Everett Thiele

Collisional effects in the multiphoton dissociation of CF2CFCl

We report results of a combined experimental and theoretical study of the effects of collisions with an inert buffer gas, on the CO2 laser induced MPD of CF2CFCl to form CF2 and CFCl. Rates of formation of the primary product CF2 have been determined, in real time using the laser excited fluorescence technique, at four IR laser intensities (Imax = 35, 47, 73, 220 MW/cm2) and a range of argon buffer gas pressures (0⩽PAr⩽500 Torr). The experimental data clearly show the effects of collisional hole filling at low pressures and V–T collisional deactivation at higher (≳100 Torr) pressures. We present a generally applicable theoretical model for collisional effects in MPD, in which two parameters (1/τ, ΔE) specify the collisional deactivation, two parameters (s, <ω≳) specify the density of states of the absorber, two parameters (A∞,Eact) specify the microscopic reaction rates and one parameter (δ) specifies the radiative pumping rates. In applying this model to CF2CFCl five of these seven parameters are readily...

The restricted quantum exchange theory of intramolecular T1 and T2 relaxation rates

We present a theory for intramolecular vibrational relaxation in polyatomic molecules. The theory postulates the existence of a restriction on the magnitudes of matrix elements connecting zero‐order states which favors coupling between vibrational modes. Specifically, the matrix elements are assumed to depend on two parameters, one determining the overall magnitude of coupling, and the other the rate of falloff with increasing quantum exchange. We use this ’’restricted quantum exchange’’ (RQE) hypothesis to derive analytic expressions for T1 (energy‐transfer) and T2 (coherence‐loss) relaxation rates which depend only on the two coupling parameters, the average molecular frequency, the number of modes, the energy in the molecule, and (for T1) the size of the energy transfer. In the derivation we obtain analytic expressions for the number of pairs of states on an energy shell related to each other by exchange of a fixed number of quantua M. The analysis has been carried out for arbitrary M, allowing us form...

Restricted quantum exchange theory for intramolecular vibrational relaxation in polyatomic molecules

Abstract We pose and solve a new quantum statistical counting problem and develop it further to obtain a theory for intramolecular vibrational coupling. Simple analytical expressions are presented for T 1 and T 2 relaxation rates. The formulas predict, in agreement with experiments, that high energy linewidths are nearly energy independent.

A quantitative test of unimolecular rate theory in the multiphoton dissociation of CF2CFCl

We have measured the distribution of total energy in reaction products for the CO2 laser‐induced MPD: CF2CFCl→CF2+CFC1. From a separate study of MPD rates as a function of laser intensity and inert buffer gas pressure, reliable estimates of the radiative pumping rates are known for this reaction. These results, when analyzed together, allow us to extract a unimolecular A factor from the MPD data. The determined value A = 3×1016 sec −1 agrees well with estimates based on independent thermal data.

Can Lasers Be Used To Break Chemical Bonds Selectively

We discuss the possibility of selective bond breaking induced by the multiple photon absorption of infrared (IR) radiation. The rate at which a reactant molecules internal vibrational energy distribution relaxes to a random distribution (intramolecular vibrational relaxation [IVR] rate) plays the central role in this discussion. It is pointed out that statistical theories of unimolecular decay, like the Rice Rampsburger Kassel (RRK) and Rice Rampsburger Kassel Marcus (RRKM) theories, preclude any possibility of selectivity because of their assumption of very rapid IVR rates. A new model of IVR and unimolecular decay which does not assume very rapid IVR rates, the restricted IVR model, is presented. Within the framework of this model the conditions necessary for selectivity are discussed quantitatively. A review of the available evidence concerning IVR rates suggests that selectivity is possible with sufficiently short and intense laser pulses. In the light of current knowledge about IVR rates we see no reason to doubt that Hall and Kaldors experiment using separately two laser pulses of widely differing frequencies to induce the simultaneous isomerization and fragmentation of cyclopropane is a genuine example of selective bond breaking.

On the nature of laser-induced energy distributions in polyatomic molecules

Abstract Necessary and sufficient conditions are found for the existence of a thermal distribution solution to the rate equations which describe radiative pumping in the quasicontinuum of closely spaced vibrational levels of a polyatomic molecule. Deviations from thermal behavior are illustrated.

Restricted intramolecular vibrational relaxation in polyatomics and laser selective effects

Abstract A theory for unimolecular dissociation applicable to selective laser excitation is presented. Expressions for lifetimes, yields, and product fragment translational energy distributions are given as a function of IVR rate. We analyze an experiment purporting to “ demonstrate” rapid IVR.

Concerning the effect of laser pulse shape on the product yield in collisionless MPD

Abstract Intrinsic to rate equation descriptions of MPD is a dependence of yield on pulse shape Upper and lower bounds are denied showing this “intrinsic” pulse shape effect to be small for overall yield, but potentially large for relative product distribution.

The unimolecular reaction of isolated CF3CN: The influence of laser fluence/intensity on the rovibronic excitation of CN(X 2Σ+) produced via infrared multiple photon dissociation

Measurements of nascent CN(X 2Σ+) rovibronic state distributions following the unimolecular reaction CF3CN‡→CF3+CN are reported. Excitation under collision free conditions is provided by IR multiple photon excitation using the focused output from a CO2 TEA laser at fluences 3–150 J cm−2. At fluences 3–20 J cm−2, the CN(X 2Σ+,v′′ = 0) rotational temperature increases monotonically from 500 to 1200 K with increasing fluence, while at fluences 30–150 J cm−2, the rotational and vibrational temperatures do not change and are TR = 1200±100 K and TV = 2400±150 K. At low fluences, the increase in V, R, T excitations with increasing fluence reflects an increase in the vibrational excitation of the dissociating parent molecules. The ultimate level of excitation that the parent molecules attain depends both on the fluence and the intensity of the laser, and the appearance time of the CN fragments decreases with increasing fluence, since molecules excited significantly above dissociation threshold decompose more rapi...

A measure of quantum chaos

From any density matrix one can calculate an entropy. If by some process a given density matrix is transformed into another density matrix with higher entropy then that process causes a loss of information. We propose that the loss of information that occurs upon time averaging the density matrix of a given initial state be taken as a relative measure of quantum chaos. We also define a weighted time average of the density matrix, based on the assumption that the time interval between preparation and observation of the state of the system is distributed as for a randomly occurring event, with frequency λ. The loss of information that occurs with this weighted time average is a monotonic function of λ. This fact suggests a definition for an intramolecular relaxation time that is basis set independent.