Non-exponential relaxation of a single quantum vibrational excitation of a large molecule in collision free gas phase at elevated temperature

Vibrational Lifetimes and Spectral Shifts in Supercritical Fluids as a Function of Density: Experiments and Theory

“…When a buffer gas or solvent is added, collisions cause the coherent evolution of the slow modes to be interrupted frequently, resulting in a narrowing of the CO asymmetric stretch 0-1 transition and possibly an averaging away of the perturbation responsible for the observed fast time dependence. Thus, the fast component observed in these experiments 9 is inherently a low-pressure, gas-phase phenomenon. Figure 6a shows a decay curve with a moderate pressure (0.05 mol/L) of Ar in the cell.…”

“…9 A small amount of solid W(CO) 6 was placed in an evacuated 1.5 cm long stainless steel cell with CaF 2 windows. The pressure in the sample cell is the vapor pressure of W(CO) 6 .…”

“…The nature of this triexponential decay will be discussed in detail elsewhere. 9 Only the qualitative features will be described here.…”

“…Calculations using the known frequencies of all of the modes 40 show that depositing ∼2000 cm -1 into the molecule raises the vibrational temperature from the initial 326 to 450 K. This produces a red shift of the absorption spectrum, which was determined by temperature-dependent absorption measurements. 9 The pumpprobe signal has two equal parts, ground-state depletion and stimulated emission. When the excited CO stretch relaxes into the density of low-frequency modes, the contribution to the signal from stimulated emission is lost.…”

“…9 In the gas-phase vibrational experiment, prior to application of the pump pulse, the initial state of the molecule is prepared by its last collision with the wall of the cell or another molecule. The initial state is a complex superposition of eigenstates.…”