We have investigated the threshold of photodetachment of Li^- leading to the formation of the residual Li atom in the $2p ^2P$ state. The excited residual atom was selectively photoionized via an intermediate Rydberg state and the resulting Li^+ ion was detected. A collinear laser-ion beam geometry enabled both high resolution and sensitivity to be attained. We have demonstrated the potential of this state selective photodetachment spectroscopic method by improving the accuracy of Li electron affinity measurements an order of magnitude. From a fit to the Wigner law in the threshold region, we obtained a Li electron affinity of 0.618 049(20) eV.Comment: 5 pages,6 figures,22 reference

We report the first observation of ionization of Rydberg atoms by subpicosecond, circularly polarized THz radiation. The field amplitude in these pulses is non-negligible for only one-quarter of an optical cycle. The experiment is performed in the short-pulse regime, where the duration of the ionizing pulse is shorter than the classical Kepler period of the Rydberg electron. We find that the ionization probability for these atoms is remarkably insensitive to the time-varying polarization of the THz field.Over the years, multiphoton ionization of atoms has been recognized as a process of fundamental importance for understanding the dynamic interaction between atoms and intense radiation fields. Very recently, a number of in vestigations have led to the discovery of novel ionization dynamics during the interaction of highly excited Rydberg atoms with strong, subpicosecond pulses of THz radiation [1][2][3][4][5][6][7]. The unipolar electric field in these pulses resembles one-half of an optical cycle of an oscillating electric field so they are commonly referred to as "half-cycle" pulses (HCPs). To date, experimental and theoretical HCP stud ies have considered only linearly polarized fields. In this configuration, HCPs provide an impulsive unidirectional "kick" that can literally push a Rydberg electron off an atom [1][2][3]8]. Although the ionization dynamics are now relatively well understood for unidirectional fields, previ ous work with multi-cycle laser [9] and microwave pulses [10], as well as classical intuition, suggests that ionization by subpicosecond far-infrared pulses might proceed very differently with circularly polarized radiation.In this Letter, we describe the results of the first experi ment on the ionization of atoms by subpicosecond circu larly polarized THz pulses. The duration of these pulses is so brief that the electric field vector rotates by only 90 ± during the pulse. In the language of classical physics, an electron exposed to this "quarter-cycle" circularly po larized pulse is subjected to a rapidly rotating force in contrast to a unidirectional kick. Therefore, one should expect to observe interesting new ionization dynamics us ing these novel pulses.The interaction of atoms with these pulses is also in teresting from the point of view of collision physics. A quarter-cycle field pulse bears a strong resemblance to the time-dependent field seen by an atom undergoing a colli sion with a charged particle, as shown explicitly in Fig. 1. In each case, the field can be described as a combination of a half-cycle cosine pulse along one axis (the transverse collision field) and a "single-cycle" sine component along an orthogonal axis (the longitudinal collision field). In contrast to true collision experiments, the relative veloc ity, impact parameter, and orientation of the colliding par ticles can be precisely controlled in a "mock collision" by adjusting the duration, field amplitude, and polarization of the quarter-cycle field pulse.It is well known that circularly polarized radiation can...

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We report on the first observation of resonance structure in the total cross section for the photodetachment of Li − . The structure arises from the autodetaching decay of doubly excited 1 P o states of Li − that are bound with respect to the 3p state of the Li atom. Calculations have been performed for both Li − and H − to assist in the identification of these resonances. The lowest lying resonance is analogous to the previously observed symmetrically excited intrashell resonance in H − but it is much broader. Higher lying resonant states are observed to converge on the Li(3p)limit. These Rydberg-like resonances are much narrower and correspond to asymmetrically excited intershell states. 31.50: Excited states, 32.80Fb: Photoionization and photodetachmentTypeset using REVT E X 1

Measurements of the photodetachment cross section of C − (2s 2 2p 3 4 S) have been performed in the vicinity of the C(2s2p 3 5 S) threshold, with photon energies ranging from 5.23-6.04 eV. The experimental cross section is nearly constant throughout this energy region, in marked contrast to recent theoretical predictions of strong resonance structures. The present data indicate that the quasibound C − (2s2p 4 4 S) state may be located at higher energies than so far predicted.