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

By Derek Ray McDowell


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. $\rm T\sb{n}\gets T\sb1$ 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:$$\rm He{<}H\sb2{<}Ne{<}D\sb2{<}Ar{<}N\sb2{<}Kr{<}Xe{<}CO{<}CH\sb4{<}CO\sb2{<}H\sb2O$$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

Topics: Physical chemistry, Molecular physics
Year: 1997
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