Narrow-band UV radiation (250–260 nm) from intracavity doubling a single-mode ring dye laser

Generation of continuous-wave, tunable UV radiation (250–260 nm) by intracavity doubling a coumarin-515 ring dye laser is described. A cooled (200–280 K) ADP crystal with end faces cut at Brester's angle is placed inside the laser ring cavity which has been compensated for astigmatism and coma. UV output powers at 254 nm of 120 μW and 60 μW are achieved with the laser operating multimode (bandwidth ? 20 GHz) and single-mode (bandwidth ? 50 MHz), respectively. Continuous single-mode scans over the 253.7 nm mercury profile demonstrate sub-Doppler resolution of the Hg 6s6p ³P^O₁ - 6s²¹S₀ transition.     

Tunable frequency-controlled laser source in the near ultraviolet based on doubling of a semiconductor diode laser

Continuously tunable ultraviolet laser radiation at 397 nm was generated by doubling the output of a semiconductor diode laser. The fundamental radiation was provided by a 150 mW AlGaAs laser diode injected by a low-power AlGaAs laser diode which was frequency stabilized by optical feedback using a new scheme of a miniature external cavity. Second-harmonic generation was produced in a lithium-triborate crystal placed in a compact enhancement cavity. The fundamental radiation was used for sub-Doppler spectroscopy of the Ar I 4s ³ P ⁰ ₀–4p ¹ P ₁ transition at 795 nm; the second-harmonic radiation was used for spectroscopy of the Ca II 4² S _1/2–4² P _1/2 transition at 397 nm.     

Oscillator strength measurements of the 4s5s 3S1 → 4snp 3Po2 Rydberg transitions of zinc

In the present experimental work, oscillator strengths of the 4s5s ³S₁ → 4snp ³P^o₂ (18?≤?n?≤?53) Rydberg transitions of zinc are reported. The stepwise laser excitation from the ground state 4s^{2 1}S₀ was achieved using three dye laser beams simultaneously pumped by the second (532?nm) and third (355?nm) harmonics of a Q-switched Nd: YAG (Neodymium-doped yttrium aluminum garnet) laser. The vapor containment and detection system was a single wire thermionic diode ion detector operating in space charge limited mode. A trend of the above-mentioned f-values from n?=?18 to 53 versus the principle quantum number n has been presented. Furthermore, continuity has been verified between the discrete f-values and the oscillator strength density at the threshold.     

Generation of cw 243 nm radiation and application to laser spectroscopy of the strontium isotopes

Intra-cavity second harmonic generation in a ring dye laser using lithium formate monohydrate has been observed to produce tunable cw UV radiation around 243 nm with single frequency powers of up to 1.4 mW. This wavelength region is of particular interest in the context of Doppler-free spectroscopy of atomic hydrogen. The radiation was used to excite an atomic beam of strontium to study the transition 5s^{2 1}S₀-4d5p ¹P₁ of Sr I. The isotope shifts were determined as follows: ⁸⁸Sr-⁸⁷Sr 123.7(8) MHz; ⁸⁸Sr 285.0(8) MHz; ⁸⁸Sr-⁸⁴Sr 600.1(9) MHz. The values are compared with results for other lines and muonic isotope shifts to obtain the specific mass shift and field shift in this transition and to study the configuration mixing effects. The hyperfine structure of ⁸⁷Sr was not resolved, the intervals being very much less than the measured linewidth of 25 MHz.     

Resonant generation of even-order harmonics in metal vapors

Generation of the second- and fourth-order harmonics of laser radiation is reported in Mg vapor at the forbidden 3s ²¹ S _o–4s ¹ S _o transition. The conversion efficiency for the second-order harmonics has been measured to be 10^–2% and for the fourth one 10^–8%.     

Measurement of cross-section of excitation transfer from 3s2 to 4s1 and 5s1 energy levels of neon

Collision induced non-radiative transitions in neon plasma have been studied using high intra-cavity radiation field of a 633 nm He-Ne laser. The transitions, induced from 3s ₂ energy level to 4s₁ and 5s₁ groups of energy levels, have been detected as changes in intensities of the spectral lines originating from these energy levels. From these intensity measurements, the quantities governing the transitions i.e. (i)S ₃ ^e /S _3RT , the ratio of the probabilities of electronic deexcitation to the total radiative deexcitation of energy level 3 (ii) 〈r ₂₃ ^e v _e〉, rate of excitation transfer per particle and (iii)S ₂₃ ^e , the total probability for excitation transfer from level 2 to level 3 at a certain value of electron density have been calculated.     

Observation of the radiative 2s02p6−2s12p5 transition in NeIII

Ion impact excited spectra of Ne contain a line at 52.82 nm which has been identified as resulting from the decay of double 2s vacancies in NeIII via the transition 2s⁰2p⁶(¹S₀)?2s¹2p⁵(¹P₁).     

Saturation of resonant frequency conversion processes in cadmium vapour

We have investigated two-photon resonant up conversion of 375 to 159 nm coherent radiation using the four-frequency process ₁₅₉=2₃₇₅+₁₀₄₀ in cadmium vapour, where the ir component (1040 nm) is internally generated by the 375 nm pump tuned to the Cd two-photon transition 5s ^{2 1} S ₀5s6s ¹ S ₀. Scaling laws and tuning behaviour of both the 159 and 1040 nm output power were measured up to 1×10⁸ W cm^–2 pump intensity and 2×10¹⁷ cm^–3 Cd number density. The results are compared to numerical calculations based on semiclassical theory in a stationary perturbation approximation up to third order. Here, in accordance with experimental results, the ir component was assumed to originate from a superposition of stimulated emission and parametric generation. The observed saturation effects turned out to be mainly due to population changes of the atomic levels involved and are qualitatively reproduced by calculations. Discrepancies with respect to absolute output levels are discussed in terms of the approximations in the theoretical formalism.     

Laser-enhanced ionization studies of the blue satellite band in the Ca−Ar system associated with the electric-quadrupole transition 4s 21 S 0→4s3d 1 D 2 of calcium

Summary The collision-assisted electric-quadrupole transition 4s ²¹ S ₀→4s3d ¹ D ₂ of calcium at 4575 ? has been observed using the laser-enhanced ionization technique for the first time. A blue satellite band due to the interaction of calcium 4s3d ¹ D ₂ state with argon¹ S ₀ has also been detected at 4527 ? which is shifted by 48 ? from the isolated atomic calcium line. The line broadening cross-section for the blue satellite has been calculated by measuring its collisional broadening at various argon pressures. Important processes involved in the generation of this blue satellite band are qualitatively discussed. The authors of this paper have agreed to not receive the proofs for correction.     

Radiation improsonment of the 1s4 resonance line in xenon

The decay of the imprisoned 1s₄ → ¹S₀ resonance radiation has been measured with an absorption method. A natural lifetime τ = 4.3 ± 0.5 ns is obtained for the 1s₄ resonance state in xenon. No evidence was found for the high collisional de-excitation measured by other authors [1].     

Nonlinear behaviour of atomic fluorescence in mercury vapours following double-resonance laser excitation

When mercury atoms, present with a buffer gas (N₂ or Ar) in a quartz cell or in a graphite furnace are excited into the 7³ S ₁ level by means of two pulsed dye lasers tuned at 253.652 nm (6¹ S _o 6³ P ₁) and 435.835 nm (6³ P ₁ 7³ S ₁), a laser-like, collimated emission is observed along the axis of the cell at the green mercury line (7³ S ₁ 6³ P ₂), 546.074 nm). This radiation exhibits a pronounced non-linear dependence upon the number density of the mercury atoms in the cell. This behaviour is interpreted as being due to amplified spontaneous emission (ASE), which occurs as a result of the transient population inversion between the 7s ³ S ₁ and 6p ³ P ₂ ^o levels. The parameters governing the population inversion can be deduced by applying a rate equation analysis to the atomic system. A time-resolved observation of both ASE and spontaneous fluorescence signals confirms the totally different characteristics of the two emission processes.     

Collisional broadening of Ba I line (553.5 nm) by He or Ar

The Doppler-free line shape of the 6s ^{2 1} S ₀ – 6s ¹6p ^{1 1} P ₁ transition at 553.5 nm in natural Ba in the presence of an argon or helium buffer gas has been measured at 744 K for gas pressures from 0.2 to 100 Torr. Using the measured data for pressures above 5 Torr, the broadening rate coefficients for the half width at half maximum (HWHM) are determined to be (4.9±0.5)10^–9 and (5.0±0.5)10^–9 s^–1 cm^–3 for helium and argon respectively.     

锶原子三重态谱线的观测及在光钟中的应用

实验中通过互组跃迁689 nm激光抽运形成三重态最低能态原子布居,引入688 nm激光改变三重态最低能态间的原子布居,利用抽运光与探测光空间分离的方法观测碱土金属锶原子的三重态能态间跃迁(5s6s)³S₁ → (5s5p)³P_j(j=0,1,2)的吸收谱线,对应三条跃迁线的激光波段为679 nm, 688 nm和707 nm.探测三重态原子跃迁谱线可以用于锶原子冷却中再抽运光707 nm和679 nm激光频率的直接锁定,相比于通常利用的腔传递技术,可以把再抽运光频率锁定在原子跃迁谱线上,有利于提高锶原子冷却中俘获原子数目的长期稳定性.     

Resonance-enhanced multiple-photon ionisation spectroscopy of excited sulphur atoms

Sulfur atoms are produced in the excited 3s ²3p ^{4 1} D ₂ and 3s ²3p ^{4 1} S ₀ levels by two-photon photodissociation of CS₂ in the gas phase in the region 285–305 nm. These excited atoms are detected by three-photon (two to resonance) ionisation at selected laser wavelengths. Many new transitions have been observed which have not been reported previously. The new dark states that have been accessed by two-photon absorption have been characterised and the energy levels with respect to the ground ³ P ₂ state have been determined. Configuration interaction between 6p ³ P ₁, 6p ⁵ P ₁, and 4p¹ P ₁ states, and also between 6p ³ P ₂, 6p ⁵ P ₂, and 4p¹ D ₂ states has been observed. It is found that intermediate states reached from the ¹ S ₀ level through two-photon absorption lie above the first ionisation potential of S⁺(⁴ S ⁰). It is proposed that autoionisation into the ⁴ S ⁰+e^– continuum is a dominant mechanism in the decay of these levels, although ionisation by a further photon absorption is not ruled out.     

Enhanced four-wave mixing in mercury isotopes, prepared by stark-chirped rapid adiabatic passage

We demonstrate significant enhancement of four-wave mixing in coherently driven mercury isotopes to generate vacuum-ultraviolet radiation at 125 nm. The enhancement is accomplished by preparation of the mercury atoms in a state of maximum coherence, i.e. maximum nonlinear-optical polarization, driven by Stark-chirped rapid adiabatic passage (SCRAP). In this technique, a pump laser at 313 nm excites the two-photon transition between the ground state 6s²¹S₀ and the target state 7s ¹S₀ in mercury. A strong, off-resonant radiation field at 1064 nm generates dynamic Stark shifts. These Stark shifts serve to induce a rapid adiabatic passage process on the two-photon transition. During the process a coherent superposition of the two states is established, which enhances the nonlinear-optical polarization in the medium to the maximum possible value. The maximum coherence permits efficient four-wave mixing of a pump laser and an additional probe laser at 626 nm. The efficiency is further enhanced, as the SCRAP process allows to stimulate the complete set of different mercury isotopes to participate in the frequency conversion process. This enlarges the effective atomic density of the medium. Thus, we observe the generation of vacuum-ultraviolet radiation at 125 nm enhanced by more than one order of magnitude with respect to conventional frequency conversion. Parallel to the frequency conversion process, we monitored the evolution of the population in the medium by laser-induced fluorescence. These data demonstrate efficient coherent population transfer by SCRAP.     

199Hg+光频标的黑体辐射频移

光学频率标准会受到环境温度的黑体辐射影响发生频移,进而影响其准确度. 本文估算了¹⁹⁹Hg⁺的超精细能级5d¹⁰6s²S_1/2 (F=0)和5d⁹6s^{2 2}D_5/2 (F=2)的极化率,得到了室温(300 K)下黑体辐射引起的相对频移为-5.4×10^-17, 最后讨论了低温环境下黑体辐射对¹⁹⁹Hg⁺光频标的影响.     

Depolarization of radiation upon coherent excitation

The possibility of cancelling the signal of linear polarization upon coherent excitation of 4s ³ S ₁ atoms via a Λ scheme from 3p ³ P ₁ and 3p ³ P ₂ levels is demonstrated using an ensemble of sulphur atoms as an example.     

The beryllium atom and beryllium positive ion in strong magnetic fields

The ground and a few excited states of the beryllium atom in external uniform magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for field strengths ranging from zero up to 2.35×10⁹ T. With changing field strength the ground state of the Be atom undergoes three transitions involving four different electronic configurations which belong to three groups with different spin projections S _z = 0, - 1, - 2. For weak fields the ground state configuration arises from the 1s ²2s ², S _z = 0 configuration. With increasing field strength the ground state evolves into the two S _z = - 1 configurations 1s ²2s2p _-1 and 1s ²2p _-13d _-2, followed by the fully spin polarised S _z = - 2 configuration 1s2p _-13d _-24f _-3. The latter configuration forms the ground state of the beryllium atom in the high field regime γ > 4.567. The analogous calculations for the Be ⁺ ion provide the sequence of the three following ground state configurations: 1s^22s and 1s ²2p _-1 (S _z = - 1/2) and 1s2p _-13d _-2 (S _z = - 3/2). Received 2 October 2000 and Received in final form 8 January 2001     

Simulation of emission spectra of the gas-discharge plasma of a krypton-xenon mixture

The narrow-band radiation observed in the range of the resonance line of xenon at 147 nm in the VUV emission spectra of the gas-discharge plasma of a krypton-xenon mixture is proposed to interpret as a manifestation of bound-bound transitions between the vibrational levels of the excited electronic states 0⁺(³ P ₁) and 1(³ P ₁) and the ground electronic state 0⁺(¹ S ₀) in the KrXe* molecule. A correction of the potential curves of the electronic states under consideration is proposed from a comparison of the calculated and experimental spectra.     

Generation of sum-frequency sideband using 964 GHz HCN laser and 70 GHz klystron radiation

Continuosly tunable submillimeter radiation above 1 THz has been generated by sum-frequency mixing of HCN laser radiation (second strongest transition at 964.3 GHz) with that of klystron (70 GHz) in the Schottky barrier diode used as the non-linear element. Generated radiation was sufficiently strong to allow the high resolution frequency measurement of 9_9,1 8_8,0 transition in³²S¹⁶O.