The objective of the project is to perform high-resolution Doppler-free two-photon vibrational spectroscopy of the H2+ molecular ion for accurate electron to proton mass ratio determination. Trapping and laser-cooling of the H2+ ions is necessary. Since H2+ molecular ions cannot be directly laser cooled, a solution is the implementation of sympathetic cooling via a laser cooled Be+ ions. In the first part of this thesis the design and implementation of a linear Paul trap is presented. This trap is suitable to confine H2+ and Be+ ions while allowing good optical access for multiple laser sources and the imaging system. The second part describes a low power, diode based laser system to provide narrow linewidth 626nm light, and a high power, fiber laser based laser system. Subsequently, a bow-tie cavity is presented to frequency double the 626nm light to obtain 313nm light for Be+ Doppler-cooling. In the last part the operational readiness of the trap and the fiber laser based 313nm laser source is demonstrated by the observation of Be+ Coulomb crystals. Electron impact ionization of H2 present in the residual gas allows to observe sympathetic cooling and shows that the developed setup can provide the experimental conditions for precision spectroscopy of H2+.
The objective of the project is to perform high-resolution Doppler-free two-photon vibrational spectroscopy of the H2+ molecular ion for accurate electron to proton mass ratio determination. Trapping and laser-cooling of the H2+ ions is necessary. Since H2+ molecular ions cannot be directly laser cooled, a solution is the implementation of sympathetic cooling via a laser cooled Be+ ions. In the first part of this thesis the design and implementation of a linear Paul trap is presented. This trap is suitable to confine H2+ and Be+ ions while allowing good optical access for multiple laser sources and the imaging system. The second part describes a low power, diode based laser system to provide narrow linewidth 626nm light, and a high power, fiber laser based laser system. Subsequently, a bow-tie cavity is presented to frequency double the 626nm light to obtain 313nm light for Be+ Doppler-cooling. In the last part the operational readiness of the trap and the fiber laser based 313nm laser source is demonstrated by the observation of Be+ Coulomb crystals. Electron impact ionization of H2 present in the residual gas allows to observe sympathetic cooling and shows that the developed setup can provide the experimental conditions for precision spectroscopy of H2+.
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)
This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.
Electron beam ion traps used for spectroscopy of highly charged ions (HCI) produce a deep trapping potential leading to high temperatures of the stored ions, and thus limiting the achievable spectral resolution. A novel device at the Max-Planck-Institut für Kernphysik, the Cryogenic linear Paul Trap Experiment (CryPTEx), attached to an electron beam ion trap, provides a new experimental platform to overcome these limitations. The trap assembly operates at a temperature of 4 K and offers optical access for quantum manipulation and imaging of the trapped ions. Since forbidden optical transitions in HCI do not support direct laser cooling, sympathetic cooling with Coulomb crystals of singly charged ions such as Be+ or Mg+ will be applied in order to reach the natural linewidth of optical forbidden transitions in HCI of interest. With the added advantage of long ion trapping times resulting from residual gas pressures of H2 at 4 K below 10−15 mbar, CryPTEx has been commissioned in collaboration with the Ion Trap Group in Århus using rovibrationally cooled MgH+ ions. Strong suppression of the black body radiation at the trap center, ion storage times of about 28 hours, and largely enhanced population of the rovibrational ground state were achieved.
The high-resolution ro-vibrational spectroscopy of the H2+ molecular ion by resonance-enhanced multiphoton dissociation (REMPD) requires a state-selective source of H2+. In this thesis we present work on a functional state selective H2+ ion source using resonance enhanced multiphoton ionisation (REMPI) with a 303 nm pulsed laser.The second part of the thesis presents numerical simulations of sympathetic cooling in linear RF traps, whose main application is the GBAR project (Gravitational Behaviour of Antihydrogen at Rest) which involves sympathetic cooling of an antimatter H̄+ ion by laser-cooled Be+ ions. We developed a GPU code using a variable timestep technique enabling a fast description of Coulomb interactions. We discuss the influence of RF heating and scaling laws between cooling times, initial energy and ion numbers in the cooling crystal. We show that the H̄ sympathetic cooling step of GBAR could be feasible using a rotationally asymmetric two-component Be+/HD+ crystal which appears more effective than a single-component Be+ crystal. We find that the H̄+ ion’s capture by this crystal could be detected experimentally by Fourier analysis of the fluorescence data.
This thesis presents the results of my graduate work which has concentrated on applying laser-cooling techniques to molecular ions, specifically CaH⁺ and BH⁺ ions for the study of their rovibronic spectroscopy. The goal of my research has been to control external and internal degrees of freedom of these two ions. Each of these molecules is spectroscopically appealing in its own right. For starters, CaH⁺ has been identified as a potential system for the test for the time variation of fundamental constants, and it's also been speculated to be in space. BH⁺ has a nearly diagonal Franck-Condon structure, and could be directly laser-cooled using only a few lasers. This structure makes it a good candidate for the cooling of internal molecular degrees of freedom. Each of the experiments described in this review involves the trapping and laser-cooling of ions. We cool the molecular ions sympathetically with laser-cooled Ca⁺. The probing techniques used to collect data are mainly: laser induced fluorescence, mass spectrometry, and resonance-enhanced multi-photon dissociation (REMPD). We have demonstrated the ground state sympathetic cooling of CaH⁺ ion to less than 16 muK, and we have used to REMPD to measure for the first time four vibronic transitions in CaH⁺ ions. Together, these results are important for future quantum logic experiments on this molecule. Furthermore, we have also made some progress towards trapping and cooling BH⁺ ions by first trapping B⁺ ions with Ca⁺ ions. We use the motional resonance method to detect the B⁺ ions, which can be reacted with H atoms to produce the molecular ions. Following the measurement of the vibronic spectrum of CaH⁺, we have narrowed the linewidth of the laser source to resolve rotational lines.
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.