Cooling of Molecular Ions

An approach that could, in principle, be used to cool the internal degrees-of-freedom of larger molecular ions is radiative cooling. Molecular ions that are confined in a low temperature ion trap eventually cool by the spontaneous emission of infrared radiation however, the cooling rate is slow [57, 58], particularly for large, flexible biomolecules, where most of the internal energy resides in low frequency vibrations, since the radiative lifetime exhibits a dependence. So while this approach is possible, it is not particularly practical. 

The most commOTily used and perhaps most viable method for cooling the internal degrees of freedom of a large molecular ion is via collisions with a cold buffer gas. Here we describe the specific situation of buffer-gas cooling of ions in an RF ion trap. 

One important consideration in the design of spectroscopic experiments in buffer-gas cooled ion traps is whether the ions are interrogated in the trap itself or are first extracted to another part of the machine. If spectroscopic detection occurs outside the trap, as is the case in several of the instruments described below, one has to consider the possibility that the ions may have collisions with buffer gas molecules as they are extracted, and these collisions can warm the ions internally. In cases where buffer gas is continuously leaked into the trap, this is necessarily the case, but it also likely occurs even when the buffer gas is pulsed, since the pump-out time of cold buffer gas can be long. If maintaining the lowest possible temperature is essential to the experiment, it is better to probe the ions before extraction from the trap. 

We give examples of implementation of buffer-gas cooling in RF ion traps in Sect. 2.5 and methods of temperature determination of ions in Sect. 2.6, but first we discuss spectroscopic techniques for detecting ion absorption of radiation. 

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Vogelius, I.S., Madsen, L.B., and Drewsen, M., Blackbody-radiation-assisted laser cooling of molecular ions", Phys. Rev. Lett., 89,173003, 2002. 

Vogelius IS, Madsen LB, Drewsen M. (2006) Rotational cooling of molecular ions through laser-induced coupling to the collective modes of a two-ion Coulomb crystal. J. Phys. B At. Mol. Opt. 39 1267-1280. 

Tong X, Winney AH, Willitsch S (2010) Sympathetic cooling of molecular ions in selected rotational and vibrational states produced by threshold photoionization. Phys Rev Lett 105 143001-1-4... 

This reaction was studied after producing cold hJ ions by electron impact ionization of neutral H2 and sympathetic cooling using laser-cooled Be" " ions and subsequent exposure to H2 gas (Figure 18.26). Because the sympathetic cooling of the product ions is so efficient, the number of molecular ions does not change appreciably... 

A quite different problem impeding the study of cold chemical reactions in ion crystals is the internal energy of the stored ions. While Coulomb interactions are very efficient at sympathetically cooling translation, the internal degrees of freedom of molecular ions are not coupled to the laser cooled atomic ions. The creation of ultracold ions in rf traps is further discussed in Chapter 6. 

Cooling all degrees of freedom of molecular ions down to a few K or even in the sub-K range has many obvious applications in spectroscopy. One example is to study rotational transitions in floppy molecular ions such as CHg. Applications of rf traps in the analysis of molecular structures together with planned extensions towards infrared or microwave absorption spectroscopy on ultracold ions are discussed in Chapter 6. 

Similarly, the proper interpretation of radio astronomical spectral lines from dense interstellar clouds requires collisional information involving ions at low temperatures. By combining the considerations reflected in Figs. 27 and 29, it is possible to obtain the pressure broadening of molecular ions at very low temperatures. This has been demonstrated for the ion HCO" and the collision partner H2. Likewise, extension of direct time-resolved measurements, as discussed in Section IV.D, has been recently carried out incorporating the collisional cooling technique discussed here. 

The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

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