Metastable Beam Research Articles

Writing nanostructures with a metastable helium beam

We have used a self-assembling monolayer of dodecanethiole molecules as the resist for a lithography technique based on a beam of metastable helium atoms. Doses as low as 3 metastable helium atoms per 10 resist molecules are enough to write patterns. The writing mechanism is based on the Penning-ionization-induced changes of the local wetting properties of the surface. An edge resolution of 30 nanometers is demonstrated.

Temperatures near the recoil limit in an atomic funnel

We report on an improved atomic funnel with transverse velocities near the recoil limit. Its central element is an atomic funnel, strongly tilted against the initial atomic beam axis, deflecting, compressing and cooling a laser decelerated metastable neon beam. In spite of a strong magnetic field gradient at 350 G/cm polarisation gradient cooling in the final section of the funnel leads to transverse temperatures far below the Doppler limit. These results were achieved for unusual small laser detunings, which cannot be explained by standard sub-Doppler theory.

The fine structure constant α: laser spectroscopy of helium and electromagnetically induced transparency

Spectroscopy of helium is discussed in the context of methods to determine the fine structure constant α. Resonances corresponding to the fine structure separation are observed in the coherent excitation of a 4He metastable beam by two laser fields. A quantitative analysis of the lineshapes is performed for the three level model ("V" scheme). A comparison with similar Doppler free line-center determinations from saturation spectroscopy is discussed. This work provides a modern demonstration in the field of coherent phenomena pioneered by George Series.

Oxidation of Zn films by irradiation with an excited metastable He beam

The oxidation of Zn films was attemmed by irradiation with an excited metastable atom beam of noble gas (He, Ne, Ar or Kr) in an oxygen atmosphere of the order of 10 −3 Pa. The excited metastable atoms were generated in a glass tube by an RF discharge and were ejected through a nozzle to a Zn film surface, to which a negative bias voltage was applied. The presence of the excited metastable atoms on the surface was confirmed by detection of the electron current ejected from a Cu plate which was irradiated with the excited metastable atom beam. For Ar and Kr, Zn films were hardly oxidized. On the other hand, transparent ZnO films were obtained for He and Ne. Judging from the fact that the energies of the excited metastable states of He and Ne are higher than the ionization potential of O 2 molecules, the oxidation of Zn films is attributed to the generation of O 2 + molecular ions on the film surface through the Penning ionization process by the excited metastable atom beam.

Apparatus for measuring electron-impact excitation cross sections using fast metastable atoms produced via charge exchange

An apparatus for measuring absolute cross sections due to electron-impact excitation out of the metastable levels of rare-gas atoms via the optical method is described with the focus specifically on excitation out of the 2 3S metastable helium level. The metastable helium target (He*) is prepared by charge exchange between 1.6 keV He+ ions and cesium vapor. An electron beam crosses the fast metastable beam target at a right angle and the fluorescence is collected at right angles to both beams. The charge transfer reaction produces He atoms mainly in the n=2 He levels. Because the target contains a negligible ground state He fraction, we can measure excitation cross sections from excitation threshold up to an arbitrarily high energy (keV regime) which represents a major improvement over previous metastable excitation cross sections measurements. The He* target density is extremely small (∼106 atoms/cm3) yielding minuscule signal rates. We describe steps taken to maximize the signal-to-noise ratio. We discuss the implications of using a fast beam target including both the finite flight time of the excited atoms across the light gathering region and the reduction of the cascade contributions to the apparent cross sections. A discussion of the identification and elimination of various systematic effects is also given. To measure absolute cross sections, we explicitly determine the spatial distributions of both the electron and metastable beams, as well as the spatially dependent response of the fluorescence gathering region. We determine the absolute flux of fast metastable atoms using a thermal detector calibrated with a He+ ion beam. As examples, we present absolute cross sections for excitation out of the 2 3S metastable level into the 3 3D and 4 3D levels.

Cross sections of electron excitation out of metastable helium levels with a fast metastable target produced via charge exchange.

Absolute direct cross sections for electron excitation out of the 2${\mathrm{}}^{3}$S level and into the 3${\mathrm{}}^{3}$D, 4${\mathrm{}}^{3}$D, and 3${\mathrm{}}^{3}$S levels of the helium atom from threshold to 500 eV and into the 3${\mathrm{}}^{3}$P level over a more limited energy range have been measured using a fast metastable atomic beam target. We produce the metastable atoms via near-resonant charge exchange between a 1.6-keV ${\mathrm{He}}^{+}$ ion beam and Cs vapor. Because this reaction is highly nonresonant with the ground state of helium, the charge-transfer process yields a primarily metastable beam. We use a thermal detector which we calibrate with ions to measure absolutely the neutral beam flux. The atomic beam is crossed by an electron beam, and we collect the resulting fluorescence at right angles to both the electron and atomic beams. We obtain the cross sections for excitation out of the 2${\mathrm{}}^{3}$S level into the various excited levels by monitoring the emission out of the excited level of interest. \textcopyright{} 1996 The American Physical Society.

Magnetic-field induced N2(B→A) emission from a beam of metastable nitrogen molecules and precise term energy of the N2(A′ 5Σ+g) state

Magnetic field anticrossing measurements have been made in order to identify unambiguously the N2(A′ 5Σ+g) level responsible for the recently observed perturbation of the N2(B 3Πg, v=10, 3Πe2(12)) level [Ch. Ottinger and A. F. Vilesov, J. Chem. Phys. 100, 4862 (1994)]. When a weak magnetic field (H=0–3000 G) was applied across the N2 metastable beam, new field-induced perturbations were observed in B,v=11, which were ascribed to interactions with A′, v=2. This multiple perturbation pattern permits a determination of the relative energy of the A′ and B states with a precision of 0.02 cm−1.

Time selection in atomic Stern-Gerlach interferometry.

As shown recently by Zawisky, Rauch, and Hasegawa in neutron interferometry [Phys. Rev. A 50, 5000 (1994)], the interference pattern obtained with a low-intensity particle beam is deeply modified when a selection is made of the time intervals between successive detected events. Experiments of the same type performed with a Stern-Gerlach interferometer using a metastable hydrogen atom beam are described. Results very similar to those of the neutron experiment are obtained, in particular a strong enhancement of the contrast when large time intervals are selected.

Simple and efficient method of spin-polarizing a metastable helium beam by diode laser optical pumping

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.

Electron capture by slow multiply charged ions from H(1s)

Over the last 11 years a considerable experimental and theoretical effort has been devoted to electron capture collisions involving slow ( v < 1 a.u.) multiply charged ions and atomic hydrogen. These collisions result in the selective population of a relatively small number of excited projectile states. Translational energy spectroscopy (TES) and the complementary technique of photon emission spectroscopy (PES) have been developed to provide state selective cross sections. In the last 5 years the merged beams technique has been used to measure total one electron capture cross sections in the energy range 1–1000 eV amu −1 enabling comparison with theory at much lower energies than was previously possible. The majority of the experimental measurements have been for fully stripped, Li-like or He-like ions. Theoretical models such as the semiclassical close coupling methods and fully quantal molecular orbital methods have been developed in parallel with experiment. In general the agreement is good although there is a need for state selective data at lower energies and for differential experimental measurements. Also experiments are needed with pure ground state or metastable ion beams.

Isotopic study of the intermolecular versus intramolecular energy transfer in the N2(W,A)+N2(X) reactions

The energy transfer from the long-lived states N2(W 3Δu, A 3Σ+u) to the radiating state N2(B 3Πg) in collisions with N2(X 1Σ+g) was studied under single collision conditions, employing a molecular beam/target gas cell arrangement. By means of using the isotopic species 14N2 in the metastable reactant beam and 15N2 in the target gas cell it was possible to differentiate between the intramolecular and the intermolecular energy transfer mechanisms, on the basis of well-resolved N2(B) product emission spectra. The overall contributions of the two reaction channels were found to be comparable, but they differ greatly in the vibrational product distributions. The intermolecular process populates preferentially the low vibrational levels of 15N2(B,v). The intramolecular process is most efficient for those 14N2(B,v) levels which are in close energy resonance with N2(A or W) vibrational levels.

Optical measurements of core density fluctuations in TEXT-U using a helium metastable beam

A compact, portable, eight-channel, filter spectrometer has been designed for beam emission spectroscopy measurements of long-wavelength density fluctuations on TEXT-U and Phaedrus-T. The system uses radially elongated image volumes which maximize the étendue of the system while still resolving the dominant fluctuations (kr&amp;lt;2 cm−1, kθ&amp;lt;6 cm−1). Because of the vertical injection geometry of the TEXT-U diagnostic neutral beam, there is little Doppler shift of the beam emissions and thus a nonhydrogenic species must be used to distinguish between the beam and edge emissions. A He0 metastable beam has been found to provide good beam penetration with little contamination of the detected beam fluorescence by edge emission. A multistep excitation model is used to quantitatively interpret the fluctuations in the intensity of the He0 triplet (3P0–3D, 587.6 nm) as plasma density fluctuations.

Near-UV sub-Doppler spectroscopy on a metastable Mg beam by a frequency-doubled diode laser

Near-UV radiation is generated by doubling the frequency of a semiconductor laser in a nonlinear crystal. The crystal is contained in a resonant cavity in order to improve the conversion efficiency. The cavity increases the efficiency by about three orders of magnitude. This radiation has been used to perform spectroscopy of metastable magnesium in an atomic beam. We observed the (3s3p)3P−(3s3d)3D transition multiplet, which is of interest for metrological applications. The isotopic shift between24Mg and26Mg was measured and new information on the hyperfine structure of25Mg was obtained. This radiation source is promising also in order to improve the Mg frequency standard.

Line shape of a Doppler-free two-photon transition in a metastable neon beam.

We present a model of the line shape of a Doppler-free two-photon transition in a metastable neon beam. This model considers the ac Stark effect, the Gaussian profile of the laser beam, and the depletion, or bleaching, of the metastable state as the atoms cross the laser. The model fit the experimental data well and was recently used to verify the time-dilation effect at an accuracy of a few ppm.

Diffraction and reflection of a slow metastable neon beam by an evanescent light grating

The first experimental observation is reported of an atomic beam diffracted by coherent transfer of two photon momenta from an evanescent standing laser field. The incident atomic beam of laser slowed metastable neon atoms had a mean velocity of 25 m/s as it was transversely compressed to a diameter below 0.15 mm and to sub Doppler temperatures. Direct images of diffraction patterns taken by a high resolution two-dimensional detector showed specular reflection of the beam of metastable neon atoms up to 74 mrad and clearly the second diffraction order from 81 to 92 mrad.

Reflection of metastable neon atoms by a surface plasmon wave

Reported is an experimental realization of an atomic mirror for a metastable neon beam whose principle is based upon the dipole force extented by a surface plasmon wave. The first experimental evidence for Doppleron resonances induced by a stationary inhomogeneous wave is demonstrated.

LIF detection of the A′ 5Σ +g state of N 2 in a beam and lifetime of the C″ 5Π u state

A beam containing N 2 in long-lived states was generated by means of a dc discharge at the nozzle. One of the metastable beam components was N 2 (A′ 5Σ + g). This species was detected by means of pulsed laser excitation in the N 2 (C″ 5Π u←A′ 5Σ + g) 2-1 band, folowed by 2-0 band fluorescence. The lifetime of the C″, ν = 2 level was measured to be 4.73 ± 0.1 μs.

Transverse laser cooling of a metastable argon beam: Dependence on the interaction time.

A pulsed metastable argon beam is transversely cooled in one dimension via the closed 4s[11/2${]}_{2}^{\mathit{o}\mathrm{\ensuremath{-}}}$4p[21/2${]}_{3}$ transition. A standing light wave, orthogonal to the atomic beam and located just at the source, provides the cooling. In all the measurements, the maximum electric field is equivalent to a Rabi frequency of \ensuremath{\sim}220 natural linewidths. The enhancement of the metastable atom flux is measured as a function of the interaction time with a time-of-flight technique. The dependence on the laser detuning is investigated and compared with theory.

Selective fragmentation and ionization of organic compounds using an energy-tunable rare-gas metastable beam source

The initial use of an energy-tunable rare-gas metastable beam source generated by a corona discharge as an ionization source for mass spectrometry is reported. The metastable atom beam is produced external to theion source and its excitation energy is controlled by the rare gas which is employed. The metastable beam source can be used routinely and provides sensitivities of the same order as those obtained by electron impact. Preliminary results involving the analysis of aromatic and aliphatic compounds with four different rare-gas metastable atom beams (krypton, argon, neon and helium) are presented in order to show the tunable fragmentation and selective ionization capabilities of the metastable source. Thus, this source can be used to gain fundamental information on the physical properties of polyatomic molecules as well as the Penning ionization process itself, and for analytical-type applications such as those encountered in LC-MS and desorption-ionization. Overall, the results presented support the potential of the metastable beam source as a new tool for a wide range of mass spectral experiments.

Stimulated Raman scattering at 53.7 nm in a singlet metastable He beam

Stimulated Raman amplification at 53.7 nm wavelength is obtained in a pulsed He beam with population inversion between the metastable 2 1S 0 state and the ground state.