Metastable beams neutral

It is not surprising that most results in atomic spectroscopy were obtained on singly charged ions which are difficult to prepare for the usual Doppler-free techniques on thermal samples. On the other hand, the fast-beam technique has certain advantages also on neutral atoms, such as the availability of metastable beams, the sensitivity, and the Doppler-tuning. 

The earlier classical publications in this field belong to Oliphant [120]. In his survey he used a beam of rapid metastable atoms of helium obtained by neutralizing ions on the walls of platinum capillary. Oliphant was the first to observe emission of electrons from a surface of magnesium and molybdenum under the action of metastable atoms, and also rebounding of metastable atoms from a molybdenum surface. 

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,... 

Gellene, G.I. and Porter, RE. Experimental observations of excited dissociative and metastable states of H3 in neutralized ion beams,/. Chem. Phys., 79 (1983), 5975-5981. 

A fast switch was discovered by Potember in CuTCNQ [41]. This was due to the bulk thermodynamic metastability (in crystals or amorphous powders) of the violet, low-conductivity ionic state (IS) Cu" TCNQ (c), relative to the yellow, low-conductivity, neutral state (NS) Cu(0) TCNQ(0)(c), with, presumably, an intermediate, mixed-valent, higher-conductivity state (CS) one could switch between the two states IS <—> CS, either with an applied voltage over a certain threshold value, or by a moderate laser beam, while heat will restore the IS. Tliis effect is found also in AgTCNQ, and in a few related systems [42]. The switching rate is fast, and could be used for optical data storage [43]. 

The LIBS technique is essentially aimed at the elemental analysis of solid and liquid materials. A pulsed laser beam is focused onto the surface of the material to be analysed and the emission from the resulting micro-plasma is collected and focused on to the slit of a monochromator equipped with an array detector capable of recording the entire spectrum from a single laser shot. The emission is initially dominated by Bremsstrahlung (white light), but this is short lived and essentially follows the laser intensity profile. This is followed by emission from atomic ions (typically 1 ps in duration) and finally by emission from neutral atomic species (very weak emission from metastable species, which decay more slowly, may... 

The observation of optical emission from electronically excited reaction products (chemiluminescence, CL) and the formation of ions in encounters between neutrals (chemiionization, Q) provide many beautiful examples of branching among the multitude of accessible states. Beam-gas or cross beam arrangements have been used, the reactants usually being atoms or atomic ions (in their ground or metastable states) plus diatomic target molecules. CL is highly product specific thanks to the spectroscopic detection, in Cl mass analysis is sometimes employed. 

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