No abstract

The temperature dependence of the collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been investigated for the temperature range 243–364 K using high resolution time resolved diode laser spectroscopy. Particular emphasis is placed on vibration to rotation-translation (V→R/T) energy transfer which leaves the CO2 vibrations unexcited and occurs predominantly through short-range repulsive forces. Vibrationally hot pyrazine is prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. For the range of experimental cell temperatures used here, the nascent rotational population distributions of the 0000 ground state of CO2 resulting from collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. The CO2 translational recoil velocity was also measured for individual rotational levels of the 0000 state. In addition, temperature dependent rate constants and probabilities were determined for energy transfer from the vibrationally hot pyrazine into individual rotational levels of the 0000 state of CO2. The rotational distributions, velocity recoils, and quenching rates exhibit a very weak temperature dependence for production of CO2 high J states, indicating that the CO2 molecules involved in these energy transfer events originate from rotational levels only slightly greater than the thermal mean J value. Based on these results, values for ΔE, the energy transfer from hot pyrazine to CO2 resulting in final CO2 0000 states J=58 through J=82, are estimated to range from 2550 to 7090 cm−1 in a single collision.

We have constructed an optical centrifuge with a pulse energy that is more than 2 orders of magnitude larger than previously reported instruments. This high pulse energy enables us to create large enough number densities of molecules in extreme rotational states to perform high-resolution state-resolved transient IR absorption measurements. Here we report the first studies of energy transfer dynamics involving molecules in extreme rotational states. In these studies, the optical centrifuge drives CO 2 molecules into states with J ∼ 220 and we use transient IR probing to monitor the subsequent rotational, translational, and vibrational energy flow dynamics. The results reported here provide the first molecular insights into the relaxation of molecules with rotational energy that is comparable to that of a chemical bond.carbon dioxide | rotational dynamics | transient spectroscopy | high-energy molecules | strong optical fields C ontrol of molecular energy for use in chemical and physical transformations requires tools for exciting specific degrees of freedom in molecules. A number of methods exist for preparing molecules with large, well-defined, and controllable amounts of energy in electronic, translational, and vibrational degrees of freedom, but until recently, it has been much more difficult to exert control over the rotational energy of molecules (1-14). Traditional methods for optically preparing rotationally hot molecules are limited by strict selection rules that constrain angular momentum changes to small values (15, 16). Microwave spectroscopy has been used to a limited degree for walking molecules up a rotational ladder, but only small amounts of rotational energy (ΔJ ∼ 5) could be imparted to molecules with this approach (17, 18). Static electric fields have been explored for orienting molecules, but this approach is impractical for rotating molecules into high-energy states due to the high angular velocity and voltages required (19). Rotational motion in molecules can be induced with strong optical fields leading to rotational recurrences, but the amount of rotational energy obtained with this method is fairly modest (19)(20)(21)(22)(23)(24)(25). In some cases, photochemical reactions and inelastic collisions can be used to produce rotationally hot molecules, but the products generally have broad and poorly controlled rotational energy distributions (26).An important development in the area of light-matter interactions is the optical centrifuge for molecules (27,28). In this device, powerful ultrafast chirped laser pulses deposit rotational excitation in molecules that is comparable to, and in some cases even exceeds, interatomic binding energies. The optical centrifuge was proposed by Corkum and coworkers in 1999 and was first demonstrated in 2000 (27, 28). They used an optical centrifuge to spin Cl 2 molecules into J ∼ 420 and used a time-of-flight mass spectrometer to detect Cl radicals that resulted from rotationally induced dissociation. The dissociation energy of Cl 2 is 3.5 eV, but rotational...

Translational and rotational excitation of the CO 2 (00 0 0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene: Evidence for impulsive collisions accompanied by large energy transfers Classical trajectory calculations of the rate of collisional energy transfer between a bath gas and a highly excited polyatomic method, and the average energy transferred per collision, as functions of the bath gas translational energy and temperature, are reported. The method used is that of Lim and Gilbert [J. Phys. Chem. 94, 72 (1990)], which requires only about 500 trajectories for convergence, and generates extensive data on the collisional energy transfer between Xe and azulene, as a function of temperature, initial relative translational energy (E T)' and azulene initial internal energy (E'). The observed behavior can be explained qualitatively in terms of the Xe interacting in a chattering collision with a few substrate atoms, with the collision duration being much too brief to permit ergodicity but with a general tendency to transfer energy from hotter to colder modes (both internal and translational). At thermal energies, trajectory and experimental data show that the root-mean-squared energy transfer per collision, (ali 2) 112, is relatively less dependent on E' than the mean energy transfer (ali). The calculated temperature dependence is weak: (AE 2) 112 0:: To. 3 , corresponding to (AE down) 0:: To. 23 • Values for the calculated average rotational energy transferred per collision (data currently only available from trajectories, and required for falloff calculations for radical-radical and ion-molecule reactions) are of the order of k B T, and similar to those for the internal energy; there is extensive collision-induced internal-rotational energy transfer. The biased random walk "model B," as discussed in text, is found to be in accord with much of the trajectory data and with experiment. This suggests that energy transfer is through pseudorandom mUltiple interactions between the bath gas and a few reactant atoms; the "kick" given by the force at the turning point of each atom-atom encounter governs the amount of energy transferred. Moreover, a highly simplified version of this model explains why average energies transferred per collision for simple bath gases have the order-of-magnitude values seen experimentally, an explanation which has not been provided hitherto.

No abstract

The relaxation of highly vibrationally excited pyrazine, C4H4N2, by collisions with CO2 that produce molecules in the vibrationally excited antisymmetric stretch state (0001) has been investigated using high resolution infrared transient absorption spectroscopy at a series of ambient cell temperatures. The vibrationally hot (Evib≊5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the vibrationally excited CO2 molecules are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions, strongly suggest that the vibrational excitation of CO2 occurs via two mechanisms. The vibrational energy transfer is dominated by a long-range attractive force interaction, which is accompanied by almost no rotational and translational excitation. However, the CO2(0001) product state distribution also reveals a smaller contribution from a short-range interaction that results in vibrational excitation accompanied by substantial rotational and translational excitation. The long-range interaction dominates scattering into low angular momentum (J) states while the short-range interaction is most important for molecules scattering into high J states. The implications of these results for our understanding of the relaxation of molecules with chemically significant amounts of vibrational energy are discussed.

The quenching of highly vibrationally excited pyrazine through collisions with CO2 is investigated as a function of initial pyrazine internal energy using a high-resolution laser transient absorption spectrometer. Experiments focus on energy exchanging collisions that result in excitation of rotations and translations in the ground vibrationless (0000) state of CO2. Highly vibrationally excited pyrazine (Evib=37 900 cm−1 or Evib=41 000 cm−1) is prepared via pulsed excitation at 266 nm or 246 nm, followed by rapid radiationless decay to the ground electronic state. The nascent CO2 rotational populations are measured by collecting the transient absorption of individual rovibrational lines at short times following the pyrazine excitation. The translational energies of CO2 recoiling from hot pyrazine are measured for numerous individual rotational levels. Energy dependent rate constants and probabilities are reported for both donor energies and results are compared with earlier studies using 248 nm excitation. These experiments reveal that for both donor energies, significant rotational and translational excitation of CO2 results from collisions with highly vibrationally excited pyrazine, as evidenced by the similarity in the observed rotational and translational distributions. Remarkably, however, the probabilities for the individual energy transfer pathways increase by as much as a factor of 3 for a 7% change in the pyrazine internal energy. The magnitudes and probabilities of energy transfer are described in terms of an energy transfer distribution function for the different donor molecule energies and implications for sequential quenching collisions are discussed.

No abstract