  This work presents an experimental protocol conceived to determine the
vibrational distribution of barium monofluoride molecules seeded in a
supersonic beam of argon. Here, as in many cases, the detection signal is
related to the number of molecules by an efficiency involving several
parameters that may be difficult to determine properly. In particular, this
efficiency depends on the vibrational level of the detected molecules. Our
approach avoids these complications by comparing different detection signals
generated by different vibrational excitations. Such an excitation is made
possible by the use of a broadband optical source that depletes a specific
vibrational level whose population is redistributed in the other levels.
