NIST MAPS ENERGY LEVELS OF FIRST ION OF MERCURY.

NIST scientists have succeeded in creating an extensive map of the electronic energy levels of the singly-ionized mercury atom, [Hg.sup.+]. The wavelengths and levels determined in this work are important for many applications, including space astronomy with the Hubble Space Telescope. Scientists at NIST (Boulder) are using the results to estimate the shift due to external fields that might be expected to occur in an optical frequency standard based on the [Hg.sup.+] transition at 281.5 nm; this is one of the most precise frequency standards currently under development.

The mapping was carried out by observing the spectrum of mercury ions over a wide wavelength region from the far ultraviolet to the near infrared. Over 100 energy levels of [Hg.sup.+] were determined in the process, some of them lying near or above the ionization limit. Nearly 500 spectral lines were measured to high accuracy and used to determine the level values. The spectral lines were excited in a special light source that used pulsed radio-frequency waves to create a plasma with a high density of mercury ions.