Measurement of the rotational temperature in a nitrogen molecular beam by REMPI

Molecular beams have been used for diagnoses of solid surfaces and gas-surface interactions. For molecular beams of polyatomic gases, it is important to clarify the distribution over the energy states of the internal degree of freedom. In this study, the rotational energy distribution in a nitrogen molecular beam was spectroscopically measured by the (2+2)N ₂-REMPI (Resonantly Enhanced Multiphoton Ionization) technique. REMPI is known to have very high detection sensitivity, which allows us to detect molecules under the very low number density condition like in a molecular beam. A REMPI spectrum indicates the rotational energy distribution of molecules, and is not affected by secondary electrons, which simplifies the analysis. The molecular beam was generated using a Kantrowitz-Grey type beam source. The measured REMPI spectra were well fitted by a theoretical spectrum with the Boltzmann distribution, and the rotational temperatures were obtained. Since the supersonic freejet was employed as a beam source, the rotational temperature in the molecular beam was analyzed in accordance with the frozen rotational temperature in a supersonic freejet using the parameter p₀d, which is a product of the source pressure p₀ and the orifice diameter d. It was found that the rotational temperature is well described as a function of the parameter p₀d under the fixed source tempearture condition.