Two-photon spectra

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... 

Figure 2. Transient two-photon spectra recorded at an excitation wavelength of 834 nm (pump) for (a)39,39 K2 and (b)39,41 K2. The ionization step (probe) was achieved by a delayed two-photon transition of the same wavelength. Despite their great similarity both molecules exhibit quite individual oscillatory behavior [7],...

Again, since the d orbitals have even parity, even if the molecule does not have an inversion center there is an approximate selection rule in which transitions that would be g -> g (or u -> u) in a parent group with inversion symmetry are allowed. The odd parity vibrations that dominate the single photon spectrum are forbidden, while the even parity vibrations are allowed, but have no advantage over the pure electronic transitions. Experimental two-photon spectra of the sharp-line transitions of Mn4+ in a Cs2Ge F6 host confirm both the simplicity of the spectrum and the relative prominence of the 0-0 lines [55],... 

Fig. 3. Continuous wave Doppler-free two-photon spectra of hydrogen and deuterium 1S-2S [11].

Despite such successes, it was obvious that only a continuous wave laser source can do justice to the extremely sharp 1S-2S transition. Production of intense cw radiation near 243 nm remained long an elusive goal. Satisfactory power levels of several mW were first achieved by B. COUILLAUD et al. [19] by summing the frequency of a 351 nm argon laser and a 790 nm dye laser in a crystal of KDP. In the first cw experiment with this source [11], the power in the observation cell was further enhanced with a standing wave build-up cavity. Fig. 3 shows two-photon spectra recorded in this way. Although the resolution is much superior to the earlier pulsed spectra, it remains limited to a few MHz by laser frequency jitter, collision effects, and transit-time broadening. A further at least millionfold improvement in resolution should ultimately be achievable. 

Two-photon spectra of A <— X transitions in ben-zonitrile and styrene. Styrene spectrum also measured using MPI in a supersonic jet... 

Hohlneicher, G., Wrzal, R., Lenoir, D., Frank, R., Two photon Spectra of Stiff Stilbenes a Contribution to the Assignment of the Low Lying Electronically Excited States of the Stilbene System, J. Phys. Chem. A 1999, 103, 8969 8975. 

Two-photon spectra of all-trans octatetraene, both in n-octane (which provides centrosymmetric mixed-crystal sites) and for isolated molecules in supersonic jets, furnish the most direct proof that the lowest energy transition must be assigned to a g g transition (Granville et al., 1979 Petek et al., 1995). 

NLO transmission, and 3D microfabrication. However, there have been rather few studies of organometallic compounds at this time, especially studies in which two-photon spectra have been determined. 

The parasitic signal called "cross-over", on the contrary, perturb the saturation spectra (and can be dangerous in the interpretation of complex molecular spectra), whereas they do not exist in two-photon spectra. 

For illustration the examples in Fig. 2.33 show the Doppler-free two-photon spectra of the 35 55" transition in the sodium atom with resolved hyperfine structure... 

In addition to the nonresonant NLO properties, one- and two-photon spectra of several of the compounds in Fig. 5.3 were also computed in Ref. [56]. The averaged two-photon absorption cross section for isotropic media in the case of two parallel linearly polarized photons <8° > is... 

The dominating importance of single excitations is based on physical reasons. They dominate the most common one-photon processes, since they usually have low excitation energies and involve large transition moments (intensities). This is different for two-photon spectra for a recent DFT study on this topic, see Ref. 18. 

S.3.2.4 Practical Example One- and Two-Photon Spectra for R3MCP In this section, we report the 5i So and S2 <— So one- and two-photon absorption, and circular dichroism (CD) spectra of R3MCP, computed according to the FCHTlAH and FCHTIVG models. 

L. Wunsch, H.J. Neusser, E.W. Schlag Polarization effects in the rotational structure of two-photon spectra in the gas phase. Chem. Phys. Lett. 38, 216 (1976)... 

The intensities of the majority of the f-f transitions vary only within a factor of 2-3 from host matrix to host matrix, but some transitions are much more host dependent. These transitions are called hypersensitive transitions. These induced electric dipole transitions obey the selection rules for electric quadrupole transitions and are therefore sometimes called pseudo-quadrupole transitions. In sect. 8, we will discuss hypersensitivity in detail. The dependence of the Qx intensity parameters on the host matrix is the subject of sect. 9. Two-photon spectra (sect. 10) and vibronic transitions (sect. 11) are discussed briefly. On the other hand, chiroptical methods will not be considered. Since the color of the lanthanide ions is related to the spectral intensities of f-f transitions, we want to give attention to the phenomenon of color (sect. 12). Finally, the intensities of actinide ions are reviewed (sect. 13). 

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