Photoassociation Spectroscopy

From measurements of the absorption probability of transitions from very high levels of the ground state the upper state potential and the dependence of the transition moment on the inter-nuclear distance R can be obtained with high precision. 

Generally the triplet ground state of alkali dimers is not directly accessible by absorption spectroscopy. Photo-association spectroscopy of cold colliding Li atoms wit a relative angular momentum / = 1 (/7-scattering) allows transitions from the state into an excited triplet state. Their measurements give precise values of high vibrational levels and allow the determination of the triplet potential. From measurements of rotational levels also the spin-spin and spin-rotation interaction can be deduced [620]. 

By exciting a pair of metastable He atoms in the metastable state 2 5 1 with a narrow-band laser at X = 1083 nm, bound vibrational levels of the helium dimer He in the electronic 0 state (2 S + 2 Pq) could be populated in a magnetic trap [619]. The inner turning point at R = ISOao and the outer turning point at 1150 0 = 57.5 nm show that these excited dimers are indeed giant molecules in terms of their size. 

Infrared-UV double resonance has proved to be very useful for the solution of the conformational structure of large bio-molecules [621]. Here the absorption of an infrared photon on a selected vibrational transition is detected by two-photon ionization of the excited vibrational level. In Fig. 5.44 the infrared UV-double resonance spectra for the two lowest energy conformations of the MAPE molecule (2-methylamino 1-phenyl ethanol) [621]. The lower level of the 4 IR-transitions A B C and D can be labelled by tuning the IR-laser on the corresponding transitions which depletes the lower levels and weakens the selected line in the absorption spectrum. 

1 A linearly polarized laser beam passes in z-direction through a sample cell. Its 

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Finally, we note that ultracold hydrogen holds enormous potential for the study of atomic interactions. For instance, scattering lengths can be determined directly from spectral line shifts [8]. Furthermore, photoassociation spectroscopy can be used to investigate the potentials for excited H% potentials, as in a recent investigation of the molecular triplet a317+ potential [13],... 

K.M. Jones, E. Tiesinga, P.D. Lett, PS. Julienne, Ultracold photoassociation spectroscopy Long-range molecules and atomic scattering. Rev. Mod. Phys. 78 (2006) 483. 

N. Vanhaecke et al.. Photoassociation spectroscopy of ultracold long-range molecules. C. R. Phys. 5, 161 (2004)... 

Tiesinga, E., Williams, C.J., and Julienne, P.S., Photoassociative spectroscopy of highly excited vibrational levels of aUcali-metal dimers Green-function approach for eigenvalue solvers, Phys. Rev. A, 57, 4257-4267,1998. 

FIGURE 5.10 Two-color lambda-type photoassociation spectroscopy of Rb2- Colliding, trapped ultracold Rb atoms are irradiated by lasers of frequencies vj and V2- Spontaneous emission from the excited level at frequency leads to loss of the atoms from the trap. Optical-optical double-resonance signals (free-bound-bound) are observed when the frequency difference V2 — vj coincides with the binding energy of a lower-state vibrational level. (From Tsai, C.C. et al., Phys. Rev. Lett, 79,1245, 1997. With permission.)... 

Wang, H., Wang, X.T., Gould, P.L., and Stwalley, W.C. Optical-optical double resonance photoassociative spectroscopy of ullracold atoms near highly excited asymptotes, Phys. Rev. Lett, 78, 4173, 1997. 

Wang, H. and Stwalley, W.C., Ultracold photoassociative spectroscopy of heteronuclear aUcali-metal diatomic molecules, J. Chem. Phys., 108, 5767,1998. 

Comparat, D., Drag, C., Fioretti, A., Dulieu, O., and PUlet, R, Photoassociative spectroscopy and formation of cold molecules in cold cesium vapor Trap-loss spectrum versus ion spectrum, J. Mol. Spectrosc., 195, 229, 1999. 

Kitagawa, M., Enomoto, K., Kasa, K., Takahashi, Y., Ciurylo, R., Naidon, R, and Julienne, P.S., Two-color photoassociation spectroscopy of ytterbium atoms and the precise determinations of s-wave scattering lengths, Phys. Rev. A, 11, 012719, 2008. 

Wang, H., Nikolov, A.N., Ensher, J.R., et al.. Ground-state scattering lengths for potassium isotopes determined by double-resonance photoassociative spectroscopy of ultracold Phys. Rev. A, 62,052704, 2000. 

FIGURE 16.2 Levels and in Cs2 molecule. (Adapted from Sainis, S., Two-color photoassociation spectroscopy of the state of Cs2, PhD Thesis, Yale University, 2005.)... 

Second the photoassociation spectroscopy of cold atoms allows the detailed investigation of the long-range part of the difference potential between... 

Three questions have motivated significant developments in cold collisions (1) How do collisions lead to loss of atom confinement in traps (2) How can photoassociation spectroscopy yield precision measurements of atomic properties and insight into the quantum nature of the scattering process itself (3) How can optical fields be used to control the outcome of a collisional encounter The first question will be largely ignored in this chapter. The answer to the second question has proved to be fruitful not only for molecular spectroscopy but has also contributed to real advances in the precision measurement of atomic properties. 

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