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arXiv:physics/0603268 · Journal of Physics B Atomic Molecular and Optical Physics

On the role of Penning ionization in photoassociation spectroscopy

E van der Zwan, D. van Oosten, Driss Nehari, P. van der Straten, Henk T. C. Stoof

Published Sep 25, 2006• 2 citations
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Abstract

We study the role of Penning ionization on the photoassociation spectra of He(^3S)-He(^3S). The experimental setup is discussed and experimental results for different intensities of the probe laser are shown. For modelling the experimental results we consider coupled-channel calculations of the crossing of the ground state with the excited state at the Condon point. The coupled-channel calculations are first applied to model systems, where we consider two coupled channels without ionization, two coupled channels with ionization, and three coupled channels, for which only one of the excited states is ionizing. Finally, coupled-channel calculations are applied to photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the model and the experimental results.

DOI
10.1088/0953-4075/39/19/s02

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We study the role of Penning ionization on the photoassociation spectra of He(^3S)-He(^3S). The experimental setup is discussed and experimental results for different intensities of the probe laser are shown. For modelling the experimental results we consider coupled-channel calculations of the crossing of the ground state with the excited state at the Condon point. The coupled-channel calculations are first applied to model systems, where we consider two coupled channels without ionization, two coupled channels with ionization, and three coupled channels, for which only one of the excited states is ionizing. Finally, coupled-channel calculations are applied to photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the model and the experimental results.
Penning ionization reactions in merged beams with precisely controlled collision energies have been shown to accurately probe quantum mechanical effects in reactive collisions. A complete microscopic understanding of the reaction is, however, faced with two major challenges-the highly excited character of the reaction's entrance channel and the limited precision of even the best state-of-the-art ab initio potential energy surfaces. Here, we suggest photoassociation spectroscopy as a tool to identify the character of orbiting resonances in the entrance channel and probe the ionization width as a function of interparticle separation. We introduce the basic concept, using the example of metastable helium and argon, and discuss the general conditions under which this type of spectroscopy will be successful.
A theoretical analysis of photoassociation in ultracold metastable helium has been undertaken for the 2(^3)S1 + 2(^3)S1 to 2(^3)S1 + 2(^3)P transition. A full multichannel calculation is presented for two aspects of the system, a) the accurate calculation of energies for the bound vibrational levels of the dimer; and b) the non-perturbative calculation of the photoassociation line shapes allowing for abitrary laser intensity and detuning. A strong emphasis is placed on the accuracy of the calculations, and the elimination of approximations that have been used in previous investigations. The results indicate that full multichannel calculations are required rather than approximate single-channel calculations in some important situations, and are especially useful for making assignments between experiment and theory. Bound levels have been previously measured from the photoassociation of cold metastable thermal gases to energies close to the three asymptotes j = 0, 1, 2 of the 2(^3)S1 → 2(^3)Pj atomic transition. Previous analysis of these measurements was made using either approximate accumulated phase techniques based on long-range single-channel potentials, or using multichannel calculations and long-range potentials. Because accurate short-range electronic potentials have been calculated only recently, this thesis presents the first multichannel calculation for the levels near the j = 1, 2 asymptotes. The new short-range potentials had previously been used in an approximate single-channel calculation, but this type of calculation ignores the effects of non-adiabatic and Coriolis couplings. Presented here are the binding energies calculated from a fully multichannel technique which includes these couplings, and some differences of up to 10% from the single-channel calculations are observed. The detailed knowledge of the bound wave functions is then exploited to determine a set of observability criteria that produce a near unique assignment of theoretical levels to experimental observations. From these unique assignments, a 1% correction to the short-range electronic potentials has been implemented that significantly improves the agreement between theory and experiment. Experimental measurements have previously been made of line shifts in the photoassociation spectra of vibrational levels in the 0+u long-range potential that asymptotes to 2(^3)S1 + 2(^3)P0 and their variation with laser intensity. These results were previously analysed by performing an approximate second-order perturbative calculation of the line shift that is valid only for low laser intensities. In this thesis a completely non-perturbative calculation of the spectra and line shapes for abitrary laser intensity is presented for photoassociation to the levels probed in experiment. The techniques of dressed state formalism and a modified radiative coupling are employed to properly treat the laser coupling for large interatomic separations. These calculations produce resonance profiles superimposed on a very significant background, a feature that is not seen in photoassociation calculations of other atomic species. From the calculated line shapes, the line shifts and line widths were determined for varying intensity and a small nonlinear intensity-dependence of up to 2.5% was found for the line shift. The effects of invoking the dipole approximation for the laser coupling for the long-range molecular system are analysed by solving the equations using the complete laser coupling that does not assume the dipole approximation. Important differences are obtained between the profiles with and without the dipole approximation, and the photoassociation resonances are found to be more discernible at larger intensities when the complete laser coupling is used.
We demonstrate a number of two-colour spectroscopy techniques where the first step is the photoassociation of laser-cooled and trapped sodium atoms to form bound states of . High-resolution spectra of are obtained. Spectra of the `purely long-range' state demonstrates the use of two-colour spectroscopy to open up an ionization channel for detecting the occurrence of photoassociation. The utility of photoassociation as a technique for producing cold molecular samples in a well defined rovibrational state is demonstrated with spectra showing and (Condon internal diffraction) transitions after the photoassociation. Starting from states in the and potentials, these transitions include upward transitions to autoionizing potentials dissociating near 3P + 3P and downward transitions back to the 3S + 3S ground-state potentials. Spectroscopic constants for six levels of a doubly excited state of are given.
We explore coherent control of Penning and associative ionization in cold collisions of metastable He∗(23S){\mathrm{He}}^{*}(2{\phantom{\rule{4pt}{0ex}}}^{3}S)He∗(23S) atoms via the quantum interference between different states of the He2∗{\mathrm{He}}_{2}^{*}He2∗​ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regimes.
Introduction If, while approaching on an unbound ground-state potential, two atoms absorb a photon and couple to an excited bound molecular state, they are said to undergo photoassociation. Figure 5.1 illustrates the process. At long range electrostatic dispersion forces give rise to the ground-state molecular potential varying as C 6 / R 6 . If the two atoms are homonuclear, then a resonant dipole–dipole interaction sets up ± C 3 / R 3 excited-state repulsive and attractive potentials. Figure 5.2 shows the actual long-range excited potential curves for the sodium dimer, originating from the 2 S ½ + 2 P 3/2 and 2 S ½ + 2 P ½ separated atom states. For cold and ultracold photoassociation processes the long-range attractive potentials play the key role; the repulsive potentials figure importantly in optical shielding and suppression, the subject of Chapter 6. In the presence of a photon with frequency ω p the colliding pair with kinetic energy k B T couples from the ground-state to the attractive molecular state in a free–bound transition near the Condon point R C , the point at which the difference potential just matches ћω p . Scanning the probe laser ω p excites population of vibration–rotation states in the excited bound potential and generates a free–bound spectrum. This general class of measurements is called photoassociative spectroscopy (PAS) and can be observed in several different ways. The observation may consist of bound-state decay by spontaneous emission, most probably as the nuclei move slowly around the outer turning point, to some distribution of continuum states on the ground potential controlled by bound–free nuclear Franck–Condon overlap factors.