Adiabatic Raman passage via two-photon resonant transitions in a five-level Λ system

We investigate a five-level Λ system for achieving the efficient population transfer between two ground-state levels without essentially populating other three intermediate excited-state levels. This five-level Λ system is found to have an approximate dark state characterized by very small (but not exactly zero) absolute eigen-values when we work in the parameter regime where all single-photon pump and Stokes transitions are well suppressed. Dynamically manipulating the approximate dark state in a proper way, we can either attain the complete population transfer from one ground-state level to another ground-state level or create the maximal coherence between the two ground-state levels. The dynamic process of a stimulated Raman adiabatic passage is implemented in fact by manipulating a two-photon resonant pump transition and a two-photon resonant Stokes transition, which is especially important when the two ground-state levels differ by Δm = ± 4 in the magnetic quantum number.     

Search for correlated two-photon emission from e+(82Sr)+Th interactions

A previously observed electron line at INS at 330.3±0.4 keV in e⁺+Th interactions has been implied by these authors to originate from the decay of an unknown neutral object X⁰. QED predicts a two-photon decay mode of X⁰ leading to a correlated two-photon coincidence peak at 841.3±0.4 keV. To search for such a peak γγ-coincidence experiments were performed. From an observed peak-like structure in the spectrum at 841.7±0.3 keV and an associated cross section of 10.6±3.1 (stat) ± 6.9 (syst) μb an upper limit for the cross section of 15.2±1.5 (syst) μb for the production and 2γ decay of the X⁰ was deduced with a statistical confidence level of 95%.     

Effect of a near-resonant intermediate level on doppler-free versus doppler-broadened two-photon absorption

The effect of a near-resonant intermediate level on the ratio of strengths of Doppler-free and Doppler-broadened two-photon absorption has been studied for the 16 601.88 cm^-1 Na₂ two-photon transition.     

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.     

The break of L-S coupling and the double Stark resonance in the spectrum of 36P → 37P two-photon transition in Rydberg atoms of sodium

New results are presented of an experimental investigation of the spectrum of 36P → 37P two-photon microwave transition in Rydberg atoms of sodium in a constant electric field. Depending on the conditions of excitation of the initial 36P state (the constant electric field is switched on either before or after the exciting laser pulse) and polarization of laser radiation, a strong variation is observed of the amplitudes of individual two-photon transitions between the fine-structure Stark components of the 36P and 37P states. This effect is an analog of the Paschen-Back effect in a strong magnetic field and is due to the break of L-S coupling and to the variation of the wave functions of Rydberg electrons in an electric field. It is also found that the break of L-S coupling affects considerably the shape of double Stark resonance arising upon intersection of the virtual intermediate level of two-photon transition with the real intermediate 37S level.     

Two-photon exchange in p⊆)p→ℓ+ℓ−X and a comparison with QCD

A thorough study of lepton-pair production from two-photon annihilation in p?)p collisions is presented. The differential cross section is calculated over a large range of energies (27?√s?800 GeV as a function of the dilepton mass M as well as the dilepton transverse momentum Q_T and the Feynman variable x_F. No kinematical approximations (such as the equivalent photon approximation) are made. For Q_T ≈ 0 the two-photon mechanism represents an important fraction of the pp→e⁺e^?X cross section already at ISR energies, whereas at ISABELLE energies it dramatically dominates in the interval 0?Q_T?1 GeV. At ISR energies these conclusions follow from a direct comparison of the two-photon contribution with pp→e⁺e^?X data. For the ISABELLE energy range the expected O(α_s) QCD contribution to pp→?⁺?^?X, corrected for soft gluon radiation to all orders (in leading bilogarithmic approximation), was taken as a reference. At larger Q_T and ISR energies the γγ contribution is negligible, whereas at √s = 800 GeV γγ/QCD? 10–20% almost everywhere. Furthermore, two-photon candidate events from the ISR are shown to be in reasonable agreement with theory. A decomposition of the γγ cross section into contributions from both proton vertices being elastic, inelastic and of mixed configuration is given. The results provide important clues for a future isolation of the two-photon mechanism.     

Stepwise ionization of the 2,4-dioxybenzaldehyde and 3,4-dimethoxypropiophenone vapors

The processes of stepwise ionization of the 2,4-dioxybenzaldehyde and 3,4-dimethoxypropiophenone vapors by radiation in the range of wavelengths down to 266 nm were studied using total-current spectroscopy, mass spectrometry, and photoelectron spectroscopy. The process of two-step ionization and the resulting generation of molecular ions are dominant if the laser-radiation intensity amounts to ≈10⁶ W/cm². These molecular ions have appreciable (up to 1.8 eV) energy of vibrational excitation. As the laser-radiation intensity increases, progressively more pronounced fragmentation occurs owing to dissociation of molecular (and, possibly, fragmentation) ions as a result of absorption of at least a single additional photon. The processes leading to fragmentation of ions are suggested. It is found that the dissociation-ionization mechanism is important for the dimethoxypropiophenone. Dissociation with a breakage of the α bond in the carbonyl group with subsequent two-photon ionization of the fragments occurs when the S ₂(¹ A′) state of the ππ* type is excited.     

双光子共振非简并四波混频测量Ba原子里德伯态的碰撞展宽和频移

采用双光子共振非简并四波混频测量了Ar缓冲气压引起的Ba原子里德伯6snd ¹D₂谱线系的碰撞展宽和频移, 计算了n=16---33的碰撞展宽截面和频移截面. 本方法是一种纯光学的测量技术, 当采用窄带激光器时可以获得里德伯态的消多普勒光谱. 与传统实验方法测量到的里德伯态纵向弛豫的碰撞展宽不同, 本方法可以研究碰撞引起的两能态间横向弛豫的展宽.     

The controlled excitation of forbidden transitions in the two-photon spectrum of strontium by using collisions and electric fields

We present experimental results involving controlled configuration mixing in two-photon spectroscopy of highly-excited states by exploiting a weak external electric field and collisions. The method has allowed new extensions to high members of the two-photon forbidden J = 3 odd-parity 5snf ¹F₃ and the J = 0, even-parity 5sns ¹S₀ Rydberg series of neutral strontium to be observed. We achieve resonant two-photon transverse excitation of a high density atomic jet by using a narrow bandwidth tunable dye laser in a heat pipe setup with sensitive ionization detection. Experimental term values are extended for the 5sns ¹S₀ series up to n = 46. By suitable exploitation of the composition and pressure of the buffer gases in conjunction with the electric field strength in the excitation region and the exciting laser beam intensity we have also extended observations up to n = 44 for the 5snf ¹F₃ series and up to n = 46 for the 5snp ¹P₁ series. Our results demonstrate a novel and remarkably simple experimental method to access high Rydberg states to which transitions are forbidden from the ground state by parity and other selection rules.     

Two-photon Rydberg resonances in lithium-7 obtained by recording reduction of resonance fluorescence

In a magneto-optical trap (MOT) of ⁷Li, two-photon Rydberg resonances are recorded by using a spectroscopic technique, which based on variation in the resonance fluorescence. The first excitation frequency is detuned by 0.59 GHz from the intermediate 2P _3/2 state. Two counter-propagating laser beams are used. The resonances are studied on two-photon transitions 2S _1/2 ? nl in a range of principal quantum number n from 38 to 120. The widths of the observed resonances are varied from 4.4 to 13 MHz for different MOT parameters.     

Lifetime Measurements of the DΠ State of HfS and the bΠ1 State of HfO

Lifetime measurements of individual rotational levels in excited states of hafnium oxide and hafnium sulfide have been performed using population probing by resonant two-photon ionization in a molecular beam. For the b³Π₁ state of HfO rotational levels in the vibrational level v=0 were found to have lifetimes in the region 650-700 ns. For the D¹Π state of HfS rotational level lifetimes in the vibrational levels v=0 and v=1 were determined to be around 240 ns. The technique used here provides very accurate results and the uncertainty in the reported values is only about 2%. Possible sources of errors are examined. The absorption oscillator strengths from the ground state are calculated.     

Precision two-photon spectroscopy of alkali elements

In this paper, we have briefly reviewed the work on two-photon spectroscopy of alkali elements and its applications. The technique of Doppler-free two-photon spectroscopy is briefly summarized. A review of various techniques adopted for measuring absolute frequencies of the atomic transitions and precision measurements of isotope shifts and hyperfine structures (HFS) is presented. Some of the recent works on precision measurements of HFS constants of 6s ² S _1/2 level of ³⁹K and ⁴¹K, 9s ² S _1/2level and 7d ² D _3/2 level of ¹³³Cs are also discussed.     

Lepton and pion pair production in γγ collisions measured near the threshold at DCI

This paper reports a complete analysis of data taken at DCI to measure lepton and pion pair production close to the threshold in two-photon processes: e⁺e⁻→e⁺e⁻(e⁺e⁻, μ⁺μ⁻,π⁺π⁻). Preliminary results have been previously published including one-half of the total statistics. Final results presented here are in good agreement with QED for lepton pair production. The measured cross section for pion pair production is twice as large as that expected from Born terms only — a two standard deviation effect.     

Calculation of the 1s–2s two-photon excitation cross-section in atomic hydrogen

The two-photon excitation cross-section of atomic hydrogen is calculated using explicit summation over intermediate states within the framework of dipole approximation. The matrix element for two-photon excitation is transformed into finite sums, consisting of the product of a radial and angular part. Nine intermediate states are employed in the calculation of the transition matrix element. The two-photon excitation cross-section obtained for the transition 1s ²S_1/2–2s ²S_1/2in atomic hydrogen is a good agreement with the literature.     

Selection rules for direct creation of biexcitons by giant two-photon absorption

The selection rules for direct creation of biexcitons by two-photon absorption processes are discussed on symmetry basis. A detailed analysis is performed for CuCl and CdS crystals, with the energy levels given by Bassani et al. It is found that only Г^biex₁ states can be created by giant two-photon absorption when the photons have the same frequency. The polarization dependence is given for the other cases.     

Electric field induced anomalies in frequency shift and collisional perturbations in the 5snd D2 Rydberg series of strontium in a heatpipe setup with ionization detection

We have examined the effect of weak DC electric field (2-20 V/cm) complimented by foreign gas collisions on the bound J = 2 even-parity 5snd ¹D₂ Rydberg states of neutral strontium. We use resonant two-photon transverse excitation, employing a narrow bandwidth tunable dye laser and an atomic jet in a heatpipe setup with ionization detection. In this paper we report certain anomalies in the observed spectra covering principal quantum number range n = 27-42 indicating a frequency shift reversal with nearly quadratic dependence on the field strength above an anti-resonance region. Furthermore, we have observed the emergence of highly localized doubly-excited 4d² states, including a remarkably broad perturber with large angular momentum, uncovering orbital contraction effect. This non-Rydberg excitation, which intrudes upon the two-photon spectrum with large energy overlap is due to single-photon excitations from the 5s5p ¹P₁ resonance level following molecular dissociation of the Sr₂ dimers suitably governed by binary atomic collisions. Our study which involves laser excitation complimented by electric field and collisions using inexpensive home-made setup opens up the possibilities for a new class of experiments, with considerable simplicity in the choice of excitation schemes for both Rydberg and non-Rydberg transitions, to reach states lying at high energies which cannot otherwise be accessed from the ground state due to parity and selection rules, while allowing one to probe localization properties of atomic wave functions.     

In vivo monitoring of multiple circulating cell populations using two-photon flow cytometry

To detect and quantify multiple distinct populations of cells circulating simultaneously in the blood of living animals, we developed a novel optical system for two-channel, two-photon flow cytometry in vivo. We used this system to investigate the circulation dynamics in live animals of breast cancer cells with low (MCF-7) and high (MDA-MB-435) metastatic potential, showing for the first time that two different populations of circulating cells can be quantified simultaneously in the vasculature of a single live mouse. We also non-invasively monitored a population of labeled, circulating red blood cells for more than two weeks, demonstrating that this technique can also quantify the dynamics of abundant cells in the vascular system for prolonged periods of time. These data are the first in vivo application of multichannel flow cytometry utilizing two-photon excitation, which will greatly enhance our capability to study circulating cells in cancer and other disease processes.     

Two-photon processes at very high energies

Formulae are presented by which a numerically stable computer program for two-photon processes above $\text{s}\text{= 100}\text{GeV}$ can be constructed. For this purpose a new method for evaluating matrix elements is introduced. It is then shown that two-photon processes should be no background to Drell-Yan signals in pp collisions. A comparison is made between two-photon physics at e⁺e^?, ep and pp collisions.     

Fine structure of two-photon absorption in CdS

We have observed the fine structure of the P-excitons (A, n = 2) in Cds through two-photon absorption. The strengths of absorption peaks of P-excitons and their dependence on the polarization of the light agree with predictions of a four-band model of two-photon absorption. Additional two-photon absorption peaks were identified as phasematched two-step two-photon processes involving second harmonic polarizations.     

Coupled-channels analysis of energy-dependent isospin violation in the 12C(d, α)10B(1.74 MeV,T = 1) reaction

We have studied the isospin non-conserving ¹²C(d, α)¹⁰B(1.74 MeV, 0⁺, T= 1) reaction at several incident energies of 9 ≦ E_d ≦ 16 MeV in terms of a coupled-channels method. The reaction processes involved in the present analysis are the successive single-nucleon pick-up processes as well as the inelastic scattering of deuterons from ¹²C. It is assumed that the isospin violation should occur in the intermediate mirror cluster states of ³He + ¹¹B and t + ¹¹C, due to the Coulomb interaction. The calculation reproduces fairly well the observed features of the reaction, i.e. the decreasing cross section with increasing incident energy, and the variation of the angular distribution. We also note that the calculation shows the energy-dependent localization of isospin violation in the angular momentum space, i.e. a specifically narrow localization at the lower incident energies studied and its broadening at the higher energies. This fact is associated with the variation of the angular distribution from a forward-backward symmetry at the lower incident energies to a forward peak at the higher energies.