Study of ortho-to-paraexciton conversion in Cu$_2$O by excitonic Lyman   spectroscopy
  Using time-resolved $1s$-$2p$ excitonic Lyman spectroscopy, we study the orthoexciton-to-paraexcitons transfer, following the creation of a high density population of ultracold $1s$ orthoexcitons by resonant two-photon excitation with femtosecond pulses.   An observed fast exciton-density dependent conversion rate is attributed to spin exchange between pairs of orthoexcitons.   Implication of these results on the feasibility of BEC of paraexcitons in Cu$_2$O is discussed. 
M. Kubouchi, K. Yoshioka, R. Shimano, A. Mysyrowicz, M.   Kuwata-Gonokami