Double resonances upon three-photon ionization of the samarium atom

The manifestation of double resonances upon the three-photon ionization of the samarium atom has been studied in a frequency range of 17200–18400 cm⁻¹. Thirty peaks caused by double resonances have been found in the dependence of the yield of Sm⁺ ions on the laser radiation frequency. The majority of these peaks greatly exceed in amplitude the peaks associated with ordinary two-photon resonances. The influence of double resonances on the three-photon ionization has been theoretically analyzed using the density matrix approximation. The calculations show that the probability of the three-photon resonance ionization can increase by more than two orders of magnitude due to the additional one-photon population of an intermediate level. The double resonance slows down the rate of increase in the probability of ionization with an increase in the laser radiation intensity compared to ordinary two-photon resonance.     

One- and two-photon resonance fluorescence of a three-level atom at high photon densities

We consider the possible physical processes that may arise in a three-level atom when only two of its levels interact with a strong electromagnetic field and when the atomic transition frequency is nearly equal to once and twice the frequency of the laser field, respectively. There have been found pronounced cooperative effects in the spectrum of the two-level system, which is in resonance with the laser field, arising from the presence of the third level. The excitation spectra describing the transitions from the first excited state into the second excited state and from that to the ground state consist, apart from the two central peaks, of two pairs of sidebands which are induced by the laser field of the neighbouring system. Detailed expressions of the spectral functions for the physical processes of one- and two-photon resonance fluorescence have been derived and discussed in the limit of high photon densities. The excitation spectrum of the low frequency modes has been considered and discussed in detail. It is found that quantum beats in spontaneous emission may appear in the spectra of the one- and two-photon resonance fluorescence arising from the interference between the two atomic transition frequencies and the frequency of the laser field. The importance of the low frequency modes that occur in the processes in question has been pointed out.     

CS_2分子激发态的紫外多光子电离——双色激光单共振离化的功率密度关系

本文报道气相CS_2紫外双色激光单共振离化的功率密度关系和对高激发态(1~Ⅱg),共振离化特性的研究结果,实验指出通过中间共振态(1~Ⅱ_g)的双光子共振激发三光子离化具有比通过中间共振态((?)A_2)的单光子共振激发三光子离子为高的总离化率,在10~7W/cm~2的功率密度下,观察到饱和效应.实验显示了双色激光单共振离化功率密度关系的研究提供了一种新的光谱学分析方法.这对研究分子高激发态的结构是很重要的.     

Two-photon interference spectra at the sum-frequency generation of a bichromatic field interacting with a three-level atom

We have calculated the spectral functions for a number of induced excitations near the sum-frequency generation of a bichromatic field interacting with a three-level atom in the cascade configuration. Although the two-photon spontaneous emission is electric dipole forbidden, there are induced peaks whose subnatural line widths and spectral heights depend on the intensities of the bichromatic field. At low intensities of both laser fields, the maximum heights of the induced peaks take positive, zero, and negative values, indicating that the physical processes of absorption (attenuation), dark resonance (nonabsorbing state), and stimulated emission (amplification) are likely to occur at the sum-frequency generation of the two laser fields, respectively. The derived results are graphically presented and discussed. PACS 32.80.-t; 42.50.Hz     

Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (～24.1 eV, ～5 TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr?dinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.     

The effect of field strength on the resonance structure of three-photon ionization spectra of the samarium atom

Amplitude ratios of resonance maxima caused by two-photon transitions from different levels of the ground term of the samarium atom are studied experimentally and theoretically as functions of the laser-radiation field strength. The results of these studies are shown to be promising for identifying the resonance structure of three-photon ionization spectra of the samarium atom. A not yet observed excited even-parity state with energy E ≈ 36789.1 cm^?1 and hypothetical values of total angular momentum J = 5, 6 has been discovered.     

Ionization of a hydrogen atom from its ground state by a coherent bichromatic laser field

We study the “coherent phase control” between the three-photon ionization by a fundamental laser field and the one-photon ionization by its third harmonic for a hydrogen atom in its ground state. The relative phase δ of the harmonic field with respect to the fundamental laser radiation “modulates” the interference between the two ionization channels, which is important near the crossing points between the ionization rates of the two individual processes. Numerical results for the total ionization rate and for the angular distribution of the photoelectrons as a function of the phase δ are presented for frequencies located in the vicinity of the atomic resonances corresponding to the absorption of two laser photons. Received 31 August 2000 and Received in final form 6 February 2001     

Lineshape asymmetry for joint coherent population trapping and three-photon N resonances

We show that a characteristic two-photon lineshape asymmetry arises in coherent population trapping (CPT) and three-photon (N) resonances, because both resonances are simultaneously induced by modulation sidebands in the interrogating laser light. The N resonance is a three-photon resonance in which a two-photon Raman excitation is combined with a resonant optical pumping field. This joint CPT and N resonance can be the dominant source of lineshape distortion, with direct relevance for the operation of miniaturized atomic frequency standards. We present the results of both an experimental study and theoretical treatment of the asymmetry of the joint CPT and N resonance under conditions typical to the operation of an N resonance clock.     

Two-photon hybrid resonance laser and photodissociation laser by two-photon pumping covering a wide wavelength region in the potassium vapor

This paper reports the results about the laser generation based on the two-photon hybrid resonance and on the photodissociation by two-photon pumping covering a wide wavelength region respectively. In the former, 3.66, 3.64, 3.16, 3.14, and 2.71 μm laser lines were obtained by collision energy transfer from a potassium dimer in an excited state to the potassium atom in the ground state, and then followed by the resonance excitation of the potassium atom from the 4P to 6S state. In the latter, 2.71, 3.14, and 3.16 μm dissociation laser lines were obtained by two-photon pumping of the potassium dimer covering a wide pumping range about 650 Å at the red wavelength region.     

Stimulated Raman adiabatic passage in fields with stochastic amplitudes

A theoretical analysis is presented of the effect of correlation between fluctuations of laser pulse amplitudes on population transfer between the states of a three-level atom coupled by the laser field. The carrier frequencies of the pulses are tuned to resonance with the transitions between the ground and excited states, |〈 and | 2〈, and the excited and metastable states, |2〈 and |3〈, in a lambda-type configuration. The laser pulses are timed so that population transfer between states |1〈 and | 3〈 is made possible by stimulated Raman adiabatic passage (STIRAP) in the absence of fluctuations. STIRAP does not occur when the laser fields are not correlated. When the fluctuations of one pulse amplitude duplicate those of the other, STIRAP can be observed for pulse amplitudes larger than those required in the absence of fluctuations.     

Tracking interacting dark states through higher order absorption resonances

A three photon resonance arising due to coherent population trapped (CPT) states in multi-level systems, is experimentally shown to be a powerful spectral marker to detect interacting CPT states. In systems showing N type or double Λ type level configurations, these absorption resonances can be used to identify spectral positions of maximal interactions between competing CPT ground states. The contrast of the absorption resonance serves to identify even partially destructive interactions between the CPT states, eliminating the need for strong resonant changes of ground state coherence for identification. We demonstrate this effect in a room temperature, gaseous collection of ⁸⁷Rb. atoms. Three laser fields interact with a double Λ configuration in the Zeeman degenerate levels of the ground state 5S_1/2S_{1/2}, F = 1 and those of the excited states 5P_3/2P_{3/2}, F = 0,1, around the D2 line. The three-photon resonance is studied in the counter-propagating third field when the other two co-propagating fields satisfy the two-photon resonance condition necessary for creation of CPT states. We envisage that this absorption feature in the third field, can become a veritable tool to quantify degradation of CPT induced effects in engineered quantum states using multi-level systems.     

Ultrafast coherent population transfer driven by two few-cycle laser pulses

We investigate ultrafast coherent population transfer driven by few-cycle pump and Stokes laser pulses in the Λ-type three-level system with the stimulated Raman adiabatic passage technique beyond the rotating-wave approximation. In contrast to the case with the rotating wave approximation, the most efficient population transfer may be realized without the satisfaction of the one-photon resonances or two-photon resonance and the transfer efficiency depends critically on the Rabi frequencies and initial optical phases of the two laser fields when the peak Rabi frequencies are much larger than the respective transition frequencies. Moreover, complete and robust population transfer can still be obtained with the variations of the Rabi frequencies, pulse durations, and one-photon or two-photon detuning in a moderate range, though a considerable transient population may reside in the excited state. These abnormal behaviors result from the counterrotating terms, which are not taken into account in the traditional rotating wave approximation.     

Optical pumping and coherence effects in fluorescence from a four level system

An effective four-level system around the D2 line of ⁸⁵Rb at room temperature, is experimentally investigated by fluorescent studies under the action of two driving fields L1 and L2. This system exhibits unique features in fluorescence as a function of frequency separation between L1 and L2. In particular, at two-photon resonance, when the Rabi frequency of L1 exceeds that of L2, signatures of Electromagnetically Induced Transperancy effect (EIT) arising from the three-level Λ sub-system is present as a sub-natural dip in fluorescence from the fourth level. At comparable strengths of L1 and L2 the fluorescence features indicate a regime, where the effects arising from optical pumping and EIT effect due to ground hyperfine level coherence coexist. We see in the coexistence regime, saturation effects arising from difference frequency crossing (DFC) resonances and optical pumping around the EIT window. At low strengths of L1, all signs of coherence vanishes from the system and the fluorescent features result from incoherent optical pumping through the Autler-Townes split states of the excited state hyperfine levels, which are split due to the stronger L2 laser. The dominant role of the L1 laser in creating a robust transparency signal even in the presence of an off-resonant excitation is brought out. The results are supported by density matrix calculations.     

Investigation of highly excited states of samarium

Highly excited even-parity states of the samarium atom in the range between 34104 and 39155 cm⁻¹ have been studied by the method of one-color three-photon resonance ionization spectroscopy. The energies and total angular momenta of 342 states have been determined. Seven new even-parity states of samarium have been discovered.     

Optical Bell state and Greenberger-Horne-Zeilinger-state analyzers through the cavity input-output process

With the ancillary one-sided cavities each trapping an alkali atom, the schemes for the analyzers of two-photon Bell states and three-photon Greenberger-Horne-Zeilinger (GHZ) states are proposed, respectively. Moreover, all of two-photon Bell states and three-photon GHZ states can be nondestructively distinguished. The influence of atomic spontaneous emission and output coupling inefficiency are discussed.     

Gd原子的共振多光子电离研究

本文首次报道了用共振多光子电离(RMPI)技术探测到四条在可见光范围内的Gd原子强双光子跃迁线.观察到双色三光子Gd原子激光同位素分离.研究了高分辨率Gd原子电离谱,并解释了电离谱的线型.     

密度泛函理论下氯普鲁卡因分子的能级结构和光谱计算

本文基于密度泛函理论,采用B3LYP方法,在6-31G(d,p)基组上对麻醉剂氯普鲁卡因的分子结构进行几何优化,在此基础上以乙醇为溶剂计算分子的前20个激发态,所有计算在Gaussian 09W-D01中进行。利用Multiwfn3.7软件绘制红外光谱图,并对其分子振动进行分析;利用Origin 2018 64Bit软件和Multiwfn3.7软件相结合绘制紫外光谱图,并计算空穴-电子来分析分子的激发态性质;通过计算前线轨道来预测氯普鲁卡因分子的活性位点。结果表明,在所计算得到的激发态中,由基态到第2、3、6、10激发态为局域激发,由基态到第19激发态为电荷转移激发。氯普鲁卡因乙氨基上的N22为亲电反应位点,苯环上的碳原子和脂基上的氧原子为亲核反应位点。本研究对更好的了解氯普鲁卡因分子的反应机理和在医学上的麻醉活性提供理论参考。     

Effect of four-photon interactions on coherent population trapping in Λ-systems

The resonance fluorescence spectrum of a Λ-system excited by two resonant light fields is calculated using a Markov analysis. Analytical formulas are derived in the strong-field limit within and beyond the rotating wave approximation. It is shown that the resonance fluorescence of the system does not vanish during coherent population trapping. Its spectrum consists of two multiplets which are similar to a triplet in the resonance fluorescence spectrum of a two-level atom and lie at the electronic transition frequencies, together with two triplets located at the frequencies of four-photon processes involving the optical excitation fields. The latter are fundamental in character and impose limits on the lower bound of the dephasing rate for the Raman resonance owing to the effect of radiative decay of the dipole transitions on the dynamics of the ground state. The effect of four-photon dephasing on the absorption spectrum of a Λ-system is analyzed and found to lead to a substantial reduction in the depth of a dip in the absorption spectrum which vanishes as the laser field strength is increased. Zh. éksp. Teor. Fiz. 113, 144–167 (January 1998)     

Spontaneous Raman scattering upon resonant three-photon ionization of a samarium atom

The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm⁺ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f ⁶6s ^{2 7} F ground term, exhibits a strong peak due to single-photon excitation of the 4f ⁵5d6s ^{2 7} D°₃ state from the 4f ⁶5d6s ⁹ H ₂ excited state. The 4f ⁶5d6s ⁹ H ₂ state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.     

Time-resolved two-color interferometric photoemission of image-potential states on Cu(100)

We describe an interferometric time-resolved photoemission technique that makes it possible to simultaneously observe the decay of optical induced polarizations and populations at surfaces in a two-color excitation scheme. In this scheme initially unoccupied electronic surface states are coherently excited by the interaction of laser pulses with frequency ω_a and the two-photon polarization which is induced by laser pulses with frequency ω_a/2. Interference is observed by changing the delay between both laser pulses using an actively stabilized two-color Mach–Zehnder interferometer. We demonstrate this technique for excitation of the n=1 image-potential state on a Cu(100) surface. PACS 78.47.+p; 79.60.Bm; 73.20.-r; 82.53.Kp; 42.50.Md