Laser-induced grating spectroscopy of highly excited overtone and combination vibrational states of methane

Laser-induced grating spectroscopy is applied to investigate weak overtone and combination absorption bands of methane. The rotationally-resolved spectra of transitions to the polyads Pn, n = 8-10, 12 recorded at pressures ranging from 0.2 to 4 bar and temperatures 298 K and 124-130 K cover a broad interval between 891 and 618 nm (11 220-16 180 cm−1) and contain a significant amount of prominent narrow lines which correspond to stationary rovibrational states. The absorption cross sections of the weakest detected methane transitions are estimated to be as small as ≈10−26 cm2 molecule−1. The technique is suitable to provide data for the development and verification of new models describing the complex energy structure of highly vibrationally excited molecules.