Rovibronic spectrum

The calculations of the rovibronic spectra verify that the T-shaped He ICl features in the LIF and action spectra can be associated with n = 0 n" = 1... 

Spectroscopic parameters of a molecule are derived from experimentally determined spectra by fitting term values to a properly chosen model Hamiltonian.161 Usually, the model Hamiltonian is an effective one-state Hamiltonian that incorporates the interactions with other electronic states parametrically. In rare cases, experimentalists have used a multistate ansatz like the supermultiplet approach162 to fit the rovibronic spectra of strongly interacting near-degenerate electronic states. The safest way of comparing theoretical data to experiment is to compute the spectrum and to fit the calculated term energies to the same model Hamiltonian as the experimentalists use. 

In product state spectra, the pump laser wavelength is fixed and the probe laser is scanned to obtain product rovibronic spectra. Analyses show how the product state distributions depend on the portion of the potential surface initially excited. Rotational state distributions of IlgHfX Z, V, N) were obtained using the A n,/2 <- X Z system. 

Using a molecular beam laser spectrometer equipped with laser-induced fluorescence and optothermal detectors, the high-resolution rovibronic spectra of large molecules, such as aniline, tetrazine, and pyridine, were measured. This technique reveals, in a direct manner, the energy transferred to vibrational and rotational degrees of freedom of the ground state after relaxation <1997JPH109>. 

As anticipated in the introduction, the methodological machinery presented in the above sections can be successfully apphed to different computational spectroscopic smdies ranging from ESR, IR/Raman, low-resolution UV—vis up to rovibronic spectra, and a large variety of physicochemical systems from small molecules in solution to macromolecules and extended systems. The following examples, which are chosen to illustrate the flexibility of the present approaches, focus on IR and vibrational analysis, optical absorption, and phosphorescence spectra. 

Simon-Parr-Finlan coordinate, and = 6 — 6 . Absolute intensities and rovibronic spectra for T = 300 K were generated with some line intensities tabulated. 

The rovibronic spectrum presented in Fig. 26 was recorded at the highest power, of 5 x 10, obtained to date with the VUV laser-driven sources described here. This represents a factor of 3 to 5 improvement over spectroscopic resolution obtained with grating instruments in this wavelength region. The 25-0 band of Fig. 26 is one of 12 vibronic bands originating from v = 0 or 1 levels of the ground state to levels v = 23 to 31 of the excited state. For each of the 24-0, 25-0, 26-0, 26-1, and 27-1 bands, three rotational branches were clearly resolved. Three branches were less conspicuous in other bands or unobservable because of blending of lines, but all bands could be analysed in terms of P, Q, and R branches. 

This pattern of transitions in the rovibrational spectrum of the diatomic is roughly similar to the appearance of a rovibronic spectrum. Equations 9.27 and 9.28 need to be adjusted to include the equilibrium term energy for the difference in energy between the potential minima for the initial and final electronic states and to separate the power series expansions in the vibrational and rotational energies. Parameters such as B (o, and depend on the potential energy curve of each individual electronic state. No simple equation relates these potential energy curves for different electronic states, and therefore distinct values for each of these parameters are given to each electronic state ... 

Relative intensities of the different rotational lines in a rovibrational or rovibronic spectrum are determined only partly by the transition moment integral, which is most sensitive to the value of / when / is small. The variation in intensities more strongly reflects the number of molecules in each rotational level, which depends on the degeneracy of the level, the symmetry of the molecule, and the temperature. The temperature-dependence of the relative intensities is no longer a function of the properties of individual molecules, however, and so it is discussed instead in Thermodynamics, Kinetics, and Statistical Mechanics, Section 6.3. 

In a rovibronic spectrum, the electronic state, as well as vibrational and rotational states, is changing. 

For an electronic spectrum that has both vibrational and rotational structure (a rovibronic spectrum), suggest a form for the complete transition moment. 

Vibrational transitions accompanying an electronic transition are referred to as vibronic transitions. These vibronic transitions, with their accompanying rotational or, strictly, rovibronic transitions, give rise to bands in the spectrum, and the set of bands associated with a single electronic transition is called an electronic band system. This terminology is usually adhered to in high-resolution electronic spectroscopy but, in low-resolution work, particularly in the liquid phase, vibrational structure may not be resolved and the whole band system is often referred to as an electronic band. 

Dissociation occurring by tunneling from a bound to an unbound rovibronic state (i.e., a state corresponding to a particular rotational energy level of a vibrational level of an electronic state). 2. The appearance of a diffuse band region within a series of sharp bands of an absorption spectrum. 

Figure 13. Rydberg spectrum of C6D6 obtained by pumping via the P transition to the Sj, 61, J = K = 1, (-/) rovibronic intermediate state. Note that Rydberg states up to n > 100 are resolved.

Radiative lifetimes of NS(B II), 6= u = l2 determined using LIF. Collisional quenching by N2 studied Fluorescence excitation spectrum of NH2(A Ai — X Bj) measured in a supersonic free jet. Lifetimes obtained for single rovibronic levels of NHjfA Aj)... 

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 specified by Eq. (8.1.1), as the photon energy hi/ is varied and either the ion or the electron is detected, a photoionization spectrum is obtained. If the monitored rovibronic state of the AB+ ion-core is known and kept fixed, as hi/ is scanned, the obtained spectrum is called a constant ionic state (CIS) spectrum. Alternatively, if hi/ is held fixed, and the e KE) is scanned, the peaks in the e (KE) spectrum correspond to successive energy levels of the ion. This is known as photoelectron spectroscopy (PES). 

The e (KE) peaks in the PES (fixed hu) provide information about the relative energies of electronic states of the AB neutral molecule (for example AsO-, Lippa et al., 1998) that, owing to spin or orbital selection rules, are not easily observed by direct study of the neutral molecule spectrum. An anion photodetachment spectrum is recorded by detecting the low-energy electrons ejected as the photon energy is scanned through the successive photodetachment thresholds associated with the energies of each neutral molecule rovibronic state. 

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