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Physical Review A

Optical nutation in a collinear fast-ion-beam–laser experiment

Anders Wännström, Anders Kastberg, Arne Arnesen, Reinhold Hallin, CARL NORDLING, O. Vogel, Staffan Linnæus

Published Jan 1, 1990• 12 citations
Publisher

Abstract

Optical nutation in Ba ii has been studied with a rapid Doppler-shifting technique in a cw collinear fast-ion-beam--laser experiment. Time-resolved nutation curves have been recorded as a function of transition, laser irradiance, laser frequency detuning, and laser polarization. Frequency spectra recorded at different, well-specified, times of interaction between the ions and the laser field reveal several interesting effects. Among them is the pronounced influence of the postacceleration potential on the line profiles. Theoretical curves, calculated in the density-matrix formalism, reproduce all the characteristics of the experimental data. Future possibilities of this technique are discussed, e.g., the prospects of determining transition probabilities through the measurements of optical nutation.

DOI
10.1103/physreva.41.440

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COHERENT OPTICAL TRANSIENT STUDIES USING FREQUENCY SWITCHING AND USING ARP EXCITATION.

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Two different time-resolved spectroscopic techniques are discussed theoretically and demonstrated experimentally in dilute gases. The first technique involves extending the advantages of Stark-effect based time-resolved spectroscopy to non-polar molecules. This involves the development of a stable, TEM₀₀ mode, cw, CO₂ laser capable of switching rapidly and controllably between two frequencies. Design problems and output characteristics are discussed. The frequency switchable laser is applied to the CO₂ 10.6 μm P(16) coincidence with the non-polar molecule SF₆. The population relaxation time, T₁, is measured using two-pulse delayed nutation. The decay of induced dipoles is studied using the phenomenon of photon echoes. It is found that the echoes decay in a manner characteristic of dephasing dominated by velocity-changing collisions. A fit of the data to a model for such decays gives values of γ(ab) ≡ 1/T₂ (the non-velocity-changing contribution to the dipole decay rate), Γ(VC) (the total probability of a velocity-changing collision per unit time), and Δu which is related to the mean velocity change of SF₆ upon a velocity changing collision. A comparison with the published results of the similar Stark experiments on C¹³ H₃F are made. The second technique involves the development of an alternative to the pulsed excitation typically used in time-resolved T₁ studies. This involves inversion of a portion of the velocity distribution by adiabatic rapid passage (ARP) techniques. The center of this portion is then probed in the manner of previous delayed nutation experiments. The system preparation is shown theoretically to be different and simpler than the pulse case. In addition, ARP preparation gives a larger signal than two-pulse delayed nutation experiments. ARP experiments on N¹⁴H₃ and N¹⁵H₃ are described and compared to two-pulse delayed notation experiments. The single exponential decay best fits to the data from the two methods are found to be in agreement. We would expect the N¹⁵H₃ results to be very similar to the N¹⁴H₃ results, though reduced rotational resonance effects in its upper state should give it an overall slower decay. It is indeed found that the decay appears to be a simple exponential as did the N¹⁴H₃ data over the time range studied. The pressure dependent single exponential decay rate for N¹⁵H₃ is however roughly 45% larger than the rate for N¹⁴H₃ in the pressure range from 0.5 to 9 mTorr.

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Nonlinear Fast-Beam Laser Interactions

U. Nielsen, Ove Poulsen

High-resolution spectroscopy performed on fast-moving absorbers has several desirable features. First, in a collinear geometry, the residual Doppler width of the fast atoms is low due to the velocity bunching taking place during acceleration. Second, the velocity of these cooled atoms can easily be changed very precisely. Third, this makes possible Doppler tuning of a laser into resonance with a particular atomic transition. Doppler shifts tip to ≃80 Å are present, dependent on the beam energy and the atom used. This, in turn, makes it possible to 'create' completely harmonic three-level atoms, using the relativistic transformation between laboratory and atom rest frame and to perform optical pumping in well defined spatial regions. Applications to velocity control, resonant three-level nonlinear field-atom interactions, high-resolution laser-r.f. double-resonance spectroscopy, and lifetime determinations are discussed.

IEEE Transactions on Nuclear Science•Apr 1, 1983•7 citations
Optical nutation in Ba ii has been studied with a rapid Doppler-shifting technique in a cw collinear fast-ion-beam--laser experiment. Time-resolved nutation curves have been recorded as a function of transition, laser irradiance, laser frequency detuning, and laser polarization. Frequency spectra recorded at different, well-specified, times of interaction between the ions and the laser field reveal several interesting effects. Among them is the pronounced influence of the postacceleration potential on the line profiles. Theoretical curves, calculated in the density-matrix formalism, reproduce all the characteristics of the experimental data. Future possibilities of this technique are discussed, e.g., the prospects of determining transition probabilities through the measurements of optical nutation.
We report the first observation of optical nutation in a collinear fast-ion-beam-laser experiment. 35-keV Ba+{\mathrm{Ba}}^{+}Ba+ ions are suddenly tuned into resonance with a strong laser field by post acceleration over a short distance. The degree of excitation as a function of time is monitored by observing the spontaneously emitted light along the ion beam. Oscillations as a function of transition, laser intensity, and laser frequency detuning have been studied. Theoretical curves have been calculated in the density-matrix formalism.
We have developed a method to measure absolute transition probabilities for ions by recording optical nutation. Ions in an accelerated beam are brought into resonance with a counterpropagating laser beam by a rapid Doppler switch. From the time evolution of the population of the upper level in the resonant transition, we determine the Rabi frequency of the transition. An accurate measurement of the electric field strength then yields the transition probability. We report measurements for five transitions in Ba ii. For three of these transitions our values are more accurate than those of earlier published results.
Time-resolved collinear fast-beam laser spectroscopy has been used to measure the lifetime of the T=23 537 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, J=9/2 odd-parity [abbreviated as (23 537)${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{9/2}$] level in Nd ii. Nanosecond pulsed excitation was achieved with the Doppler switching technique. The fast ions were selectively excited from the metastable 4${\mathit{f}}^{4}$5d 6^{6}6K9/2{\mathit{K}}_{9/2}K9/2​ level to the (23 537)${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{9/2}$ level by the light of a cw dye laser. The decay of the resonance fluorescence photons emitted in the transition to the ground 4${\mathit{f}}^{4}$6s 6^{6}6I7/2{\mathit{I}}_{7/2}I7/2​ level, as a function of the flight distance, was observed. The decay length was scaled into the decay time by the ion velocity, which was determined using the optical resonance itself. The lifetime is 27.7(9) ns.
Two different time-resolved spectroscopic techniques are discussed theoretically and demonstrated experimentally in dilute gases. The first technique involves extending the advantages of Stark-effect based time-resolved spectroscopy to non-polar molecules. This involves the development of a stable, TEM₀₀ mode, cw, CO₂ laser capable of switching rapidly and controllably between two frequencies. Design problems and output characteristics are discussed. The frequency switchable laser is applied to the CO₂ 10.6 μm P(16) coincidence with the non-polar molecule SF₆. The population relaxation time, T₁, is measured using two-pulse delayed nutation. The decay of induced dipoles is studied using the phenomenon of photon echoes. It is found that the echoes decay in a manner characteristic of dephasing dominated by velocity-changing collisions. A fit of the data to a model for such decays gives values of γ(ab) ≡ 1/T₂ (the non-velocity-changing contribution to the dipole decay rate), Γ(VC) (the total probability of a velocity-changing collision per unit time), and Δu which is related to the mean velocity change of SF₆ upon a velocity changing collision. A comparison with the published results of the similar Stark experiments on C¹³ H₃F are made. The second technique involves the development of an alternative to the pulsed excitation typically used in time-resolved T₁ studies. This involves inversion of a portion of the velocity distribution by adiabatic rapid passage (ARP) techniques. The center of this portion is then probed in the manner of previous delayed nutation experiments. The system preparation is shown theoretically to be different and simpler than the pulse case. In addition, ARP preparation gives a larger signal than two-pulse delayed nutation experiments. ARP experiments on N¹⁴H₃ and N¹⁵H₃ are described and compared to two-pulse delayed notation experiments. The single exponential decay best fits to the data from the two methods are found to be in agreement. We would expect the N¹⁵H₃ results to be very similar to the N¹⁴H₃ results, though reduced rotational resonance effects in its upper state should give it an overall slower decay. It is indeed found that the decay appears to be a simple exponential as did the N¹⁴H₃ data over the time range studied. The pressure dependent single exponential decay rate for N¹⁵H₃ is however roughly 45% larger than the rate for N¹⁴H₃ in the pressure range from 0.5 to 9 mTorr.
High-resolution spectroscopy performed on fast-moving absorbers has several desirable features. First, in a collinear geometry, the residual Doppler width of the fast atoms is low due to the velocity bunching taking place during acceleration. Second, the velocity of these cooled atoms can easily be changed very precisely. Third, this makes possible Doppler tuning of a laser into resonance with a particular atomic transition. Doppler shifts tip to ≃80 Å are present, dependent on the beam energy and the atom used. This, in turn, makes it possible to 'create' completely harmonic three-level atoms, using the relativistic transformation between laboratory and atom rest frame and to perform optical pumping in well defined spatial regions. Applications to velocity control, resonant three-level nonlinear field-atom interactions, high-resolution laser-r.f. double-resonance spectroscopy, and lifetime determinations are discussed.