Absorption intensity ratio, time dependence

Figure 10.2 Time dependence of absorption intensity ratio of 330 nm to 630 nm (/330//630) in UV-Vis spectra of EB/NMP solution (mole ratio of EB/Fe = 1/10, measured in 3.5 wt% NaCl solution at 25 °C) [45].

Pyrene derivatives are the widest used probes for qualitative solubilization [365] by virtue of the solvatochromic shifts of the absorption bands [255], the excimer formation [145,186], the polarity dependent quantum yields [197] and fluorescence life-times [185-187, 196, 197, 202, 215, 292], and the pyrene fluorescence fine structure [65, 74,78,103,112,167, 224, 363, 371] the intensity ratio of the fluorescence bands I at 372 nm and III at 383 nm is a convenient measure for the polarity of the environment of the pyrene label ( py -scale I/III values increase with polarity, cf. Fig. 27). As, however, the fluorescence of pyrene is very sensitive to the experimental set-up [372], absolute I/III values reported by different groups are difficult to compare. 

Undoubtedly the most powerful method of study in academic laboratories has been ir spectroscopy, advanced rapidly in recent years by the development of FTIR and microspectroscopy and by rapid improvement in sampling methods. Infrared spectroscopy can easily detect the formation of hydroxyl and carbonyl species, and a common technique is to monitor the time dependence of the so-called carbonyl index, the ratio of the intensity of the carbonyl absorption envelope to that of a chosen reference band. 

If the laser pulse applied to the sample molecules is sufficiently long and intense, a molecule (represented by a two-level system) will be driven back and forth between the two levels at the Rabi flopping frequency (2.134). The time-dependent probability amplitudes a (t) and a (t) are now periodic functions of time and we have the situation depicted in Fig.2.30. Since the laser beam is alternatively absorbed (induced absorption E ) and amplified (induced emission E2-> E ), the intensity of the transmitted beam will display an oscillation. Because of relaxation effects this oscillation is damped and the transmitted intensity reaches a steady state determined by the ratio of induced to relaxation transitions. According to (2.133) the Rabi frequency depends on the laser intensity and on the detuning molecular eigenfrequency o) 2 laser frequency co. This detuning can... 

Fortunately in a number of cases there is experimental information on these points from broad band pump/probe experiments when the anharmonicity A is larger than the linewidth but much smaller than the bandwidth 8(o of the laser. Then the 0-1 transition is seen as a bleaching signal and the 1-2 (66,67,71) as well as the 2-3 and often higher quantum number transitions (68,95) appear as new absorptions to an extent that depends on the pump intensity. A direct comparison of the total linewidths (1/T2) of these transitions, and the population relaxation times for the v = 1, v = 2 and perhaps higher levels can be obtained from such data. For N3 we found that ratio of the state to state relaxation from v = 2 to v = 1 was 1.8 times that for v = 1 to v = 0, not far from the harmonic value of 2 (50,95). However, the bandwidth of both transitions was roughly the same. 

Now there is a theorem of Kirchhoff s (1859) which states that the ratio of the emissive and absorptive powers of a body depends only on the temperature of the body, and not on its nature otherwise radiative equilibrium could not exist within a cavity containing substances of different kinds. (By emissive power is meant the radiant energy emitted by the body per unit time, by absorptive power the fraction which the body absorbs of the radiant energy which falls upon it.) By a black body is meant a body with absorptive power equal to unity, i.e. a body which absorbs the whole of the radiant energy falling upon it. The radiation emitted by such a body—called black radiation —is therefore a function of the temperature alone, and it is important to know the spectral distribution of the intensity of this radiation. The following pages are devoted to the determination of the law of this intensity. 

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