New views of collisional vibrational relaxation: Energy removal rates and energy distributions of triplet state pyrazine

[摘要] Collisional energy removal rates from vibrationally excited T$sb1$ pyrazine are measured using the refined and validated Competitive Radiationless Decay (CRD) method. Optical excitation followed by intersystem crossing prepares a vibrationally excited vapor sample of T$sb1$ pyrazine. $m Tsb{n}gets Tsb1$ transient absorption kinetics, measured with a S/N ratio of ca. 1000, provides the collisional dependence of the average triplet radiationless decay rate constant. Using a calibration between this decay constant and the triplet vibrational energy, the collisional history of the sample;;s vibrational energy content is deduced. This leads to the rate of collisional energy removal as a function of the triplet pyrazine;;s vibrational energy content. Results with a variety of small relaxers comprise the most useful database to date on collisional vibrational relaxation of a triplet state polyatomic. We find the following order of relaxer effectiveness per collision:$$m He{<}Hsb2{<}Ne{<}Dsb2{<}Ar{<}Nsb2{<}Kr{<}Xe{<}CO{<}CHsb4{<}COsb2{<}Hsb2O$$These triplet state energy removal rates exceed those recently reported for vibrationally excited ground state pyrazine by a factor of ca. 7. In addition, a new method for determining the distribution of vibrational energy contents in an excited polyatomic sample is applied to vibrationally excited T$sb1$ pyrazine. The T$sb1$ population decays with a distribution of rate constants corresponding to the underlying distribution of vibrational energies. This rate constant distribution is extracted from decay kinetics through the use of a multi-Gaussian distribution model. The calibration between decay constant and triplet vibrational energy is used to deduce the molecular vibrational energy distribution, providing the first experimental view of an excited sample;;s vibrational energy distribution. Relatively narrow nascent vibrational energy distributions are progressively broadened during the early collisional encounters with a relaxer. These new vibrational energy distributions and the collisional energy removal results suggests a threshold for enhanced relaxation near 2000 cm$sp{-1}$ of donor vibrational energy. These intriguing results should stimulate further theoretical and experimental research into the collisional relaxation of electronically excited molecules.

[发布机构] Rice University

[效力级别] Molecular physics