| The hydrogen molecular ion(HD+)is a simplest heteronuclear molecular ion,and its rovibrational transition spectra can be accurately obtained by experimental measurements.The measured transition frequencies contribute to the determination of mp/me with the aid of quantum electrodynamics(QED)theory.HD+ions cannot be laser cooled directly due to the absence of a closed optical cycling transition caused by the complex internal state structure of HD+ions.To reduce the Doppler broadening during the spectroscopic measurements,the external temperature of HD+ ions are sympathetically cooled to the order of millikelvin by the co-trapped laser-cooled Be+ions using Coulomb interaction.However,due to the uncoupling of the internal and external degrees of freedom,the populations of the internal states still maintain the thermal equilibrium distribution of ambient temperature.For HD+ions at room temperature,almost all the ions are populated in the v+=0 vibrational state of the 1sσelectronic ground state,but only 10%are populated in the v+=0,J+=0 rovibrational ground state.Therefore,it is not favorable for the measurements of high-precision rovibrational spectra based on the rotational ground state.In addition,there are other challenged to prepare ultracold HD+ ions populated in the rovibrational ground state:1.The preparation of HD+ ions by electron impact ionization will produce many impurity ions due to the ionization energy of neutral HD molecules up to 15 eV;2.It is difficult to trap HD+ions stably by an ion trap due to their lighter mass,which requires a high amplitude and frequency trapping potential;3.The population distribution of HD+ions is susceptible to the blackbody radiation(BBR).In order to overcome the above difficulties,an experiment to prepare HD+ions by[2+1’]resonance enhanced threshold photoionization(RETPI)is designed and stateselected HD+ions are generated efficiently.The above research work is elaborated in this thesis and main content is categorized as follows:1.An experimental system for the preparation of ultracold HD+ions in rovibrational ground state are designed and constructed,including a pulsed supersonic molecular beam(SMB)apparatus,a time-of-flight mass spectrometer(TOFMS)and a segmented linear ion trap.The main features are:the rovibrational temperature of the pulsed molecular beam generated by SMB apparatus reaches 12 K;a mass resolution of 104 in the TOFMS is achieved;the ion trap,driven by a 15.1 MHz radio frequency electric potential,can trap both Be+ and HD+ ions simultaneously.2.The photoionization mechanism of neutral HD molecules is investigated in TOFMS.The yield of HD+ions,produced by one-color[2+1]and two color[2+1’]resonance enhanced multiphoton ionization(REMPI)process,are proportional to the cubic and quadratic power of the excitation laser energy,respectively.The stateunselected[2+1]REMPI is suppressed by controlling the excitation laser energy.3.HD+ions created by[2+1’]REMPI are trapped in an ion trap,and then sympathetically cooled by co-trapped laser-cooled Be+ ions to form the Be+-HD+bicomponent Coulomb crystal.The number of HD+ions loaded is determined with an accuracy of a single ion by a comparison between the experimental and simulated images of Be+-HD+bi-component Coulomb crystals.The average loading rate of 4.3(2)HD+ ions per second in the ion trap is obtained.4.The rotational ground state HD+ions are produced in an ion trap by the[2+1’]RETPI.The frequency of the threshold ionization laser is determined by taking into account the effect of electric field of the ion trap on the RETPI process.The rotational ground state population of HD+ions is detected by the state-selected[1+1’]resonance enhanced multiphoton dissociation(REMPD).Combined with the evolution of the rotational state populations caused by BBR coupling,it is inferred that the initial ground state population of HD+ ions produced by RETPI is 0.93(12).The average ground state population of HD+ ionic Coulomb crystal prepared during the cumulation time of 5 s is 0.77(8). |
PDF Full Text Request