Pyrazine excitation spectrum

Figure 2. The vibronic excitation spectrum of the lBJU transition in pyrazine.

Figure 3. The rotational excitation spectrum of the lBiu (0-0) transition of pyrazine.

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

The photoelectron spectrum of pyridazine is similar to those of pyrazine, pyrimidine, and triazine i.e., the lowest ionization potential corresponds to ionization of a lone-pair electron. The ionization potential is in agreement with the calculated value. These spectra were recorded also of pyridazine 1-oxide and 1,2-dioxide, and it was found that the perturbation of the t-system by the N—O group results in the separation of the lower excited states of the AT-oxide ions. ... 

Comparison of the results of the previous effort, where it was assumed that excitation and absorption spectrum coincided,7 with the present results shows that there is really not much of a difference. Because of the higher sensitivity, MEs belonging to P(l) are detected over a range of almost 8 GHz instead of 3.5 GHz as found previously, and in accordance with that their number has increased. We now have 35 triplet states over 7.6 GHz, which leads to a density of 140 states per cm-1. The calculated density of pyrazine triplet vibronic states around the singlet energy appears to be around 100 cm-1,13-15 but if we want a comparison, we should take into account that they are triplets (times 3) and that nuclear symmetry permits interaction only between equal symmetry species. Table II gives the nuclear spin symmetry species of pyrazine and their statistical weight. The J = 0, K = 0 has the symmetry At and therefore can only interact with Ae triplet rovibronic states, which constitute th of the triplet manifold. We therefore expect about 3 x 17/48 x 100 a 106 triplet vibronic states per cm-1 to be available for the interaction, which compares favorably to the density of 140 per cm-1 as found from the density of the ME spectrum. 

As we outlined in the theoretical section, the time dependence of the fluorescence is critically dependent on the type of exciting source used. We therefore list the experiments as a function of increasing laser width. We will first limit ourselves to the photodynamics of the J = 0, K = 0 rotational state of the lB3u of pyrazine, since only of that state we know the ME spectrum. 

Luminescent coordination compounds continue to attract considerable attention. Zink recently reported a new mixed-ligand copper(I) polymer that shows interesting photoluminescence (232). The complex [CuCl(L44)Ph3P] consists of a one-dimensional chain lattice of metal ions bridged by both Cl" ions and pyrazine molecules. The compound shows conductivity of less than 10-8 S cm 1. The absorption spectrum of the complex shows a band at 495 nm, which could be interpreted as the promotion of an electron from the valence band to the conduction band. On the basis of resonance Raman spectra, the lowest excited state in the polymer is assigned to the Cu(I)-to-pyrazine metal-to-ligand charge-transfer excited state. 

Fig. 38 The solution spectrum of 0.1 mol 1 pyrazine and 0.1 mol 1 NaCl04, and the SER spectra of pyrazine adsorbed on bare nickel electrodes at various potentials in solutions of 0.01 mol 1 pyrazine and 0.1 mol 1 NaCl04. Excitation line 632.8 nm.

The Si[ Bsu nn )] and S2[ B2u (tttt )] excited states of pyrazine represent a classic example of vibronic coupling in aromatic systems.The gas-phase absorption spectrum of pyrazine exhibits typical features of such... 

Interestingly, a conical intersection very similar to that of the S i(n7r ) and S 2(7T7r ) neutral excited states has been found for the n and n hole states of the pyrazine cation.A linear vibronic-coupling model has been constructed for the and states of the pyrazine cation employing many-body Green s function methods for the calculation of the vibronic-coupling parameters. The ah initio calculated photoelectron spectrum of the Ag n ) and states is compared in Fig. 7 with the... 

Pig. 9. The effect of a bath on the spectrum of the 4-mode model for the S1/S2 manifold of the pyrazine molecule after initial excitation to the S 2 state. Taken from Ref. 14. (a) No bath present, (b) 5 bath modes, (c) 10 bath modes, (d) 20 bath modes. The spectra are calculated from the Fourier Transform of the autocorrelation function. No phenomenological broadening has been added, but a cutoff function is used to remove artefacts due to the finite time-length (240 fs) of the autocorrelation function. 

Our calculations show that these second order terms are important for a quantitative description of nonadiabatic systems. This is demonstrated in the pyrazine S1/S2 system, where a reduced 4-mode model provides a qualitative picture with the main peaks of the spectrum in the correct places. The addition of second order terms and all degrees of freedom, results in the correct spectral envelope also being produced by the model. Also in the allene A/B system, the second order terms are required, not only for the correct description of the Duschinsky rotation in the excited state, but also for the high spectral density between the two bands. Even in the butatriene X/A system, in which second order terms play a minor role in the description of the spectral band, the inclusion of these terms means that the ab initio data could be taken with minimal adjustment, whereas a reduced dimensionality model required significant adjustment of the expansion parameters. 

Calculations of the ion yield in dependence on the pulse delay time At and on the parameters of the laser pulses have been performed in Refs. 86 and 93 for simple one-dimensional models of excited-state vibrational motion and vibronic coupling. It has been found that for the vibronic-coupling examples considered and for suitably chosen pulse parameters, the ion signal as a function of At maps very well the adiabatic electronic population probability. As an example of a molecular system comprising conical intersections. Sec. 5.1 presents a calculation of the time-resolved photoelectron spectrum of pyrazine. 

Reactive MMCT states cannot only be generated by direct light absorption into MMCT bands but also by other processes [60]. The absorption spectrum of (NH3)5Co -pyz-Fe (CN)5 (pyz = pyrazine) does not display a MMCT bad which seems to be obscured by an intense Fe(II) pyz MLCT absorption. Nevertheless, MLCT excitation leads to the same products which should be obtained by direct MMCT excitation. 

The S ( fl3 (n7r )] and 52[ 2a(rr r )] excited states of pyrazine represent a classic example of vibronic coupling in aromatic systems. The gas-phase absorption spectrum of pyrazine exhibits typical features of such systems, namely a well-resolved vibronic structure in the lower energy band, corresponding to the Si-So transition, and a largely structureless spectral profile for the S2-S0 transition. The S2 state is characterized by a lack of detectable emission, amounting to a fluorescence quantum yield

[Pg.3176]

The absorption spectrum of the 52-5o and 5 -5o electronic transitions of pyrazine has been calculated at various levels of sophistication. " The most elaborate treatment relies on the CASSCF/MRCI data mentioned above and includes seven vibrational modes, V[, V4, vg< u% V o , and V14. " The linear vibronic coupling approach, equation (31), has been shown to be appropriate for the totally symmetric modes vj, Vfin, vgtf. Consequently, it has been adopted for these modes in the calculation of the spectrum. For the coupling mode V (v the significant reduction of vibrational frequency in the excited states has been accounted for. " The two additional nontotally symmetric modes, V4 and V 4, are characterized by relatively... 

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