Multi-resonance isotope

Kr. Among the noble gas radioisotopes with the potential for application in hydrology, Kr is certainly the most exotic. Due to the enormous effort of Kr analysis, there are so far only two examples of actual Kr-measurements on ground-water samples. Lehmann et al. (1991) were able to determine the Kr/Kr ratio in one sample from the Milk River aquifer (Canada) to be (82 18)% of the modern atmospheric ratio. The measurement was done by laser resonance spectroscopy following an elaborate multi-step isotope enrichment procedure. In view of the achieved accuracy, which yielded only an upper limit of 140 kyr for the age of the sample, this first measurement was hardly more than a demonstration of the feasibility of Kr-dating. 

The multi-photon dissociation process is illustrated in Fig. 10.34. Because of the anharmonicity of the vibrational potential a photon energy that is resonant in the first vibrational step will successively pull out of resonance higher up in the vibrational energy level ladder. However, the molecule can be excited in a multi-photon process (Sect.9.1.3c) until it is dissociated. SFg then disintegrates into SF5 and F. By making the first step resonant for one isotopic molecule the probability of subsequent dissociation for this molecular species is significantly increased. 

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