  On 21 March 2013 high-resolution slit spectrographs of comet C/2011 L4
(Panstarrs), at the heliocentric distance r = 0.46 Astronomical Units (AU),
were obtained at the Osservatorio Astronomico Campo dei Fiori, Italy. Emission
lines of sodium were the strongest in the spectrum as is common in comets, but
also potassium lines were detected. These have been rarely observed in comets
since the apparition of brightest comet C/1965 S1 (Ikeya-Seki). Lithium was not
detected, and stringent upper limits of its abundance compared to other alkali
were derived. We obtain abundance ratios Na/K = 54 +/-14 and Na/Li > 8000. As
well as in Mercury exosphere (Leblanc & Douressoundiram 2011)}, we show that
photoionization at the beginning of the alkali tails may increase the solar
ratio Na/K = 15.5 (Asplund et al 2009) by a factor 3, close to that required to
match the observed value. In the same tail position the Na/Li ratio increases
by a factor 2 only, very far from the factor > 8 required to match an original
meteoritic ratio. We apply the same model to similar alkali data (Preston 1967)
of comet C/1965 S1 (Ikeya-Seki), obtaining consistent results. An original
solar Na/K ratio fits the observed value at the beginning of the alkali tails
within the slit size, whereas Li is depleted by a factor > 8.
