Elastic scattering of electrons by metastable hydrogen atoms in the presence of a resonant laser field

Within the framework of the rotating wave approximation the elastic scattering of electrons by metastable 2s state of hydrogen atoms is studied in the presence of a resonant laser field. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. Variation of the cross section with laser intensity and with incident electron energy (50-150 eV) is investigated. Received: 18 July 1997 / Received in final form: 5 December 1997 / Accepted: 19 January 1998     

Lifetime of metastable fluorine atoms

For laser collimation of neutral F atoms, a resonance transition cycle between the metastable and the upper excited states (3s⁴ P _5/2?3p⁴ D ⁰ _7/2) can be used as a two-level closed system. We have determined the lifetime of the metastable state (3s⁴ P _5/2) in F atoms by measuring the decay curve of the fluorescence intensity as a function of distance from the plasma source. Combining the measured velocity of F radicals from the Doppler shift of the fluorescence peak, we have obtained the lifetime of the F metastable state as 3.7±0.5 μs. With this short metastable lifetime of F radicals, the simple Doppler cooling method using spontaneous light force is not practical for laser collimation of F radicals. Use of stimulated light force may be necessary to collimate F radical beams in a short distance. Received: 4 July 2000 / Published online: 13 September 2000     

A robust method for frequency stabilization of 556-nm laser operating at the intercombination transition of ytterbium

A frequency-stabilized 556-nm laser is an essential tool for experimental studies associated with 1 S 0-3 P 1 intercombination transition of ytterbium (Yb) atoms.A 556-nm laser light using a single-pass second harmonic generation (SHG) is obtained in a periodically poled MgO:LiNbO 3 (PPLN) crystal pumped by a fiber laser at 1111.6 nm.A robust frequency stabilization method which facilitates the control of laser frequency with an accuracy better than the natural linewidth (187 kHz) of the intercombination line is developed.The short-term frequency jitter is reduced to less than 100 kHz by locking the laser to a home-made reference cavity.A slow frequency drift is sensed by the 556-nm fluorescence signal of an Yb atomic beam excited by one probe beam and is reduced to less than 50-kHz by a computer-controlled servo system.The laser can be stably locked for more than 5 h.This frequency stabilization method can be extended to other alkaline-earth-like atoms with similar weak intercombination lines.     

锶原子互组跃迁荧光探测系统的研制

锶原子单态和三重态间的互组跃迁(5s2)1S0-(5s5p)3P1辐射率远小于一般的电偶极跃迁,共振跃迁荧光信号微弱。介绍了一种应用于探测该互组跃迁荧光谱的直流偏置探测器。该探测器选用极低输入偏置电流运算放大器作为前置放大,具有信噪比高、增益高、偏置可调等优点。此探测器探测增益为106 V/W量级,-3dB带宽为1MHz。实验中利用该探测器对锶原子互组跃迁(5s2)1S0-(5s5p)3P1微弱共振荧光进行探测,获得信噪比很好的共振荧光谱,且无直流偏置,并由此获得高信噪比的鉴频曲线。应用该探测器观测到了饱和荧光谱线以及对应的鉴频曲线,可用于689nm激光锁频,应用于锶光钟系统。     

Penning collisions of laser-cooled metastable helium atoms

We present experimental results on the two-body loss rates in a magneto-optical trap of metastable helium atoms. Absolute rates are measured in a systematic way for several laser detunings ranging from -5 to -30 MHz and at different intensities, by monitoring the decay of the trap fluorescence. The dependence of the two-body loss rate coefficient β on the excited state ( 2³ P ₂) and metastable state ( 2³ S ₁) populations is also investigated. From these results we infer a rather uniform rate constant K _sp = (1±0.4)×10^-7 cm³/s. Received 8 September 2000 and Received in final form 19 December 2000     

钡原子三重态的碰撞诱导受激辐射

在XeCl激光光泵的Ba蒸气中,首次观察到波长为551.9nm、606.3nm及611.1nm的受激辐射,它们分别对应于Ba原子的三重态跃迁:6s6d~3D_2—6s6p~3P_1、5d6p~3P_2-6s5d~3D_3及5d6p~3P_1—6s5d~3D_2.对于Ba原子的自旋禁戒跃迁6s7p~1P_1—6s6d~3D_2的可能性进行了探讨.     

A diode-laser optical frequency standard based on laser-cooled Ca atoms: Sub-kilohertz spectroscopy by optical shelving detection

We report an optical frequency standard at 657 nm based on laser-cooled/trapped Ca atoms. The system consists of a novel, compact magneto-optic trap which uses 50 mW of frequency-doubled diode laser light at 423 nm and can trap >10⁷ Ca atoms in 20 ms. High resolution spectroscopy on this atomic sample using the narrow 657 nm intercombination line resolves linewidths (FWHM) as narrow as 400 Hz, the natural linewidth of the transition. The spectroscopic signal-to-noise ratio is enhanced by an order of magnitude with the implementation of a “shelving" detection scheme on the 423 nm transition. Our present apparatus achieves a fractional frequency instability of in 1 s with a potential atom shot-noise-limited performance of and excellent prospects for high accuracy. Received 2 November 1998     

Patterning nonanethiol protected gold films by barium atoms

Self assembled monolayers (SAM) formed from nonanethiols on thin gold films were exposed to a beam of ground state and metastable neutral barium atoms through a nickel mask. The interaction of the Ba atoms with the nonanethiol layer, followed by an etching process, creates well defined structures on the gold film, with features below 100 nm. We compared the interaction of ground state Ba atoms and SAM molecules with respect to metastable Ba atoms, finding that by using metastable atoms the Ba dose per SAM molecule is reduced. The results indicate that nanofabrication in the nanometer range with barium atoms is feasible. PACS 07.77.Gx; 42.82.Cr; 81.16.Ta     

Multifrequency radiation pressure laser,isotope separation

Isotope separation by laser deflection of an atomic beam is limited in its efficiency by the accumulation of atoms in metastable states. This restriction can be removed with the use of lasers which excite metastable atoms to states from which spontaneous decay to the ground state is allowed. This is demonstrated in the separation of barium isotopes, where efficiency was improved from a lower limit of 70% to at least 83% using a second laser. Efficiency approaching 100% can be achieved in barium with a second laser but the required wavelength is not available.     

Decay rate measurement of lithium in a magneto-optical trap

A detailed account of various experimental techniques developed during the study on the decay rate coefficient of laser trapped ⁷Li atoms are presented. The frequency of a dye laser is stabilized using a simple sealed-off cell specially designed for Li vapor. The accurate number of trapped atoms are obtained by measuring the fluorescence intensity and the population ratio between the ground and the excited states by absorption coefficient measurement. The absolute value of the collisional lossrate coefficient of trapped ⁷Li atoms is determined by analyzing the temporal change of the fluorescence intensity when the supply of the Li beam is turned off.     

Energy pooling in caesium vapour:

The total rate coefficient k for the energy pooling process has been measured in dependence on the ratio of the number densities of the Cs 6P fine-structure sublevels. The measurements were performed applying the laser fluorescence method. From the set of the obtained data for k, the rate coefficients for the particular collisions were derived. The obtained values for the and collisions at T = 600 K are , and , respectively. The value for k₁ and the estimate for k₃ are consistent with the data previously published by other authors. The rate coefficient k₂ is reported for the first time. Received: 10 June 1997 / Revised: 13 October 1997 / Accepted: 5 January 1998     

Observations of four-wave mixing processes in barium vapour

Emission at fifty discrete wavelengths is observed when Ba I is excited by two laser beams, a dye laser tuned to the λ 7911 Å intercombination line, 6s²¹S₀-6s6p ³P₁ and a ruby laser. The wavelengths range from 2312 Å to 8027 Å. Most of the emission lines can be attributed to four-wave mixing processes in barium.     

利用飞行时间法测定锶原子5s5p3P1亚稳态能级寿命

本文结合TOF法和吸收光谱测量技术,利用连续光场测定了锶原子亚稳态能级的寿命。实验中,利用689nm激光连续泵浦基态锶原子至5s5p3P1亚稳态能级,然后在距泵浦区域不同距离处观测处于5s5p3P1态的原子对688nm探测光的吸收强度,得到锶原子5s5p3P1态的能级寿命,我们利用TOF法实验得到的能级寿命为21.9±0.8μs。本文给出的方法具备实验装置简单的优点,同时可获得较高的测量精度。     

低He气压下Rb(5P3/2)激发态有效辐射率的计算与测量

计算和测量了Rb-He混合蒸气中Rb(5P3/2)共振能级的有效辐射率,使用单模半导体激光器(泵浦激光)将Rb原子激发至5P3/2态,另一调谐到5P3/2→7S1/2的单模激光束(检测激光)与泵浦束反平行通过样品池,并在池的直径方向平行移动,通过对检测激光束的吸收测定了激发态原子密度及其空间分布.由于辐射陷获存在,有效辐射率是自然辐射率与透射因子(发射的光子在探测区域内没有被吸收的平均概率,它与吸收截面及激发态原子密度和空间分布有关)的乘积.5P3/2原子密度及其空间分布结合5P3/2←5S1/2跃迁线的碰撞增宽计算了透射因子,从而得到了不同He气压下,Rb D2线的有效辐射率.从5P3/2→5S1/2跃迁线强度I780的测量,得到的不同He气压下有效辐射率的比值与理论计算得到的比值相符.     

Spectral line suppression and sideband narrowing via quantum interference

We reveal that for a realistic system, interference effects are obtained such as the suppression of central line and inner sidebands and the narrowing of the outer fluorescence sidebands. For this purpose, we consider a spontaneous decay from an excited state to a metastable state when the excited and metastable states are resonantly coupled to an auxiliary metastable state by a laser field and a microwave field, respectively. The fluorescence spectrum evolves from a five-peaked structure into a doublet of ultrasharp lines as the ratio of the laser field Rabi frequency to the microwave Rabi frequency is decreased. The physical origin is presented in terms of dressed states.     

Isotope separation by laser deflection of an atomic beam

Separation of isotopes of barium has been accomplished by laser deflection of a single isotopic component of an atomic beam. With a tunable narrow linewidth dye laser, small differences in absorption frequency of different barium isotopes on the 6s^{2 1}S₀₋ 6s6p¹P₁ 5536 Å resonance were exploited to deflect atoms of a single isotopic component of an atomic beam through an angle large enough to physically separate them from the atomic beam. It is shown that the principal limitation on separation efficiency, the fraction of the desired isotopic component which can be separated, is determined by the branching ratio from the excited state into metastable states. In barium, repeated absorptions and emissions on the 5536 Å transition eventually result in decay from the 6s6p¹P₁ state to the metastable 6s5d¹D₂ state. This was observed to occur for all but 3% of the¹³⁸Ba atoms. As a result, the efficiency of separation was about 0.7 for the 8 mrad atomic beam divergence employed. (Throughput was nearly 1 mg/day. No attempt was made to maximize this value.) The isotopic purity of the separated atoms was measured to be in excess of 0.9, limited only by instrumental uncertainty. The effects of near resonant atomic scattering and excitation exchange on isotopic purity are considered. Work performed under the auspices of the U.S. Energy Research & Development Administration.     

Kinetics of Er3+-doped fluorozirconate optical fiber upconversion fluorescence and laser emission under 800 nm excitation

Applying single- and double-pulse excitation at 800 nm, the kinetics of the upconversion fluorescence in the green, as well as the upconversion laser at 543 nm was studied. No significant delay between the pumping pulse and the laser emission was found. In the erbium doped (1000 ppm) optical fiber, the mechanism responsible for the upconversion is purely of the excited state absorption (ESA) type. The double-pulse technique enables also a determination of the lifetime of the intermediate metastable state ⁴ I _11/2 (7±0.3 ms). Some other basic properties of the upconverted fluorescence and of the laser itself (fluorescence spectrum, optical gain, laser threshold) are also described. Received: 11 December 2000 / Revised version: 18 February 2001 / Published online: 18 July 2001     

Isotope shifts and hyperfine structure in the transitions in calcium II

The isotope shift and hyperfine structure in the three - transitions in Ca II have been studied by fast ion beam collinear laser spectroscopy for all stable Ca isotopes. The metastable 3d states were populated within the surface ionization source of a mass separator with a probability of about 0.1%. After resonant excitation to the 4p levels with diode laser light around 850 nm the uv photons from the transitions to the ground state were used for detection. Hyperfine structure parameters A and B for the odd isotope ⁴³Ca, as evaluated from the splittings observed, agree well with theoretical predictions from relativistic many-body perturbation theory. Field shift constants and specific mass shift constants were extracted from the measured isotope shifts and are discussed in comparison with expectation values from theory. Received: 19 September 1997 / Revised: 5 December 1997 / Accepted: 27 January 1998     

Superfluorescence from europium atom at 629.977 nm

Visible superfluorescence at 629.977 nm is observed in europium atom with very high optical conversion efficiency on the transition 5d6p ¹⁰F_7/2 → 5d6s ¹⁰D_7/2. The peak intensity of fluorescence varies as square of the number of atoms in the excited state (N), which shows the superfluorescence character of the transition. The ratio of average superfluorescence power to excitation laser power is observed to be ∼15% in the forward direction.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.