Sodium dimer , fluorescence

This represents a formidable practical problem, as one is very unlikely to find isolated atoms with two nonorthogonal dipole moments and quantum states close in energy. Consider, for example, a V-type atom with the upper states 11), 3) and the ground state 2). The evaluation of the dipole matrix elements produces the following selection rules in terms of the angular momentum quantum numbers J — J2 = 1,0, J3 — J2 = 1,0, and Mi — M2 = M3 — M2 = 1,0. Since Mi / M3, in many atomic systems, p12 is perpendicular to p32 and the atomic transitions are independent. Xia et al. [62] have found transitions with parallel and antiparallel dipole moments in sodium molecules (dimers) and have demonstrated experimentally the effect of quantum interference on the fluorescence intensity. We discuss the experiment in more details in the next section. Here, we point out that the transitions with parallel and antiparallel dipole moments in the sodium dimers result from a mixing of the molecular states due to the spin-orbit coupling. 

Spectroscopic Basics. The sodium dimer s first singlet electronic state A 17+ has been studied by different cw techniques using laser-induced fluorescence, optical-optical double resonance, and Fourier transform spectroscopy [319, 320, 330-332]. Ro-vibrational levels could be numbered and... 

The method of Kato and Nakai (27) for determining protein surface hydrophobicity was adapted for evaluating procyanidin binding to BSA and Gl. The procedure is based on the fact that the fluorescence quantum yield of cis-parinaric acid increases 40-fold when cis-parinaric acid enters a hydrophobic environment from a hydrophilic environment. The digestion of BSA by trypsin in the presence of procyanidin dimer, procyanidin trimer and black bean procyanidin polymer was evaluated by discontinuous sodium dodecyl sulfate (SDS) slab gel electrophoresis and a picryl sulfonic acid (TNBS) assay (28). 

In the course of similar experiments in which the liquid nitrogen baffle above is removed, an intriguing phenomena has been observed. We have been able to induce sodium D-line fluorescence upon single photon pumping at energies far lower than that required to simultaneously dissociate dimeric sodium and produce emission from excited atoms. The atomic fluorescence and its characterization will be the subject of our following discussion. Briefly, we have observed and characterized laser induced atomic fluorescence upon photodissocia-tlon of sodium trimers formed under a variety of conditions. 

The dissociation energy of Na2 to two S sodium atoms is 6022 cm (AA). Therefore the photodissociation of dimeric sodium to produce the onset of D-llne fluorescence would require that Na2 possess 5500 cm of internal excitation (X Zg", v" A5). 

If the first laser beam is interrupted at the time to for a time interval At that is short compared to the transit time T = z2 — Z )/v (this can be realized by a Pock-els cell or a fast mechanical chopper), a pulse of molecules in level /) can pass the pump region without being depleted. Because of their velocity distribution, the different molecules reach zi at different times t = to- -T. The time-resolved detection of the fluorescence intensity /fi(0 induced by the second, noninterrupted laser beam yields the distribution n T) = n(Az/v), which can be converted by a Fourier transformation into the velocity distribution n(v). Figure 4.13 shows as an example the velocity distribution of Na atoms and Na2 molecules in a sodium beam in the intermediate range between effusive and supersonic conditions. If the molecules Na2 had been formed in the reservoir before the expansion, one would expect the relation Up(Na) = V2up(Na2) because the mass m(Na2) = 2m(Na). The result of Fig. 4.13 proves that the Na2 molecules have a larger most probable velocity Up. This implies that most of the dimers are formed during the adiabatic expansion [410]. 

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