Photoassociation of ultracold metastable helium

Cocks, Daniel Graeme (2010) Photoassociation of ultracold metastable helium. PhD thesis, James Cook University.

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Abstract

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.

Item ID: 19030
Item Type: Thesis (PhD)
Keywords: multichannel calculations, ultracold atomic collisions, photoassociation, metastable helium, atomic scattering, atomic physics, wave functions, non-adiabatic couplings, Coriolis couplings, perturbation theory, resonance profiles
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Cocks, D.G., Whittingham, I.B., and Peach, G. (2010) Effects of non-adiabatic and Coriolis couplings on the bound states of He(2 ^3S) + He(2 ^3P). Journal of Physics B: Atomic, Molecular and Optical Physics 43(13): 1-12.

Cocks, D.G., and Whittingham, I. (2010) Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers. Physical Review A: Atomic, Molecular and Optical Physics 81(3): 1-5.

Cocks, D.G., and Whittingham I.B. (2009) Theoretical studies of photoassociation in ultracold metastable helium. Journal of Physics: Conference Series 185(1): 1-4.

Cocks, D.G., and Whittingham, I.B. (2009) Laser-intensity dependence of photoassociation in ultracold metastable helium. Physical Review A: Atomic, Molecular and Optical Physics 80(2): 1-13.

Cocks, D., Whittingham, I.B., and Peach, G. (2009) Bound states of He(2 3S) + He(2 3P). XXVI International Conference on Photonic, Electronic and Atomic Collisions (XXVIth ICPEAC) Kalamazoo, Michigan 2009.

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Date Deposited: 28 Nov 2011 22:53
FoR Codes: 02 PHYSICAL SCIENCES > 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics > 020201 Atomic and Molecular Physics @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970102 Expanding Knowledge in the Physical Sciences @ 100%
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