Photon absorption spectra

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

Fu J, Padilha LA, Hagan DJ, Van Stryland EW, Przhonska OV, Bondar MV, Slominsky YL, Kachkovski AD (2007) Experimental and theoretical approaches to understanding two-photon absorption spectra in polymethine and squaraine molecules. J Opt Soc Am B 24 67-76... 

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. 

The nuclear quadrupole resonance absorption spectra of 2,2 -bipyridine and 4,4 -bipyridine have been measured, and the photoelectron spectra of the same bipyridines and 2,2 6, 2"-terpyridine have been investigated. Two-photon absorption spectra of 2,2 -bipyridine have been taken. ... 

Fig. 9 Two photon absorption spectra (from 2PIF measurements) of distyrylbenzene derivatives with various substitution patterns, q.3 data from [80,90] q.4 from [94] q.5 from [95,96]...

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). 

Fig. 23.12 Ba two-photon absorption spectra 6s45p—>nfn t". Narrow resonance lines correspond to ionic resonances of Ba+ due to coincidences between atomic (Ba) and ionic (Ba+) resonances. They are indicated by an arrow. Note the evolution to a continuous spectrum at high photon energies (from ref. 34).

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. 

Two-photon absorption spectra of NO in the region 374 55 000—60 000 cm. Identification of previously unobserved 3p Rydberg state... 

Multiple-photon absorption spectra of SF, NFI3, and C2H4 determined using a new optothermal detector... 

Figure 3.52. Two-photon absorption spectra for 112a (AF450), 112b (AF455), and 112c (AF457). (From Ref. [508] with permission of the American Chemical Society.)...

Typical CNDO/S linear and two-photon absorption spectra of polyphenyls are displayed in Fig. 33 [157]. The low energy optical response of all compounds is accurately described by a three-level model with parameters that are summarized in Table 6. Transition energies are red-shifted, and transition dipole moments increase with the monomer munber n. 

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...

Fig. 39 Two-photon absorption spectra of V-shape and dendrimeric compounds.

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. 

Figure 11 SDMRG two-photon absorption spectra (corresponding to the dipole transition moments from 1B to different Ag states) for N=6-16 (arbitrary units).

Fig. 36 Two-photon absorption spectra of 73 (filled triangles) and 79 (filled circles) measured by the femtosecond Z-scan method...

As mentioned in Section 9.2, there is an important distinction between action spectroscopies based on the absorption of multiple photons, as is possible with FEL excitation, and those based on single photon absorption, such as messenger spectroscopy [26,27], laser-induced reaction spectroscopy [73] or indeed direct absorption experiments. When the signal (in this case, the depletion of the parent ion and creation of product ious) depends on the absorption of multiple photons, the signal intensity may not be compared directly to computed linear (single-photon) absorption spectra, as is the normal fashion. There are several reasons why intensities may differ. Thus, baudpositiou (frequency) is considered usually to be more reliable than band intensity when comparing computed and experimentally-measured spectra. 

Time-dependent density functional theory studies of the photoswitching of the two-photon absorption spectra in stilbenes were reported [15]. The one- and two-photon absorption characteristics of the open- and closed-ring isomers of 1-3 have been... 

Figure 3.8 Vibronic two-photon absorption spectra of Sila-1f and 2 2a-2f of 4-dimethylamino-4 -Me-trans-stilbene calculated with B3LYP at the HF geomet7. Franck-Condon contribution, Herzberg-Teller contribution, and total spectra are shown for lifetime broadening of 0.10 eV (left panels) and 0.01 eV (right panels), respectively. The dashed line corresponds to the vertical transition [18]. (Reproduced with permission.)...

Micron- and submicron-scale lateral structures of optically nonlinear organic films comprised of substituted trons-stilbene derivatives (Ri = OCH3, R2 = CN) was characterized [45]. Second harmonic generation (SHG), optical microscopy, and atomic force microscopy (AFM) were used in this investigation. The third-order nonlinear optical properties and two-photon absorption ofdifferent types of stilbene derivatives (D-tt-D, A-7t-A, D-7t-A) were investigated [46]. Using the INDO/CI method, the UV-vis spectra were explored and the position and strength of the two-photon absorption were predicted by sum-over-states expression. Relationships of the structures, spectra, and nonlinear optical properties have been examined. Two-photon absorption spectra (650-1000 nm) of a series of asymmetrically substituted stilbenoid chromophores... 

Two-photon absorption spectra (650-1000 nm) of a series of stilbene chromophores were measured via a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond white light continuum... 

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