Beatriz M. Toselli

Vibrational relaxation of highly excited toluene

The collisional loss of vibrational energy from gas‐phase toluene, pumped by a pulsed KrF laser operating at 248 nm, has been observed by monitoring the time‐resolved infrared fluorescence from the C–H stretch modes near 3.3 μm. The fragmentation quantum yield of toluene pumped at 248 nm was determined experimentally to be ∼6%. Energy‐transfer data were obtained for 20 collider gases, including unexcited toluene, and analyzed by an improved inversion technique that converts the fluorescence intensity to the bulk average energy, from which is extracted 〈〈ΔE〉〉, the bulk average amount of energy transferred per collision. Comparisons are presented of these results with similar studies of benzene and azulene, and with the time‐resolved ultraviolet absorption study of toluene carried out by Hippler et al. [J. Chem. Phys. 78, 6709 (1983)]. The present results show 〈〈ΔE〉〉 to be nearly directly proportional to the vibrational energy of the excited toluene from 5000 to 25 000 cm−1. For many of the colliders at hig...

Isotope effects in the vibrational deactivation of large molecules

Collisional deactivation of highly vibrationally excited gas phase toluene‐d8 and benzene‐d6 pumped at 248 nm, has been investigated by monitoring the time resolved infrared fluorescence from the C–D stretch modes near 4.3 μm. For toluene‐d8, energy transfer data were obtained for about 20 collider gases, including unexcited toluene‐d8; for benzene‐d6, only a few colliders were investigated. For both systems the data were analyzed by an inversion technique that converts the fluorescence decay to the bulk average energy, from which is calculated the average energy transferred per collision, 〈〈ΔE〉〉inv. Data obtained earlier for benzene‐d0 were reanalyzed and the revised results are reported. Results for both normal and deuterated excited species show 〈〈ΔE〉〉inv to be nearly directly proportional to the vibrational energy 〈〈E〉〉inv of the excited molecule from 5 000 to 25 000 cm−1. However, for pure toluene‐d8, benzene‐d6, and a few other collider gases at high energies, the slope of the 〈〈ΔE〉〉inv vs 〈〈E〉〉inv ...

Time dependent thermal lensing measurements of V–T energy transfer from highly excited NO2

The time dependent thermal lensing technique has been used to measure the vibrational relaxation of NO2 (initially excited at 21 631 cm−1) by Ar, Kr, and Xe. The energy transfer analysis was carried out in terms of 〈〈ΔE〉〉, the bulk average energy transferred per collision. This quantity was found to have a very strong dependence on vibrational energy, with a marked increase at energies greater than about 10 000 cm−1, where several electronic excited states (2B2, 2B1, and 2A2) mix with the ground state (2A1). This effect may be due to large amplitude vibrational motions associated with the coupled electronic states. Even at low energies, deactivation is faster than in other triatomic systems, probably because NO2 is an open shell molecule and electronic curve crossings provide efficient pathways for vibrational deactivation. The V–T rate constant for deactivation of NO2(010) by argon is estimated to be (5.1±1.0)×10−14 cm3 s−1. Results obtained for NO*2–NO2 collisions gave 〈〈ΔE〉〉 values in good agreement wi...

Excitation of CO2 by energy transfer from highly vibrationally excited benzene derivatives

The time‐resolved infrared fluorescence technique has been used to study V–V and V–T/R energy transfer to carbon dioxide from highly excited benzene, benzene‐d6, toluene, and toluene‐d8. The highly vibrationally excited aromatics in the electronic ground state are obtained by radiationless transitions after pumping with a KrF laser at 248 nm to the S1 excited electronic level. The V–V energy transfer from the excited parent to the asymmetric stretch mode of CO2 was measured by observing the characteristic emission of CO*2 near 4.3 μm. From these measurements, the probability per collision of formation of CO*2 was determined as a function of the internal energy in the excited aromatic. In all cases investigated, this probability is ≤0.1% at the initial excitation energy of 40 000 cm−1 and it is approximately directly proportional to the vibrational energy of the excited aromatic. The total concentration of CO*2 produced as a result of the many collisions needed to totally deactivate the excited aromatic am...

Quantum effects in large molecule collisional energy transfer

Recently, Gilbert and Zare proposed that dynamical quantum effects might explain the poor performance ofclassical trajectory calculations in simulating the vibrational deactivation of excited azulene by the lighter noble gases. They proposed an experimental test: a comparison of 3He and 4He deactivation of azulene. In this Letter, the collisional deactivation of benzene, toluene and toluene-ds by ‘He and ‘He has berm investigated by infrared fluorescence to assess the importance of dynamical quantum effects. The results show that the proposed dynamical quantum effect is not important for these systems over the range of vibrational energies from x 8000 to ?5 35000 cm-‘.

Reactant states model: Predicted k(E,J) for NO2(2A1)→O(3P)+NO(2Π), based on spectroscopic data

High‐order spectroscopic data for the reactant are used exclusively to determine both the sum of open reactive channels and the density of states, which are used in a statistical theory to predict dissociation rate constants. Practical methods are introduced for calculating sums of reactive channels and densities of states, when couplings among all degrees of freedom are included. An empirical method is described for reconciling spectroscopic parameters with known dissociation energies (also determined spectroscopically). The predicted k(E,J)’s and thermal k∞(T) for NO2 dissociation are in good agreement with experimental data, especially when the effects of electronically excited states are included. The predicted low pressure thermal rate constants are generally in fair agreement with experiment, although a slightly different temperature dependence is calculated; this discrepancy is probably due to the absence of unknown higher order spectroscopic terms and to the crude corrections made for excited elec...

RO-Vibrational state densities based on spectroscopic data for non-separable systems

A Monte Carlo procedure for calculating the density of states is described. The method is based on an expansion of the eigenstate energies as term values, and coupling among vibrations and rotations is explicitly included. The accuracy of the technique for state densities is demonstrated by comparisons with “exact” results obtained form the differentiated sum of states. The applicability of the method is general, when reliable high order spectroscopic data are available. Calculations are presented for NO,, H20, HOCi and CHsO for energies ~20000 cm-l. 1. Intloduction The calculation of the densities of vibrational and rotational energy levels and of the corresponding sum of states is important for numerous theoretical applications, including theories of unimolecular reactions, energy transfer, radiationless transitions, multiphoton absorption processes, and spectroscopic measures of molecular chaos. The current techniques available for the calculation of densities of states, N(E), include direct count [ 11, semiempirical [ 21 and inverse Laplace transform [ 31 methods. The Beyer-Swinehart algorithm [ 41, which has very high efftciency, and its extension by Stein and Rabinovitch [ 5 ] enable the exact calculation of N( E) for separable degrees of freedom. Indeed, inherent in most of these methods is the assumption that the system can be treated as a collection of independent (separable) oscillators. This is not because the couplings among the different degrees of freedom (DOF) are thought to be unimportant, but because they are difftcult to evaluate. Methods for evaluating state densities that can incorporate the effects of non-separability include those based on classical mechanics and a recently described Monte Carlo method for sums of quantum states [ 6 1; “exact” densities of

Deactivation of highly excited CS2 and SO2 by rare gases

The time dependent thermal lensing (TDTL) technique has been used to study collisional energy transfer from highly excited CS2 in baths of Xe, Kr, and Ar, and from highly excited SO2 in Kr and Ar. Bath gas pressures ranged from about 50 to about 600 Torr. The data were analyzed by simulating the observed TDTL signals with a unified hydrodynamic TDTL theory. The results are expressed in terms of 〈ΔE〉, the bulk average energy transferred per collision as a function of 〈E〉, the mean energy content. The results show that 〈ΔE〉 increases dramatically at 〈E〉≈17 500–23 500 cm−1 for CS2 deactivation, and at 〈E〉≈18 000–22 500 cm−1 for SO2 deactivation. This enhancement of energy transfer, which was observed previously in NO2 and CS2 deactivation, has been linked to the presence of nearby excited electronic states. Furthermore, at lower energy, our results reveal an unusual systematic dependence of 〈ΔE〉 on bath pressure; energy transfer per collision is significantly more efficient at lower collision frequency. Thes...

MEMORY EFFECTS DURING COLLISIONAL ENERGY TRANSFER FROM HIGHLY EXCITED CS2

Abstract Time dependent thermal lensing has been used to monitor energy transfer from CS 2 (optically excited at 31250 cm −1 ) to Kr gas at 50–600 Torr. The results show that the energy transferred per collision is significantly more efficient at lower collision frequencies: a memory effect. This can be explained with a model which includes collision-free radiationless transitions between excited electronic states and collisional vibrational relaxation within each electronic state. The model also explains why the bulk average energy transfer exhibits a dramatic transition in energy transfer efficiency at bulk average energies corresponding to the triplet state origin.