Noble-gas atoms can be placed in a metastable state, in which, for example, the atoms exist in an excited state for lengths of time (depending on the state) ranging from nanoseconds to tens of seconds. These metastable gases have uses, too: for example, metastable Krypton (Kr*) is essential in some techniques for sum-frequency generation of tunable vacuum-ultraviolet (vacuum-UV) radiation, and Kr* can also be used for radiocarbon dating. Traditionally, though, methods for creating Kr* are very inefficient. For example, direct-current or radio-frequency discharge sources can produce Kr*, but at efficiencies of only around 10-4, while conventional methods of optical excitation to produce Kr* have similarly low efficiencies or other limitations.
Now, scientists from the University of Adelaide (Adelaide, Australia), Griffith University (Nathan, Australia), and the Defence Science and Technology Group (Canberra, Australia) have optically generated Kr* in the long-lived (tens of seconds) 1s5 state with an efficiency of 2% and with the possibility of a 30% production efficiency. The group uses laser excitation via a pulsed, frequency-doubled optical parametric oscillator (OPO) emitting at a 215 nm wavelength and with a pulse energy of 2 mJ. The light is focused using a calcium fluoride (CaF2) lens to a 100 µm spot in a volume of Kr gas at a 0.05 mbar pressure to excite the gas. Improving the duty cycle and reducing the pressure of the Kr should boost the production efficiency. Reference: M. A. Dakka et al., arXiv:1805.05669v1 [physics.atom-ph] (May 15, 2018).