Duncan A. Tate

High-sensitivity frequency-modulation spectroscopy with a GaAlAs diode laser

A high-sensitivity spectroscopic system has been constructed with a GaAlAs diode laser by using two-tone frequency-modulation spectroscopy. We have demonstrated an absorption sensitivity of 3 × 10−7 in a 0.84-Hz bandwidth by using the Doppler-broadened water vapor absorption line at 12 238.32 cm−1. The sensitivity is limited by laser excess noise and is approximately five times less than the sensitivity expected on the basis of detector noise.

Radiative lifetime measurements of rubidium Rydberg states

We have measured the radiative lifetimes of ns, np and nd Rydberg states of rubidium in the range 28 < n < 45. To enable long-lived states to be measured, our experiment uses slow-moving Rb atoms in a magneto-optical trap (MOT). Two experimental techniques have been adopted to reduce random and systematic errors. First, a narrow-bandwidth pulsed laser is used to excite the target Rydberg state, resulting in minimal shot-to-shot variation in the initial state population. Second, we monitor the target state population as a function of time delay from the laser pulse using a short-duration, millimetre-wave pulse that is resonant with a one- or two-photon transition. We then selectively field ionize the monitor state, and detect the resulting electrons with a micro-channel plate. This signal is an accurate mirror of the target state population, and is uncontaminated by contributions from other states which are populated by black body radiation. Our results are generally consistent with other recent experimental results obtained using a less sensitive method, and are also in excellent agreement with theory.

Fine structure and isotope shift of tritium in the Balmer-α transition

By means of saturated absorption spectroscopy in a discharge tube containing hydrogen isotopes, fine-structure intervals in the Balmer- alpha line of tritium have been measured, and for each of four components of that line the tritium-hydrogen and tritium-deuterium isotope shifts have been measured. The results agree with theory within the experimental limits of error, which are typically +or-2 MHz. The profiles of the components of Balmer- alpha have been analysed in detail and in particular a Stark-induced crossover signal has been revealed.

Controllable Interactions between Rydberg Atoms and Ultracold Plasmas

We discuss the control of dipole-dipole interactions in a frozen assembly of Rydberg atoms. We report the evidence of dipole blockade of the Rydberg excitation for two configurations: dipole blockade induced by electric field and dipole blockade in Forster resonance. We demonstrate that two individual atoms separated by ~ 4 μm can act as a collective dipole if their interaction is strong enough to be in the dipole blockade regime. This observation is crucial for the quantum entanglement of two or more atoms using dipole-dipole interaction. The dipole-dipole interactions between Rydberg atoms are also responsible for Penning ionization leading to the formation of an ultracold plasma. We have demonstrated that Penning ionization of np Rydberg cesium atoms can be prevented by considering repulsive dipole-dipole interactions.

Method of concentration of power in materials for x-ray amplification.

Recent experimental and theoretical results indicate that a new technique for the controlled concentration of power in materials may be feasible. The power levels that are potentially achievable are sufficient for the generation of amplification of x-ray wavelengths in the kilovolt range. The method of power concentration involves the combination of (1) a new ultrahigh brightness subpicosecond laser technology, (2) multiphoton coupling to atoms and molecules, and (3) a new channeled mode of electromagnetic propagation. The energy scaling of this approach is the most important consideration, and it is shown that the control of the propagation is the key factor that enables high levels of amplification in the kilovolt regime to be achieved with a total excitation energy of ~1 J.

Multiphoton double ionization of barium with intense picosecond pulses

The authors have observed the production of Ba2+ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba2+ production can be attributed to Ba+ transitions originating in the 6s1/2 or 5dj states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba2+ production is strong throughout the region studied, at times comparable to that of Ba+. The authors have also looked for Ba2+ production in the range 280-325 nm, but no Ba2+ signal was observed in this part of the ultraviolet.

Precision cw Laser Spectroscopy of Hydrogen and Deuterium

The 1S–2S transition of atomic hydrogen has attracted considerable attention because of its extremely narrow natural width (1 Hz). High resolution Doppler-free spectra of this transition, obtainable by two-photon absorption, offer the prospect of precise tests of bound-state quantum electrodynamics (QED) and more accurate values for fundamental constants /1/. The two-photon absorption was first observed by HANSCH et al. /2/ at Stanford, in the first of a series of progressively more accurate measurements /3/. Their latest result is a measurement of the IS Lamb shift in hydrogen using continuous-wave radiation /4/.

High sensitivity near‐infrared diode laser spectroscopy of hydrogen sulfide

We have used high‐sensitivity diode laser spectroscopy to measure the positions of some 130 rotation‐vibration transitions in hydrogen sulfide (H2S). The absorption lines lie in the range 12,100–12,230 cm−1 (818–826 nm), corresponding to transitions from the ground vibrational state to upper levels with ν1, ν2, ν3=4,2,0; 3,2,1; 5,0,0; 4,0,1; 1,4,2; and 0,4,3. We believe that this is the first time these spectral features have been observed. Approximately 5 mW of light from a GaAlAs diode laser was frequency modulated at 1 GHz±10 MHz (two‐tone FM) and double passed through a 2 m long cell containing approximately 100 Torr of H2S. The transmitted light was detected with a fast photodiode and the signal demodulated using heterodyne techniques. The frequencies of the spectral features were measured with an estimated uncertainty of 0.03 cm−1.

External Control of Electron Temperature in Ultra‐Cold Plasmas

We discuss our progress towards achieving external control of the electron temperature and the Coulomb coupling parameter of ultra‐cold plasmas. Using a Littman dye laser, we create the plasma by partially photoionizing a dense, cold sample of rubidium atoms in a magneto‐optical trap (MOT). At a controllable time delay, we excite neutral atoms in the plasma to a specific Rydberg state using a narrow bandwidth pulsed laser. We have made very qualitative measurements of the electron temperature as a function of delay from the exciting laser pulses. Some of our results suggest that the plasma is stabilized by the presence of the Rydberg atoms, with a longer lifetime and slower expansion rate than a plasma that evolves directly from a dense Rydberg sample.

High precision CW laser measurement of the 1S−2S interval in atomic hydrogen and deuterium

We have excited the 1S‐2S transition in atomic hydrogen and deuterium by two‐photon absorption of cw 243 nm light. The transition frequency has been measured by comparison with calibrated lines in the spectrum of the 130Te2 molecule, providing new precise values for the 1S Lamb shifts or the Rydberg constant.