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)     

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.     

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.     

Multiwave excimer lamps using XeF/XeCl/KrF/KrCl molecules

Results are presented of an investigation of the spectral and temporal characteristics of an electricdischarge excimer lamp emitting simultaneously on the 351 nm XeF, 308 nm XeCl, 249 nm KrF, and 222 nm KrCl transitions. The He/Kr/Xe/SF₆/HCl working mixtures were excited in a transverse discharge with ultraviolet spark preionization at a total pressure of 25–100 kPa. In order to obtain the same brightness the concentration ratio [Kr]/[Xe] for the excimer molecular transitions was 8/0.8 kPa, and that for the halogen-containing molecules was [SF₆]/[HCl][=[0.06–0.12]/[0.08–0.16] kPa. The duration of the radiation pulses for the excimer molecular transitions at atmospheric pressure was 100–200 ns and when the total mixture pressure was reduced to 250 kPa, this was doubled or trebled. The service life of the spontaneous radiation for B-X transitions in excimer molecules was 10⁵ pulses. Zh. Tekh. Fiz. 68, 64–67 (December 1998)     

Coherent transients in gases

A great variety of coherent transient processes in atomic and molecular gases is investigated. The degeneracy with respect to magnetic sublevels, which is typical of working levels of atomic and molecular transitions in gases, leads to new phenomena. In particular, the non-Faraday rotation of the coherent-response polarization vector at the 0 ↔ 1 transition is investigated in the presence of the longitudinal magnetic field in the ¹⁷⁴Yb vapor for both photon echo and stimulated photon echo. A specific relaxation channel that involves the depolarizing collisions emerges in a gas due to the degeneracy of working levels. Such collisions are especially important for atoms and are also observed in molecules. The anisotropy of the depolarizing collisions leads to a new phenomenon of the collision photon echo, which is experimentally demonstrated for the 0 ↔ 1 transition in the ¹⁷⁴Yb mixtures with atomic buffers. The velocity-dependent relaxation rates are observed for various coherent phenomena in molecular gases. The coherent control of a few coherent transient processes is implemented.     

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.     

Dynamic and stimulated rf echo in high-Tc superconducting powders

Two-pulse and three-pulse echoes in powdered yttrium and bismuth high-T _c superconductors are investigated to determine the dependence of the signal amplitude on the magnetic field, the temperature, and the gas pressure. The temperature is measured as a function of the relaxation time of the echo signal. The properties of the long-lived rf echo are studied in detail; it exhibits a persistent (lasting more than several hours) memory of a time series of write pulses and a cumulative storage effect. The experimental results can be explained qualitatively within the framework of the theory proposed by Asadullin [Sverkhprovodimost’ 6, 545 (1993)] to account for the nonlinear motion of vortices associated with sample defects. Zh. éksp. Teor. Fiz. 111, 1032–1046 (March 1997)     

Nutation electron spin echo ofE-centers in quartz

Attenuation of the signals of the nutation echo of E’-centers in crystalline and glass quartz was experimentally investigated at room temperature. The nutation echo in EPR was excited in the single-photon regime and formed by two Zeeman-field pulses with duration t₁ and t₂ (t₁<t₂) and time interval between them τ. The echo signal was recorded during the second pulse at t≈2t₁+τ. It was established that this signal attenuation, measured as t₁ increased, follows an, exponential law, its rate Γ is much less than 1/T₂ (T₂ is the transverse relaxation time), and it linearly increases with an increase in the amplitude of the exciting SHF field B₁. The parameters of the dependence of Γ on B₁ correlate with the parameters of the analogous dependence revealed previously for attenuation of nutation of E’-centers in quartz in the two-photon regime at T=4.2 K. At the same time, the value of Γ measured with different values of τ is independent of B₁ and is equal to 1/T₁, where T₁ is the longitudinal relaxation time. A. N. Sevchenko Scientific Research Institute of Applied Physical Problems, 7, Kurchatov St., Minsk, 220064, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 405–411, May–June, 1998.     

Phonon echo in L-alanine

Phonon echo signals have been observed in a finely-dispersed crystalline powder of the L-alanine amino acid. Measurements of the relaxation time T ₂ have revealed a phase transition in L-alanine crystals at a temperature of about 170 K. Fiz. Tverd. Tela (St. Petersburg) 40, 2119–2120 (November 1998)     

Photon echo relaxation in LaF3:Pr3+

We have observed deeply modulated photon echoes from Pr³⁺ ions in LaF₃ reflecting nuclear hyperfine interactions in both the ground ³H₄ and excited ³P₀ states. We infer splittings of 0.75 MHz for the ³P₀ state. Inhomogeneous broadening of the terminal levels is found to lead to echo modulation damping. A surprising concentration dependence of the photon echo relaxation rate is observed as well as a dramatically long-lived (3 min) stimulated photon echo.     

Large contribution of virtual Delbrück scattering to the emission of photons by relativistic nuclei in nucleus–nucleus and electron–nucleus collisions

Delbrück scattering is the elastic scattering of a photon in the Coulomb field of a nucleus via a virtual electron loop. The contribution of this virtual subprocess to the emission of a photon in the collision of ultra-relativistic nuclei, Z₁Z₂→Z₁Z₂γ, is considered. We identify the incoming virtual photon as being generated by one of the relativistic nuclei involved in the binary collision and the scattered photon as being emitted in the process. The energy and angular distributions of the photons are calculated. The discussed process has no infrared divergence. The total cross section obtained is 14 barn for Au–Au collisions at the RHIC collider and 50 barn for Pb–Pb collisions at the LHC collider. These cross sections are considerably larger than those for ordinary tree-level nuclear bremsstrahlung in the considered photon energy range, m_e⪡E_γ⪡m_eγ, where γ is the Lorentz factor of the nucleus. Finally, photon emission in electron–nucleus collisions, eZ→eZγ, is discussed in the context of the eRHIC option.     

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.     

Cross relaxation in the rotational inversion doublets of ammonia in the far infrared

The effect of cross relaxation due to inelastic collisions on the inversion doublets of ammonia in the far infrared was studied by interferometric spectroscopy. Line widths and cross-relaxation rates were measured for several rotational transitions, using CH₂ = CF₂, C₂H₆, CF₄ and SF₆ as perturbing gases in the pressure range from 1 to 17.5 atm. The J-dependence of the observed cross-relaxation rates was calculated using a first Born approximation.     

Stochastic excitation of a stimulated spin echo

The excitation of a stimulated spin echo by two coherent pulses of Gaussian white noise and a δ-function-like radio pulse is studied. The mean value of the complex envelope of the echo is obtained by solving the Bloch equations. The behavior of the mean value of the envelope of the stimulated echo is investigated for a Lorentzian shape of the inhomogeneously broadened absorption line. It is shown that in an excitation regime which is linear for noise pulses the envelope corresponds to the Fourier transform of the line-shape function. Nonlinear distortions of the mean value of the envelope of a stimulated echo can arise outside the linear excitation regime. Zh. Tekh. Fiz. 67, 100–104 (October 1997)     

Pressure-shock-controlled pulsed molecular beams

A method of controlling the duration of pulses of intense molecular beams is suggested. The idea of the method is the shortening of an initial molecular beam pulse by producing a pressure shock in front of a solid surface through which the beam passes. Experiments on shortening H₂, He, SF₆, SF₆/H₂(1/10), and SF₆/He(1/10) molecular beam pulses are reported. The parameters of the beams incident on, and transmitted through, the surface are studied. The gas density in the initial beam and in the pressure shock before the surface is estimated. The intensity and duration of shortened molecular pulses are found as a function of the initial intensity, angle of incidence, and the diameter of a hole on the surface through which the beam passes. It is established that the duration of the shortened beam decreases greatly with increasing incident intensity and decreasing hole diameter. It is shown that intense pulsed H₂, He, SF₆, SF₆/H₂(1/10), and SF₆/He(1/10) molecular beams with a pulse duration of ≤10–15 μs and an extent of ≤1–2 cm can be generated with the method suggested.     

Production of collimated positronium by positron scattering from CO<Subscript>2</Subscript>, N<Subscript>2</Subscript> and Xe

The efficiency for collimated positronium production by charge-exchange in positron collisions with gaseous targets has been investigated in the range 20–396 eV. At 250 and 396 eV, CO₂ has been found to be approximately twice as efficient as N₂, the previous best neutralising gas at high energies. The efficiency from Xe, whilst lower at low energies, becomes comparable to that from H₂ at around 100–120 eV; at ∼250 eV, it is an order of magnitude lower.     

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.     

Experimental investigations of the formation of XeF*(B 2II1/2) excimer molecules during the injection of SF6 into a xenon plasma flow

The luminescence of XeF*(B ²II_1/2) molecules from a zone formed in the injection of SF₆ gas into a freely flowing xenon plasma jet is investigated. The experiments show that both the energy characteristics of the luminescence and the geometrical dimensions of the plasmachemical reaction zone can be controlled by varying the power input to the plasmatron and the mass flow rates of SF₆ and the xenon plasma. It is shown that the main contribution to the production of XeF*(B ²II_1/2) excimer molecules under the given experimental conditions is from two-particle ion-ion reactions involving Xe ions and SF ₆ ⁻ and SF ₅ ⁻ molecular ions. Zh. Tekh. Fiz. 68, 36–42 (February 1998)     

Observation of simultaneous ν1(SF6) + ν3(NF3) and ν2(SF6) + ν3(NF3) transitions enhanced by the resonance dipole-dipole interaction with the ν1 + ν3 and ν2 + ν3 states of the SF6 molecule

IR spectra of the solution of SF₆ molecules in liquid NF₃ at 84 K have been recorded. In a solvent transmission window of 1500–1750 cm⁻¹, two wide absorption bands with pronounced peaks in the high-frequency part are observed. The profile of these bands is explained by the influence of the resonance dipole-dipole (RDD) interaction of the states of the simultaneous transition ν₁(SF₆) + ν₃(NF₃) and ν₂(SF₆) + ν₃(NF₃) with the states (ν₁ + ν₃) and (ν₂ + ν₃) of the SF₆ molecules, respectively. The use of three isotopic modifications ³²SF₆, ³³SF₆, and ³⁴SF₆ has allowed us to vary the resonance detuning and thus to change the strength of the RDD interaction. With the liquid near the melting point being represented as a close-packed cubic crystal, the profile was calculated and its spectral characteristics were determined. The frequencies of the main peaks coincide with the experimental values accurate to the error.     

Inelastic atom scattering from layers of atoms and molecules on Cu(110)

The vibrational properties of adsorbed layers have been studied using inelastic He atom scattering. The substrate was Cu(110) viewed along the [001] and [11&#x0304;0] azimuth. The adsorbates included, Kr, Xe, CH₄, CD₄, C₂H₆, CO, CO₂ and O₂ and covered the region of both physisorption and chemisorption. Despite a range of binding energies and mass ratios the vibrational frequencies showed no dependence on the momentum change, i.e. scattering was dispersionless, although intensity changes occurred in the inelastic peaks. There seemed to be no dependence of the adsorbate spectra on coverage below a monolayer. The peaks of CH₄ and C₂H₆ appeared broader than those of Kr and Xe suggesting molecular motions. Further, CH₄ and C₂H₆ gave very similar spectra. Both energy gain and loss events were observed in the inelastic spectrum. For those adsorbates which gave multilayer adsorption (Xe, C₂H₆) changes in the spectra were observed as the second layer developed. In the latter category also, mixed layers (Xe on C₂H₆, Xe on CO and C₂H₆ on Xe) were studied.