Laser-intensity dependence of photoassociation in ultracold metastable helium

Abstract

Photoassociation of spin-polarized metastable helium to the three lowest rovibrational levels of the J =1, 0 u + state asymptoting to 2s 3 S 1 +2p 3 P 0 is studied using a second-order perturbative treatment of the line shifts valid for low laser intensities, and two variants of a nonperturbative close-coupled treatment, one based upon dressed states of the matter plus laser system, and the other on a modified radiative coupling which vanishes asymptotically, thus simulating experimental conditions. These nonperturbative treatments are valid for arbitrary laser intensities and yield the complete photoassociation resonance profile. Both variants give nearly identical results for the line shifts and widths of the resonances and show that their dependence upon laser intensity is very close to linear and quadratic, respectively, for the two lowest levels. The resonance profiles are superimposed upon a significant background loss, a feature for this metastable helium system not present in studies of photoassociation in other systems, which is due to the very shallow nature of the excited state 0 u + potential. The results for the line shifts from the close-coupled and perturbative calculations agree very closely at low laser intensities.

Effects of non-adiabatic and Coriolis couplings on the bound states of He(23S)+He(23P)

“…Two possibilities are the LS coupling scheme L = L1 + L2 , Ŝ = Ŝ1 + Ŝ2 and Ĵ = L + Ŝ + l and the jj coupling scheme ĵ1 = L1 + Ŝ1 , ĵ2 = L2 + Ŝ2 , ĵ = ĵ1 + ĵ2 and Ĵ = ĵ + l. The LS coupling scheme diagonalizes Ĥ el whereas the jj coupling scheme diagonalizes Ĥ fs . We choose to use the body-fixed jj coupled states [9] |γ 1 γ 2 j 1 j 2 j j wJ m J…”

“…The approximate coupling between the experimental collision channels and excited states is calculated from the matrix element between the metastable state and the excited basis states of the interaction Ĥ int ∼ λ • d for radiation of circular polarization λ with the molecular dipole operator d. The matrix element between basis states of the form (10) has been derived in [9] and is given by where λ = 0, ±1 represents π and σ ± polarization respectively in the space-fixed frame, b labels the polarization components in the molecular frame, I is the laser intensity and d sp at is the reduced matrix element of the dipole operator between the 2s and 2p atomic states. The basis states |Sm S lm l that are relevant to experiment, of the system He(2s 3 S 1 )+He(2s 3 S 1 ), are obtained through the unitary transformation |Sm S lm l = J m J j j δ S,j (−1) j − j × C jJ l j − j 0 C SlJ m S m l m J |j 1 j 2 j j J m J , (B.2)…”

“…The matrix elements of the various contributions to the Hamiltonian in this basis are derived in [9]. We list here the required elements using the abbreviated notation |a = | a (R, q) , where a = {γ 1 , γ 2 , j 1 , j 2 , j, j , w, J, m J }.…”

Long-range states in excited ultracold3He*–4He* dimers

“…Unfortunately there is not a clear and obvious choice for the preparation of a heteronuclear experimental gas mixture, so we have not included a set of observability criteria in this paper. It would be desirable, in such a case, to perform a scattering calculation similar to [15], which would consider the appropriate incoming 2s + 2s channels and laser coupling terms The full scattering matrix could then be obtained, along with various cross sections relevant to experiment.…”