Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping


Spin-exchange optical pumping (SEOP) has been widely used to produce enhancements in nuclear spin polarisation for hyperpolarised noble gases. However, some key fundamental physical processes underlying SEOP remain poorly understood, particularly in regards to how pump laser energy absorbed during SEOP is thermalised, distributed and dissipated. This study uses in situ ultra-low frequency Raman spectroscopy to probe rotational temperatures of nitrogen buffer gas during optical pumping under conditions of high resonant laser flux and binary Xe/N2 gas mixtures. We compare two methods of collecting the Raman scattering signal from the SEOP cell: a conventional orthogonal arrangement combining intrinsic spatial filtering with the utilisation of the internal baffles of the Raman spectrometer, eliminating probe laser light and Rayleigh scattering, versus a new in-line modular design that uses ultra-narrowband notch filters to remove such unwanted contributions. We report a ~23-fold improvement in detection sensitivity using the in-line module, which leads to faster data acquisition and more accurate real-time monitoring of energy transport processes during optical pumping. The utility of this approach is demonstrated via measurements of the local internal gas temperature (which can greatly exceed the externally measured temperature) as a function of incident laser power and position within the cell.

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We thank S. Fitzgerald, W. Twigger, R. Senghani, and A. Knowles (Horiba Jobin–Yvon) for experimental assistance and loan of equipment, as well as Prof. A. Compaan (U. of Toledo) for helpful discussions. H. Newton is funded by an EPSRC studentship. N. Whiting is an NSF International Research Fellow (OISE 0966393). B.M. Goodson is a Cottrell Scholar of Research Corporation. Work at Southern Illinois University Carbondale is supported by DoD CDMRP (W81XWH-12-1-0159/BC112431) and SIUC OSPA. M.J. Barlow acknowledges the School of Clinical Sciences, University of Nottingham and ESPRC grant EP/G003076/1.

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Correspondence to Michael J. Barlow.

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Newton, H., Walkup, L.L., Whiting, N. et al. Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping. Appl. Phys. B 115, 167–172 (2014).

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  • Notch Filter
  • Raman Line
  • Raman Spectrometer
  • Alkali Metal Atom
  • Nuclear Spin Polarisation