text: Gerhard Herzberg was the first to find spectroscopic lines of neutral H when he was 75 years old in 1979.  Later he announced that this observation was one of his favourite discoveries. The lines came about from a cathode discharge tube. The reason that earlier observers could not see any H spectral lines, was due to them being swamped by the spectrum of the much more abundant H. The important advance was to separate out H so it could be observed alone.  Separation uses mass spectroscopy separation of the positive ions, so that H with mass 3 can be separated from H with mass 2.  However there is still some contamination from HD, which also has mass 3.
  The spectrum of H is mainly due to transitions to the longer lived state of 2pA".  The spectrum can be measured via a two step photo-ionization method.
  Transitions dropping to the lower 2sA' state are affected by its very short lifetime in what is called predissociation.  The spectral lines involved are broadened. In the spectrum there are bands due to rotation with P Q and R branches.  The R branch is very weak in H isotopomer but strong with D (trideuterium).
  The symmetric stretch vibration mode has a wave number of 3213.1 cm for the 3sA' level and 3168 cm for 3dE" and 3254 cm for 2pA". The bending vibrational frequencies are also quite similar to those for .