The properties of molecular beams of ICl and IBr built by means of the seeded beam technique were investigated experimentally. The axial velocity distribution was measured in a pseudorandom timehyphen;ofhyphen;flight experiment and the rotational state distribution was estimated from the focusing properties of the beam in an alternate gradient focusing system. From a comparison between the computer simulation of a focusing experiment and actual measured results, it can be stated that the population of the very lowest rotational states with rotational quantum numbersJ=0, 1, 2 do not deviate significantly from a Boltzmann distribution. The rotational temperature is not very different from the translational temperature and has a value between 1 and 3 K. Collision numbers which are a measure of the translational rotational relaxation, are estimated from the experimental results.
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