We have developed a photoionization method for complete conversion of a quantum-selected population to ionization, making possible sensitive and absolute measurement of the selected populations in a gas. Each photoionization involves the absorption of two photons (from a pulsed dye laser), one of which is resonant with an intermediate state. In this demonstration we measured the absolute number of
He(21S) states per ion pair following interaction of pulses of 2-MeV protons with He.
A new method for obtaining the cross section for photoionization of photoexcited cesium is presented. The salient feature of this experiment is to use three crossed beams, i.e., two light beams intersecting a beam of cesium atoms. The cross section is determined by counting the ions produced. The excitation light sources used in obtaining this cross section were rf resonance lamps or GaAs lasers. The ions were extracted from the interaction region and detected with a channel electron multiplier. (Author)
We experimentally demonstrate a detection scheme suitable for state analysis of single optically trapped atoms in less than 1 μs with an overall detection efficiency η exceeding 98%. The method is based on hyperfine-state-selective photoionization and subsequent registration of the correlated photoion-electron pairs by coincidence counting via two opposing channel electron multipliers. The scheme enables the calibration of absolute detection efficiencies and might be a key ingredient for future quantum information applications or precision spectroscopy of ultracold atoms.
In recent years there has been increasing interest in quantum atom optics, in which the discretenature of atoms plays an important role. As such, the ability to resolve atoms at the singleatom level has become important.This thesis investigates detecting atoms using photoionisation and ion detection. Thismethod is applicable to all atoms in common use in atom optics experiments, enables theirdetection with high e±ciency, allows atom counting with sub-Poissonian resolution (¾N < pN)and is applicable for large atom numbers (up to » 105). Two approaches towards atom detectionvia photoionisation and ion detection are examined, di®ering in the rate of photoionisation. Fastphotoionisation of individual atoms can be used for detecting a series of single atoms, such asmight be produced with an atom laser. Slow ionisation is required for counting large numbersof atoms contained in a single region, due to the necessity of being able to resolve the individualions with the ion detector. The application of these methods to 87Rb is examined, althoughthey can be adapted to other atoms.The scheme for quickly detecting single atoms uses STImulated Raman Adiabatic Passage(STIRAP) to excite 87Rb atoms to the 5D5=2 level followed by ionisation from this level usinga Nd:YAG laser pulse. Simulations and experiments were carried out and used to predict thatexcitation to the 5D5=2 level can be achieved with close to 100% e±ciency, and to show theexperimental parameters required. The photoionisation cross-section of 87Rb in the 5D5=2 stateusing 1064 nm light was measured in order to predict the photoionisation e±ciency that maybe achieved, which was found to approach 100%.The scheme for atom counting uses 2£778 nm photon excitation to raise atoms to the5D5=2 state followed by ionisation from this state using a cw Nd:YAG laser. The statisticsof atom counting are examined and used to ¯nd the counting accuracy as a function of thedetection e±ciency and its uncertainty. For a precisely calibrated detector, sub-Poissoniancounting is possible for detection e±ciencies greater than 50%. The e®ect of losses and the useof the scheme for measuring number squeezing are examined. Achieving low count uncertaintiesrequires measuring the ion detection e±ciency to high accuracy. A method for doing this isproposed and analysed, and experiments were carried out to measure the ion detection e±ciencyof an ion detector in a mock experimental setup. A detailed scheme is proposed and analysed for measuring number di®erence squeezing ina RF induced double well potential using atom counting via photoionisation and ion detection.Simulations of the double well potential were performed to show how even splitting can beachieved and the degree of squeezing expected to occur. A design for an experimental setup isdescribed and analysed in detail, in order to show the accuracy with which number di®erencesqueezing can be measured.
Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of He(
21S) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and by using proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and time resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. Resonance ionization spectroscopy has application to classical phenomena such as the diffusion of free atoms, the chemical reaction of free atoms with a gas, the measurement of concentration fluctuations in a dilute vapor, and a variety of other atomic fluctuation phenomena. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states. Modern applications include the detection of single atoms in individual ionization tracks, such as those created by the binary fission of a parent atom, and the extension of this technique to the detection of stable or unstable daughter atoms in time coincidence with the decay of a parent atom. These methods are beig developed for use in low-level counting to determine, for example, the flux of solar neutrinos on the earth.