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Chirped fields for Rb plus Cs photoassociation

A. Homer, G. Roberts

Published Dec 31, 2008• 0 citations
Publisher

Abstract

This paper presents model numerical simulations of photoassociation and ionization of cold Rb+Cs atoms steered by broadband optical pulses operating within the weak-field limit over a picosecond time scale. The primary focus of the work is on generation of RbCs molecules in bound levels of the (1)0(+) (X-1 Sigma(+)) electronic ground state through a sequence of pump-dump transitions between (1)0(+) and (4)0(+) states driven by a field centered at 811 nm with different phase modulations. It is found that chirped fields generate substantially bound oscillator levels of the X-1 Sigma(+) state by avoiding promotion of amplitude to vibrational levels in the upper state whose Franck-Condon factors for stimulated emission to levels in the X-1 Sigma(+) state are detrimentally low, but which otherwise come into play when the driving field is transform limited. Optimal generation of molecules in the electronic ground state irrespective of vibrational level selectivity is calculated to occur when the temporal phase of the driving field is modulated by the classical energy difference for promotion of (4)0(+)(1)0(+) stimulated emission. Driving fields phase modulated by the classical difference potential or linearly down-chirped enhance molecule formation vis-a-vis the thermal collision; fields whose temporal phase is modulated according to the shape of the (1)0(+) potential energy curve or which are linearly up-chirped suppress molecule formation relative to the thermal probability, but generate deeply bound X-1 Sigma(+) vibrational levels. Application of a bichromatic field comprising temporally overlapped components centered 811 and 622 nm also improves the probability of generating deeply bound vibrational levels, but at the expense of increased ionization. Model calculations of transition probabilities between neutral and ionic states of the collision lead to the conclusions that: transitions to individual vibrational levels of the X-1 Sigma(+) state occur with a higher probability than resonant three-photon ionization prompted by a single-color field at 811 nm; and application of bichromatic light at 811 and 622 nm preferentially ionizes the incipient molecule via a two-color two-photon absorption, the likelihood of which masks the occupation of deeply bound X-1 Sigma(+) oscillator levels.

DOI
10.25405/ncl.34105002

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We investigate the possibility of forming deeply bound ultracold RbCs molecules by a two-color photoassociation experiment. We compare the results with those for Rb 2 in order to understand the characteristic differences between heteronuclear and homonuclear molecules. The major differences arise from the different long-range potential for excited states. Ultracold 85 Rb and 133 Cs atoms colliding on the X 1 Σ + potential curve are initially photoassociated to form excited RbCs molecules in the region below the Rb(5S)+Cs(6P 1/2 ) asymptote. We explore the nature of the Ω=0 + levels in this region, which have mixed A 1 Σ + and b 3 Π character. We then study the quantum dynamics of RbCs by a time-dependent wavepacket (TDWP) approach. A wavepacket is formed by exciting a few vibronic levels and is allowed to propagate on the coupled electronic potential energy curves. We calculate the time dependence of the overlap between the wavepacket and ground-state vibrational levels. For a detuning of 7.5 cm −1 from the atomic line, the wavepacket for RbCs reaches the short-range region in about 13 ps, which is significantly faster than for the homonuclear Rb 2 system; this is mostly because of the absence of an R −3 long-range tail in the excited-state potential curves for heteronuclear systems. We give a simple semiclassical formula that relates the time taken to the long-range potential parameters. For RbCs, in contrast to Rb 2 , the excited-state wavepacket shows a substantial peak in singlet density near the inner turning point, and this produces a significant probability of de-excitation to form ground-state molecules bound by up to 1500 cm −1 . The short-range peak depends strongly on non-adiabatic coupling and is reduced if the strength of the spin–orbit coupling is increased. Our analysis of the role of spin–orbit coupling concerns the character of the mixed states in general and is important for both photoassociation and stimulated Raman de-excitation.
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We demonstrate the direct formation of vibronic ground state RbCs molecules by photoassociation of ultracold atoms followed by radiative stabilization. The photoassociation proceeds through deeply bound levels of the (2)3Π0+ state. From analysis of the relevant free-to-bound and bound-to-bound Franck–Condon factors, we have predicted and experimentally verified a set of photoassociation resonances that lead to efficient creation of molecules in the v = 0 vibrational level of the X1Σ+ electronic ground state. We also compare the observed and calculated laser intensity required to saturate the photoassociation rate. We discuss the prospects for using short-range photoassociation to create and accumulate samples of ultracold polar molecules in their rovibronic ground state.