One-Photon Absorption Spectra

In Fig. 3.17 the UV-Vis-NIR spectra of the studied fullerene derivatives are presented. The spectra of bis-fullerene material 3 shows the characteristic absorptions of a fulleropyrrolidine monoadduct at 433 nm as well as that of TPhA at the UV region (see Figs.3.17 and 3.18). The assignment of the band located at 433nm has 

Spectra of lb. The feature at 250 nm, however, is not clearly visible in the simulated 

the nonlinear relfactive parameter 7 have been determined according to the following relations [151]  

From the determined 7 and / values, the real and imaginary parts of the third-order susceptibility i e. and Imx can be obtained using the following 

Finally, the second-order hyperpolarizability 7 has been deduced according to the 

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Selection rules are different from one-photon selection rules, leading to facile selective excitation of one component in a mixture of compounds with overlapping one-photon absorption spectra. 

Fig. 3.19 One-photon absorption spectra of 1 (top) and 2 bottom) simulated at the PBEO/6-3 lG(d) level of theory...

One very important aspect of two-photon absorption is that the selection ndes for atoms or synnnetrical molecules are different from one-photon selection ndes. In particular, for molecules with a centre of synnnetry, two-photon absorption is allowed oidy for g g or u u transitions, while one-photon absorption requires g-f u transitions. Therefore, a whole different set of electronic states becomes allowed for two-photon spectroscopy. The group-theoretical selection ndes for two-photon spectra are obtained from the synnnetries... 

Octatetraene is the shortest unsubstituted polyene that exhibits fluorescence. The X kg —> 2 kg transition is clearly seen in one- and two-photon absorption spectra and the 2 kg is unambiguously identified to be the lowest singlet state1. 

Table 6 contains two other examples of multibranched structures consisting of one, two, or three branches around a common electron-rich tripheny-lamine core. In the first case, oc.6-8, the linear units belong to class III of Fig. 8 and share one of the terminal amine groups [122]. The 2PA spectra of these three compounds, studied by Yoo et al, exhibit a peak at the same wavelength (AmL = 840 nm), while a small red shift was observed in the position of the one-photon absorption band (473, 492, and 495 nm for 0C.6, OC.7, and oc.8, respectively) [122]. The authors also reported an enhancement in the 2PA cross section going from one to three branches, by a factor larger than the increase in the number of branches (the ratio of 5max for 0C.6, OC.7, and oc.8 are 1 2.3 3.7, while the number of branches increases as 1 2 3). 

Fig. 19 Top One-photon absorption (solid line), two-photon excitation ( ), and relative acid-yield efficiency (o) spectra of p.2 plotted versus transition wavelength. Bottom Logarithmic plot of [ff ] against power of the excitation beam at 745nm/80fs pulses the best-fit line has a slope of 2.3, indicating an approximate quadratic increase of acid yield with excitation intensity. Reproduced from [214] with permission from AAAS...

Simulations of the experimental signal were performed using Equation 1 without adjustable parameters. The spectrum of the pulse and the absorption spectrum of HPTS were measured experimentally. An examination of the molecular structure of HPTS shows that it has no center of symmetry. Since parity restrictions may be relaxed in this case, the similarity between one-photon and two-photon absorption spectra is expected. The spectral phase

phase mask was the same used for the simulations. Both experimental and theoretical data were normalized such that the signal intensity is unity and the background observed is zero. The experimental data (dots) generally agree with the calculated response (continuous line) of the dyes in all pH environments (see Fig. 2). 

A two-photon allowed absorption of poly(dihexylsilane) has been observed at 0.9 eV above the one-photon excitation63 -65, in contrast to the two-photon absorption of linear polyenes, which is about 0.5 eV below the intense one-photon absorption. Absorption spectra and the dynamics of other high-lying excited states of the polysilane have been investigated by means of femtosecond time-resolved excited-state absorption spectroscopy66. 

All physical properties of pyrylium salts (unsubstituted or substituted with alkyl and/or aryl groups) prove the aromaticity of these cations vibrational spectra [66], mass-spectral fragmentations [67], magnetic properties [68-70], and electronic absorption spectra [71]. It should be mentioned that there is a close similarity between the electronic absorption bands of pyrylium salts and those of benzene, easily recognized by the marked bathochromic effect of substituents in y-position of pyrylium salts on one of these bands. Two-photon absorption spectra of 2,4,6-triarylpyrylium cations [72] may be used in optical data storage, lasing, and photodynamic therapy. 

In good agreement with the model described in Fig. 2b for push-pull molecules, TPA spectra match one-photon absorption curves this confirms that the value of cttpa is dominated by the dipolar term of Eq. 5 and that dipolar molecules efficient in quadratic NLO processes (see Eq. 6) should present high TPA responses. This trend has been illustrated by work on azo-aromatic... 

A short mathematical analysis of formulae Eq. 13 and Eq. 14 shows that the absorptions corresponding to p = 1 and p = 1 are highly dominating the one-and two-photon absorption spectra. 

Fig. 36 Experimental one- and two-photon absorption spectra of 9,9 -dihexylfluorene oligomers in dichloromethane. Top Absorption coefficient per monomer. Bottom Two-photon absorption cross-section per monomer. Nx indicates an oligomer containing x monomeric units...

Figure 11 One-photon absorption, fluorescence, and two-photon excitation spectra of diphen-ylhutadiene in ERA at 77 K. The two-photon excitation spectrum is plotted versus where is the wavelength of the laser excitation (Reproduced by permission from J. Chem. Phys., 1980, 73, 4231).

From (2)-(7) one can deduce the properties of two-photon spectroscopy. First and foremost is the fact that selection rules are different from those pertaining to one-photon absorption. For instance, in centrosymmetric molecules the selection rules are u<- u and gv u, that is, the opposite of the one-photon selection rules. Thus, two-photon spectroscopy in this case is complementary to one-photon spectroscopy much in the same way as infrared and Raman spectroscopy are in the domain of molecular vibrations. Another property is the fact that 8 depends on the polarization of the radiation even in fluid samples, that is, for complete orientation randomization. The form of 8 for all point groups at various polarization combinations of the two photons has been derived and tabulated. Consequently, polarization studies may be used to obtain structural information. Another unique feature of two-photon absorption is the possibility to observe spectra free of Doppler broadening by using two counter propagating beams. ... 

Absorption.—The studies reported here are conventional one-photon absorption studies on ground-state molecules and atoms. The use of a rapidly tuneable CW dye laser for direct absorption spectroscopy has been described.98 Transition probabilities in the spectra of Ne (I),97 and the classification of the 650.4 nm line of Xe97 have been discussed. Pressure-broadening coefficients for the atomic iodine 2P4-2Pj transition for C02, N2, He, Ne, Ar, Kr, and Xe have been measured as 7.4 0.7, 6.2 0.8, 3.6 0.3, 4.3 0.4, 5.1 0.5, 4.4 0.4, and 3.0 0.3, respectively.98 Measurements of the polarizabilities of alkali-metal atoms have been described,99 and hyperfine interactions in the excited states of sodium discussed.100 Excitation parameters in the vacuum-u.v. region... 

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