Light-induced ESR

Light-induced ESR (LESR) can only detect those spins that are generated by optical excitation of the sample investigated. The LESR experimental procedure consists of a comparison between two measurements ... 

Fig. 1.21. (a) Light-induced ESR intensity as a function of the 3-factor in an MDMO-PPV/PCBM blend, = 9.5 GHz, T = 100 K, Aexc = 488 nm, P = 20 lW, 20 mW, and 200 mW. (b) A doubly integrated LESR signal of the prompt contribution as a function of the excitation power dependence. Squares correspond to the positive polaron signal and circles to Cg0... 

Fig. 5.7. Light-induced ESR spectra of p-type and n-type a-Si H, showing the induced g = 2.0055 resonance which is absent in the dark (Street and Biegelsen 1982).

Light induced ESR measures the density of band tail electrons and holes, and provides a different method of measuring the recombination... 

Fig. 9.20. Dark and light-induced ESR spin density for a-Si H/a-Si,N4 H multilayers with different numbers of repeat periods (Tsai et at. 1986b).

A light-induced ESR signal of P-682 resembling that of P-700 has been detected at cryogenic temperatures [74,75] its spectral characteristics (g = 2.002 and line-width of 6-8 G) are similar to those observed for a ligated chlorophyll a cation radical in aprotic solvents [76]. It is unclear, therefore, if P-682 is a dimeric structure or a single chlorophyll a molecule ligated to a metal ion (see Fig. 4.4). 

Definitive evidence of charge transfer and charge separation was obtained from light-induced ESR (LESR) experiments [149,178,180]. Upon illuminating the... 

Analysis of light-induced ESR signals of sensitizing dyes on the surface of AgBr microcrystals in photographic emulsions revealed that positive holes trapped by the dyes were responsible for the ESR signals and desensitization caused by the dyes. It was demonstrated from the ESR and sensitometric measurements that positive holes trapped by the dyes could react in several minutes with latent images on the surface of the microcrystals. 

Figure 1. The lowest vacant electronic energy level (n,) and the highest occupied one (c, ) of dyes as correlated with the appearance of the light-induced ESR signal, desensitization, and latent image fading by the corresponding dyes.

Figure 2. Light-induced ESR signal appeared at g = 2.005 of 0.7 yjn AgBr emulsion to which 7 X lO 5 mol Dye I/mol AgBr was added.

Figure 3 shows decay curves of the light-induced ESR signals of various dyes in emulsions. As shown in Figure 4, the decay time was in the order of several minutes, depending upon the kind of dyes, and increased with the increase of the height of the highest occupied level of dyes. 

It should be noted that the observed light-induced ESR signal of dyes was sharp and lacked any hyperfine structure. The signal, which was broad and Gaussian in shape at liquid nitrogen temperature, became sharper as the temperature of the ESR measurement was increased, and was Lorentzian in shape at room temperature. 

Figure 3. Decay curves of light-induced ESR signals oj various dyes in 0.7 pin AgBr emulsion. The numbers in this figure denote the dyes listed in Table I. The amount of the dye added to the emulsion was 7 X I0 s mol/mol AgBr.

Figure 4. Decay time of light-induced ESR signal of dyes in 0.1 pm AgBr emulsion as a function of the height of the highest occupied electronic enery level th of the corresponding dyes with reference to the top of the valence band of AgBr.

Figures 6 and 7 show the dependence of the intensity of the light-induced ESR signals of Dye 3 in emulsions upon reduction sensitization by use of stannous shloride and upon addition of Rh ( III ) to the emulsions. In accord with the view that small silver specks formed during reduction sensitization capture positive holes ( K), 11 ), the above-stated reduction sensitization decreased the intensity of the ESR signal, which were related to dye positive holes. In accord with the view that Rh ( III ) captures photoelectrons in AgBr ( 12 ), thus accelerating the separation between photoelectrons and positive holes, the addition of Rh ( III ) to emulsions increased the intensity of the ESR signal.

Metastable, light-induced ESR responses have been observed (Street et al, 1981 Dersch et al., 1981a-c Pontuschka et al, 1982) in a-Si H after irradiation with band-gap light at greater intensities (>100 mW cm" ). Some of these resonances are due to unpaired spins associated with the silicon atoms and some are associated with impurities. 

The temperature dependence of the light-induced ESR is relatively weak, especially below 100°K where the intensity is essentially constant (Biegelsen and Knights, 1977). The ESR signals increase rapidly after the light is... 

In the second approach the light-induced ESR is explained in terms of postively or negatively charged states of two-coordinated silicon atoms (Tj and Tj), which are assumed to be a positive-(7 system where the ground states are charged. The ESR responses at g = 2.013 and 2.004 are attributed... 

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