Rotational energy distribution in a nitrogen molecular beam was experimentally studied by (2+2) N2-REMPI(Resonantly Enhanced Multiphoton Ionization). REMPI is known to have high detection sensitivity, which allows obtaining the signal under the very low number density condition like a molecular beam, successfully. Obtained REMPI spectrum was fitted by a theoretical spectrum to determine rotational temperature. The spectrum was well fitted, showing the rotational energy distribution obeyed the equilibrium Boltzmann distribution. The rotational temperature in a molecular beam must be similar to the frozen rotational temperature in a free jet, since a free jet is used as a beam source in the Kantrowitz-Grey type. The parameter p0d, which is a product of the source pressure p0 and the orifice diameter d, is known to characterize a free jet. Therefore, the rotational temperature in a molecular beam was analyzed in terms of the parameter p0d. The rotational temperature was able to be described by the power of the parameter p0d. The rotational temperature and the obtained function were compared with the value in literatures, which showed good agreement.
