  We report on results obtained with the NEXT-DEMO prototype of the NEXT-100
high-pressure xenon gas time projection chamber (TPC), exposed to an alpha
decay calibration source. Compared to our previous measurements with alpha
particles, an upgraded detector and improved analysis techniques have been
used. We measure event-by-event correlated fluctuations between ionization and
scintillation due to electron-ion recombination in the gas, with correlation
coeffcients between -0.80 and -0.56 depending on the drift field conditions. By
combining the two signals, we obtain a 2.8 % FWHM energy resolution for 5.49
MeV alpha particles and a measurement of the optical gain of the
electroluminescent TPC. The improved energy resolution also allows us to
measure the specific activity of the radon in the gas due to natural
impurities. Finally, we measure the average ratio of excited to ionized atoms
produced in the xenon gas by alpha particles to be $0.561\pm 0.045$,
translating into an average energy to produce a primary scintillation photon of
$W_{\rm ex}=(39.2\pm 3.2)$ eV.
