By means of a time-resolved microwave spectroscopy experiment using the Stark switching technique, the authors have pointed out an exchange of rotational coherence from a level pair (a-a') of an A molecule to a level pair (b-b') of a B molecule in a gas at low pressure. The experiment exploits the linear Stark effect of the (a-a') line, the frequency of which is successively switched on resonance with the applied CW field to bring a coherence on the (a-a') level pair or with the (b-b') line frequency which is Stark insensitive and always off resonance with the applied CW field. The coherence exchange efficiency is maximum when the two line frequencies (a-a') and (b-b') are switched in exact resonance during a time around the transverse relaxation time T2. The radiation trapping effects are calculated and explain fairly well the experimental results obtained in a long cell. The collisional effects are shown to give a contribution which differs from the previous one only by its phase and its cell-length dependence. An experiment in a short cell is exploited to give a tentative upper limit for the collisional coherence transfers.
By means of a time-resolved microwave spectroscopy experiment using the Stark switching technique, the authors have pointed out an exchange of rotational coherence from a level pair (a-a') of an A molecule to a level pair (b-b') of a B molecule in a gas at low pressure. The experiment exploits the linear Stark effect of the (a-a') line, the frequency of which is successively switched on resonance with the applied CW field to bring a coherence on the (a-a') level pair or with the (b-b') line frequency which is Stark insensitive and always off resonance with the applied CW field. The coherence exchange efficiency is maximum when the two line frequencies (a-a') and (b-b') are switched in exact resonance during a time around the transverse relaxation time T2. The radiation trapping effects are calculated and explain fairly well the experimental results obtained in a long cell. The collisional effects are shown to give a contribution which differs from the previous one only by its phase and its cell-length dependence. An experiment in a short cell is exploited to give a tentative upper limit for the collisional coherence transfers.
Resonant microwave coherence transfer between two levels pairs (a−aprime) and (b−bprime) of two molecular species A and B has been observed. A and B are either different molecules or the same molecule in states uncoupled by collision induced transitions. The frequencies of the two transitions arightarrowaprime and brightarrowbprime are nearly equal and can be transiently brought in coincidence by applying a suitable Stark field. The coherence transferred from A-molecules to the B-molecules is detected by the techniques of time resolved spectroscopy. The observed phenomena may be interpreted either by collision induced effects between collision partner molecules or by collective effects. The contribution of these two effects are roughly estimated.
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.
Each of three basic coherent transient phenomena (optical nutation, free polarization decay photon echo) can be generated by sufficiently abrupt modulation of the interaction between the matter and radiation. This can be performed by rapid commutation of either radiation parameters, or the parameters of the matter. I present here comparative analysis of these two approaches (the first one, using electro-optical switching of the resonant laser power to create the pulses for the gas excitation and the second one, based on the Stark switching of vibro-rotational levels in a gas, excited by CW laser radiation), used in our experiments in molecular gases, from the point of view of both their capability to generate various coherent transients and their sensitivity to the intensity of laser radiation.
effects, due to the propagation of laser light in optically dense media, have been observed and studied. The use of the method of Stark switching [6] has been of much assistance in overcoming experimental difficulties. In that method, time-varying coherent effects are observed through the use of cw lasers, and the interaction between molecules and laser light can be switched on and off rapidly by changing the energy interval between levels through the use of the Stark effect. The idea of changing the energy separation was subsequently implemented in ESR [7], where the use of cw microwave radiation and pulsed changes in a polarizing magnetic field (Zeeman switching) have also made it possible to detect the basic time-varying effects. Many of the new properties of time-varying phenomena which have been studied in the field of optical resonances stem from the partial excitation of a spectral line (~o 1 a, this signal is reflected by a sinusoidal signal with a frequency specified by the generalized Rabi frequency (Wl 2 + b2)1/2.
Coherent transient phenomena can be generated by the abrupt changes of parameters either of the electromagnetic field or of the matter. Comparative analysis of these two approaches, made on the base of experimental research in molecular gases SF6, 15NH3, HFCO and 13CH3F, excited by the radiation of CW CO2 and 13CO2 lasers, is presented. The first approach of photon echo (and its modifications) generation, based on electro-optic pulses formation from the resonant CW exciting radiation, has proven to be successful for intense infrared molecular transitions. The coherent responses are sufficiently powerful to be applied in high resolution spectroscopy, the spectral markers creation ,and molecular collisions investigations. However, this technique is not very effective for the free polarization decay investigations. Another approach based on the Stark switching molecular levels of polar gas, irradiated by CW laser, was demonstrated for free polarization decay and photon echo generation in 13CH3F. Free polarization decay has shown the pronounced frequency shift because of dynamic Stark effect in the CW radiation field. This specific feature of coherent transients, created by Stark switching, limits the high resolution spectroscopy by the low intensity range of exciting radiation. From other viewpoint, it gives unique possibility to investigate coherent transients generated at the dressed molecular levels.
Polar molecules offer a new platform for quantum simulation of systems with long-range interactions, based on the electrostatic interaction between their electric dipole moments. Here, we report the development of coherent quantum state control using microwave fields in 40 Ca 19 F and 87 Rb 133 Cs molecules, a crucial ingredient for many quantum simulation applications. We perform Ramsey interferometry measurements with fringe spacings of ∼1 kHz and investigate the dephasing time of a superposition of N = 0 and N = 1 rotational states when the molecules are confined. For both molecules, we show that a judicious choice of molecular hyperfine states minimises the impact of spatially varying transition-frequency shifts across the trap. For magnetically trapped 40 Ca 19 F we use a magnetically insensitive transition and observe a coherence time of 0.61(3) ms. For optically trapped 87 Rb 133 Cs we exploit an avoided crossing in the AC Stark shifts and observe a maximum coherence time of 0.75(6) ms.