Rotational contours

This general behaviour is characteristic of type A, B and C bands and is further illustrated in Figure 6.34. This shows part of the infrared spectrum of fluorobenzene, a prolate asymmetric rotor. The bands at about 1156 cm, 1067 cm and 893 cm are type A, B and C bands, respectively. They show less resolved rotational stmcture than those of ethylene. The reason for this is that the molecule is much larger, resulting in far greater congestion of rotational transitions. Nevertheless, it is clear that observation of such rotational contours, and the consequent identification of the direction of the vibrational transition moment, is very useful in fhe assignmenf of vibrational modes. 

Figure 7.45 Computed (a) type A and (b) type C rotational contours for 1,4-difluorobenzene using the same rotational constants as for Figure 7.44(b). (Reproduced, with permission, from Cvitas, T. and Hollas, J. M., Mol. Phys., 18, 793, 1970)...

The narrowest PFI bands in the present study are 3 cm-1 FWHM, using a 0.5 V/cm extraction field with the lasers attenuated to minimize effects of space charge. We measure band positions at the intensity maxima. These are reproducible to better than 1 cm-1. The bandwidth is limited by the rotational contour and also by the ionization process. A major advantage of ZEKE-PFI over more traditional photoelectron techniques is that the energy calibration is that of the tunable dye lasers, which are quite stable from day to day. In contrast, both electrostatic analyzers and time-of-flight photoelectron spectrometers require frequent calibration. 

For UV spectra of parent and substituted 1,2,3-triazoles and benzotriazoles, see CHEC-I <84CHEC-1(5)684 >. The UV spectra of benzotriazole, 1-methyl- and 2-methyl-benzotriazole in the gas phase at 90°C have been recorded <94JOC2799>. 1-Alkyltriazolines show two A ax in acetonitrile, 239-242 and 263-266 nm, both with log e w 3.50 <93JOC2097>. The UV spectra of bicyclic triazolines (754) have been recorded <9lJOC4463>. The Si-So electronic absorption spectrum of 1/f-benzotriazole at 286 nm has been studied by computer simulation of the rotational contours. The result shows that the benzotriazole band is an almost pure type-5 band <93JSP(158)399>. 

Finally, So (X) — Tj (a) transitions to the three spin sublevels of the Tj state were observed by fluorescence-detected S2 < l i < S(l OODR experiments, and confirmed by rotational contour analyses. The transition to the Tz spin sublevel is much stronger and broader in linewidth, than the transitions to the Tx and Ty spin sublevels, consistent with the theoretical expectations. Highly differing Tx S0 interaction strengths for the three spin sublevels leads to Tj decay, which appears to be biexponential with vastly different lifetimes. 

In the preceding sections vibrational spectra were used as a source of information on H-bonding. The rotational fine structure of the bands could not be resolved. Only the rotational contours, the breadth of unresolved P, Q, R branches entered occasionally the discussion. In order to resolve rotational fine structure much lower... 

For the dimethylether. HF complex Thomas also recorded the spectrum from 50 to 8 cm-1 and found rotational contour around 10 cm-1 belonging to the complex meaning that the complex has a lifetime of at least three picoseconds. 

Phosphorescence of s-triazine has been observed by Ohta et al. following excitation of the 6o band of the Si — So transition. Values for the phosphorescence lifetime and quantum yield were reported. The effects of rotational excitation on the yields and decays of the fast and slow components of Si state s-triazine fluorescence have been studied. Excitation along the rotational contours of the 6j and 6o bands revealed that the fast component showed little rotational level dependence in contrast to the slow component. This behaviour was interpreted in terms of an increase in the number of triplet levels coupled to the optically prepared singlet levels with increasing angular momentum quantum number, J. A broad emission feature present in addition to narrowline fluorescence from rovibronic levels of 6 or 6 in S, s-triazine has been observed and the rotational level dependence of its quantum yield and decay over a range of pressures reported... 

Analysis of the vibrational structure and rotational contours of the LIF emission and excitation spectra offers the possibility of studying the conformational changes of the molecules and clusters upon excitation. However, the assignment of the low-frequency modes observed in the LIF excitation spectra of the investigated compounds is difficult in view of the complexity of the dialkylamino group which may... 

Figure 11. Low-iesolution rotational contour of (6,0) band of BaO as produced by reaction with three SO2 beams. In (a) beam contained 20% dimers, in (b) 3%, and in (c) no dimers could be detected. Rotational contour varies clearly as a function of dimer concentration. Dimer spectrum shows a rotational energy distribution which corresponds to 100 K while monomeric reaction produces a distribution that can be fitted to 400 K.

Very importantly, Pearson and Innes found distinctive rotational contour in the 0-0 band of ethane-dg. The band contour analysis using computer simulation showed that the transition moment for the 0-0 band is perpendicular to the C-C bond. This indicates that the transition is Aig — This is believed to be definite. It is however, still compatible with more than one orbital assignment. It will be further discussed below. 

Very little is known about the nature of rotational energy transfer in a collision between an electronically excited molecule and a ground-state atom or molecule. In the few reported studies the experimental method is fundamentally the same as that described at the beginning of Section III.A. An initial rotational distribution is established by narrow-band excitation. The fluorescence emission contour is recorded twice, under collision-free and thermal equilibrium conditions, and then again under conditions such that there is one collision during the lifetime of the excited state. The differences in the rotational contours of the three emission spectra are then used to infer the pathway of rotational energy transfer, and the rate of that transfer. Some examples of the emission spectra recorded under these conditions are shown in Fig. 22. Because of the small spacings between the rotational levels of polyatomic molecules most excitation sources prepare nonthermal superpositions of rotational states rather than pure rotational states, and this complicates interpretation of the observations. 

Collision-induced intramolecular vibration-to-rotation energy transfer appears to be inefficient. The evidence for this inference comes from the study of rotational contours in the one collision-induced transition 7 0° in glyoxal. It is found that the emission from 0° has a distribution over rotational transitions that is close to the thermal distribution. But the vibration v-j in glyoxal is a torsional motion, and the axis of torsion very nearly coincides with the smallest axis of inertia of the molecule, so if collision-induced intramolecular vibra-tion-to-rotation transfer were efficient the emission from 0 should have a nonthermal distribution in the quantum number K (which describes quantization of the motion about the smallest axis of inertia). Note, however, that the collision partner used in this experiment was... 

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