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IBM Journal of Research and Development

Two-Photon Coherent Transients

M. M. T. Loy

Published Sep 1, 1979• 3 citations
Publisher

Abstract

Two-photon nutation, free-induction decay, and population inversion by adiabatic rapid passage have been studied in NH3. These effects are easily visualized with a vector model. Relaxation time T1and T2have been measured.
DOI
10.1147/rd.235.0504

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This Letter reports the first experimental observation of population inversion of a two-photon transition in NH3{\mathrm{H}}_{3}H3​ using the adiabatic rapid-passage technique. The population inversion was demonstrated by the observation of stimulated two-photon gain and emission. By use of this technique, the two-photon population relaxation time T1{T}_{1}T1​ was measured.
The observation of two-photon optical nutation and free-induction decay is reported. The experimental result, on a two-photon transition in NH3{\mathrm{H}}_{3}H3​ with all relevant parameters known, is shown to be in quantitative agreement with the prediction of the two-photon vector model.
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.