Spin Polarization of Rb and Cs $np$ ${{}^{2}P}_{\frac{3}{2}}$ ($n=5$, 6) Atoms by Circularly Polarized Photoexcitation of a Transient Diatomic Molecule

We report the selective population of Rb or Cs $np$ ${{}^{2}P}_{\frac{3}{2}}$ ($n=5$, 6; $F=4$, 5) hyperfine states by the photodissociation of a transient, alkali-rare gas diatomic molecule. Circularly polarized (${\sigma }^{-}$), amplified spontaneous emission (ASE) on the $D_2$ line of Rb or Cs (780.0 and 852.1 nm, respectively) is generated when Rb-Xe or Cs-Xe ground state collision pairs are photoexcited by a ${\sigma }^{+}$-polarized optical field having a wavelength within the $D_2$ blue satellite continuum, associated with the $B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}\leftarrow X{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ ($\text{free}\leftarrow \text{free}$) transition of the diatomic molecule. The degree of spin polarization of Cs ($6p$ ${}^2{P}_{\frac{3}{2}}$), specifically, is found to be dependent on the interatomic distance ($R$) at which the excited complex is born, a result attributed to the structure of the $B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ state. For Cs-Xe atomic pairs, tuning the wavelength of the optical field from 843 to 848 nm varies the degree of circular polarization of the ASE from 63% to almost unity because of the perturbation, in the $5\le R\le 6\AA$ interval, of the ${{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ potential by a $d\sigma$ molecular orbital associated with a higher ${}^2\mathrm{\Lambda }$ electronic state. Monitoring only the Cs $6p$ ${}^2{P}_{\frac{3}{2}}$ spin polarization reveals a previously unobserved interaction of CsXe ($B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$) with the lowest vibrational levels of a ${}^2\mathrm{\Lambda }$ state derived from Cs $(5d)+\mathrm{Xe}$. By inserting a molecular intermediate into the alkali atom excitation mechanism, these experiments realize electronic spin polarization through populating no more than two $np$ ${{}^{2}P}_{\frac{3}{2}}$ hyperfine states, and demonstrate a sensitive spectroscopic probe of $R$-dependent state-state interactions and their impact on interatomic potentials.

Figure 1

Partial energy level diagram for the Cs-Xe diatomic complex, illustrating the photoassociation of ground state Cs-Xe collision pairs with a LHCP (${\sigma }^{+}$) optical field, thereby forming a transient molecule in the $B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ state. Subsequent dissociation of the excited complex yields a spin-polarized, alkali atomic fragment. For clarity, the bound $A{{}^2\mathrm{\Pi }}_{\frac{1}{2}}$ and $A{{}^2\mathrm{\Pi }}_{\frac{3}{2}}$ molecular states are not shown, but an expanded view of the hyperfine structure of the Cs 6 ${{}^{2}S}_{\frac{1}{2}}$ and 6 ${}^2{P}_{\frac{3}{2}}$ states is given at right (not to scale). The $X$ and $B$ interatomic potentials shown are those calculated on the basis of absorption spectra (Ref. [14]).

Figure 2

Schematic diagram of the experimental arrangement with which the energies of the horizontal and vertical components of the backward wave ASE are measured.

Figure 3

Photoexcitation spectra recorded for Cs-Xe when the linearly polarized pump is tuned over the $B{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}\leftarrow X{{}^2\mathrm{\Sigma }}_{\frac{1}{2}}^{+}$ band of the diatomic complex (i.e., the $D_2$ blue satellite). Spectra acquired by monitoring the energy of detector 2 (red points) or detector 3 (blue) are identical.

Figure 4

Excitation spectra similar to those of Fig. 3 but recorded when the pump is circularly polarized (${\sigma }^{+}$). The inset compares the normalized profiles of both spectra.

Figure 5

(a) (Black) $P(\lambda )$: Dependence on pump wavelength of the contribution of ${\sigma }^{-}$ polarized ASE (at 852.1 nm) to the total energy of the backward-propagating pulse; (red) derivative of $P(\lambda )$ with respect to $\lambda$ in the 842–850 nm interval which corresponds to $5.3\lesssim R\lesssim 6.2\AA$. (b) Polarization data obtained for $\lambda =842.7\mathrm{nm}$ by detectors 2 (horizontal) and 3 (vertical).