Molecular ions, cooling

Because of the low collision rate in the high vacuum environment of a Fourier transform mass spectrometer (FTMS), vibrationally excited molecular ions cool predominantly by IR fluorescence. For typical IR transition dipole moments and frequencies in the mid-IR, spontaneous emission is expected to occur at a rate in the range of 1-100 s To energize an ion efficiently using IR multiple-photon excitation (MPE), the rate of photon absorption - the product of absorption cross section and photon flux - should exceed the emission rate. From such a back-of-an-envelope estimate, one finds that radiation sources producing several Watts/cm are required to induce efficient dissociation [141], Note that the demands on laser power may further increase because of the limited residence time of the ions in the laser field, collisional deactivation in traps at higher pressures, limited spectral overlap between molecular absorption and laser emission profiles, etc. 

Quite apart from thermolysis occurring before fragmentation, the temperature of the ion source may have a marked effect on the appearance of a mass spectrum. Comparison of mass spectra obtained with hot and cooled ion-sources and of spectra obtained by photon impact or field ionization show by the increased amount of fragmentation that a molecular ion possesses a greater excess of internal energy when formed in a hot, electron-impact source. Possible origins of this excess internal energy are collision with or radiation from surfaces. Some effects of hot and cold ion sources are discussed. 

Vibrationally hot molecular ions that are injected into the ring can cool internally after a few seconds of storage owing to emission of infrared radiation. This is the case for heteronuclear diatomic molecules and infrared active modes in polyatomic molecules. 

In Section 10.2 is discussed the principal idea of our novel SCSI-MS (Sympathetically-Cooled Single Ion Mass Spectrometry) technique for performing single-ion mass measurements in the light of already well-established techniques. This discussion is followed by both a description of the current experimental arrangement, used for single molecular ion experiments (Section 10.3), and an account of the... 

A schematic of the SCSI-MS technique is presented in Figure 10.1. The technique relies on the measurement of the resonant excitation frequency of one of the two oscillatory modes of a trapped and crystallized linear two-ion system consisting of one laser-cooled atomic ion of known mass and the a priori unknown atomic or molecular ion, whose mass is to be determined. From this measured frequency, the mass of the unknown ion can be deduced from a simple relation between the frequency and the relative masses of the two ions (see Section 10.3). 

In Figure 10.4c and d, images of two laser-cooled atomic ions and one atomic and one molecular ion (CaO +) are presented when applying a driving force of a fre-qnency close to co +. The smearing of the fluorescence as compared to Figure 10.4a and b, shows clearly that the ions are excited motionally. 

Molecular ions can be produced, trapped and cooled by the application of a variety of techniques. Within this laboratory, we can use essentially three methods (1) reaction of trapped and laser-cooled atomic ion species with neutral molecules in a gas leaked into the vacuum chamber of the ion-trap apparatus (11) electron impact ionization, using an electron gun, of neutral molecules leaked into the ion-trap center (see Figure 10.3) and (III) photoionization of leaked-in neutral molecules. 

Doppler laser-cooling is an essential ingredient in the SCSI-MS technique. First, it provides the necessary damping force to cool directly and sympathetically the atomic and molecular ions, respectively, such that a cold and strongly-coupled two-ion system is formed. Second, it gives rise to the fluorescence photons used in the detection process. Third, the radiation pressure force can be modulated to excite the common motion of the ions. 

The main effect of background gas (residual gas or reactant gas) is due to ion/neutral collisions of background gas with the cold atomic and molecular ions. Some collisions are manifested by a sudden disappearance of fluorescence light because the ions acquire sufficient kinetic energy to move away from the ion-trap axis. They are, however, typically not expelled from the pseudo-potential well of depth ca 1 eV and, after a while, they become laser-cooled sufficiently to be re-aligned along the ion-trap axis and they resume fluorescing. 

In reaction experiments as described in Section 10.4.2.2, where two laser-cooled ions are trapped and one of them reacts to form an unknown molecular ion species, a reference measurement of cOi with two laser-cooled ions can be carried out easily in appropriate time intervals before the reaction occurs. In this way, cOi is measured shortly before 0)+ such that a slow drift of cOi, due to changing DC voltage for example, has negligible influence. Similarly, in experiments where a single laser-cooled ion is trapped and a second unknown ion is loaded, a fragment of a complex molecular ion [33] or a super-heavy ion species for example [39-41], a reference measurement of cOi can be made with the single ion. 

Part 3. Ion Spectroscopy. In Chapter 9, we return to the theme of ion photodissociation, which was included also in Volume IV, Part 6, in an exploration of trapped-ion photodissociation, electron photodetachment, and fluorescence. Trapped-ion fluorescence may offer an alternative approach for the elucidation of ion conformation. Whereas these spectroscopic experiments require high ion densities, much attention is directed to the spectroscopic study of single ions confined in an ion trap. Chapters 10 and 11 are illustrative of such studies, with the former devoted to the study of a single molecular ion in a linear ion trap and the latter to a single atomic ion in Paul-type ion traps. While both types of studies require extensive cooling of the subject ion, once such cooling has been achieved, the ions can remain confined for many hours. 

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]. 

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