Steady-state absorption

The model (9.73)—(9.75) was presented as an initial value problem We were interested in the rate at which a system in state 0) decays into the continua L and R and have used the steady-state analysis as a trick. The same approach can be more directly applied to genuine steady state processes such as energy resolved (also referred to as continuous wave ) absorption and scattering. Consider, for example, the absorption lineshape problem defined by Fig. 9.4. We may identify state 0) as the photon-dressed ground state, state 1) as a zero-photon excited state and the continua R and L with the radiative and nonradiative decay channels, respectively. The interactions Fyo and correspond to radiative (e.g. dipole) coupling elements between the zero photon excited state 11 and the ground state (or other lower molecular states) dressed by one photon. The radiative quantum yield is given by the flux ratio Yr = Jq r/(Jq r Jq l) = Tis/(Fijj -F F1/,). 

Note that in such spectroscopic or scattering processes the pumping state 0) represents a particular state of energy Eq out of a continuous manifold. In most cases this state belongs to one of the manifolds R and L. For example, in the absorption lineshape problem this photon-dressed ground state is one particular state of the radiative (R) continuum of such states. 

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We have seen that in a steady field Hq a small excess, no, of nuclei are in the lower energy level. The absorption of rf energy reduces this excess by causing transitions to the upper spin state. This does not result in total depletion of the lower level, however, because this effect is opposed by spin-lattice relaxation. A steady state is reached in which a new steady value, n, of excess nuclei in the lower state is achieved. Evidently n can have a maximum value of o and a minimum value of zero. If n is zero, absorption of rf energy will cease, whereas if n = no, a steady-state absorption is observed. It is obviously desirable that the absorption be time independent or. in other words, that s/no be close to unity. Theory gives an expression for this ratio, which is called Zq, the saturation factor ... 

Figure 8.11a shows steady-state absorption spectra of the CdTe quantum dots in water. Each spectrum in the figure exhibits a distinct peak at a different band corresponding to its size, indicating that all of these suspensions include mono-dispersed nanocrystals. This mono-dispersibility is also supported by their emission spectra with different peak bands corresponding to particle size, as in Figure 8.11b. 

AMMONAB - Steady-State Absorption Column Design... 

Seminal studies on the dynamics of proton transfer in the triplet manifold have been performed on HBO [109]. It was found that in the triplet states of HBO, the proton transfer between the enol and keto tautomers is reversible because the two (enol and keto) triplet states are accidentally isoenergetic. In addition, the rate constant is as slow as milliseconds at 100 K. The results of much slower proton transfer dynamics in the triplet manifold are consistent with the earlier summarization of ESIPT molecules. Based on the steady-state absorption and emission spectroscopy, the changes of pKa between the ground and excited states, and hence the thermodynamics of ESIPT, can be deduced by a Forster cycle [65]. Accordingly, compared to the pKa in the ground state, the decrease of pKa in the... 

Replacing the nitrile group by a benzothiazole produces an important subclass of fluorescent compounds represented by thioflavin T (25, Fig. 10). It is not clear if this compound undergoes deactivation via intramolecular rotation that would meet the criterion for a molecular rotor. The steady-state absorption and emission properties of thioflavin T has been attributed to micelle formation [53, 54], dimer and excimer formation [55, 56], and deactivation through intramolecular rotation [57]. 

Steady-State Absorption Column Design 471 Oxidation of O-Xylene to Phthalic Anhydride 324 Continuous Stirred Tank Reactor Model of Activated 577... 

The photochemical stabihty of the molecules is characterized by the quantum yield of photodecomposition, (P = N/Q [69], where N and Q are the numbers of decomposed molecifles and absorbed photons, respectively. The photochemical properties of the fluorene derivatives were investigated in different organic solvents (hexane, CH2CI2, ACN, and polyTHF) at room temperature by the absorption and fluorescence methods and comprehensively described [70-72]. These methods are based on measurements of the temporal changes in the steady-state absorption and fluorescence spectra during irradiation. For the absorption method, the quantum yield of the photodecomposition under one-photon excitation, c >ipa, can be obtained by the equation [73] ... 

Our experimental set-up (described in ref. 7), allows us to record steady state absorption and emission spectra over a wide range of densities (10 5 to 20 at/nm3) in the Ar supercritical domain (Tc = 150.8 K, Pc = 49 bar). Representative absorption and emission spectra are shown in figure 1. These spectra could be reproduced with a good accuracy by means of equilibrium MD simulations performed with a standard procedure [8], In these simulations, the NO X-Ar and Ar-Ar interaction potentials were taken from the literature [9], We extracted an analytical NO A-Ar pair potential by an iterative fit of the experimental spectra, valid for the whole supercritical domain. 

Figure 1 NO X(v=0)-A(v =0) representative steady-state absorption (left) and emission (right) spectra. The solid lines correspond to MD simulations and the dots to experimental results.

The wavelengths of the steady state absorption (Aabs) and fluorescence (A,) spectral peaks and magnitudes of the fluorescence Stokes shift (Av) of these PYP analogues in comparison with those of w.-t. PYP, denatured PYP and several site-directed mutants are given in Table 1. The observed fluorescence decay curves at various wavelengths of these PYP analogues are... 

Fig. 1. (a) Differential absorbance spectra of native PYP, after excitation at 430 nm, at different pump-probe delays. The scattered pump light around 430 nm has been masked. Steady-state absorption and fluorescence spectra are represented with dotted lines, (b) Kinetics extracted from the transient spectra at selected wavelengths... 

Fig. 3. Differential absorbance (AA) spectra of pCT in water (pH 10.2) after excitation at 430 nm. The spectral region corresponding to the scattered pump light is masked. Steady-state absorption and spontaneous emission spectra are also represented in dotted lines...

Fig. 1-left gives a general overview of the differential absorption spectra recorded for the free chromophore, oxyblepharismin, dissolved in DMSO for reference the steady-state absorption and (uncorrected) fluorescence spectra are also given below, in dotted lines. At all pump-probe delay times, the overall picture is a superposition of the structured bleaching and gain bands, as expected from the steady-state spectra, and broad transient absorption bands around 530 nm and 750 nm (weaker). These apparently homothetic spectra are very similar to... 

The steady state absorption and emission spectra of poly(A), poly(dA), and the absorption spectrum of the ribonucleotide monomer adenosine 5 -monophosphate (AMP) are shown in Fig. 1. The absorption spectra of poly(A) and poly(dA) are essentially identical. The AMP absorption spectrum is similar to the polymer spectra, but subtle differences exist. The absorption maximum of both homopolymers is shifted to the blue by several hundred wavenumbers, while the low energy band edge is red-shifted with respect to AMP. Similar shifts are observed at 77 K [15]. 

Fig. 4. (left) Transient-absorption spectra of the B-DNA helix. The dotted line is the steady-state absorption spectrum, the arrow indicates the pump frequency at 1670 cm"1, the red edge of the guanine CO-stretch band, (right) Transient-absorption spectra of the B-DNA helix. The dotted line is the steady-state absorption spectrum, the arrow indicates the pump frequency at 1685 cm 1 (the center frequency of the guanine CO-stretch band). 

The steady-state absorption spectra of the compounds are characterized by a strongly absorbing Soret band at 429 nm (So — S2) and two minor Q-bands at 558 and 598 nm (So —> Si) (Fig. 2). Increasing size of the dendrimers does not produce any significant changes in the absorption spectra. This indicates weak interactions between the porphyrin units, i.e. the energy transfer can be described by FSrster theory. Excitation of the dendrimers in the Soret band results in two fluorescence bands (not shown here) with maxima at 600 and 655 nm (Si —> So for both transitions). Again, only minor differences can be observed dependent on size. 

The steady state absorption and fluorescence spectra of both dendrimer generations 1 and 2 are depicted in Fig. 2. The former are merely superpositions of the absorption spectra of both chromophores involved. In the fluorescence, however, the peryleneimide part is almost completely quenched compared to the model compound. Instead, the fluorescence at wavelengths longer than 650 nm almost completely resembles the emission spectrum of the terrylene-diimide model compound 3. This feature is a strong indication that within these dendrimers the excitation energy is efficiently transferred from the peryleneimide to the terrylenediimide. 

Fig. 2. Normalized steady state absorption (left part) and fluoresecence spectra (right part) of first generation dendrimer (solid line) and second generation dendrimer (dotted line).

Fig. 1. Top Reference spectra for femtosecond transient absorption measurements S-S abs. in solution (thin solid lines), oxidized dye (dye+) abs. in solution (thick solid line), fluorescence for solution (dotted line), steady-state absorption ofNKX-2311/ZnO (dotted-dashed line), and absorption of electrons in the conduction band (dashed line). Bottom Transient absorption spectra of NKX-23ll/ZnO in the spectral range between 600 and 1350 nm at the 2 (thick solid line), 10 (dotted line), 100 ps (thin solid line) delay times after excitation at 540 nm by the femtosecond pulse with the intensity of about 10 pJ.

Fig. 1 shows the steady state absorption and fluorescence emission spectra of C343 in bulk water and in an aqueous solution of Zr02 nano-particles. In the presence of Zr02 the dye forms strong bonds to Zr atoms at the surface (as shown in the inset) and we obtain complete adsorption to the Zr02-surface. Upon adsorption both, the absorption as well as the fluorescence band, show a pronounced red-shift. This bathochromic shift is an indication that C343 is covalently bound to the surface, while in bulk water it is found in its ionized form. 

In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

Lightfoot, E. N. A.l.Ch.E.J. 4 (1958) 499. Steady state absorption of a sparingly soluble gas in an agitated tank with simultaneous first order reaction. 

The ensemble of the experimental results briefly reviewed here, e.g. steady-state absorption and fluorescence spectra, fluorescence decays, fluorescence anisotropy decays and time-resolved fluorescence spectra, allow us to draw a qualitative picture regarding the excited state relaxation in the examined polymeric duplexes. Our interpretation is guided by the theoretical calculation of the Franck-Condon excited states of shorter oligomers with the same base sequence. 

Fig. 11 Illustration of the excited state relaxation derived from experimental results obtained for poly(dA).poly(dT) by steady-state absorption and fluorescence spectroscopy, fluorescence upconversion and based on the modeling of the Franck-Condon excited states of (dA)io(dT)io. In red (full line) experimental absorption spectrum yellow circles arranged at thirty steps represent the eigenstates, each circle being associated with a different helix conformation and chromophore vibrations.

Figure 5.4, one can easily understand why the interfacial electron transfer should take place in the 10-100 fsec range because this ET process should be faster than the photo-luminescence of the dye molecules and energy transfer between the molecules. Recently Zimmermann et al. [58] have employed the 20 fsec laser pulses to study the ET dynamics in the DTB-Pe/TiC>2 system and for comparison, they have also studied the excited-state dynamics of free perylene in toluene solution. Limited by the 20 fsec pulse-duration, from the uncertainty principle, they can only observe the vibrational coherences (i.e., vibrational wave packets) of low-frequency modes (see Figure 5.5). Six significant modes, 275, 360, 420, 460, 500 and 625 cm-1, have been resolved from the Fourier transform spectra of ultrashort pulse measurements. The Fourier transform spectrum has also been compared with the Raman spectrum. A good agreement can be seen (Figure 5.5). For detail of the analysis of the quantum beat, refer to Figures 5.5-5.7 of Zimmermann et al. s paper [58], These modes should play an important role not only in ET dynamics or excited-state dynamics, but also in absorption spectra. Therefore, the steady state absorption spectra of DTB-Pe, both in...

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