Lifetimes of excited atoms

This phase-shift method is a general technique and is e.g. also applied to the measurement of lifetimes of excited atoms or molecules (see Sect. 6.3). The combination of CRDS with Fourier-spectroscopy gives for a fixed detection time a better signal-to noise ratio, because now all absorption lines within the covered spectral range are detected simultaneously. In Fig. 1.22 a possible experimental arrangement for this combined spectroscopic technique is schematically depicted. The transmitted laser... 

We will at first discuss techniques for the generation and detection of short laser pulses before their importance for different applications is demonstrated by some examples. Methods for measuring lifetimes of excited atoms or molecules and of fast relaxation phenomena are presented. These applications illustrate the relevance... 

Measurements of lifetimes of excited atomic or molecular levels are of great interest for many problems in atomic, molecular, or astrophysics, as can be seen from the following three examples ... 

Principle of the technique. The delayed-coincidence technique, which is well known in nuclear physics, was first applied to the measurement of the lifetimes of excited atoms by Heron et al, (1954, 1956). However, it was not widely used until Bennett (1961) improved the method... 

Another technique for measuring the lifetime of excited activator atoms in solid-state lasers has been published by Gilrs h If the pulsed laser is operated close above threshold, only a single spike (i.e. a short pulse of induced emission) appears, whereas many spikes are emitted when the laser ist running well above threshold. This... 

LIFETIMES OF EXCITED ELECTRONIC STATES OF ATOMS AND MOLECULES... 

The natural broadening which results from the finite lifetime of excited states. The energy of a state and its lifetime are related by the principle of uncertainty (section 2.2) which implies a minimal spread of the actual energy of any excited state of finite lifetime this gives an absolute limit to the width of atomic spectral lines. 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

The energy levels and eigenfunctions, obtained in one or other semi-empirical approach, may be successfully used further on to find fairly accurate values of the oscillator strengths, electron transition probabilities, lifetimes of excited states, etc., of atoms and ions [18, 141-144]. 

There are numerous needs for precise atomic data, particularly in the ultraviolet region, in heavy and highly ionized systems. These data include energy levels, wavelengths of electronic transitions, their oscillator strengths and transition probabilities, lifetimes of excited states, line shapes, etc. [278]. 

Expressions for a number of main moments of the spectrum may be utilized to develop a new version of the semi-empirical method. Evaluation of the statistical characteristics of spectra with the help of their moments is also useful for studying various statistical peculiarities of the distribution of atomic levels, deviations from normal distribution law, etc. Such a statistical approach is also efficient when considering separate groups of levels in a spectrum (e.g. averaging the energy levels with respect to all quantum numbers but spin), when studying natural widths or lifetimes of excited levels, etc. 

The lifetime of a separate atom in its ground state is infinite, therefore the natural width of the ground level equals zero. Typical lifetimes of excited states with an inner vacancy are of the order 10-14 — 10 16 s, giving a natural width 0.1 — 10 eV. The closer the vacancy is to the nucleus, the more possibilities there are to occupy this vacancy and then the broader the level becomes. That is why T > Tl > Tm- Generally, the total linewidth T is the sum of radiative (Tr) and Auger (T ) widths, i.e. 

This methodology has been applied in many areas, such as the measurement of lifetimes of excited nuclear states and nuclear magnetic moments, the investigation of electric and magnetic fields in atoms and crystals, in the analysis of special relativity, the equivalence principle, and also in other applications [50-57],... 

The gain term comes from the process of stimulated emission, in which photons stimulate excited atoms to emit additional photons. Because this process occurs via random encounters between photons and excited atoms, it occurs at a rate proportional to n and to the number of excited atoms, denoted by A (t). The parameter G > 0 is known as the gain coefficient. The loss term models the escape of photons through the endfaces of the laser. The parameter k>0 is a rate constant its reciprocal T = lk represents the typical lifetime of a photon in the laser. 

Stepwise laser excitation and ionization techinques can be used to determine lifetimes of excited levels in atoms. Oscillator strengths or transition... 

Penkin, N. P. and Komaravskii, V. A., "Oscillator strengths of spectral lines and lifetimes of excited levels of atoms of rare earth elements with unfilled 4f shells,"... 

Ordinarily, the lifetime of an atom or molecule excited bv absorption of radiation is brief because there are several relaxation processes that permit its return lo the ground stale. 

Natural line broadening is the consequence of a finite lifetime of an atom in any excited state. The absorption process is very fast being about 10 s. The lifetime of the excited state is longer (about 10 s), but sufficiently short that the Heisenberg s Uncertainty Principle is appreciable. If the mean time the atom spends in an excited state, E, is Af, then there will be an uncertainty, in the value of Ei. 

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