Observation of strong forward emission due to UV multiphoton-dissociative excitation of CH4:N-2

The multiphoton dissociative excitation in a high pressure mixture of CH4:N-2 has been investigated using a third harmonics of a Nd:YAG laser under various experimental conditions, namely at a fixed pump energy (10mJ) and CH4 pressure (300PSI), varying the N-2 pressure; at a fixed pump energy (10mJ) and N-2 (30PSI) varying CH4 pressure; at fixed CH4 and N-2 pressure of 300PSI and 30PSI respectively, varying the pump energy. We have observed the first positive system, B-3 pi(g)-->A(3) Sigma(u)(+) (v'=10;v ''=5) at 544nm; second positive system, C-3 pi(u)-B-3 pi(g) at 434.4nm (v'=0;v ''=4) and at 405.9nm (v'=0;v ''=3) due to neutral N-2. We also have observed CN band B-2 Sigma-->X(2) Sigma(v'=11;v ''=11) at 392nm. These emitted signals have the same spectral directionality as the pump beam. The transition at 434.4nm is due to four photon excitation with a rate-limiting step that scales as a two photon process. The CN band is due to three photon processes. Spontaneous emission for the (B-A); (C-B) of N-2 and CN (B-X) band systems were observed but the signal was very weak for our optical detection system to record the spectrum.