounding solvent molecules. The concentration of solvated electron increases quadratically with the intensity of the excitation laser beam. This indicates that the electron ejection occurs in two steps.
Introduction P-Naphtholate ion (I) is known to produce a solvated
woc I
electron by the irradiation of a N2 l a ~ e r . l - Using ~ a nanosecond laser photolysis apparatus, Klaning et al.4 measured the absorption spectrum of the solvated electron and found a broad band with a peak at -750 nm. Goldschmidt and Stein3 measured the lifetime of the solvated electron as 300 ns at room temperature, but the formation rate was too rapid to be measured by an apparatus with nanosecond time resolution. In the present study, we measured the formation rate by use of a mode-locked ruby laser with 20-ps pulse width. Moreover, we found that the ionization is a biphotonic process.
Experimental Section P-Naphthol was purified by vacuum sublimation after repeated recrystallizations from water. Wako reagent grade NaOH was used without further purification. The details of the picosecond laser photolysis system were described previously.6 A single pulse with -20-ps width was obtained by the combination of a ruby laser, a Pockels cell composed of a KDP crystal and a polarizer, and an amplifier. A second harmonic of the ruby laser was used as a pulsed excitation light source. A SPM (self-phase modulation) light generated by irradiating aqueous phosphoric acid was used as a monitpring light. The monitoring light was monochromatized with a Jarrell-Ash J E 25 monochromator after passing the sample cell and was detected by a Vidicon (PAR 1205D)-OMA(PAR 1205A) system. A N2 laser (Molectron UV 1000) was used for nanosecond laser photolysis as the excitation source of sample and dye solutions. A superradiant light beam from a mixed dye solution of a rhodamine B-nile blue A perchlorate or rhodamine B-oxazine 1 perchlorate was used as the monitoring light source covering the wavelength region 600-950 nm. The monitoring light passing the sample cell was monochromatized with a Spex 1700 monochromator *Institute for Solid State Physics, The University of Tokyo. 0022-3654/78/2082-1201$01 .OO/O
and was detected by an EM1 62568 photomultiplier. The signal-to-noise ratio was improved with a PAR Model 160 boxcar integrator. The details of the nanosecond laser photolysis apparatus were described previ~usly.~
Results and Discussion By use of the nanosecond laser photolysis apparatus, a transient absorption was observed for an aqueous alkaline M 0-naphtholate ion. The solution containing 1.0 x result is shown in Figure 1. We can see that the spectrum consists of a broad band with the peak at -760 nm. It agrees fairly well with the absorption spectrum of the solvated electron observed by Klaning et aL4 Therefore, the transient absorption observed in the present study is concluded to result from the solvated electron. By use of the picosecond laser photolysis apparatus, the time dependence of the transient absorption intensity was observed for an aqueous alkaline solution containing 1.0 x M P-naphtholate ion, excitation being performed at 347.2 nm and the absorption intensity being observed at 600 nm. The result is shown in Figure 2. Plots of the logarithms of the observed absorbances vs. time are given by the open circles in this figure. The plots are composed of an ascending part corresponding to the formation process of the solvated electron and a decay part. The differences between the points of the ascending curve and the corresponding points of the extrapolation of the decay curve give a straight line, the slope of which corresponds to the formation rate of the solvated electron. Thus the formation rate of the solvated electron was determined as (1.8 f 0.5) X 1O1O s-l at 23 "C, corresponding to a rise time of 55 ps. It is known that in alcohols a trapped electron in a shallow potential well, e;, is formed very quickly at the first stage and thereafter is converted into a solvated electron in a fully relaxed well, eso
