The threshold region of the methane carbon K-absorption spectrum

The carbon K-absorption of CH4 is analyzed with the help of an ab initio calculation of the Rydberg states of la1-1 hole state. The strong absorption peak at 288.3 eV is identified as the first Rydberg 2t* ← la1, whose calculated ionization potential is in good agreement with experiment. Also, the oscillator strength for this transition was calculated to be f = 1.9 × 10 -2, which is within the estimated margin of error of the experimental f = 0.6 × 10-2. The weak peak at 287.2 eV is interpreted as a vibronic transition to the 3a1 (3s) Rydberg. An estimate of its oscillator strength as 10% of the allowed 2t(3p) Rydberg line is consistent with experiment. For the case of CD4, this weak forbidden transition was estimated to be 20% of the allowed 2t(3p) Rydberg line. © 1973.