Thomas Greytak - Academia.edu (original) (raw)

Papers by Thomas Greytak

Applied Physics B: Lasers and Optics, 1999

The addition of atomic hydrogen to the set of gases in which Bose-Einstein condensation can be ob... more The addition of atomic hydrogen to the set of gases in which Bose-Einstein condensation can be observed expands the range of parameters over which this remarkable phenomenon can be studied. Hydrogen, with the lowest atomic mass, has the highest transition temperature, 50 µK in our experiments. The very weak interaction between the atoms results in a high ratio of the condensate to normal gas densities, even at modest condensate fractions. Using cryogenic rather than laser precooling generates large condensates. Finally, two-photon spectroscopy is introduced as a versatile probe of the phase transition: condensation in real space is manifested by the appearance of a high density component in the gas, condensation in momentum space is readily apparent in the momentum distribution, and the phase transition line can be delineated by following the evolution of the density of the normal component.

Developments with Ultracold Hydrogen

We present an overview of current and planned experiments in the Ultracold Hydrogen Group at MIT.... more We present an overview of current and planned experiments in the Ultracold Hydrogen Group at MIT. One project is high resolution spectroscopic measurements of 2S-nS two-photon transitions on our ultracold, trapped hydrogen sample. Our goal is an improvement in the value of the Rydberg constant and the Lamb shift by an order of magnitude. To probe the 2S-nS transitions, we

Le Journal de Physique Colloques, 1980

RBsum6.-Nous dQcrivons des premiers rQsultats experimentaux concernant l'obtention de H4 et son c... more RBsum6.-Nous dQcrivons des premiers rQsultats experimentaux concernant l'obtention de H4 et son confinement en champ magndtigue QlevQ. Nous avons rQussi 2 maintenir 3x10~' atomes dans un volume de 4 cm3, 21 0,3 K et dans un champ de 8 T. Dans ces conditions, la constante de temps de dQcroissance de la densit6 atomique Qtait de l'ordre de l'heure. Nous donnons des details concernant notre technique expsrimentale et discutons les espoirs possibles concernant l'obtention de densitQs plus BlevBes.

Spin-polarized hydrogen maser

Physical Review A, 1986

Maser action on the hydrogen hyperfine transition has been achieved at a temperature of 0.3 K in ... more Maser action on the hydrogen hyperfine transition has been achieved at a temperature of 0.3 K in a liquid-helium-lined resonator. Polarized atoms are provided by magnetic ejection from a low-temperature discharge in a high magnetic field. The maser demonstrates the feasibility of a proposed low-temperature frequency standard. Operation of the atomic hydrogen source represents an essential step toward magnetic trapping of hydrogen at very low temperatures. Operated continuously, the source provides 5 x 10Â¹Â² atoms/sec to the maser. In a 1 sec single pulse it can provide up to 2 x 10Â¹Â³ polarized atoms.

Physical Review Letters, 2008

We have trapped large numbers of copper (Cu) and silver (Ag) atoms using buffer-gas cooling. Up t... more We have trapped large numbers of copper (Cu) and silver (Ag) atoms using buffer-gas cooling. Up to 3 Â 10 12 Cu atoms and 4 Â 10 13 Ag atoms are trapped. Lifetimes are as long as 5 s, limited by collisions with the buffer gas. Ratios of elastic to inelastic collision rates with He are * 10 6 , suggesting Cu and Ag are favorable for use in ultracold applications. The temperature dependence of the Ag-3 He collision rate varies as T 5:8AE0:4. We find that this temperature dependence is inconsistent with the behavior predicted for relaxation arising from the spin-rotation interaction, and conclude that the Ag-3 He system displays anomalous collisional behavior in the multiple-partial wave regime. Gold (Au) was ablated into 3 He buffer gas, however, atomic Au lifetimes were observed to be too short to permit trapping.

Physical Review Letters, 1996

Observation of Three-Body Recombination in Spin-Polarized Hydrogen

Physical Review Letters, 1983

Three-body recombination in spin-polarized hydrogen has been observed. Doubly polarized hydrogen ... more Three-body recombination in spin-polarized hydrogen has been observed. Doubly polarized hydrogen was compressed by a piston into a thin cell and the temperature and pressure were independently monitored. Measurements were carried out in the temperature range 0.3 to ...

Physical Review Letters, 1981

The nuclear relaxation rate has been measured in spin-polarized hydrogen. The rate is so low that... more The nuclear relaxation rate has been measured in spin-polarized hydrogen. The rate is so low that a gas composed of atoms initially in the bvo nuclear spin states spontaneously acquires a high nuclear polarization as one of the states recombines. The remaining state is stable against recombination, but decays slowly by nuclear relaxation. Nuclear polarization greater than 99Vo has been achieved. The absorption energy and the recombination rate of hydrogen have also been measured on liquid 4He.

Physical Review Letters, 1979

Physical Review Letters, 1993

Measurements of the sticking probability s(T) for H on bulk liquid He reveal the onset of the uni... more Measurements of the sticking probability s(T) for H on bulk liquid He reveal the onset of the universal JT dependence expected at very low atom temperatures. Studies of s(T) as a function of film thickness clearly demonstrate the influence of the van der Waals-Casimir force due to the substrate, in agreement with recent theories.

$Research paper thumbnail of Hydrogen in the submillikelvin regime: Sticking probability on superfluid ^{4}He$

Physical Review Letters, 1991

We have measured the sticking probability of atomic hydrogen on a superAuid He surface for atom e... more We have measured the sticking probability of atomic hydrogen on a superAuid He surface for atom energies between 100 pK and 1 mK. The sticking probability shows little variation with energy over this interval, and is close to the value 0.2. The studies were carried out using a magnetic trap that produced a density of 8x10' cm ' at a temperature of 100 pK.

Physical Review A, 2003

We report the first detailed decay studies of trapped metastable (2S) hydrogen. By two-photon exc... more We report the first detailed decay studies of trapped metastable (2S) hydrogen. By two-photon excitation of ultracold H samples, we have produced clouds of at least 5 × 10 7 magnetically trapped 2S atoms at densities greater than 4 × 10 10 cm −3 and temperatures below 100 µK. At these densities and temperatures, two-body inelastic collisions of metastables are evident. Experimental values for the total two-body loss rate constant are K 2 = 1.8 +1.8 −0.7 ×10 −9 cm 3 /s at 87 µK and K 2 = 1.0 +0.9 −0.5 ×10 −9 cm 3 /s at 230 µK. These results are in the range of recent theoretical calculations for the total 2S-2S inelastic rate constant. The metastable clouds were excited in a gas of ground state (1S) hydrogen with peak densities reaching 7 × 10 13 cm −3. From the one-body component of the metastable decay, we derive experimental upper limits for K 12 , the rate constant for loss due to inelastic 1S-2S collisions.

$Research paper thumbnail of Zeeman relaxation of cold atomic iron and nickel in collisions with ^{3}He$

Physical Review A, 2010

We have measured the ratio γ of the diffusion cross section to the angular momentum reorientation... more We have measured the ratio γ of the diffusion cross section to the angular momentum reorientation cross section in the colliding Fe-3 He and Ni-3 He systems. Nickel (Ni) and iron (Fe) atoms are introduced via laser ablation into a cryogenically cooled experimental cell containing cold (<1 K) 3 He buffer gas. Elastic collisions rapidly cool the translational temperature of the ablated atoms to the 3 He temperature. γ is extracted by measuring the decays of the atomic Zeeman sublevels. For our experimental conditions, thermal energy is comparable to the Zeeman splitting. As a result, thermal excitations between Zeeman sublevels significantly impact the observed decay. To determine γ accurately, we introduce a model of Zeeman-state dynamics that includes thermal excitations. We find γ Ni-3 He = 5 × 10 3 and γ Fe-3 He 3 × 10 3 at 0.75 K in a 0.8-T magnetic field. These measurements are interpreted in the context of submerged shell suppression of spin relaxation, as studied previously in transition metals and rare-earth-metal atoms [

Physical Review A, 2011

The collisional magnetic reorientation rate constant g R is measured for magnetically trapped ato... more The collisional magnetic reorientation rate constant g R is measured for magnetically trapped atomic dysprosium (Dy), an atom with large magnetic dipole moments. Using buffer gas cooling with cold helium, large numbers (>10 11) of Dy are loaded into a magnetic trap and the buffer gas is subsequently removed. The decay of the trapped sample is governed by collisional reorientation of the atomic magnetic moments. We find g R = 1.9 ± 0.5 × 10 −11 cm 3 s −1 at 390 mK. We also measure the magnetic reorientation rate constant of holmium (Ho), another highly magnetic atom, and find g R = 5 ± 2 × 10 −12 cm 3 s −1 at 690 mK. The Zeeman relaxation rates of these atoms are greater than expected for the magnetic dipole-dipole interaction, suggesting that another mechanism, such as an anisotropic electrostatic interaction, is responsible. Comparison with estimated elastic collision rates suggests that Dy is a poor candidate for evaporative cooling in a magnetic trap.

The Journal of Chemical Physics, 1992

The discovery of H + 3 in the diffuse interstellar medium has dramatically changed our view of th... more The discovery of H + 3 in the diffuse interstellar medium has dramatically changed our view of the cosmic-ray ionization rate in diffuse molecular clouds. However, another surprise has been that the ortho:para ratio of H + 3 in these clouds is inconsistent with the temperature derived from the excitation of H 2 , the dominant species in these clouds. In an effort to understand this discrepancy, we have embarked on an experimental program to measure the nuclear spin dependence of the dissociative electron recombination rate of H + 3 using the CRYRING and TSR ion storage rings. We have also performed the first measurements of the reaction H + 3 + H 2 → H 2 + H + 3 below room temperature. This reaction is likely the most common bimolecular reaction in the universe, and plays an important role in interconverting ortho-and para-H + 3. Finally, we have constructed a steady-state chemical model for diffuse clouds, which takes into account the spindependence of the formation of H + 3 , its electron recombination, and its reaction with H 2. We find that the ortho:para ratio of H + 3 in diffuse clouds is likely governed by a competition between dissociative recombination and thermalization by reactive collisions.

Japanese Journal of Applied Physics, 1987

We have confined over 5X10' atoms of hydrogen in a static magnetic trap. The atoms are loaded int... more We have confined over 5X10' atoms of hydrogen in a static magnetic trap. The atoms are loaded into the trap by precooling with a dilution refrigerator. At operating densities near 1X 10' cm the gas is observed to be electron and nuclear polarized in the uppermost hyperfine state. The long lifetime of the trapped gas (over 20 min) suggests that it is thermally decoupled from the wall and has evaporatively cooled to a temperature of about 40 mK. The residual decay of the gas density is consistent with spin relaxation induced by dipolar interactions between atoms.

Magnetic trapping of copper and silver using buffer gas loading

Atomic silver and coppper are magnetically trapped using buffer gas loading. Copper (Cu) is trapp... more Atomic silver and coppper are magnetically trapped using buffer gas loading. Copper (Cu) is trapped in the 4s;^2S1/2, mj=1/2 state with lifetimes as long as 8s. Silver (Ag) is trapped in the 5s;^2S1/2, mj=1/2 state with lifetimes as long as 2.3 s. Lifetimes are limited by collisions with background ^3He. Inelastic Zeeman state-changing collisions are observed between Ag and ^3He.

Multidimensional Evaporative Cooling of Trapped Atomic Hydrogen

Bose-Einstein condensation of spin-polarized atomic hydrogen has been pursued for many years. Pre... more Bose-Einstein condensation of spin-polarized atomic hydrogen has been pursued for many years. Previous attempts(J.M. Doyle, Ph.D. thesis, Massachusetts Institute of Technology, 1991.) to evaporate hydrogen into the quantum degenerate regime have been thwarted by inefficient one dimensional evaporation(P.W.H. Pinkse, Ph.D. thesis, University of Amsterdam, 1997.). We have a new apparatus that incorporates RF induced evaporation. Samples have already been produced

Physical Review Letters, 1980

Magnetic confinement is used to obtain spin-polarized atomic hydrogen (Ht) in an openended helium... more Magnetic confinement is used to obtain spin-polarized atomic hydrogen (Ht) in an openended helium-coated chamber, at densities up to 0.8x 10 17 atoms/cm 3. At a field of 10 T and temperature of 0.3 K, confinement times as long as 4 h are achieved. A quantitative study of the confinement times as a function of magnetic field is in excellent agreement with the expected escape from the magnetic potential well accompanied by a very slow loss process. Heat transport measurements confirm that the Hf is a gas and an upper limit is placed on the three-body recombination rate.

Growth of a Hydrogen Bose Condensate

We have observed in detail the formation of a hydrogen Bose-Einstein condensate. Hydrogen atoms w... more We have observed in detail the formation of a hydrogen Bose-Einstein condensate. Hydrogen atoms were evaporatively cooled close to the onset of Bose-Einstein condensation and then suddenly quenched below the transition temperature. The subsequent growth of the hydrogen condensate was monitored nearly non-destructively using high-resolution spectroscopy of the 1S-2S transition. We used the mean-field density shift of this transition to