Radiative lifetimes metastable levels

Here vq is the inverse value of the mean lifetime of the excited state q. For levels in which a decay by an allowed radiative transition can take place the lifetime is of the order of 10-8 s. When no radiative transitions are allowed we have metastable levels (e.g. Ar 11.5 and 11.7 eV), which only can decay by collisions. Therefore, such levels in the case of low pressure discharges may have very long lifetimes (up to 10 1 s). 

A state-of-the-art example [28] for trapped-ion optical frequency standards is the case of a laser with a line-width of less than 25 Hz locked to a electric quadrupole transition at 282 nm in a single laser-cooled Hg ion. The inherent stability of this trap based on the radiative lifetime of the metastable upper level of this transition is calculated to be about 1,5 x 10" x[29]. This is an exceptional example - in most cases as yet, the lasers used as oscillators for optical frequency standards based on ion traps often do not have a stability which matches the spectral sharpness of the trapped ion reference resonance. 

The first and the most fundamental effect is the radiative broadening due to the spontaneous decay of an excited state (the half-width of the spectral line is r = 7rad = 7). A spectral line due to spontaneous decay has a Lorentzian shape (eqn 2.5). The lifetime of an excited atomic or molecular state depends on the oscillator strength of the transition and the radiation wavelength. For electronic transitions of atoms and molecules in the visible region of the spectrum, l/7rad — s. For metastable atomic and vibrational molecular levels, the radiative lifetime may be much longer, namely... 

Fig. 1. Energy levels of the antiproton in pHe+. The p is captured by replacing one of the Is electrons, which corresponds for the p to a state with principal quantum number no JW /m, where M is the reduced mass of the atomcule, and m the electron mass. About 3% of antiprotons are captured in metastable states (black lines) at high angular momenta L n — 1, for which deexcitation by Auger transitions is much slower than radiative transitions. The lifetimes of these states is in the order of /is. The antiprotons follow predominantly cascades with constant vibration quantum number v = n — L — 1 (black arrows) until they reach an auger-dominated short-lived state. The atomcule then ionizes within < 10 ns and the pHe++ is immediately destroyed in the surrounding helium medium. The overall average lifetime of atomcules is about 3 — 4 ps...

Trivalent neodymium absorbs strongly in broad bands in the green, red and near infra-red with reasonable coincidence in the output of a xenon flash lamp. Nearly all the electrons excited into the pump bands revert to the metastable upper level 4F3/2 through non-radiative decay which has a lifetime of 230 ps. Radiative transition then occurs to a set of lower energy levels 4I)5/2,4Ii3/2,4I 1/2,4l9/2- All these levels are multiplets and the lowest of 4l9/2 is... 

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