One-photon absorption

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... 

All nonlinear (electric field) spectroscopies are to be found in all temis of equation (B 1.3.1) except for the first. The latter exclusively accounts for the standard linear spectroscopies—one-photon absorption and emission (Class I) and linear dispersion (Class II). For example, the temi at third order contains by far the majority of the modem Raman spectroscopies (table B 1.3.1 and tableBl.3.2). 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Figure 13 shows the images of masses m/e = 42, corresponding to C3D3, at various delay times. The image of m/e = 42 was disk-like and its width did not change with the delay time. Therefore, it was totally from the dissociation of hot benzene after the ionization. Photolysis laser fluence dependence study showed that it was from one-photon absorption. 

Kamada K, Ohta K, Iwase Y, Kondo K (2003) Two-photon absorption properties of symmetric substituted diacetylene drastic enhancement of the cross section near the one-photon absorption peak. Chem Phys Lett 372 386-393... 

Kleinschmidt J, Rentsch S, Tottleben W, Wilhelmi B (1974) Measurement of strong nonlinear absorption in stilbene-chloroform solutions, explained by the superposition of two-photon absorption and one-photon absorption from the excited state. Chem Phys Lett 24 133-135... 

Extensive electron delocalization along the chain direction leads to an electronic transition energy for one-photon absorption (E0) of typically 15 000-16 000 cm-l for an unstrained backbone (.7). This value is very close to that of polyacetylene (or... 

The strong one-photon absorption is observed for polarizaion parallel to the chain axis (7). Our polarization studies of the three-wave mixing demonstrate that the two-photon absorption observed in these experiments is also polarized along the chain axis. 

One-part solvent-releasing butyl sealant formulation, 22 43t One-part urethane sealants, 22 36-37 One-pass clarifier, 22 61 One-photon absorption, 77 455 One-pot oxidation polymerization, 23 713 One-stage separation plants, 75 840 One-step cameras/processors, 79 276-278 One-step Ziegler process, 77 715-718 One-way memory, of shape-memory alloys, 22 341, 345-346... 

Fig. 1 Energy diagrams illustrating the process of one-photon absorption (IPA) and two-photon absorption (2PA) for (left) a molecule without an inversion center (right) a molecule with an inversion center. The length of each arrow is proportional to the photon energy. The dotted arrows represent possible deexcitation pathways...

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...

Two-photon absorption is an example of a process that causes transmission of light through a medium to be power dependent. In the absence of one-photon absorption, the transmission T through a two-photon absorbing sample is... 

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. 

The situation can differ significantly in the presence of a pulse of UV or X-ray radiation. Indeed, a weakly bound electron submitted to a UV pulse is readily ionized, via one-photon absorption, even at low or moderate peak intensities. However, if the peak intensity of the radiation field is increased, several model simulations show that, very counter-intuitively, the ionization yields can decrease, [17]. This situation can be pictured as resulting of a dichotomy of the electron wave function strongly driven away from the... 

According to the selection rules, one-photon absorption occurs only if the change in angular momentum (change in L) is +1 or -1 (Al = 1, A/ = 0, 1 (0 o 0 not allowed), AL = 0, 1, AS = 0) (Al is according to the hydrogenic atom model, whereas AL is for multielectron atoms). The selection rules allow transition in one-photon absorption only to the p states from the s ground state as a result only even-to-odd parity is allowed. 

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