Beyond adiabatic elimination: A hierarchy of approximations for multi-photon processes Vanessa Paulisch, Han Rui, Hui Khoon Ng and Berthold-Georg Englert Eur. Phys. J. Plus, 129 1 (2014) 12 Published online: 28 January 2014 DOI: 10.1140/epjp/i2014-14012-8