Intermediate state resonance enhancement

Variations of arm and of the intermediate state resonance enhancement ISRE, the ratio between the nondegenerate TPA cross-section and the degen-... 

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. 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

Resonantly enhanced two-photon dissociation of Na2 from a bound state of the. ground electronic state occurs [202] by initial excitation to an excited intermediate bound state Em,Jm, Mm). The latter is a superposition of states of the A1 1+ and b3Il electronic curves, a consequence of spin-orbit coupling. The continuum states reached in the two-photon excitation can have either a singlet or a triplet character, but, despite the multitude of electronic states involved in the computation reported J below, the predominant contributions to the products Na(3s) + Na(3p) and Na(3s) + Na(4s) are found to come from the 1 flg and 3 + electronic states, respectively. The resonant character of the two-photon excitation allows tire selection of a Single initial state from a thermal ensemble here results for vt = Ji — 0, where vt,./, denote the vibrational and rotational quantum numbers of the initial state, are stJjseussed. 

It is known that the position of the Raman D-band shows excitation energy dependence [44]. The 790- and 395-nm excited D-bands indeed appeared around 1,300 and 1,390 cm , respectively [27]. According to the D-band position in the HRS spectmm, both fundamental and harmonic resonances to intermediated states were thought to contribute to the enhancement of the HRS intensity. Conversely, the G-band shape, which is decomposed into G" " and G , suggested the contribution of harmonic resonances to the transition. The observed HRS G -band was able to be fitted with Breit-Wigner-Fano (BWF) type line shape, which is characteristic in Raman spectra of metallic SWNTs [45]. 

Resonance Raman spectra have been recorded for transient phenoxyl radicals by pulse radiolysis. From the spectra recorded for various substituted and deuteriated radicals, it was possible to analyze the peaks in terms of the C—O and C—C stretching modes and C—C—C bending modes and to draw conclusions about the structures of the radicals. For example, the frequency assigned to the C—O bond in benzosemiquinone was found to be intermediate between those for the C=0 in benzoquinone and the C—O in hydroquinone, which led to the conclusion that the bond order in semiquinone is about 1.5. Raman spectroscopy also yielded information on the excited states of the radicals by examining which frequencies are resonance-enhanced ° ° . Furthermore, time-resolved experiments with Raman spectroscopy allowed kinetic measurements on a specific intermediate unmasked by changes in other species. ... 

Two-photon photoemission spectroscopy is known for its capability to reveal not only occupied but also unoccupied electronic density of states [10]. In this scheme, one photon excites an electron below the Fermi level to an intermediate state. A second photon then excites the electron from the intermediate state to a final state above the vacuum le vel. The photoelectron yields are strongly enhanced if the excitation photon energy is tuned to the resonance conditions, and the photoelectron spectrum reflects the electron lifetime in the intermediate states as well as their density of states. It is necessary to keep the employed photon energy below the work function of the sample, otherwise one photon photoemission signal becomes excessive and buries the 2PPE signals. 

In atoms and molecules, shakeup satellites, corresponding to internal electronic transitions, are routinely observed using photoelectron and resonant Raman spectroscopy. In particular, shakeup satellites can be observed in the two particle spectrum, i.e., when two holes are left in the final state of an atom after electron emission. Satellite s strength can be strongly enhanced in the presence of a resonant intermediate state. For example, in copper atoms, the incident photon can first excite the core 3p electron to the 4s shell the core hole then decays to the 3d shell through the Auger process (with electron ejected from 3d shell) leaving two 3d holes in the final state [48]. For recent reviews of extensive literature the reader is referred to Refe. [49,50]). 

The frequency denominators in the eight terms of equation B1.5.30 introduce a resonant enhancement in the nonlinearity when any of the three frequencies (co j, CO2, co ) coincides with a transition from the ground state g) to one of the intermediate states n ) and n). The numerator of each term, which consists of the product of the three dipole matrix elements reflects, through its tensor character, the structural... 

On comparing H NMR spectra obtained for the reaction of para-H2 with X at different reaction times (fig. 10), it can be seen that the H2 signal starts as an enhanced negative value (due to the initial transfer from para-H2), and then reaches its thermal equilibrium value by a complex mechanism basically determined by the relaxation rates of the protons in the ruthenium complex. Clearly, the exchange rate of H2 over the triruthenium cluster should be faster than the relaxation rate of the hydride ligands, otherwise no memory of the intermediate state could have been revealed in the molecular hydrogen resonance. [56]... 

Two of the three laser ionization methods have already been discussed, namely one-photon PI and multiphoton MPI. The third type is resonance enhanced MPI, or REMPI. In the latter method the laser is tuned so that an intermediate state of the molecule is excited with one, two, or perhaps three photons. The excitation of the intermediate state determines the overall cross section for the process because the absorption of additional photons to reach the ionization continuum is generally rapid. In contrast to PI and MPI, REMPI is state selective if the absorption process is resonant between two bound and reasonably long-lived states of the molecule. It is an extremely sensitive method for product detection because the result of the REMPI process is an ion which can be detected with near 100% efficiency. Not only is the ion collection efficiency of the detector (e.g., by channeltron electron multiplier or a multichannel plate detector) extremely high (ca. 50%), but all ions regardless of their initial velocity vector can be collected by the application of appropriate electric fields. This is a major advantage... 

To obtain information as to which vibrations are subject to resonance enhancement, we apply (following Albrecht ) the adiabatic Bom-Oppen-heimer approximation of separability of electronic and vibrational wave functions for g), r), and ]/) states in Eq. (2). Thus, by taking the initial and final states to belong to the ground electronic state and the intermediate state to the resonant excited electronic state, and dropping the nonreso-nant term, Eq. (2) can be rewritten as... 

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