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

Abstract

We report measurements of rate coefficients at T ≈ 600 K for rotationally inelastic collisions of NaK molecules in the 2(A)Σ 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)Σ 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)Σ 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)Σ with argon do not show a ΔJ = even propensity. The experimental results are compared with new theoretical calculations of collisions of NaK 2(A)Σ 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.

The effect of collisions on the rotational angular momentum of diatomic molecules studied using polarized light

“…This approximation is justified by the theoretical calculations of Price [46] (in particular, see Ref. [33], Fig. 6 and the related discussion).…”

“…According to Jones et al [33] Eq. ( 5), the fluorescence intensity for a transition from excited (upper) level e to final (lower) level f is given by…”

“…Following Jones et al [33] we make the assumption that for a particular perturber (argon, helium, or lithium) the quenching rates are approximately equal for the four levels (Ji = 1, 3, 5, and 7) under consideration in this work. This approximation is justified by the theoretical calculations of Price [46] (in particular, see Ref.…”

“…Many of these experiments were carried out with the lighter alkali dimers Li2 [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and Na2 [19][20][21][22][23] due to their relatively simple electronic structure and the convenient transition wavelengths for laser excitation in the visible and near IR regions. Examples of other homonuclear as well as heteronuclear diatomic molecules that were used in such studies include I2 [24][25][26], Br2 [27,28], N2 [29], NaLi [30], NaK [31][32][33], NaCs [33], CsF [34], and BaO [40], among others. Theoretical studies of rotationally inelastic collisions have also been carried out [41][42][43][44][45][46][47][48], and good agreement between calculated and measured cross sections was obtained for the ( )…”

“…However, in these experiments the lithium density was sufficiently high that depolarization due to lithium collisions was probably also significant. Similarly, Wolfe et al [32] and Jones et al [33] used polarization spectroscopy to determine the fraction of orientation lost in NaK inelastic J-changing collisions with noble gas atoms. However, because initial J values used in these experiments ranged from 14 to 44, this technique also did not provide information on specific M to M  transitions.…”