This paper reports on two experimental setups that allow the cooling of molecular ions over a wide molecular mass range. In the experiments they use /sup 138/Ba/sup +/ and /sup 9/Be as directly laser cooled ion species. /sup 9/Be/sup +/ is used as coolant for very light molecules, in particular molecular hydrogen ions. /sup 138/Ba+ can cool small di-and triatomic molecules and also offers the prospect of cooling complex, highly charged molecules with masses up to 20000 amu.
This paper reports on two experimental setups that allow the cooling of molecular ions over a wide molecular mass range. In the experiments they use /sup 138/Ba/sup +/ and /sup 9/Be as directly laser cooled ion species. /sup 9/Be/sup +/ is used as coolant for very light molecules, in particular molecular hydrogen ions. /sup 138/Ba+ can cool small di-and triatomic molecules and also offers the prospect of cooling complex, highly charged molecules with masses up to 20000 amu.
Ultracold molecular hydrogen (D/sub 2//sup +/) is produced by sympathetic cooling in a linear Paul trap. With this method the translational degrees of freedom of the molecules are cooled by Coulomb interaction with laser-cooled /sup 9/Be/sup +/. This work shows that sympathetic cooling and crystallization is possible down to mass ratios of 0.45. Using molecular dynamics simulations, the mass range for sympathetic cooliing is predicted. By employin a light and a heavy species of laser-cooled ions (/sup 9/Be/sup +/ and e.g. /sup 137/Ba/sup +/) the mass range from 2 to 20 000 amu, i.e. from the simplest molecule to complex proteins, can be sympathetically cooled.
This thesis reports on experimental and theoretical studies of the sympathetic cooling of complex molecular ions demonstrating that this general method for cooling atomic and molecular ions is reliable and efficient.
For this purpose, complex molecular ions and barium ions have been confined simultaneously in a linear Paul trap. The complex molecular ions are generated in an electrospray ionization system and transferred to the trap via a 2 m long octopole ion guide. These molecular ions are pre-cooled by room temperature helium buffer gas so that they can be captured by the trap. The atomic barium ions are loaded from a barium evaporator oven and are laser-cooled by a 493 nm cooling laser and a 650 nm repumping
laser. Due to the mutual Coulomb interaction among these charged particles, the kinetic energy of the complex molecular ions can be reduced significantly.
In our experiments we have demonstrated the sympathetic cooling of various molecules (CO2, Alexa Fluor 350, glycyrrhetinic acid, cytochrome c) covering a wide mass range from a few tens to 13000 amu. In every case the molecular ions could be cooled down to millikelvin temperatures.
Photo-chemical reactions of the 138Ba+ ions in the (2P1/2) excited state with gases such as O2, CO2, or N2O, could be observed. If the initial 138Ba+ ion ensemble is cold, the produced 138BaO+ ions are cold as well, with a similar temperature as the laser-cooled barium ions (a few tens of millikelvin). The back-reaction of 138BaO+ ions with neutral CO to 138Ba+ is possible and was observed in our experiments as well.
A powerful molecular dynamics (MD) simulation program has been developed. With this program dynamic properties of ion ensembles, such as sympathetic interactions or heating effects, have been investigated and experimental results have been analyzed to obtain, for example, ion numbers and temperatures. Additionally, the feasibility of nondestructive spectroscopy via an optical dipole excitation and the lineshape of rovibrational
transitions of 1.4 and 5.1 μm have been studied.
We have produced ensembles of cold 16O+2, 40Ar+, 12C16O+2, and various isotopes of barium ions (135Ba+, 136Ba+ and 137Ba+) via sympathetic cooling with laser-cooled 138Ba+ in a linear radiofrequency trap. The sympathetically cooled species were embedded in the centre of large 138Ba+ Coulomb crystals containing up to 2000 ions and were identified by motional resonance excitation. Crystals with molecular fractions exceeding 70% were obtained. The observed multi-species crystal structures agree well with results from molecular dynamics simulations. The simulations were also used to deduce an upper limit for the translational temperature of the molecular ions, as low as 20 mK.
This work presents various novel results in the field of experimental and theoretical trapped ion studies. All investigations involve laser-cooled 138Ba+ ions confined in a linear quadrupole ion trap, which serve as coolant for complex molecular ions, as target for collisions with neutral atoms, or as model system in theoretical analyses. The special feature of the apparatus used in this work, is a molecular ion source based on electrospray ionization that allows for the production of gas-phase molecular ions of almost arbitrary species with a maximum mass-to-charge ratio of 2,000 Da. Trapped together with laser-cooled 138Ba+ ions, the molecular ions can be cooled down, kept cold and investigated for many minutes and in principle for up to hours in a nearly collisionless environment.
In this work, the sympathetic cooling of complex molecular ions to translational temperatures below 1 K is demonstrated for different types of molecules covering a mass range from 182 to 12,400 Da. Using the example of the protein cytochrome c, with a mass of about 12,400 Da the most massive molecular species sympathetically cooled in an ion trap so far, the methods for detection of successful trapping and cooling, and temperature determination are presented in detail. In one case, multiply protonated molecules of the protein cytochrome c have been cooled to less than 0.75 K. Furthermore, two methods for measuring photodestruction rates of cold, trapped complex molecular ions have been developed that can, for example, be applied in destructive spectroscopy schemes. Both techniques are demonstrated using the example of singly protonated glycyrrhetinic acid molecules (GAH+) dissociated by an ultraviolet (UV) laser. Measurements of the photodestruction rate were performed at different intensities of the UV laser, and rates as low as 0.05 s-1 and less have been determined, which is only possible due to the long ion storage times that can be achieved with this apparatus. For the UV laser wavelength of 266 nm, a destruction cross section of (1.1±0.1)∙10E-17 cm2 for GAH+ has been determined.
In this work, feasibility studies towards internal cooling of trapped complex molecular ions have been performed. Such an additional internal cooling could be realized via collisions with laser-cooled neutral atoms. To test their suitability, neutral atoms of Li, Na, K, Rb, Cs, and Yb have been collided with trapped laser-cooled and not cooled Ba+ ions as well as with complex molecular ions. For all species charge exchange reactions with Ba+ ions have been observed excluding these species for collisional cooling. While K, Rb, and Cs showed charge exchange reactions with ground state and laser-excited Ba+ ions, the charge exchange rates of Yb and Ba+ were found to be state dependent. Collisions of neutral Yb atoms were found to be reactive and lead to the formation of adducts and fragments, in contrast to the alkali metals for which no such reactions have been observed. In addition, the motion of cooled, trapped ions has been studied theoretically via simulations. When the translational energy of a trapped ion ensemble is sufficiently reduced, a first-order phase transition from a liquid to a solid state occurs with the ion ensemble changing from a disordered ion cloud to an ordered ion crystal. As ions in a linear quadrupole trap arrange in shell structures, a free motion in radial direction is disabled, and ions can only move to neighboring shells when their kinetic energy is high enough to overcome a certain potential barrier. The simulations have shown that intershell diffusion rates in the considered ion crystals increase exponentially when the ion temperatures exceed values that well agree with the predictions of other established models. Ion ensembles of different ion numbers (300, 500, and 1000 138Ba+ ions) and symmetries (prolate and spherical) have been investigated showing a clear tendency for the phase transition temperatures: The higher the ion number, the lower is the transition temperature.
In this work, the setup of an experiment aimed at high-precision laser spectroscopy of ultracold molecular hydrogen ions, started at the University of Konstanz in 1999 [1,2], was continued and first results were obtained. In particular, a new linear rf trap and a new trap supply electronics were built, the latter being capable of driving the linear rf trap with a maximum rf amplitude of > 1000V at a frequency of 14.2MHz. This made possible the simultaneous trapping of a large mass range (2u For laser cooling of Be+ ions, doubly resonant sum-frequency generation (DR-SFG) of 313nm laser light was performed between a resonantly frequency-doubled monolithic Nd:Yag laser and a Ti:sapphire laser, emitting at 532 and 760nm wavelength, respectively. An output power of P_UV > 100mW was achieved at 313nm wavelength, with a high spectral purity (rms short-term frequency fluctuations With this experimental setup, sympathetic cooling of N+, BeD+, BeH+, 4He+, HD+, and D+ ions by the laser-cooled Be+ ions was demonstrated, using laser-cooled fluorescence mass spectrometry (LCF-MS) to detect the sympathetically cooled species. Accordingly, sympathetic cooling at a minimum mass ratio of m(H2+)/m(Be+) = 0.22 was shown to be possible with both the sympathetically and laser-cooled ions in a cloud-like state. This points out the incorrectness of the theoretical lower limit m_SC/m_LC = 0.53 which was predicted in Ref.[3]. Here, the subscripts SC and LC denote the sympathetically and the laser-cooled species, respectively.
In a further experiment, almost pure Be+ ion plasmas were laser-cooled down to temperatures in the milli-Kelvin range, leading to the crystallization of the Be+ ion plasmas. The outer shapes of different-sized Be+ ion crystals in a fully anisotropic trap potential were compared with the charged fluid predictions, showing good agreement at sufficiently large numbers of Be+ ions. By fitting Voigt profiles to the measured fluorescence signal, a typical temperature T(Be+) of 30mK was estimated for the Be+ ions.
Finally, an ultracold sample of 4He+ ions [T(He+) approximately 40mK] was produced for the first time by sympathetic crystallization of the 4He+ ions. This may open up perspectives for the high-precision laser spectroscopy of 4He+ (and 3He+).
[1] T.K.W. Boley, Diplomarbeit, Universitat Konstanz (1999)
[2] H. Schnitzler, Dissertation, Universitat Konstanz (2001)
[3] T. Baba and I. Waki, Appl.Phys.B, 74, 375-382 (2002)