28 



The red phosphorescence of rhodamin has a wavelength which 

 is 35 m/i longer than that of its fluorescence, triplet levels being 

 usually somewhat lower than singlet levels. As has been sym- 

 bolized in Fig. 5, in the unfrozen state the rhodamin gets excited 

 by our UV to its higher S2 level, drops in its "internal conver- 

 sion" to Si, and from here to G emitting its excess energy as 

 fluorescent light. What happens in the frozen part of the second 

 tube in Fig. 5 is that the electron goes from S2 to Ti and then 

 returns from here to its ground state G, emitting energy in the 

 form of phosphorescent light. Whether it gets from S2 to Ti via 

 Si or T2 can be left an open question. 



On closer observation this second tube in Fig. 6 with the rhoda- 

 min-thiamine mixture is found to have three zones, a fluorescent 

 liquid one on top, a phosphorescent one on the bottom, and a zone 

 which separates the two and emits no light. This intermediar)' 

 zone has an intermediate temperature. We can observe this transi- 

 tory nonluminous phase also by gradually cooling our solution. 

 On cooling, the fluorescence disappears before the phosphorescent 

 light emission appears. Only when the tube reaches a low tem- 

 perature does the phosphorescent light emission set in. It becomes 

 stronger when we go from dry ice to liquid N2 which is about 

 100°C colder. One would have expected radiation to become 

 weaker on cooling and stronger on heating. What happens is the 

 opposite, showing that the light emission was not dependent on 

 activation energies. 



The middle tube in Fig. 6 contained a 10"^ Al solution of ribo- 

 flavine- 5 '-phosphate.^ The situation is analogous to that found in 

 the first tube with rhodamin : the upper part shows the well known 

 brilliant yellowish-green fluorescence, while the lower frozen part 

 shows no light emission at all. In this tube the O2 was expelled 



^ It is necessary to use fresh solutions. The riboflavine is light sensitive but 

 even stored in the dark refrigerator it loses its phosphorescence without di- 

 minishing its fluorescence. The former seems to be linked to subtle qualities 

 of the molecule. 



