Vibrational Self-relaxation Research Articles

Classical dynamics of H2O vibrational self-relaxation.

The vibrational self-relaxation rate constants of the (001), (100), (020), and (010) states of H2O from 295 to 2500 K are calculated, using ∼1.6 × 10(6) classical trajectories with Gaussian binning for determining product vibrational quantum numbers. The calculations use a new H2O-H2O potential surface obtained by fitting 1.25 × 10(5) ab initio geometry points at the CCSD(T)//cc-pvtz level of theory. The resulting vibrational self-relaxation rate constants are generally within a factor of 2 of the measured data, which are large in magnitude and tend to increase with decreasing temperature. At lower temperatures, the calculations show long-lived (20 ps and longer) H2O-H2O collision complexes which accompany vibrational relaxation. Product rotational and translational energy distributions are investigated, and joint vibrational state and molecule-specific relaxation rate constants are presented.

Ab Initio Energies and Product Branching Ratios for the O + C3H6Reaction

Intermediate and transition-state energies have been calculated for the O+C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O-C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300-3000 K temperature and 0.001-760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H+CH3CHCHO is favored over the entire temperature range, although the competing H2CO+CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol(-1), largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p<or=1 atm are negligible compared to the dissociation rates.

Vibrational-rotational energy transfer in H2-H2 collisions: III. Ortho-ortho collisions

A recently proposed[1999, J. chem. Phys., 111, 2401 and 1999, Chem. Phys. Lett., 312, 530] new semi-classical decoupling procedure for rotational projection states in ro-vibrationally inelastic atom-diatom and diatom-diatom collisions is applied to inelastic collisions in molecular hydrogen. The role of initial rotational excitation of both collision partners in the ro-vibrational transitions, attached to the vibrational (10 -> 00) transition in ortho-H2, is analysed in detail. The computed vibrational self-relaxation rate constant for ortho-H2 (as earlier for para-H2, too) is in good quantitative agreement (within a factor of 2) with experimental data over the whole experimentally investigated temperature range, 50-3000 K. This also indicates that the more detailed (non-measured) rate constants for ro-vibrational state-to-state transitions in molecular hydrogen, calculated by our new model, are sufficiently accurate for astrophysical applications.

A simplified approach to the vibrational self-relaxation of simple molecules through convolution of their velocities

A theoretical approach has been developed for computing collisional self-relaxation probabilities of the first excited level in the lowest vibrational mode of simple molecules. The bending (n2) vibration in triatomic molecules, in which the average translational and rotational velocities are of the same order of magnitude, was examined. The approach was based on the assumption that both the velocities should be taken into account as a convolution of the corresponding Maxwell's distribution functions. The model was checked for the SO2 molecule in the temperature range from 130.1100 K. The calculated temperature dependence curve (the Landau-Teller plot) exhibits a minimumat about 150K.The data obtained is discussed in relation to some experimental results. The comparison indicates that the problem was treated in correct manner. Some additional aspects of the relaxation, like intermolecular interactions and the steric factor, are also briefly considered. It is believed that this approach offers quite a good basis for further improvements of theoretical treatments.

Vibrational–rotational energy transfer in H 2–H 2 collisions : II. The relative roles of the initial rotational excitation of both diatoms

A recently proposed [J. Chem. Phys. 111 (1999) 2401] new semi-classical decoupling procedure for rotational projection states in ro-vibrationally inelastic atom–diatom and diatom–diatom collisions is extended and applied to inelastic collisions in molecular hydrogen. The role of initial rotational excitation of both collision partners in the ro-vibrational transitions, attached to the vibrational (10 → 00) transition in para-H 2, is analyzed in detail. The computed vibrational self-relaxation rate constant for para-H 2 is in a good quantitative agreement (within a factor of 2) with experimental data over the whole experimentally investigated temperature range, 50–3000 K.

Vibrational-rotational energy transfer in H2-H2 collisions. I. Semiclassical decoupling approximation

A new semiclassical decoupling procedure for rotational projection states in rovibrationally inelastic atom-diatom and diatom-diatom collisions is developed. Computed vibrational self-relaxation rate constants for para-H2 and ortho-H2 are in good quantitative agreement (within a factor of 1.5, except for the lowest temperatures) with experimental data over the investigated temperature range 50–2000 K. This allows us to hope that also more detailed (nonmeasured) rate constants for rovibrational state-to-state transitions in molecular hydrogen, calculated by our new model, are sufficiently accurate for astrophysical applications.

Jump in depletion rates of highly excited O 2: reaction or enhanced vibrational relaxation?

A simple model is used to calculate vibrational self-relaxation rates of highly excited oxygen. In contrast to previous theoretical treatments, the model reproduces the experimental observation of a sharp increase in depletion rates above a critical value of v. It is proposed that the observed jump in O ‡ 2(v) depletion rates is partially due to inelastic collisions, rather than to ozone formation. It is also shown that the presence of the reactive transition state region in the potential energy surface is the key for the enhancement of the relaxation rates.

Rate coefficients for the vibrational self-relaxation of NO(X 2Π, ν = 3) at temperatures down to 7 K

Infrared-ultraviolet double resonance (IRUVDR) experiments have been implemented in the super-cold environment provided by a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus. This method has enabled us to measure rate coefficients for the vibrational self-relaxation of NO(X 2Π; ν = 3), i.e. NO( ν = 3) + NO → NO( ν < 3) + NO. Using different Laval nozzles, results have been obtained at six different temperatures between 85 and 7 K. The rate coefficients increase strongly as the temperature is lowered. The endothermicity of single quantum vibrational-vibrational (V-V) energy exchange ( δE/ hc = 55.9 cm −1) means that the rate of this process must decrease markedly at the lowest temperatures of our experiments. Therefore, the high relaxation rates which are observed must be due to vibrational-translational (V-T) energy transfer. It is proposed that this process is efficient because of the transient formation of (NO) 2 collision complexes in which intramolecular vibrational energy transfer occurs at rates competitive with re-dissociation of the dimer to NO( ν = 3) and NO( ν = 0).

Infrared–ultraviolet double resonance measurements on the temperature dependence of rotational and vibrational self-relaxation of NO(X2Π, υ = 2,j)

Infrared–ultraviolet double resonance experiments have been performed to measure the rates of rotational and vibrational self-relaxation in NO at three temperatures: 295 K, 200 K, and 77 K. Pulses of tunable infrared radiation from an optical parameteric oscillator have been used to excite molecules into selected rotational levels (j = 0.5, 6.5, or 15.5) in the [Formula: see text] vibronic component of the X2Π electronic ground state of NO. Loss of population from the initially excited level was observed by making time-resolved laser-induced fluorescence measurements on appropriate lines in the A2Σ+ − X2Π(2,2) band. The rate constants for removal of population from specific rovibronic levels are essentially independent of j and at 295 K agree well with previous direct measurements on a range of υ, j levels. The rotationally thermalized population in υ = 2 relaxes by vibration–vibration (V–V) energy exchange, NO(υ = 2) + NO(υ = 0) → 2 NO(υ = 1), at a rate which is almost independent of temperature and which seems to be uninfluenced by the presence of spin-orbit degeneracy in, and attractive forces between, the NO collision partners.

Direct observation of vibrational relaxation in highly excited HCN

Direct measurements of state-specific vibrational self-relaxation rates for highly excited HCN are reported at energy contents which range between 6500–10000 cm −1. By combining single photon overtone vibration excitation with a laser-induced fluorescence probe of the vibrationally energized species the following rate constants were obtained ( k/cm 3 molecule −1 s −1): k (1,2,1)=(69±16)×10 −12; k (3,0,1)=(35±4)×10 −12); k (0,0,3)=(14±4)×10 −12. The role of intermolecular V—V transfer is discussed and the possibility of deactivation by complex formation is examined.

Rates and pathways of vibrational self-relaxation of HF(v=2) between 300 and 700 K

The temperature dependencies of the total self-relaxation rate constants for the vibrational deactivation of HF(v=2) and HF(v=1) and the state-to-state vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V-T,R) energy transfer components of the HF(v=2) self-relaxation process are measured using the overtone vibration excitation-laser double resonance technique. The total self-relaxation rate constants vary inversely with temperature. The much weaker temperature dependence of HF(v=2) self-relaxation compared to that of HF(v=1) arises from the significant role of the V–V energy transfer route. Competition between energetics and collision duration results in a weaker inverse variation with temperature for the slightly endothermic V–V route than for the exothermic V-T,R route for HF(v=2). The branching ratio for V–V energy transfer increases slightly with temperature and the data suggest that two quantum relaxation processes constitute no more than 10% of the total self-relaxation of HF(v=2). The available temperature dependence data on self-relaxation of HF(v=1–5) form a consistent picture in which the energetics of the V–V and V-T,R relaxation pathways control their relative contributions to the total energy transfer.

Temperature dependence of the vibrational self-relaxation (? 2) of group VI hydrides and deuterides

Self-relaxation probabilities for vibrational (ν2, 1 → 0) deactivation of Group VI hydrides and deuterides in the gas phase have been calculated for temperatures from 55 to 3000 K. A semi-classical (V–R) approach has been used. Parabolic curves with a minimum at a specific temperature have been found for all the systems. The temperatures of the minima have been plotted against the potential well depths of the examined molecules and a relation between them has been established. The theoretical data have been analysed and discussed with reference to experimental results available in the literature.

Vibrational self-relaxation of group VI hydrides. A simple theoretical approach

The vibrational–collisional self-relaxation process (1–0) of the bending mode of Group VI hydrides has been investigated theoretically at room temperature. A simple (V–R) model was developed assuming head-on collisions between an excited harmonic oscillator and a rotator. The role of translation has been neglected. This approach has given good agreement between the calculated and experimental values of relaxation probabilities especially for their relative ratios and the isotope effect. It reflects the importance of (V–R) transfer and roles of moments of inertia, frequencies of transitions and intermolecular forces for the vibrational deactivation of such molecules.

Ultrasonic detection of vibrational relaxation of ClCN and BrCN

A modified Sell-type ultrasonic transducer, utilizing Teflon materials and the hard-anodization process for the aluminum back plates, shows significant improvement in operating characteristics over earlier models. The vibrational self-relaxation times of ClCN ( pτ=29 ns bar) and BrCN ( pτ=15 ns bar) are measured for the first time. The results are discussed in relation to the empirical Lambert–Salter plot for V–R,T energy transfer.

Electronic to vibrational energy transfer from I(5 2P1/2). II. H2O, HDO, and D2O

Time-resolved infrared fluorescence from I* (*=2P1/2) and vibrationally excited water has been observed following pulsed laser dissociation of I2 at 500 nm. The time dependence of the fluorescence signals has been used to determine the rates for the total quenching of I* by H2O, HDO, and D2O as well as for the vibrational self-relaxation of two modes of H2O and D2O. H2O and HDO are roughly 50 times more efficient in quenching I* than D2O. Analysis of relative fluorescence amplitudes shows that the quenching of I* by H2O and HDO is due entirely to an electronic-to-vibrational mechanism which produces two quanta of stretching vibration in the water molecule. The E→V mechanism is also likely to be the dominant mechanism for quenching of I* by D2O.

Characteristic behaviour of molecules at low temperatures: Vibrational self-relaxation of CHF 3 and CF 4

The vibrational self-relaxation of CHF 3 has been studied in the range 299-163 K by ultrasonic absorption technique. For CF 4 the measurement of self-relaxation times has been extended from 183 to 139 K. The characteristics low temperature behavior of these two molecules will be discussed and compared with that of BF 3.