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Lawrence Berkeley National Laboratory

LIFETIME OF THE METASTABLE 23S1 STATE IN STORED Li+ IONS

Randall D. Knight

Published Jun 14, 2010• 0 citations

Abstract

A laser-induced fluorescence technique combined with the observation of spontane ous magnetic dipole photons from the highly metastable 2 3 S t state of Li + has been used to measure the radiative lifetime of this state.The ions are created by electron impact on a lithium A. Direct Determination of the Spontaneous Decay Curve .... 18 B. Direct Determination of the Number of Remaining Metastables 20 C. Indirect Determination of the Radiative Lifetime 23 IV.Ion Trap Theory and Performance 29 A. The RF-Quadrupole Ion Trap 29 B. The Ion Detection System 36 C. Ion Trap Performance 43 1.Ion storage 43 2. Ion distribution 50 V.Experimental Methods 56 A. The Vacuum System 56 B. The Lithium Oven and Hot Wire Detector 60 C. The Ion Trap and Electron Gun 62 D. The Magnetic Field 66 1. Detector operation ..... 73 2. Detector calibration 76 6.The Laser and Optical System 81 H.The Computer System 84 1. Timing and control .84 2. Operating and analysis programs 88 VI.Experimental Results 93 A. The Detection of Spontaneous Ml Photons 93 B. The Detection of Laser-Induced Photons 96 C. The Metastable Lifetime and Nonradiative Quenching .... 99 D. Procedures in Measuring the Radiative Lifetime 104 E. Data Reduction and Error Analysis Ill VII.Conclusion 119 Appendix A. Optical Pumping Calculations .121 A. The Magic Angle 121 B. The Optical Pumping Equations 123 C. Results of the Calculations .126 Appendix B. Measurements of the Ion Density Distribution 128 A. The Laser Scanning Technique 128 B. Experimental Results 130 Acknowledgements ....

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A laser-induced fluorescence technique combined with the observation of spontane ous magnetic dipole photons from the highly metastable 2 3 S t state of Li + has been used to measure the radiative lifetime of this state.The ions are created by electron impact on a lithium A. Direct Determination of the Spontaneous Decay Curve .... 18 B. Direct Determination of the Number of Remaining Metastables 20 C. Indirect Determination of the Radiative Lifetime 23 IV.Ion Trap Theory and Performance 29 A. The RF-Quadrupole Ion Trap 29 B. The Ion Detection System 36 C. Ion Trap Performance 43 1.Ion storage 43 2. Ion distribution 50 V.Experimental Methods 56 A. The Vacuum System 56 B. The Lithium Oven and Hot Wire Detector 60 C. The Ion Trap and Electron Gun 62 D. The Magnetic Field 66 1. Detector operation ..... 73 2. Detector calibration 76 6.The Laser and Optical System 81 H.The Computer System 84 1. Timing and control .84 2. Operating and analysis programs 88 VI.Experimental Results 93 A. The Detection of Spontaneous Ml Photons 93 B. The Detection of Laser-Induced Photons 96 C. The Metastable Lifetime and Nonradiative Quenching .... 99 D. Procedures in Measuring the Radiative Lifetime 104 E. Data Reduction and Error Analysis Ill VII.Conclusion 119 Appendix A. Optical Pumping Calculations .121 A. The Magic Angle 121 B. The Optical Pumping Equations 123 C. Results of the Calculations .126 Appendix B. Measurements of the Ion Density Distribution 128 A. The Laser Scanning Technique 128 B. Experimental Results 130 Acknowledgements ....

The radiative lifetime of

1s2s^{3}S_{1}

{\mathrm{Li}}^{+}

ions has been measured by a comparison of the count rate of spontaneous decay photons (210 \AA{}) to a laser-induced fluorescence signal proportional to the total number of metastable ions. The ions were confined in a rf-quadrupole trap, and the fluorescence signal used was the intercombination transition

1s2p^{3}P_{1}

to $1{s}^{2}^{1}S_{0}$ at 202 \AA{}. The

^{3}P_{1}

state was populated from

^{3}S_{1}

by absorption of laser photons at 5485 \AA{}. The result is ${\ensuremath{\tau}}_{r}=58.6\ifmmode\pm\else\textpm\fi{}12.9$ sec (95% confidence level); theory predicts ${\ensuremath{\tau}}_{r}=49.1$ sec.

The radiative lifetime of Li II

2^{1}S_{0}

has been measured by counting decay photons versus time from an ensemble of metastable Li ions stored in a simple electromagnetic trap of the Penning variety. The result is ${\ensuremath{\tau}}_{\mathrm{expt}}=503(26)$ \ensuremath{\mu}sec. The error represents one standard deviation from the mean of a series of 34 separate determinations. This result is in agreement with the theoretical value ${\ensuremath{\tau}}_{\mathrm{th}}=513$ \ensuremath{\mu}sec.

The natural lifetime of the metastable

4d\phantom{\rule{0.2em}{0ex}}^{2}D_{5∕2}

state in

^{88}\mathrm{Sr}^{+}

has been measured using a single laser-cooled, trapped ion. The ion was prepared in the

4d\phantom{\rule{0.2em}{0ex}}^{2}D_{5∕2}

state by laser excitation of the $5s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}\ensuremath{-}4d\phantom{\rule{0.2em}{0ex}}^{2}D_{5∕2}$ quadrupole transition at 674 nm. The decay time was measured by using Dehmelt's electron shelving method to monitor the ion's electronic state. Two separate investigations were conducted under slightly different experimental conditions. 160 000 decay times from the

4d\phantom{\rule{0.2em}{0ex}}^{2}D_{5∕2}

state were observed in total, and two different methods for determining the statistical value of the measured lifetime have been compared. Systematic effects have also been estimated, and including these effects yields a value of $\ensuremath{\tau}=390.8(1.6)\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$ ($1\ensuremath{\sigma}$ uncertainty) for the natural lifetime.

A time-of-flight technique has been used to measure the radiative lifetime of the

2 ^{1}S_{0}

state of helium. After passing from a cooled source, helium atoms are excited by a pulsed anti-parallel electron beam. The atomic beam then contains not only ground-state helium atoms but also metastable atoms in both the

2 ^{3}S_{1}

and the

2 ^{1}S_{0}

states. The metastable atoms are detected at two positions, 1.9 and 6.7 m from the electron gun, by Auger ejection of an electron from a copper target, the first of which is a 60% transmitting mesh. Ground-state helium atoms cannot cause an electron to be ejected and hence are not detected. The time-of-flight distribution for the metastable

2 ^{1}S_{0}

state is separated from that of the

2 ^{3}S_{1}

state by illuminating the beam with an rf-discharge helium lamp. The

2 ^{1}S_{0}

state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the

2 ^{1}P_{1}

state, which then decays preferentially to the

^{1}S_{0}

ground state; the

2 ^{3}S_{1}

state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the

2 ^{1}S_{0}

atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of

2 ^{1}S_{0}

metastables within a given velocity interval at the two detectors determines the number which decay in flight and yields a value for the two-photon radiative lifetime. The value of the singlet lifetime for both

{\mathrm{He}}^{3}

and

{\mathrm{He}}^{4}

is 19.7 \ifmmode\pm\else\textpm\fi{} 1.0 msec, agreeing with the theoretical value of 19.5 msec, but disagreeing with the value 38 \ifmmode\pm\else\textpm\fi{} 8 msec measured by Pearl using a movable detector. The 1.0-msec error is an estimate of the remaining systematic errors in the experiment.