Long-Range Predissociation in Two-Color Photoassociation of Ultracold Na Atoms

Abstract

We report two-color photoassociative ionization of sodium in a magneto-optical trap. The experimental results yield information on both singly and doubly excited states. We find that the highest bound vibrational levels (y . 20) of the singly excited 0 2 g state predissociate into the 3 2 P 3͞2 1 3 2 S 1͞2 ͑F g 1͒ dissociation continuum due to avoided crossings of the hyperfine components of this potential with other molecular symmetries. Based on symmetry and energy consideration we argue that a doubly excited 1 u state remains autoionizing even when excited only a few GHz above the dissociation continuum. [S0031-9007(96) PACS numbers: 33.80. Gj, 32.80.Pj, 33.15.Mt, 34.50.Rk The rapid development of laser cooling techniques (for a review see [1]) during the last decade has opened the path for study of cold collisions. The narrow thermal energy distribution in which the atoms can be prepared allows spectroscopy with extremely high resolution. One of the reactions that has been studied is the photoassociative ionization reaction for sodium (Na 1 Na 1hv ! Na 2 ‫ء‬ , followed by Na 2 ‫ء‬ 1hv ! Na 2 1 1 e 2 ). The structures in the production of ions as a function of the frequency of the exciting light in these experiments reflects the boundstate structure of the singly excited states ͑Na 2 ‫ء‬ ͒. The second step, which is a bound-free transition, displays no observable structure in these experiments [2][3][4][5][6]. The reaction mechanism at those low energies displays some characteristics different from those in the reaction mechanism at typical thermal temperatures. At thermal energies the associative ionization (AI) reaction involves two atoms prepared in excited states, which due to their thermal motion reach the reaction region where autoionization takes place. At temperatures that can be reached with laser cooling techniques the velocity of the excited atoms is so low (typically a few cm͞s) that they propagate only a few a 0 before they spontaneously emit a photon and decay to the ground state. So the colliding excited atom pair will not survive to the reaction region.Until now, the reaction at low temperature has been described as a three step process [5]. First, the two colliding atoms are excited at long range to a bound singly excited molecular state (Fig. 1). Second, due to the attraction in this bound intermediate state the atoms are effectively accelerated to each other and reach the short internuclear distances where direct photoionization takes place. To distinguish this reaction from the associative ionization reaction this mechanism is referred to as photoassociative ionization (PAI).A number of groups has studied this reaction using only a single exciting laser for the two successive excitation steps [5,6]. Since the last step in this reaction is insensitive to the exciting laser frequency, they found that the production of ions as a function of frequency reflects the structure of the bound singly excited sates. Unfortunately, this excitation scheme is restricted to those singly excited states...

Photoassociation of Ultracold Atoms: A New Spectroscopic Technique

“…Ladder-type two-color photoassociative OODR experiments have been carried out for K 2 (37,63,88) and for Na 2 (49,83,84,86,105). Figure 18 shows representative direct ionization signals for three (4s ϩ 6s, 4s ϩ 5d, 4s ϩ 6d) of the five asymptotes studied (4s ϩ 4d and 4s ϩ 7s are not shown).…”

Long-range interaction of the 39K(4s)+39K(4p) asymptote by photoassociative spectroscopy. I. The g− pure long-range state and the long-range potential constants

“…[2][3][4][5][6][7][8][9][10] Recently, the rapid development in laser cooling, trapping and Bose-Einstein condensation has again triggered many new investigations of the long-range interactions of the alkali dimers. [9][10][11][12][13][14][15][16] This is because accurate knowledge of the long-range potential curves ͑especially correlated with the ground and the first-excited asymptotes such as 4sϩ4s and 4sϩ4p for K 2 ͒ is crucial in the interpretation of many new physical phenomena associated with ultracold atoms, such as cold collision dynamics, 17,18 photoassociative ionization, [19][20][21] fine-and hyperfine predissociation, [22][23][24] optical suppression of inelastic collisions, [25][26][27] as well as the stability of Bose-Einstein condensates. [28][29][30] Since long-range interactions are always associated with highly excited molecular vibrational levels and very weak binding energies ͑typically a few eV͒, determination of long-range potential energy curves based on conventional molecular spectroscopy is extremely difficult because direct Franck-Condon transitions are not able to access these vibrational levels near dissociation from the thermally populated region of the bound ground state.…”

Accurate studies on dissociation energies of diatomic molecules

“…For example, accurate physical information of D e is usually used as an important physical quantity in investigating the spectra of long-range molecules and the collision between cold atoms [2][3][4][5][6] . Moreover, accurate high-lying vibrational energies and dissociation energy play important roles in the studies of deriving long-range interatomic potentials from spectroscopic data [7] .…”