Ultracold collisions

Atoms in an optical trap (Doppler cooling Wineland, et al., 1978 optical molasses Chu, et al., 1986 magneto-optic trap Steane and Foot, 1991 Helmerson, et al., 1992) are confined and cooled to translational temperatures on the order of << 1 mK. Ultracold collisions between such trapped atoms permit the recording of bound<—free spectra with resolution limited only by the translational temperature (1 mK, which corresponds to a frequency resolution of 7 x 10-4 cm-1) (Julienne and Mies, 1989 Lett, et al., 1995 Burnett, et al., 2002). This makes spectroscopically accessible the extremely long-range regions of potential energy curves (R >10A 5Re) and otherwise only indirectly observable weakly bound or repulsive electronic states. 

Lett P D, Jessen P S, Phillips W D, Rolston S L, Westbrook C I and Gould P L 1991 Laser modification of ultracold collisions experiment Phys.Rev.Lett. 67 2139-42... 

Advances in Ultracold Collisions Experiment and Theory, J. Weiner... 

The construction of such a TDMEP was in harmony with femtosecond experiments of "transition state spectroscopy" that had been done in the late 1980s [58, 59]. It is also relevant to phenomena in current ultracold collision physics that go under the name "photoassociation" [60]. 

In this chapter, we give an overview of recent studies of ultracold atom-molecule collisions, focusing on nonreactive and reactive systems and the effect of vibrational excitation of the molecule on the collisional outcome. We will discuss both tunneling-dominated and barrierless reactions and examine recent efforts in extending these studies to ionic systems as well as molecule-molecule systems. We consider mostly the novel aspects of collisional dynamics of atom-diatom systems at cold and ultracold temperatures with illustrative results for specific systems. For more comprehensive discussion of cold and ultracold collisions including reactive and nonreactive processes and the effect of external fields we refer the reader to several review articles [6,8,13-15] that have appeared in the last few years. For details of the theoretical formalisms we refer to the chapters by Hutson and by Tscherbul and Krems. 

Quantum caleulations of rotational relaxation of CO in cold and ultracold collisions with H2 have recently been performed by Yang and colleagues [132,133]. They reported quenching rate coefficients for y = 1 to 3 of the CO molecule in collisions with both ortho- and para-H2 [132]. Due to the relatively deep van der Waals interaction potential for the H2-CO system the cross-sections exhibit a number of narrow resonances for collision energies between 1.0 and 40.0 cm . The signatures of these resonances are present in the temperature dependence of the rate coefficient, which shows broad oscillatory features in the temperature range of 10 to 50 K [132]. 

Avdeenkov and Bohn also studied ultracold collisions between OH [135,136] and OD radicals [137] in the presence of an applied electric field. They showed that elastic scattering is more efficient than inelastic processes for ultracold collisions between fermionic OD molecules [137], inhibiting state-changing collisions. The energy dependence of elastic and inelastic cross-sections for OH + OH and OD + OD collisions is illustrated in Figure 3.24 for an applied electric field of e = lOOV/cm. While the elastic cross-sections approach finite values in the Wigner regime for the bosonic system of OH molecules and the fermionic system of OD molecules, the... 

GuiUon, G., Stoecklin, T., and Voronin, A., Spin-rotation interaction in cold and ultracold collisions of with He and He, Phys. Rev. A, 75,... 

Cvitas, M.T., Sold i, R, Hutson, J.M., Honvault, R, and Launay, J.-M., Ultracold collisions involving heteronuclear alkali metal dimers, Phys. Rev. Lett., 94,200402,2005. 

FIGURE 4.4 Zero-temperature rate eonstant for Zeeman relaxation in collisions of rotation-ally ground-state NH( S) moleeules in the maximally stretched spin level with He atoms. Such field dependence is typical for Zeeman or Stark relaxation in ultracold collisions of atoms and molecules without hyperfine interaction. The variation of the relaxation rates with the field is stronger and extends to larger field values for systems with smaller reduced mass. (Adapted from Krems, R.V., Int. Rev. Phys. Chem., 24, 99, 2(X)5. With permission.)... 

Ultracold collisions of molecules are predominantly determined by the long-range part of the interaction potential between the collision partners. As discussed in Chapter 2, the long-range interaction between polar molecules placed in an electric field is dominated by the dipole-dipole forces, which can be tuned by varying the external electric fields. Figure 4.8 illustrates a characteristic dependence of rate... 

There is a massive literature on PA of ultracold atoms (primarily like atoms) recently and admirably reviewed by Jones and colleagues [13], Early work emphasizing Na2 and Rb2 PA is reviewed in Refs. [14,15], The well-studied example of K2 is treated in considerable detail in Ref. [7], while Cs2 is the focus of the review of Masnou-Seeuws and Pillet [16], The field of cold and ultracold collisions (not just PA) is admirably reviewed in Ref [17], We can only touch on a few topics here. 

WrighL M.J., Pechkis, J.A., Carini, J.L., Kallush, S., Kosloff, R., and Gould, P.L., Coherent control of ultracold collisions with chirped light Direction matters, Phys. Rev. A, 75, 05401, 2007. 

As the heavy atoms are identical fermions, we have (R, r) = — h (—R, r). Combined with Equation 10.61, this leads to the condition I (R,r) = h(—R, —r). Therefore, h(R,r) describes atom-dimer scattering with even angular momenta, and for ultracold collisions we have to solve an -wave atom-dimer scattering problem. 

Finally connections to ultracold molecular ions, to tests of fundamental laws, to quantum computing with ultracold polar molecules, to ultracold collisions and chemistry, and to more speculative areas are mentioned. 

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