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 ....
We have measured the lifetime of the
3P1 metastable state of Mg and obtained a value of 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 msec. The level was excited with a dye laser tuned to the intercombination line ($3s3p^{3}P_{1}\ensuremath{\leftrightarrow}3{s}^{2}^{1}S_{0}$). In addition to the
3P1 state, the
3P2 and
3P0 levels were populated by mixing collisions owing to the presence of an inert buffer gas. The populations of all three metastable levels were monitored as a function of time by observing the absorption of Mg resonance radiation ($^{3}P_{0,1,2}\ensuremath{\rightarrow}^{3}S_{1}$) out of each level. The mixing time was short compared to the
3P1 lifetime, so that the populations of the three metastable levels were in thermal equilibrium with each other. This produced an apparent lengthening of the
3P1 lifetime, which would introduce a large systematic error if the degree of mixing were not known. Cenventionally, one observes the fluorescence from the excited levels to obtain their lifetimes. Because the lifetimes of the
3P2 and
3P0 states are too long, fluorescence from these states cannot be observed, and the observation of fluorescence from the
3P1 state alone provides insufficient information to determine its lifetime.
The radiative lifetime of Li II
21S0 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 radiative lifetime of the
1S0 metastable state of
Xe2+ has been measured with a different type of electrostatic ion trap. The ion trap stores ion beams of a few keV using purely electrostatic fields and provides a large field-free region where the stored ions oscillate. A photomultiplier working in the single-photon counting mode was used to observe the 380 nm photons arising from the magnetic dipole
(M1) transition ${}^{1}{\stackrel{\ensuremath{\rightarrow}}{{S}_{0}}}^{3}{P}_{1}$ of
Xe2+. The present measurement yields a value of $4.46\ifmmode\pm\else\textpm\fi{}0.08 \mathrm{ms},$ which is in good agreement with previous measurements, but is almost four times more accurate.
A time-of-flight technique has been used to measure the radiative lifetime of the
21S0 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
23S1 and the
21S0 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
21S0 state is separated from that of the
23S1 state by illuminating the beam with an rf-discharge helium lamp. The
21S0 state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the
21P1 state, which then decays preferentially to the
1S0 ground state; the
23S1 state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the
21S0 atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of
21S0 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
He3 and
He4 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.
We performed laser spectroscopy of
In+ ions stored in a radio-frequency trap. The 5
s2 1S0\ensuremath{\rightarrow}5s5p
3P1 transition at 230.6 nm was excited and its hyperfine splitting and isotope shift have been measured. The lifetime of the
3P1 state was measured using intensity modulated laser excitation: \ensuremath{\tau}
(3P1)=0.44(4) \ensuremath{\mu}s. A single indium ion was laser cooled using this intercombination line and dark periods in the fluorescence were observed when the metastable
3P0 state was populated by a magnetic dipole decay from
3P1. The radiative lifetime of the forbidden transition
3P0${\ensuremath{\rightarrow}}^{1}$
S0 was determined from the duration of the dark periods: \ensuremath{\tau}
(3P0)=0.14(2) s. This value is compared to calculations based on perturbation theory. Prospects for an optical frequency standard using the narrow 5
s2 1S0\ensuremath{\rightarrow}5s5p
3P0 resonance at 236.5 nm of a single laser-cooled indium ion are discussed.