K. Bergmann

Population transfer between molecular vibrational levels by stimulated Raman scattering with partially overlapping laser fields. A new concept and experimental results

The feasibility of a novel technique for efficient and selective population transfer from a thermally populated level 1 via an intermediate state 2 to level 3 is experimentally demonstrated. It is shown for sodium dimers that the process of on‐ or near‐resonance stimulated Raman scattering with only partially overlapping laser beams is, in particular, useful for the selective population of high vibrational levels of particles in a molecular beam. This is achieved when the interaction with the Stokes laser, coupling levels 2 and 3, begins earlier than the interaction with the pump laser. The phenomenon, which is closely related to the formation of ‘‘trapped states,’’ is quantitatively explained using the basis of eigenstates of molecules strongly coupled to the radiation fields. The similarity and difference to related techniques such as rapid adiabatic passage phenomena in two‐level systems, off‐resonant stimulated Raman scattering, or stimulated emission pumping is briefly discussed.

Population switching between vibrational levels in molecular beams

Abstract We consider the transfer of population in a three-level system by near resonant stimulated Raman scattering. Numerical results indicate that complete transfer of population is possible with cw lasers of moderate power provided a suitable spatial displacement of the pump and stimulating laser is introduced. These conclusions are supported by experimental results. The proposed and demonstrated technique does not require laser intensity modulation, laser frequency chirping or level shifting. It is particularly suited for molecular beam experiments.

State‐to‐state differential cross sections for rotationally inelastic scattering of Na2 by He

State‐to‐state differential cross sections for rotational transitions of Na2 in collisions with He are measured in the electronic and vibrational ground state at thermal collision energies using a new laser technique. Single rotational levels ji are labelled by modulation of their population via laser optical pumping using a dye laser. The modulation of the fluorescence induced by an Ar+ laser tuned to the level jf=28 is proportional to the cross section for collisional transfer ji→jf and is detected at the scattering angle ϑ. A single optical fiber and a fiber bundle provide a flexible connection between the detector and the laser and photomultiplier, respectively. Transitions as large as Δj=20 are observed. At small angles elastic scattering is dominant, but rotationally inelastic processes become increasingly important at larger scattering angles. Rotational rainbow structure causing a steep onset of the cross section with the scattering angle ϑ (at fixed Δj) or a sharp cutoff with Δj (at fixed ϑ) is f...

Robust creation and phase-sensitive probing of superposition states via stimulated Raman adiabatic passage (STIRAP) with degenerate dark states

We describe a method for creating an arbitrary coherent superposition of two atomic states in a controlled and robust way by using a sequence of three pulses in a four-state system. The proposed technique is based on the existence of two degenerate dark states (i.e. states having no component of the excited state) and their interaction. The mixing of the dark states can be controlled by changing the relative delay of the pulses, and thus an arbitrary superposition state can be generated. It is shown that the method is robust against small variations of parameters (e.g. the area of the pulses) and is insensitive to radiative decay from the intermediate excited state. A time reversed version of the technique makes possible the determination of phase occurring in a superposition of two atomic states.

Molecular beam diagnostics with internal state selection: Velocity distribution and dimer formation in a supersonic Na/Na2 beam

Abstract The velocity distribution parallel and perpendicular to the molecular beam axis has been determined for molecules in well defined quantum states using TOF — via optical pumping and the Doppler-shift method. It has been found that the flow velocity as well as the speed ratio changes with the internal energy of the molecule. The flow velocity increases with increasing internal energy at low pd values ( p is the pressure in the oven, d is the nozzle diameter) while the opposite is true at high pd values. The parallel speed ratio is smaller for molecules in vibrationally excited states and the perpendicular velocity distribution shows excessive tails that are more pronounced for molecules in higher lying levels. The population of individual levels has been monitored via laser induced fluorescence. It does not change monotonically with pd . The population distribution is not in thermal equilibrium and can only be approximately described by a temperature of T υ , ≈ 150 K. On the basis of these results a simple model for the influence of the recombination of atoms on the expansion is derived: Molecules are initially neither formed in the υ = 0 vibrational level nor with high internal excitation but probably with ⩾ 1000 cm −1 of internal energy. The recombination leads to fast atoms and molecules. It is the incomplete deceleration of these fast particles together with an efficient quenching process for the internal energy that determines the flow velocity of molecules in individual quantum states at low pd values. At high pd values the acceleration of molecules with much internal energy is incomplete because those molecules have necessarily made only few collisions.

Population transfer by stimulated Raman scattering with delayed pulses: Analytical results for multilevel systems

Stimulated Raman scattering involving adiabatic passage (STIRAP) is a proven technique for population transfer in three‐level systems with strong oscillator strengths. We show that the STIRAP process should also yield high transfer efficiencies when the densities of states near the intermediate and final levels are high, provided certain criteria for the experimentally controllable parameters are met. We also show that high transfer efficiencies may even be possible when the pump and Stokes lasers can access levels outside of the three‐level system. The criteria are derived with an approach that emphasizes the adiabatic nature of the transfer process in an eigenvector of the interaction Hamiltonian that resembles a trapped state. The results are compared to density matrix calculations.

Energy dependence and isotope effect for the total reaction rate of Cl+HI and Cl+HBr

A laser initiated chemical reaction method has been used to determine the total reaction rates for Cl+HI and Cl+HBr and the dependence of the rates on collision energy and H–D isotopic substitution. The rate constants are k=1.64×10−10 cm3 molecule−1⋅sec−1 (σ=33.5 A2) for Cl+HI and k=7.4×10−12 cm3 molecule−1⋅sec−1 (σ=1.44 A2) for Cl+HBr. Measurements were all done at 295 °K in slowly flowing gases. Substituting H by D decreases the rate constant by a factor of 1.84 in the case of HI and by 1.5 in the case of HBr. The isotope effect may be a result of tunneling on corner cutting trajectories. The cross section decreases with increasing collision energy. This, together with the large reaction cross section, indicates the importance of an attractive potential in these systems.

Efficient adiabatic population transfer by two-photon excitation assisted by a laser-induced Stark shift

We demonstrate and analyze a novel scheme for complete transfer of atomic or molecular population between two bound states, by means of Stark-chirped rapid adiabatic passage (SCRAP). In this two-laser technique a delayed-pulse laser-induced Stark shift sweeps the transition frequency between two coupled states twice through resonance with the frequency of the population-transferring coupling laser. The delay of the Stark-shifting pulse with respect to the pulse of the coupling-laser Rabi frequency guarantees adiabatic passage of population at one of the two resonances while the evolution is diabatic at the other. The SCRAP method can give a population-transfer efficiency approaching unity. We discuss the general requirements on the intensity and timing of the pulses that produce the Rabi frequency and, independently, the Stark shift. We particularly stress extension to a double-SCRAP technique, a coherent variant of stimulated emission pumping in the limit of strong saturation. We demonstrate the success ...

Population transfer by stimulated Raman scattering with delayed pulses using spectrally broad light

The feasability of selective and complete population transfer between atomic or molecular levels by stimulated Raman scattering with delayed pulses involving spectrally broad light with characteristics typical for pulsed lasers is investigated. In extension of previous work, the effect on the transfer efficiency of phase fluctuations and of the detuning ΔR of the laser frequencies from the two‐photon resonance is analyzed. The minimum pulse energy Pmin required to achieve a transfer efficiency of nearly unity is derived analytically, with some restrictions imposed on the type of phase fluctuations. Pmin increases approximately proportional to the square of the bandwidth of the laser radiation and to the square of ΔR. The conclusions are confirmed by the results of extensive numerical calculations. These studies also reveal a high sensitivity of the transfer efficiency to the autocorrelation of the fluctuating light.

Effect of reagent electronic excitation on the chemical reaction Br(2P1/2,3/2)+HI

Br(2P3/2)+HI are found to react at room temperature to give HBr+I(2P3/2) with a rate constant of (1.0±0.3) ×10−11 cm3 molecule−1⋅sec−1. For Br+HI, electronic excitation of Br inhibits the reaction to HBr+I. Collisions with HI remove excited Br(2P1/2) with a rate of about 0.25×10−11 cm3 molecule−1⋅sec−1. This rate includes electronic quenching and electronic‐to‐vibration energy transfer as well as reaction. From correlation arguments the reaction of excited Br(2P1/2) with HI is expected to be much slower than that of ground state. The observed rate is comparable to that for quenching plus electronic‐to‐vibration transfer for Br(3P1/2) by HCl and HBr.