Depolarizing collision anisotropy and collision echo in ytterbium vapor

A photon echo induced exclusively by collisions of ytterbium atoms with buffer gas atoms has been observed at a 0 ? 1-type¹ S ₀(6s ²)-³ P ₁(6s6p) ¹⁷⁴Yb transition. The polarization properties of a collision echo and the buffer gas density dependence of its intensity agree with theoretical predictions of a model of depolarizing collisions that takes into account the dependence of a relaxation matrix on the velocity of active particles. Thus, direct experimental evidence of the relaxation anisotropy due to depolarizing collisions has been obtained.     

Velocity effects in atomic and molecular collisions: Study by coherent transients

The effect of translational velocity of active atoms and molecules on the properties of photon echo is investigated using the technique of coherent transient processes. A variation in the photon-echo decay with a frequency detuning of the excitation radiation relative to the center of the vibrational-rotational transition 0 ? 1 ν₃ R(4, 3) is observed in a mixture of ¹³CH₃F with atomic buffers. The results are interpreted using the dependence of the echo decay rate on the magnitude of the translational velocity of active particles. The dependence of the relaxation matrix on the direction of the velocity of active atoms results in a new phenomenon of the collision-induced echo, which is investigated at the transition 0 ? 1 ¹⁷⁴Yb in mixtures with atomic buffers.     

Photon echo generated at the transition 0–1 in ytterbium vapor

Photon echo generated at the inter-combination transition (6s ²) ¹ S ₀ ? (6s6p) ³ P ₁ of ¹⁷⁴Yb was investigated for pure ytterbium vapor and for its mixtures with atomic buffers. In pure ytterbium vapor, the polarization of photon echoes at this 0?C1 transition coincides with the polarization of the second exciting pulse for all combinations of linear and circular polarizations of exciting radiation pulses. Photon echo does not appear either for linear orthogonal or for opposite circular polarizations of exciting pulses in pure ytterbium. In mixtures of ytterbium with atomic buffers (Kr, Xe), collision induced photon echo arises only for exciting pulses of linear orthogonal polarizations, its power is essentially less than that of the ordinary echo generated by pulses with parallel polarizations in the same mixture. Polarization of collision induced echo is linear, and it coincides with polarization of the first exciting pulse. Experimental results agree with calculations, and they confirm that the collision induced photon echo at this transition arises exclusively due to anisotropy of depolarizing collisions.     

Effect of the translational velocity of active particles on the properties of a photon echo

The photon echo method was used to detect an increase in the relaxation rate in the vibrational-rotational transition of fluoromethane in ¹³CH₃F gas and a ¹³CH₃F-argon mixture upon an increase in the velocity of active particles. For a photon echo in the 0–1 transition in a Yb + Xe mixture, the dependence of the relaxation parameters on the direction of the translational velocity of Yb leads to qualitative changes and a collision photon echo arises.     

Depolarizing collisions in ytterbium vapor: Isotropic and anisotropic relaxation

Isotropic depolarizing collisions are studied using a stimulated photon echo with a specific polarization of the excitation radiation pulses in a mixture of ytterbium with krypton for the J = 1 ? J = 0 transition of ¹⁷⁴Yb. The difference between the relaxation rates of orientation and alignment γ _b ⁽²⁾ ? γ _b ⁽¹⁾ of the ³ P ₁(6s6p) ¹⁷⁴Yb level is measured as a function of the krypton pressure. The collision photon echo at the J = 1 ? J = 0 transition induced by the anisotropic relaxation is studied for the Yb + Xe mixture. The power of the collision echo increases from zero with the addition of a buffer gas to ytterbium, reaches an optimal level, and decreases with an increase in the buffer gas pressure. The polarization of this collision-induced echo differs from the polarization of the conventional echo. The experimental results are in qualitative agreement with theoretical predictions.     

Study of the rates of collisional decay of population, orientation, and alignment by stimulated photon echo in a molecular gas

A stimulated photon echo technique with specially selected linear polarizations of the coherent resonant driver pulses is used to study depolarizing collisions in the molecular gas SF₆ and in mixtures of it with buffer He and Xe. The collisional decay rates of the population, orientation, and alignment in an ensemble of gas particles are determined for the first time in a single experiment. These relaxation rates are measured as a function of the longitudinal translational velocities of the resonant particles. To within the experimental accuracy, no significant dependence of the collisional decay rates on the translational velocities of the particles was observed. This result confirms the conventional theoretical approach to depolarizing collisions. In pure SF₆ the decay rates for the orientation and alignment were lower than the relaxation constant for collisions involving a change in the longitudinal velocity (elastic collisions) that is known from experimental observations of the ordinary photon echo. This means that only some of the elastic collisions participate in destroying the multipole moments of the levels. Evidence is found that the relaxation of the multipole moments created by polarized radiation in a resonant medium of molecular SF₆ gas depends on j, the total angular momentum of the level. Zh. éksp. Teor. Fiz. 113, 826–833 (March 1998)     

Formation of stimulated photon echo in ytterbium vapours in a longitudinal magnetic field

Intensity and polarization are derived for stimulated photon echo produced in a longitudinal magnetic field by transitions between resonance levels with the angular momenta 0 and 1. It is shown that stimulated photon echo generated in a longitudinal magnetic field can be used to obtain spectroscopic data on the times of relaxation of population, orientation, and alignment of the level ³P₁ of a ¹⁷⁴Yb atom.     

Conventional and collision photon echo in ytterbium vapor

The polarization properties of the photon echo generated by two linearly polarized pulses of resonant radiation at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition of ¹⁷⁴Yb are investigated. A complicated polarization behavior of the photon echo versus an angle between the polarization vectors of the excitation pulses is revealed in a mixture of ytterbium vapor with inert gas. For the angles ranging from 0° to 75°, a conventional echo with its linear polarization coinciding with the second excitation pulse dominates and the echo amplitude decreases with an increasing angle. For the angles ranging from 75° to 89°, the photon echo is elliptically polarized. Finally, for an angle of 90°, the conventional echo disappears and the collision echo becomes linearly polarized along the first excitation pulse.     

Photon echo and stimulated photon echo study of various collisional-relaxation channels

Collisional relaxation in SF₆ gas and its mixtures with He and Xe is studied by photon echo and stimulated photon echo methods from the standpoint of the possibility of identifying the contributions of different types of collisions. The nonexponential nature of the kinetic curve of the photon echo is clearly observed for pure SF₆, it is weaker in the mixture SF₆+Xe, and it is virtually completely absent for high degrees of dilution of SF₆ with helium. These features can be explained on the basis of estimates, made from experimental data, of the critical delay between the exciting pulses (for which the nonexponential behavior should be most strongly manifested). In pure SF₆ it is possible to distinguish the contribution of the inelastic channel (rotational relaxation) and the contribution of weak collisions. To distinguish successfully the relaxation channels in mixtures with buffer gases a heavier buffer gas and a much better time resolution must be used. It is shown that data obtained on the orientation and alignment relaxation rates by the stimulated photon echo method can serve as an upper limit for the rates of inelastic processes which cannot be measured by the photon echo method. The combined use of photon echo and stimulated photon echo methods made it possible to obtain data on the cross sections for elastic and inelastic scattering of the collisional pairs SF₆–SF₆, SF₆–Xe, and SF₆–He. Zh. éksp. Teor. Fiz. 116, 47–56 (July 1999)     

Study of relaxation processes in a gas with the aid of coherent transient processes

The paper is devoted to the study of two types of coherent transient processes, viz., coherent radiation in time-separated fields (CRSF) and photon echo (PE) in a traveling wave in SF₆ gas. Study of CRSF signals produced at instants of time t = T + nT (T is the delay time between the exciting pulses and n is an integer) has made it possible to measure the relaxation rates of the polarization and of the populations in SF₆ gas and draw qualitative conclusions concerning the character of the collisions. Damping of PE signals as a function of the delay time is nonlinear in the region of short T and permits measurement of the rates of rotationally inelastic processes, as well as of the contribution of other collisions with small velocity change.Translated from Lazernye Sistemy, pp. 143–154, 1982.     

Slow cross-symmetry phase relaxation in complex collisions

We discuss the effect of slow phase relaxation and the spin off-diagonal S-matrix correlations on the cross-section energy oscillations and the time evolution of the highly excited intermediate systems formed in complex collisions. Such deformed intermediate complexes with strongly overlapping resonances can be formed in heavy-ion collisions, bimolecular chemical reactions, and atomic cluster collisions. The effects of quasiperiodic energy dependence of the cross sections, coherent rotation of the hyperdeformed ?(3 : 1) intermediate complex, Schrödinger cat states, and quantum-classical transition are studied for ²⁴Mg + ²⁸Si heavy-ion scattering.     

Semiclassical approach to resonance-optics polarization effects in coherent and squeezed fields

Equations are derived for the atomic density matrix and relaxation operator for a broadband squeezed field in an arbitrary polarization state and resonance atomic energy levels with an arbitrary degree of degeneracy. It is shown that suppression of the relaxation of the quadrature component of the atomic polarization depends strongly on the type of resonance transition and the polarization state of the squeezed and coherent perturbing fields. When the resonance levels are strongly degenerate, the relaxation of the quadrature component of the atomic polarization under conditions of maximum suppression is nonexponential in character. The mathematical apparatus developed here makes it possible to calculate polarization-related aspects of the multifrequency optical behavior of atomic and molecular systems resonantly excited both by coherent light and by broadband squeezed fields. Zh. éksp. Teor. Fiz. 111, 25–43 (January 1997)     

Photon echo in the presence of a magnetic field: New prospects for data accumulation and processing

The possibility for echo signal switching-off and the switching of echo polarization between the ±45° positions is demonstrated for the photon echo (PE) and stimulated photon echo (SPE) generated in Yb vapor at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition by two pulses of the resonant linearly polarized radiation at appropriate experimental parameters in the weak magnetic field limit. The experimental data are in qualitative agreement with the theoretical analysis and the calculations made for the 1 ? 0 transition. The strong magnetic field limit leads to unpolarized PE and SPE signals generated by linearly polarized radiation pulses. The possibility of the generation of a long-lived echo in ytterbium vapor due to the magnetic field induced mixing of the upper working level with the metastable level is discussed. The results can be employed for the optical data storage and processing.     

New possibilities for producing photon echoes from a homogeneous ensemble of atoms and with the use of a single light pulse

In investigations of predicted new types of photon echoes — an echo from a homogeneous ensemble of atoms and an echo of a single light pulse — have established that the magnitude and form of these new types of photon echoes are completely determined by the type of optical transition and by the “area” and polarization of the exciting light pulses (and the form can be controlled by varying the magnetic field). It is also established that the echo amplitude decreases for both small (compared to 1) and large light-pulse areas, and the optimal areas for which the maximum echo is obtained have been found. Investigations show that such photon echoes can also appear under conditions when an ordinary photon echo is absent (in atomic or molecular gases at high pressure, in the far-IR region of the spectrum, from cooled trapped atoms or ions, and so on). Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 738–743 (25 May 1997)     

与光场依赖强度耦合多光子通道中原子周期量子回声的产生和控制

研究了与光场依赖强度耦合多光子通道中原子态保真度演化,探讨了原子周期量子回声的产生和控制。通过分别考察原子相干分布角、光场平均光子数以及原子跃迁时吸收（或发射）的光子数对原子态保真度演化的影响,获得了产生和控制原子周期量子回声的系统参数,并揭示了原子态高保真输出的物理实质。     

Cold beam of isotopically pure Yb atoms by deflection using 1D-optical molasses

We demonstrate the generation of an isotopically pure beam of laser-cooled Yb atoms by deflection using 1D-optical molasses. Atoms in a collimated thermal beam are first slowed using a Zeeman slower. They are then subjected to a pair of molasses beams inclined at 45^° with respect to the slowed atomic beam. The slowed atoms are deflected and probed at a distance of 160 mm. We demonstrate the selective deflection of the bosonic isotope ¹⁷⁴Yb and the fermionic isotope ¹⁷¹Yb. Using a transient measurement after the molasses beams are turned on, we find a longitudinal temperature of 41 mK.     

Observation of a new coherent transient in NMR — Two-pulse nutational stimulated echo in the angular distribution of gamma-radiation from oriented nuclei

A new coherent transient in NMR, the two-pulse nutational stimulated echo is reported for the ferromagnetic system⁵⁰CoFe, observed by monitoring the nuclear spin dynamics as a function of the second pulse duration via anisotropic gamma quanta from thermally oriented radioactive nuclei,⁶⁰Co. The mechanism of echo formation under strong Larmor inhomogeneous broadening and the secondary but important role of inhomogeneity associated with the rf amplitude (Rabi freqeuncy) due to skin-effect are investigated via the method of concatenation of perturbation factors in the statistical tensor formalism. For those experiments performed on time scales short compared with irreversible relaxation the theoretical predictions and subsequent experimental time-domain signals are in excellent accord. Remarkable constancy of amplitude of the new gamma-detected two-pulse echo with increase of interpulse time interval is observed, the longitudinal relaxation being manifest in the off-echo signals. Comparisons are made with NMRON four-pulse stimulated off-echo decay (analogous with conventional NMR three pulse stimulated echo) which is also sensitive to longitudinal relaxation, and with three-pulse on-echo decay (analogous with conventional NMR two-pulse Hahn echo) which measures transverse relaxation.     

Characteristics of the coherent excitation of an impurity atom in a photonic crystal

It is shown that when an optically allowed transition is coherently excited in an impurity atom in a photonic crystal under conditions where one of the normal relaxation channels is suppressed by the spectral characteristics of the photonic crystal, new relaxation mechanisms are activated involving a coherent field quantum. These mechanisms substantially alter the dynamics of the atomic system, leading to filling of levels of the impurity atom which do not belong to the coherently excited atomic transition. Under certain conditions this leads to population inversion as a result of an optically allowed transition which does not interact with the coherent pump and at a frequency where no photonic band gaps can exist. Zh. éksp. Teor. Fiz. 116, 1963–1978 (December 1999)     

Stimulated photon echo study of Na(32S12)-CO velocity-changing collisions

Using the recently developed single-state stimulated photon echo technique, we have studied the velocity changes experienced by ground-state Na atoms in collisions with carbon monoxide molecules. We find that the ground-state Na atoms have a total cross section of ≈420 Ų for velocity-changing collisions with CO, and that they experience an average velocity change of ≈600 cm/s. A scattering kernel of exponential form is shown to be consistent with our results. We discuss the essential features of the estimated photon echo technique.     

Atomic coherent states and reduced quantum fluctuations in optical transients

We show that the atomic coherent states (the Bloch states) commonly encountered in coherent transient phenomena exhibit squeezing. We illustrate this property calculating the degree of squeezing and the normalized correlation coefficient g⁽²⁾ for a system of N two-level atoms interacting with a cw laser electric field.