A temperature-drift technique for locating moderately excited states

In this report a temperature-drift (TD) technique is described which allows one to determine the energy content of moderately excited states. The basic equations were derived for a primitive model, which proves to be not overly restricted. The application of this procedure was illustrated by determining the energy of the band of states just above the barrier summit which separates the syn/anti isomers of methyl nitrite. Rapid changes can be induced in the IR absorption spectrum of methyl nitrite when the gas (pressure range 5-30 torr) is irradiated with pulses of a CO₂ laser. In the unperturbed gas, there is a well-resolved doublet corresponding to the syn (1620 cm^-1) and anti (1677 cm^-1) isomers. The transition involved is v = 0 → 1 of the N=O stretching mode. CO₂ laser pulses induce the growth of an intermediate absorption peak (ṽ = 1652 cm^-1). The induced absorption shows an exponential decay after the laser is shut off. The decay time increases linearly with the pressure, indicating that it is due to thermal cooling of the gas. At any specified pressure, the decay time of the transient absorption is clearly shorter than the decay time of the fluorescence which originates from the transition v = 1 → 0 of the N=O stretching mode. This fluorescence decay time also increases linearly with the pressure. Comparison of the two decay times allows one to estimate the energy of excited species when that of the fluorescing state is known. The value of 11.5 ± 2.5 kcal/mol thus obtained may be identified with an intermediate configuration in the syn ⇄ anti isomerization reaction; it corresponds to a free rotator around the OCabrian (era) sign;N=O bond. An independent measurement of the barrier height for the gas-phase isomerization of methyl nitrite (via NMR), in the second-order regime, gave a value of 10.5 kcal/mol.