On the role of Penning ionization in photoassociation spectroscopy (original) (raw)
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Photoassociation Spectroscopy of Cold He(2 3S) Atoms
Physical Review Letters, 2000
We observe vibrational states by photoassociation spectroscopy of cold He͑2 3 S͒ atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 3 S 1 -2 3 P 2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed. PACS numbers: 33.80.Eh, 32.80.Pj, 33.20.Tp Photoassociation spectroscopy of cold atoms is a new and powerful technique to investigate long-range interactions between atoms . Experiments in alkali-atom traps have revealed precise information on the long-range part of molecular potentials. This has resulted in the precise determination of s-wave scattering lengths for these systems, of high interest for studies of Bose-Einstein condensation in dilute trapped gases . In these experiments a probe laser beam is sent through a cloud of cold atoms in a trap and the trap loss as a function of the laser frequency is detected. When the laser frequency is tuned to a photoassociative resonance, trap losses increase through the process of radiative escape or the formation of molecules, which are not trapped . In some cases the excited molecules can also be photoionized by absorption of a second photon and the produced ions can be detected with high efficiency [1]. In view of this success it is obvious that application of the same method to cold rare gas metastables is of great value. Especially in the case of metastable He͑2 3 S͒ atoms ͑He ء ͒, the detailed information that can be obtained on the long-range interactions of collision systems in ground and photoexcited states is of fundamental interest. Metastable rare gas systems differ from alkali systems mainly by the possibility of Penning ionization occurring at small internuclear distances. A molecule formed by photoassociation would contain an internal energy of 40 eV and its lifetime due to Penning ionization is expected to be short compared with a vibrational period, thereby excluding photoassociation spectroscopy for this system. For He͑2 3 S͒ atoms, close collisions in the molecular 1 S 1 g or 3 S 1 g potentials have ionization probability close to unity [6], while for the 5 S 1 g potential the ionization probability is reduced by orders of magnitude [7] due to the total spin conservation selection rule. An ionization rate constant of 1.3 3 10 210 cm 3 ͞s in laser-cooled unpolarized He ء atomic clouds [8] is attributed to these close collisions. It is to be expected that, for the molecular potentials relevant for the photoassociation process, i.e., potentials that asymptotically belong to He͑2 3 S͒-He͑2 3 P͒, the situation at small internuclear distances is equivalent: the singlet and triplet states decay with high probability, while the quintet states are stable. However, the fine-structure interaction implies the possibility of spin mixing at large distances. It is thus an open question how this mixing affects the formation and detection of photoassociated molecules. An earlier measurement of the ion rate as a function of the detuning did not show any vibrational resonances [9]. However, this measurement was aimed at a determination of the general shape of the ion rate curve and does not exclude the existence of such resonances due to insufficient statistics. In this paper we demonstrate that photoassociation spectroscopy is indeed possible on the He͑2 3 S͒-He͑2 3 P͒ system.
Absolute Photoionization Cross Sections of Excited He States in the Near-Threshold Region
Physical Review Letters, 1999
The absolute photoionization cross sections of the excited, short-lived He ء 1s2p 1 P and 1s3p 1 P states are determined experimentally in the region close to the He 1 1s 2 S threshold (from 0 to 2 eV). The intermediate He ء states are prepared by photoabsorption of a high-order harmonic of an intense picosecond tunable laser and subsequently ionized by absorption of photons of several fixed frequencies, ranging from the near infrared to the ultraviolet. Our experimental results quantitatively confirm earlier theoretical work. [S0031-9007(99)09309-6]
Some recently found interchannel coupling effects in atomic photoionization processes
Radiation Physics and Chemistry, 2004
The present article highlights some recently found interchannel coupling effects that must be taken into account to explain an atomic photoabsorption process. The importance of interchannel coupling effects is illustrated in a few select cases of recent interest: (a) sustenance of interchannel coupling effects at photon energies high enough where the independent particle approximation was believed to hold good; (b) demonstration of the generality of a new spin-orbit activated correlation effect as an essentially many-body collective event; (c) recognition of the scope and limitations of the indispensable approximation techniques to account for each 'kind' of correlation effect in configuration interactions between 'discrete' and 'continuum' channels; and (d) the recognition in some recent studies on atomic mercury, that contrary to the earliest expectations, dynamical interchannel coupling between dipole and quadrupole channels is important as found in the studies of photoionization of atomic xenon as well. r
Calculation of the photoionization with deexcitation cross sections of He and helium-like ions
Journal of Experimental and Theoretical Physics, 2006
We discuss the results of the calculation of the photoionization with de-excitation of excited He and helium-like ions Li + and B 3+ at high but non-relativistic photon energies ω. Several lower 1 S and 3 S states are considered. We present and analyze the ratios R + * d of the cross sections of photoionization with de-excitation, σ + * (d) (ω), and of the photo-ionization with excitation, σ + * (ω). The dependence of R + * d on the excitation of the target object and the charge of its nucleus is presented. Apart to theoretical interest, results obtained can be verified using such long living excited state as 2 3 S of He.
Photoionization electron spectra in a system interacting with a neighboring atom
Physical Review A, 2011
Photoelectron ionization spectra of a system interacting with a neighbor two-level atom are investigated using the Laplace-transform method. These spectra are typically composed of several peaks. Photoelectron ionization spectra conditioned by the measurement on the two-level atom show oscillations at the Rabi frequency. The presence of spectral zeros occurring periodically with the Rabi period is predicted. This phenomenon is analyzed in detail.
Photodetachment study of the 1 s 3 s 4 s 4 S resonance in He −
Physical Review A, 1997
A Feshbach resonance associated with the 1s3s4s 4 S state of He − has been observed in the He(1s2s 3 S) + e − (ǫs) partial photodetachment cross section. The residual He(1s2s 3 S) atoms were resonantly ionized and the resulting He + ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy E r = 2.959 255(7) eV and a width Γ = 0.19(3) meV, in agreement with a recent calculation.
Line shape analysis of two-colour photoassociation spectra on the example of the Cs ground state
The European Physical Journal D, 2002
Two-photon photoassociation spectra in a Λ-type excitation scheme are analysed under the systematically varied experimental conditions of frequency detunings and laser intensities. Line shape fits are presented as well as the investigation of intensity and detuning dependent line shifts. From both we determine the attained spectroscopic precision, that is corrected for a systematic line shift due to the thermal distribution of atoms in the trap. An energy correction for this effect is given. Information about the feasibility of generating translationally cold molecules in a well defined rotational and vibrational level by the photoassociation process is derived from the analysis.
Physical Review A, 2002
The photodetachment of the metastable He Ϫ 1s2s2p 4 P o state has been calculated in two photon energy regions of interest: the first, named here as energy region I, below the double photoionization threshold He ϩ (nϭ1), involving outer-shell ionization and doubly excited states of He Ϫ , and the second, named here as region II, above the He Ϫ 1s ionization threshold and below the He ϩ (nϭ2) threshold, involving K-shell detachment and triply excited states of a He Ϫ ''hollow ion.'' We have implemented an ab initio three-electron configuration-interaction method in the LS-coupling scheme combined with complex scaling to obtain resonance positions and widths and the photodetachment cross sections. We have revisited region I, although widely studied before, as a test of our method. Notwithstanding some small discrepancies, our complex scaling results compare well with the previously published results and also add new understanding to some features in the cross section. Our emphasis is given to K-shell photodetachment in photon energy region II, where comparison is made with two other recent theoretical calculations that use completely different methods, and that were in dispute. We also compare with a very recent experiment for the He Ϫ K-shell photodetachment, which displays three major features; a broad nonresonant hump after the He 2s2p 3 P o threshold and two other peaks. A complex scaling analysis of prominent structures in the photodetachment spectra in region II, previously claimed to be nonresonant structures, leads to a different conclusion; i.e., they are all true triply excitedstate resonances, and two of them correspond to the peaks observed experimentally.
Photodissociation of the HeH^{+} ion into excited fragments (n=2,3) by time-dependent methods
Physical Review A, 2009
The total and partial photodissociation cross sections of the molecular ion HeH + are computed by timedependent methods for fragmentation into the excited shells n =1,2,3 up to a photon energy of 40 eV. 1 ⌺ + and 1 ⌸ states are considered for parallel and perpendicular transitions for different initial rotational or vibrational excitations. Nonadiabatic radial and rotational couplings are taken into account. The results from coupledchannel equations are compared with the Born-Oppenheimer approximation. A time-dependent calculation with a femtosecond laser pulse is carried out to simulate a recent crossed beam photodissociation imaging experiment with vacuum ultraviolet free-electron laser ͓H. B. Pedersen et al., Phys. Rev. Lett. 98, 223202 ͑2007͔͒. The dominance of photodissociation perpendicular to the photon polarization is confirmed.