Spectroscopy in Cold Ion Beams

In the second part of the interaction zone an additional voltage AU is applied, which changes the velocity of the ions. If the laser-induced fluorescence is monitored by PM2 as a function of A17, a Lamb dip will be observed at Af/ = 0 because the absorbing level i) has already been partly depleted in the first zone. 

If several transitions are possible from level /) with frequencies a within the Doppler tuning range 

Fast ion and neutral beams are particularly useful for very accurate measurements of lifetimes of highly excited ionic and neutral molecular levels (Sect. 6.3). 

Although the velocity spread of ions in fast beams is reduced by acceleration cooling, their internal energy (Avib, E t, Ae). which they acquired in the ion source, is generally not decreased unless the ions can undergo radiative transitions to lower levels on their way from the ion source to the laser interaction zone. Therefore, other techniques have been developed to produce cold ions with low internal energies. Three of them are shown in Fig. 4.34. 

Instead of gas discharges, electrons emitted from hot cathodes can be used for ionization (Fig. 4.34c). With several cathodes arranged cylindrically around a cylindrical grid acting as an anode, a large electron current can be focused into the cold molecular beam. Because of the low electron mass, electron impact ionization at electron energies closely above ionization threshold does not much increase the rotational energy of the ionized molecules, and rotationally cold molecular ions can be formed from cold neutral molecules. Rotational temperatures of about 20 K have been reached, for instance, when supersonically cooled neutral triacetylene molecules were ionized by 200 eV electrons in a seeded free jet of helium [484]. 

Cold ions can also be formed by two-photon ionization directly behind the nozzle of a supersonic neutral molecular beam [9.92]. These cold ions can then be further investigated by one of the laser spectroscopic techniques discussed above. The combination of pulsed lasers and pulsed nozzles with time-of-flight spectrometers gives sufficiently large signals to study not only molecular excitation but also the different fragmentation processes [9.93-9.95]. 

Although the velocity spread of ions in fast beams is reduced by acceleration cooling, their internal energy (E ji, E ) which they have acquired 

Therefore other techniques have been developed to produce cold ions with low internal energies. Three of them are shown in Fig. 9.28. 

Instead of gas discharges electrons emitted from hot cathodes can be used for ionization (Fig. 9.28c). With several cathodes arranged cylindrically 

The combination of pulsed lasers, pulsed molecular beams and time-of-flight mass spectrometry represents a powerful technique for studying the selective excitation, ionization and fragmentation of wanted molecules out of a large variety of different molecules or species in a molecular beam [9.83-88]. The technique, developed by Boesl et al. [9.83] is illustrated by Fig.9.29 rotationally and vibrationally cold neutral parent molecules M in a supersonic molecular beam pass through the ion source of a time-of-flight mass spectrometer. A pulsed laser LI forms molecular ions M+ by resonant enhanced multiphoton ionization. By selecting special intermediate states of M the molecular ion M can often be preferentially prepared in a selected vibrational level. 

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The most common RF ion trap is a Paul trap [42], a 3-D quadrupole device in which ions are confined in a small volume of typically a few tens of millimeters [2] between a hyqterbolically shaped inner surface of a ring electrode and two end-cap electrodes, also of hyperbolic shape (Fig. 1). Elach end-cap electrode has a central hole for loading and ejection of irais. As these traps are compact, commercially available, and allow mass-selection of stored ions, they have become an increasingly popular technically simple solution for cryogenic ion spectroscopy. Paul traps have several drawbacks for cold-ion spectroscopy, however inefficient ion injection an intrinsically limited ability to cool ions low storage volume and inconvenient optical access to the ions by laser beams. 

In order to improve the signal-to-noise ratio in collinear laser spectroscopy, an ion source with bunched beam release was tested successfully. For this purpose, the temperature of a cold trap" inside the ion source is reduced for storage of reaction products, which are released from the trap during a subsequent period of increased temperature. The release of indium was found to occur with a FWHM of approximately 0.5s, corresponding to a... 

In order to determine ion temperatures more directly, laser based techniques have been used and new schemes of action spectroscopy are under development. The Doppler profile of an observed transition provides direct information on the ion velocity distribution along the laser beam. In addition a quantitative analysis of the population of rotational or other suitable internal states can provide information about how cold the ions are internally. The first successful application based on the laser induced reaction... 

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