Transient absorption optical density

Delaire et al. [124] have reported that laser photolysis of an acidic solution (pH 2.8) containing PMAvDPA (23) and MV2 + allows the formation of surprisingly long-lived MV + - and DPA cation radicals with a very high charge escape quantum yield. The content of the DPA chromophores in PMAvDPA is as low as less than 1/1000 in the molar ratio DPA/MAA. Figure 20 shows a decay profile of the transient absorption due to MV + monitored at 610 nm [124]. The absorption persists for several milliseconds. As depicted in Fig. 20, the decay obeys second-order kinetics, which yields kb = 3.5 x 10s M 1 s. From the initial optical density measured at 610 nm, the quantum yield for charge escape was estimated to be 0.72 at 0.2 M MV2 +. ... 

Transient absorption spectra were measured with PBS in dioxane solution. In aerated solution the absorption decayed by two processes, a rapid one with f 1/2 50 /rs and a slow one with fj/2 20ms. In argon-saturated solution, a relatively weak absorption which decayed with f1/2 20ms was observed. The rate of the rapid decrease in the optical density agrees with that of the decrease in the intensity of the scattered light. 

The poly(542) pristine film, representative of the more crystalline polymers, exhibited a broader transient photoinduced absorption band around 900-1000 nm. This transient optical density decayed rapidly on the... 

Fig. 8 Transient absorption spectrum measured 20 Jis after 470-nm excitation of equimolar Ru(Me4-bpy)-imidazole complex and P450cam (9.6 JiM). Observed changes in optical density are chiefly due to the conversion of ferric to ferrous haem, with comparatively minor contributions from Ru(II) to Ru(III) [63]...

Fig. lla-d. Transient absorption spectra of reaction intermediates (a-c) and final product (d) observed upon photopolymerization of a TS-6 crystal with a 308 nm laser pulse. Parameters are the temperatures and the delay times (At) between the laser pulse and spectra recording. In b and c the time window of the detection circuit was 100 ps. AOD is the difference of the optical densities after and before the UV-flash (from Ref. 

The electronic state of the switch is often read by using optical transient absorption and fluorescence emission spectroscopies. Fluorescence is a much more sensitive technique, and can be used even at the single molecule level. It also requires a much smaller density of photons to provide a stable signal however, very few charge-separated ion pair states exhibit radiative charge recombination. In these cases transient absorption is often used because it allows direct nondestructive observation of a molecule s electronic configuration. Problems with this technique... 

Decay of Transient Absorption. The optical absorption decayed significantly over a few milliseconds only at the longer wavelength side of the maximum absorption—i.e., in the region of 300 m/. (The behavior of the absorption on appreciably longer time scales was obscured by the combination of low optical density and fluctuations in lamp intensity.)... 

Identification of the Transients. Neutral Solutions. In deaerated 5 X 10"5M p-nitrophenol solution at pH 7, the absorption spectrum with a maximum at 290 n.m. was observed about 20 /xsec. after the end of the radiation pulse (Figure 1-A) when all the hydrated electrons, H and OH radicals have reacted with the p-nitrophenol. The spectra shown in Figure 1-A and 1-B were corrected for the decrease in optical density owing to solute destruction. The G-value for the destruction of p-nitro-phenol was assumed to be equal to the sum of G(e"aq) + G(OH) + G(H) = 6.0 (2, 15, 19). The corrections had a maximum value at 400 n.m. namely, 103% and 130% of the observed optical densities and decreased to 40% and 81% at 290 n.m. for spectrum A and B, respectively. When hydrated electrons are converted to OH radicals by reaction with N20 (II), the absorption at 290 n.m. decreases and the maximum shifts towards 300 n.m. (Figure 1-B). It can be concluded, therefore, that the transient species produced by hydrated electrons as well as OH radicals have an optical absorption in the same wavelength region. 

Formation, decay, and absorption spectra of transient species produced by reaction of OH free radicals with DNA and nucleic acid constituents have been investigated by pulse radiolysis. Dilute aqueous N20 saturated solutions were exposed to 2-500 nsec, pulses (750-1700 rads/pulse) of 10 Mev. electrons, and changes in optical density were measured. The rate constant for formation of the DNA transient (nucleotide M.W. 350) is 6 X 108 M I seer1. The transient persists for more than 1 msec, with no significant change in spectrum. Thymine, uracil, deoxyribose, and thymidine transients have complex decay patterns which vary with pH. Comparisons of transient spectra confirm that the sites of attack by OH on thymine, 5-methylcytosine, and thymidine depend on pH. Abstraction from 5,6-dihydro-thymine is slower than addition to thymine. 

Figure 6. Absorption spectra of transient species formed on irradiation of N20 saturated 5 X 10 M solutions of pyrimidine bases. The optical density is the maximum observed during formation and decay of the transient. Doses were 800 rads/pulse for thymine (A and B) and 5-methyl-cytosine (C), and 1070 rads/pulse for uracil (C and D)...

Figure 8a. Absorption spectra of transients produced by 10 nsec, pulses in N20 saturated solutions of A) 5 X I0"4M deoxyribose. Optical densities were adjusted to 1000 rads/pulse...

Transient grating spectroscopy is relatively easily handled compared with the transient absorption spectroscopy, and is often used to study carrier dynamics at semiconductor electrodes [32]. Figure 14 schematically shows the principle of transient grating spectroscopy. A femtosecond laser pulse for sample excitation is split into two beams, which are crossed again at the semiconductor surface to produce an optical striped interference pattern. The interference pattern produces a striped pattern of the densities of photo-generated electrons and holes near the semiconductor surface. The latter striped pattern gives rise to a striped pattern of optical refractive index near the semiconductor surface, which is monitored by measuring a diffraction pattern of a second probe laser... 

A less-used form of transient absorption is the monitoring of the electron density within the metal oxide electrode by measuring changes in the optical density in the red-IR, where Ti + states are... 

Transient absorption spectra for dye-sensitized Ti02 films covered with polymer electrolytes based on P(EO-EPI), MI/I2 (M = Li or Na) with GBL (black line) and 7.5 wt% or 20 wt% of Lil, or without plasticizer (gray) (OD = optical density). 

The first method extracts / based on optical transient absorption results using a laser only pump-probe approach, where the photoexcitation conditions are set to be as close as possible to those in the XTA measurements. The/ values can be obtained by comparing AOD (change in optical density) and the total OD in the spectral region of the ground state bleach, as long as minimal interference from other spectral features is present in the same region. 

These short pulses induce a non-equilibrium situation in a very short time scale, such that a sufficiently high concentration of transient free radical species is formed. These short-lived free radical species are detected in their lifetimes, by following the changes in their characteristic properties such as optical absorption, electrical conductivity, spin density, Raman spectroscopy, etc. Pulse radiolysis has been found to be extremely useful in studying several of these free radical reactions. Although modern pulse radiolysis techniques are capable of producing much shorter pulses seconds), most of the relevant... 

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