Energy Partitioning in the IR Multiphoton Dissociation of Molecules: Energy of XCF2 and XCFCl from CF2CFCl

Document Type

Article

Journal/Book Title

Journal of Chemical Physics

Publication Date

1980

Volume

72

First Page

1161

Abstract

We have measured the vibrational (v), rotational (J,K), and translational energy, (E T ), of the X̃ CF2 and X̃ CFCl fragments formed in the CO2 laser induced multiphoton dissociation of CF2CFCl (chlorotrifluoroethylene): CF2CFCl→CF2 (v,J,K)+CFCl(v,J,K)+E T (v,J,K), which was the only detectable reaction path for CF2CFCl. More vibrational energy (E v ) appears in CF2 than in CFCl. Direct spectroscopicmeasurements of populations in levels 0<ν2<7 show that E v is distributed statistically in the bending mode (ν2) of CF2 according to P (E v ) =exp(−E v /k T v ), where P (E v ) is the probability of a CF2 product being formed with a particular amount of energy in ν2, and the vibrational temperature which characterizes the nascent distribution is T v (ν2) =1860±250 °K. A vibrational relaxation method was used to accurately determine f o , the fraction of CF2 and CFCl molecules initially formed in the ground vibrational level. The measurements of f o showed that the energy in the stretching modes (ν1 and ν3) of CF2 is not characterized by this T v (ν2); if the energy in ν1 and ν3 is also thermal, it must be characterized by a lower temperature: T v (ν1 and ν3) ≃1100 °K. For the CFCl product, direct spectroscopicmeasurement of the relative populations in ν2=1 and ν2=0 are consistent with T v (ν2) =1550±300 °K. However, the measuredf o for CFCl was consistent with a thermal distribution characterized by a lower vibrational temperature. Values of f o for CF2 were measured as a function of laser fluence for the condition where the reactant was extremely dilute (X CF 2 CFCl<10−5) in a high pressure (119 Torr) of Ar buffer gas. These measurements showed that the fraction of CF2 product molecules formed in vibrationally excited states decreased from 76% to 53% as the fluence decreased by a factor of 5.5, from 30 J cm−2 to 5.7 J cm−2. This decrease in E v reflects a change in the ratio of laser excitation rate to vibrational deactivation rate for the CF2CFCl reactant. Under conditions where collisions are unimportant, the initial rotational energy in the CF2 was probed and found to be consistent with a thermal distribution characterized by a rotational temperature T R =1550±150 °K. The translational energyE T was the same for CF2 fragments formed with no vibrational energy and for those formed in the ν2=5 level with E v =3320 cm−1, and E T was also the same for products formed with little rotational excitation (E R ≃40 cm−1) and for those born with substantially higher rotational energy (E R =240 cm−1). The kinetic energy of the products is less than that observed in the photodissociation of CF2HC1.

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