Fluorescence spectra ground electronic state

The ground electronic state of 139La160 is X2S+ audits electronic spectrum involving the excited B2Y,1 has been studied by Doppler-free laser-induced fluorescence by Bacis, Collomb and Bessis [85] and by Bernard and Sibai [86]. Both states have therefore been well characterised and the system is ideal for radiofrequency/optical double resonance, as described by Childs, Goodman, Goodman and Young [87]. They used a collimated molecular beam, with the laser pump/probe technique described elsewhere in this chapter. 

Between the A and B regions, a radioffequency field was applied to induce fine-structure transitions within the v" = 1 level of the ground electronic state, split by the nuclear hyperfine interaction. The selection rules for these transitions, which ranged in frequency from 360 to 7700 MHz, were A J = 1, AF = 0, 1. They were detected through resonant changes in the fluorescence intensity an example of a radioffequency double resonance line is shown in figure 11.53. The observed spectrum involved N values from 1 to 27. 

The fluorescence spectrum shows a vibrational structure giving the vibrational frequency of the molecule in the ground state. This is in contrast to the electronic state. This follows that the fluorescent spectrum and electronic spectrum should appear as mirror images of each other, but this description should be taken literally. 

The resonance Raman and resonance fluorescence contributions are seen separated in the closer look seen in Fig. 18.9. The observed spectrum is assigned to emission from the origin of the electronic excited state onto a particular vibronic level in the ground electronic state. For reasons irrelevant to our discussion the Raman line appears as a doublet in this high-resolution spectrum. The emission... 

A fluorescence spectrum is usually the mirror image of the absorption spectrum and a study of the fluorescence spectrum of a substance yields information concerning its molecular structure of essentially the same nature as that derived from its absorption spectrum. When vibrational structure appears in the fluorescence spectra of molecules in solution, a more or less complete vibrational analysis of the ground electronic state may be made analogous to that derived from Raman or infra-red spectra. 

Zare and co-workeis have demonstrated the high sensitivity of las -induced fluorescence in determination of internal state distributions of a wide range of reaction products formed under molecular beam conditions. As well as having made systematic lifetime determinations, Zare et al. could obtain a laser excitation spectrum by continuously scanning the wavelength of a tunable dye laser which excites a particular state of a reaction-product ground electronic state to a... 

Providing ancillary spectroscopic data is available, equation (17) permits extraction of relative ground-state vibrational populations from intensities of the laser excitation spectrum. Wavelength resolution of the fluorescence enables Franck-Condon factors to be calculated from relative intensities observed in emission from a single v level to a manifold of levels v" in the ground electronic state. 

The LIF of polyatomic molecules opens possibilities to recognize perturbations both in excited and in ground electronic states. If the upper state is perturbed its wave function is a linear combination of the BO wave functions of the mutually perturbing levels. The admixture of the perturber wave function opens new channels for fluorescence into lower levels with different symmetries, which are forbidden for unperturbed transitions. The LIF spectrum of NO2 depicted in Fig. 1.56 is an example where the forbidden vibrational bands terminating on the vibrational levels (ui, U2, U3) with an odd number U3 of vibrational quanta in the asymmetric stretching vibrational mode in the electronic ground state are marked by an asterisk [182]. 

FIGURE 3.10 Time-dependent fluorescence spectra upon the reaction of photoisomerization of pentadiene-l,3into 3-methylcyclobutene corresponding to transitions from the resonant level to the vibronic levels of the ground electronic states of pentadiene-1,3 (upper spectrum) and 3-methylcyclobutene (lower spectmm).Each spectrum is normalized to the maximum intensity value. The frequency co is counted from the frequency of the 0-0 transition. 

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