Multipole relaxation and transfer rates in the impact approximation: application to the resonance interaction

Rotationally inelastic collisions of excited NaK and NaCs molecules with noble gas and alkali atom perturbers

We report measurements of rate coefficients at T ≈ 600 K for rotationally inelastic collisions of NaK molecules in the 2(A)¹Σ⁺ electronic state with helium, argon, and potassium atom perturbers. Several initial rotational levels J between 14 and 44 were investigated. Collisions involving molecules in low-lying vibrational levels (v = 0, 1, and 2) of the 2(A)¹Σ⁺ state were studied using Fourier-transform spectroscopy. Collisions involving molecules in a higher vibrational level, v = 16, were studied using pump/probe, optical-optical double resonance spectroscopy. In addition, polarization spectroscopy measurements were carried out to study the transfer of orientation in these collisions. Many, but not all, of the measurements were carried out in the "single-collision regime" where more than one collision is unlikely to occur within the lifetime of the excited molecule. The analysis of the experimental data, which is described in detail, includes an estimate of effects of multiple collisions on the reported rate coefficients. The most significant result of these experiments is the observation of a strong propensity for ΔJ = even transitions in collisions involving either helium or argon atoms; the propensity is much stronger for helium than for argon. For the initial rotational levels studied experimentally, almost all initial orientation is preserved in collisions of NaK 2(A)¹Σ⁺ molecules with helium. Roughly between 1/3 and 2/3 of the orientation is preserved in collisions with argon, and almost all orientation is destroyed in collisions with potassium atoms. Complementary measurements on rotationally inelastic collisions of NaCs 2(A)¹Σ⁺ with argon do not show a ΔJ = even propensity. The experimental results are compared with new theoretical calculations of collisions of NaK 2(A)¹Σ⁺ with helium and argon. The calculations are in good agreement with the absolute magnitudes of the experimentally determined rate coefficients and accurately reproduce the very strong propensity for ΔJ = even transitions in helium collisions and the less strong propensity for ΔJ = even transitions in argon collisions. The calculations also show that collisions with helium are less likely to destroy orientation than collisions with argon, in agreement with the experimental results.

Collisional transfer of population and orientation in NaK

Collisional satellite lines with |ΔJ| ≤ 58 have been identified in recent polarization spectroscopy V-type optical-optical double resonance (OODR) excitation spectra of the Rb(2) molecule [H. Salami et al., Phys. Rev. A 80, 022515 (2009)]. Observation of these satellite lines clearly requires a transfer of population from the rotational level directly excited by the pump laser to a neighboring level in a collision of the molecule with an atomic perturber. However to be observed in polarization spectroscopy, the collision must also partially preserve the angular momentum orientation, which is at least somewhat surprising given the extremely large values of ΔJ that were observed. In the present work, we used the two-step OODR fluorescence and polarization spectroscopy techniques to obtain quantitative information on the transfer of population and orientation in rotationally inelastic collisions of the NaK molecules prepared in the 2(A)(1)Σ(+)(v' = 16, J' = 30) rovibrational level with argon and potassium perturbers. A rate equation model was used to study the intensities of these satellite lines as a function of argon pressure and heat pipe oven temperature, in order to separate the collisional effects of argon and potassium atoms. Using a fit of this rate equation model to the data, we found that collisions of NaK molecules with potassium atoms are more likely to transfer population and destroy orientation than collisions with argon atoms. Collisions with argon atoms show a strong propensity for population transfer with ΔJ = even. Conversely, collisions with potassium atoms do not show this ΔJ = even propensity, but do show a propensity for ΔJ = positive compared to ΔJ = negative, for this particular initial state. The density matrix equations of motion have also been solved numerically in order to test the approximations used in the rate equation model and to calculate fluorescence and polarization spectroscopy line shapes. In addition, we have measured rate coefficients for broadening of NaK 3(1)Π ← 2(A)(1)Σ(+)spectral lines due to collisions with argon and potassium atoms. Additional broadening, due to velocity changes occurring in rotationally inelastic collisions, has also been observed.

Self-broadening in singlet spectral lines of helium

The emission profiles of five lines in the singlet spectrum of helium, λλ492.2, 501.5, 504.7, 667.8 and 728.1 nm, have been ’studied under high resolution to test some aspects of line-broadening theory, particularly as it is applied to the resonance interaction. The source was a weak d.c. discharge at 77 or 273 K; the pressure range was 0-10 Torr. Self-absorption was shown to be negligible in separate experiments. Profiles were recorded digitally and analysed by computer into Lorentzian and Gaussian components. The results show that the observed profiles of all the lines are influenced to varying degrees by excitation mechanisms; a discussion of these effects is given. They prevent a quantitative interpretation of λλ492.2 and 667.8 nm, and limit the accuracy of the results for the other lines. Of these, λ728.1nm shows the closest approach to pure resonance broadening; the measured broadening constants are 7.7 ± 0.2 (77K) and 9.3 ± 0.9 (273 K), both in units of 10 -20 cm -1 cm 3 per atom. This line also shows at 77 K a shift to the red of 0.11 ± 0.03 in the same units; this and the temperature dependence are attributed to a non-resonant contribution to the interatomic potential. Preliminary results of recent calculations based on a model potential are in fair agreement with our data. The Lorentzian width of λ728.1nm extrapolated to zero density is found to exceed the radiation width by (1.2 ± 0.5) 10~ 3 cm_ 1 , in agreement with the results of earlier workers, but the possibilities of systematic error (apparatus function, excitation mechanisms) reduce the significance of this finding. Possible experiments to circumvent the difficulties encountered in the present work are discussed.