Transient absorption, recording

Figure 17.6 A series of transient absorptions recorded at 1661 cm-1 for tRNAphe. Qualitatively, the data at 1620 cm 1 are the same. The initial temperature was held constant at 47 °C and the magnitude of the T-jump was varied up to a maximum of 20 °C. Each transient was fit to a three exponential model from t = 100 ns to the maximum transient absorption (t = 1 ms).

It is not often that the triplet state properties of cofacial dimers are reported but, in an interesting variation on the normal theme, this has been done for a series of naphthalenophanes. Here, the triplet state is accessed via sensitization with benzophenone and it is shown that the excited state behaviour depends markedly on the conformation of the superstructure. Both monomer and excimer states are apparent in the transient absorption records, according to the extent of... 

Excitation using high-intensity 532 nm laser pulses led to photoionization of these dyes [67]. The transient absorption recorded following the laser flash excitation of 3 is shown in Fig. 5. The linear dependence of the absorption at... 

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

A further spectroscopic approach was employed in the work by Ding et al. [43], in which the absorption transient is recorded after a potential step. For the case of TCNQ, the absorption transient is given by... 

FIGURE 8.7 Transient absorption spectra recorded 3ps following excitation for zeaxanthin (a) and ACOA (b). The spectra were measured with excitation at 400 nm (H-aggregates), 485 nm (monomers), and 525 nm (J-aggregates). 

Irradiation of a benzene solution of DABA at room temperature with a nitrogen laser (Horn and Schuster, 1982) gives the transient absorption spectrum shown in Fig. 3. This spectrum was recorded 50 ns after irradiation of the diazo-compound and decays over a period of ca 250 ps by a path exhibiting complex kinetic behavior. This transient spectrum is essentially identical with the low temperature optical spectrum described above, and thus is similarly assigned to 3BA. 

Fig. 3 Transient absorption spectrum recorded 50 ns after irradiation of DABA in benzene solution. The region between 370 and 440 nm is obscured by the absorption of the diazo compound...

Figure 8.7. Delayed fluorescence and diffuse reflectance transient absorption spectroscopy on scattering substrates. Example terthicnyl on silica gel excited with = 354 nm (neodymium/yttrium-aluminum-garnet) (Nd/YAG) laser pulse of 10 nsec, 20 mj), recorded with a gated diode array spectrometer.

The differential absorption spectra obtained in the presence of these two nucleotides are indeed similar to those obtained after reduction electrolysis of the complex in the first reduction wave, and obtained by pulse radiolysis. The prerence of the deprotonated radical cation GMI —H) can also be detected by recording the transient absorption after reaction of the reduced complex with O2. 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

Figure 1 Transient absorption spectra of c-S and t-S (S = r -4 ) recorded at time t after eT during PR of c-S and t-S (S = 1-4) with [S] = 5.0 x 10 M in DCE at r.t. Insets kinetic traces illustrating the time profiles of the D2 band at 1 as a function of t. Both t and the observed decay rate constant (kobs) and are mentioned in the figure.

Figure 13 Transient absorption spectra recorded before hvs i (O) and immediately after ( ) and 200 nsec (A) and 1 ps (A) after hvs32 during PR-LFP of c-St (5x10 M) in DCE (a). Kinetic traces illustrating time profiles of AO.D.480 (b) and AO.D.515 (c) as a function of time after e. ...

Figure 14 Transient absorption spectra recorded immediately after e (O), and before ( ) and immediately after (A) hvs22 during PR-LFP of a mixture of -St (5 x 10 M) and Bp (1 x 10 M) in DMF. The delay time of hv i from e was 75 nsec.

Fig. 1a,b Transient absorption spectra of 2AP (0.1 mM) in deoxygenated 20 mM phosphate buffer (pH 7) solutions recorded after 308-nm XeCl excimer laser pulse excitation (70 mj pulse" cm" ) [10]. The decay of hydrated electrons was recorded at 650 nm (a) and bleaching of the 2AP band at 310 nm (b). Reprinted with permission from the J Phys Chem, Copyright (1999) American Chemical Society... 

Analysis of the transient absorption spectra recorded at 100 ns allows for the determination of the prompt relative yields of the G /G(-H) radicals, I g (<100 ns). This yield is probably due, in part, to the oxidation of the guanines by the 2AP + radical cations [11]. Assuming that the 2AP radical cations decay only via the deprotonation of 2AP (rate constant ku) and hole transfer from 2AP + to guanine (fct), the prompt yield Og may be expressed as follows ... 

The general time dependence of the transient absorption signals, A(t), recorded at two representative wavelengths, 315 and 365 nm (Figs. 3, 5, 6), can be represented by the following equation ... 

Fig. 1. Transient absorption spectra recorded in ethylene glycol after photoionisation of the solvent by a 263 nm laser pulse.

The disappearance of an absorption band in the near IR domain is corroborated by the transient absorption signals recorded at three different wavelengths after photoionisation of... 

Fig. 3. Transient absorption signals recorded at 715 nm after photoionisation of ethylene glycol at 263 nm with three laser power densities. Inset the same signals after normalisation.

In order to study the influence of electron concentration on the observed dynamics, we performed experiments with different laser power densities. As an illustration, the transient absorption signals recorded at 715 nm in ethylene glycol upon photoionisation of the solvent at 263 nm with three different laser power densities are presented in Fig.3. As expected for a two-photon ionization process, the signal intensity increases roughly with the square of the power density. However, the recorded decay kinetics does not depend on the 263 nm laser power density since the normalised transient signals are identical (Cf. Fig.3 inset). That result indicates that the same phenomena occur whatever the power density and consequently that the solvation dynamics are independent of the electron concentration in our experimental conditions i.e. we are still within the independent pair approximation as opposed to our previous work on hydrated electron [8]. 

The transient absorption spectrum of DMABN-F4 in acetonitrile, recorded at 1-ps delay shows an absorption band near 360 nm (Fig. 3a). This band can be attributed to the CT state by comparison with that reported for DMABN in acetonitrile at lOOps (Fig. 3b). For the latter, both the LE state decay and CT state risetime were found to be 6 ps in time-resolved fluorescence measurements with a 4-ps time-resolution streak-camera [6]. From various studies, the CT formation time is now well known to be 4-6 ps [1] so that, at 100 ps, only the CT state is present. Fig. 3a shows that, for DMABN-F4, the CT state is populated in less than 1 ps in acetonitrile. We can thus conclude from the present observation that the access to the CT state for DMABN-F4 is significantly faster than for DMABN in acetonitrile. 

Figure 4b shows the measured transient difference absorption spectrum as a function of the x-ray probe energy E, recorded 50 ps after laser excitation (data points with error bars) for a sample containing 80 mmol/1 solution of [Ru"(bpy)3]2+ in H2O. This transient contains all the electronic changes from the reactant state absorption spectrum, R E), to the product state absorption spectrum, P(E,t), at the time t after photoexcitation. WithXO being the fraction of excited state species at time t, the transient absorption spectrum T(E,t) is given by... 

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