A commercial Spectra Diode Laboratories, 1.083
μm diode laser has been used to efficiently spin polarize an atomic beam of He(2$\sp3$S) metastable atoms by optical pumping on the $\rm 2\sp3S\sb1\rightarrow2\sp3P\sb1$ (D1) transition. The diode laser is an InGaAs/GaAs strained multiquantum well with a distributed Bragg reflector to insure single longitudinal mode operation. The laser has been frequency stabilized to less than 1 MHz/hr drift. The helium metastable atom beam is produced by electron impact and the polarization is measured using a Stern-Gerlach analyzer. Atomic polarizations exceeding 97% have been obtained with $\sim$25 mW of power from the diode laser. A number of experiments currently under construction may also make use of the diode laser. These include the production of a spin polarized helium ion beam by optically pumping the metastable atoms in an rf discharge, and the trapping of helium metastable atoms in a magneto-optical trap (MOT).
A time-of-flight technique has been used to measure the quenching of the
21S0 metastable state of helium by a static electric field. Neutral ground-state helium atoms effusing from a cooled source slit are immediately excited to the
21S0 and
23S1 metastable states by a pulse of antiparallel magnetically focused electrons. The metastable beam is collimated before passing through a uniform electric field region 0.5 m long and is then preferentially detected at the end of the time-of-flight region, 1.825 m from the electron gun. The time-of-flight distribution for the
21S0 state is separated from that of the
23S1 state by illuminating the beam before the electric field region with an rf-discharge helium lamp. The
21S0 state is quenched by resonant absorption of a 20 581-\AA{} photon, raising the atom to the
21P1 state, which then decays to the
1S0 ground state; the
23S1 state remains unaffected because it is the ground state for the triplet system. The
21S0 time-of-flight distribution is therefore obtained from the difference between the full beam and the quenched beam. The number of
21S0 atoms arriving at the detector in specific velocity intervals with the electric field off is compared to the number with the field on to determine the quenching rate (
=kE2). The result for the quenching constant
k for both
He3 and
He4 with
E in kV/cm is $k=0.933\ifmmode\pm\else\textpm\fi{}0.005$; this value is in good agreement with theory and with an earlier less accurate experiment. The error in the present experiment arises from a 0.5% uncertainty in the effective length of the electric field region.
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 present experimental results that demonstrate—for the first time—the Zeeman deceleration of helium atoms in the metastable 2 3 S 1 state. A more than 40% decrease of the kinetic energy of the beam is achieved for deceleration from 490 m s −1 to a final velocity of 370 m s −1 . Metastable atom generation is achieved with an electron-impact-excitation source whose performance is enhanced through an additional discharge-type process which we characterize in detail. Metastable helium is efficiently generated even in a mixture of other noble gases that have much lower excitation energies. Comparison of deceleration data at different electron beam pulse durations confirms that a matching between the initial particle distribution and the phase-space acceptance of the decelerator is crucial for the production of a decelerated packet with a well-defined velocity distribution. The experimental findings are in good agreement with three-dimensional numerical particle trajectory simulations.
Diode laser optical pumping to produce a highly spin-polarized metastable He beam to be used in a spin-polarized metastable atom deexcitation spectroscopy experiment on magnetized surfaces is described. Efficient pumping of the beam is performed by means of an SDL-6702 distributed Bragg reflector diode laser which yields 50 mW of output power in a single longitudinal mode at 1083 nm, the resonance wavelength for the 23 S→23 P0,1,2 (D0, D1, and D2) transitions of He*. The light is circularly polarized by a quarter-wave plate, allowing easy change of the sense of atomic polarization. The laser frequency can be locked to the atomic transition for several hours by phase-sensitive detection of the saturated absorption signal in a He discharge cell. Any of the three transitions of the triplet system can be pumped with the laser but the maximum level of atomic polarization of 98.5% is found pumping the D2 line.
Pulsed lasing from optically pumped rare gas metastable atoms (Ne, Ar, Kr, and Xe) has been demonstrated previously. The laser relies on a three-level scheme, which involves the (n+1)p[5/2](3) and (n+1)p[1/2](1) states from the np(5)(n+1)p electronic configuration and the metastable (n+1)s[3/2](2) level of the np(5)(n+1)s configuration (Racah notation). Population inversions were achieved using relaxation from ((n+1)p[5/2](3) to (n+1)p[1/2](1) induced by collisions with helium or argon at pressures near 1 atm. Pulsed lasing was easily achieved using the high instantaneous pump intensities provided by a pulsed optical parametric oscillator excitation laser. In the present study we examine the potential for the development of a continuous wave (CW) optically pumped Ar laser. We report lasing of the 4p[1/2](1)→4s[3/2](2) (912.547 nm) transition following CW diode laser excitation of the 4p[5/2](3)←4s[3/2](2) line (811.754 nm). A pulsed discharge was used to generate Ar 4s[3/2](2), and the time-resolved lasing kinetics provide insights concerning the radiative and collisional relaxation processes.