Vibrational Energy Transfer Rates Research Articles

The interaction of rotation and local mode tunneling in the overtone spectra of symmetrical hydrides

In the ‘‘local mode limit’’ where the tunneling time for vibrational energy exchange is long compared to the classical rotational period, one expects that the effective rotational Hamiltonian will reflect the reduced symmetry of the local mode state. Hamiltonians in the local mode basis are given for interaction of rotation and local mode tunneling for molecules of the XH2, XH3, and XH4 type. Transformation of these Hamiltonians to a symmetrized basis (which diagonalizes the vibrational problem), produces rotational couplings between the vibrational states. Relations between the spectroscopic constants are derived that are less restrictive than those given earlier by Halonen and Robiette, but reduce to them when the assumptions of their model are met. The present algebraic procedure can be easily extended to include higher order terms. The effect of these couplings is to reduce the size of the pure vibrational splittings. This is due to the fact that in the rovibrational problem, in general, one must reorient the angular momentum vector in the body frame as well as transfer the vibrational action between bonds. This increases the length of the tunneling path and thus decreases the rate of vibrational energy transfer. Model calculations show that a simple semiclassical picture can rationalize the observed trends.

Laser-induced fluorescence of silicon monoxide in a glow discharge and an atmospheric pressure flame

The laser excitation spectra of several bands of the A 1Π-X 1Σ + system of SiO produced in a glow discharge and an atmospheric pressure flame have been observed. Spectral simulations were used to assign and extract temperatures from the congested excitation spectra. Resolved fluorescence measurements gave an estimate of the ratio of the rates of vibrational energy transfer and electronic quenching in the A state and indicate that the A—X transition moment shows a strong variation as a function of r-centroid.

Studies of Solar Heating by CO2in the Upper Atmosphere Using a Non-LTE Model and Satellite Data

Solar heating and thermal cooling rates by the CO2 near-infrared bands in the mesosphere and lower thermosphere are derived from measurements of the CO2 4.3 μm atmospheric emission by the Stratospheric and Mesospheric Sounder on Nimbus 7. A detailed analysis of the relaxation of the solar energy initially absorbed by the different bands, before it escapes to space or is thermalized, is included. The isotopic and hot bands of CO2 near 4.3 μm play an important role since they produce a significant heating in the mesosphere and are important emitters of the solar energy absorbed at 4.3 μm and 2.7 μm around the mesopause. The pathways followed by the fraction of the electronic energy of O(1D) that is transferred into the CO2(00°1)-N2(1) system have been studied, resulting that an important fraction of this energy is emitted by the CO2 4.3 μm fundamental band in the lower thermosphere. Sensitivity studies of the net heating rates to the atomic oxygen and carbon dioxide concentrations and to the rate of vibrational energy transfer are also presented. Global distributions of solar heating rates by CO2 for solstice and equinox situations are shown. They present a double peak structure with maxima situated around 95 and 75 km; the latter being particularly important because other components of the radiative balance have at a minimum at the same region.

Electronic quenching and vibrational relaxation of no A2Σ(υ′=1 and υ′=0) by collisions with H 2O

Temporally-resolved measurements of laser-induced flourescence from the A 2Σ state of nitric oxide in a series of low-pressure (19, 38, and 76 torr), stoichiometric H 2/O 2/Ar flames were used to obtain electronic quenching and vibrational energy-transfer rates. Using a three-level rate-equation system to model the evolution of the flourescence signal from the directly excited A 2Σ(υ′=1) state and the collisionally-populated A 2Σ(υ′=0) state, we have determined the electronic quenching cross sections for the υ′=1 (36±2 å 2) and υ′=0 (35 ± 3 å 2) levels, as well as the vibrational energy transfer (υ′=1–0) cross section (1.8 ± 0.4 å 2) for NO due to collisions with H 2O at 1340 K. With the temperature lowered from 1340 to 1050 K, the electronic quenching cross sections remain nearly constant while the vibrational energy transfer cross section increased to 3.2±0.5 å.

Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids

Using intense ultrashort excitation pulses in the infrared (IR) tuned to the CH or oligomeric OH absorption bands of ethanol dissolved in CCl4, vibrational predissociation and partial reassociation of hydrogen bonds are observed on the picosecond time scale. Induced absorption at the OH stretching frequency 3500 cm−1 of proton donor end groups serves as a spectroscopic probe of the broken H bridges and is studied as a function of time by the help of delayed, tunable interrogation pulses. Different values for the effective dissociation rates and total quantum yields provide strong evidence for mode specifity of the IR photodissociation in the liquid at ambient temperature. The experimental results on vibrational population decay and energy transfer rates suggest a V–V transfer mechanism for CH to OH modes for the vibrational predissociation after CH excitation.

Rovibrational energy transfer in D 2CO: Evidence of an intramolecular mechanism from studies of D 2CO/N 2O collisions

Infrared-ultraviolet double-resonance spectroscopy shows that the high rate of vibrational energy transfer between the ν 6 and ν 4 vibrational modes of D 2CO in D 2CO/D 2CO collisions is paralleled by efficient transfer in D 2CO/N 2O collisions. This implies that the mechanism of collision-induced ν 6→ν 4 transfer in D 2CO is predominantly intramolecular, in which vibrational excitation remains within the state-selected species, rather than intermolecular.

ChemInform Abstract: Vibrationally Excited Formaldehyde. The Relationship Between Vibrational Structure and Collisional Properties

Collisional energy transfer has been investigated in highly vibrationally excited H2CO(X1A1) at Evib≈ 11 300 cm–1 using the method of stimulated emission pumping-transient absorption spectroscopy (SEP–TAS). The experiments yielded total depopulation rate constants as well as state-to-state rate constants of several specific channels for selected rotational levels belonging to the 2444 vibrational state. Total depopulation rate constants at room temperature for M = H2CO and He were found to be near k(H2CO)≈ 3× 10–9 cm3 molecule–1 s–1 and k(He)≈ 3.7 × 10–10 cm3 molecule–1s–1, for levels of the type 2444, JKaKc=J1,J–1. The relaxation was found to proceed primarily via purely rotational transitions. An analysis of the state-to-state data with a master equation approach revealed the dominance of ΔJ=±1, ΔKa= 0, and ΔKc=ΔJ steps, which account for ≈50% of the total decay. The observed propensity rule can be rationalized in terms of a simple dipole–dipole interaction. The results indicate that on the timescale of the present study, at Evib≈ 11 300 cm–1, where ρvib≈ 0.42 per cm–1, collisions conserve the vibrational character despite the fact that the energy spacing of two adjacent vibrational states is only a fraction of the transferred rotational energy. The possible role of Coriolis coupling in highly vibrationally excited molecules for enhancing vibrational energy transfer rates is discussed.

Vibrationally excited formaldehyde. The relationship between vibrational structure and collisional properties

Collisional energy transfer has been investigated in highly vibrationally excited H2CO(X1A1) at Evib≈ 11 300 cm–1 using the method of stimulated emission pumping-transient absorption spectroscopy (SEP–TAS). The experiments yielded total depopulation rate constants as well as state-to-state rate constants of several specific channels for selected rotational levels belonging to the 2444 vibrational state. Total depopulation rate constants at room temperature for M = H2CO and He were found to be near k(H2CO)≈ 3× 10–9 cm3 molecule–1 s–1 and k(He)≈ 3.7 × 10–10 cm3 molecule–1s–1, for levels of the type 2444, JKaKc=J1,J–1. The relaxation was found to proceed primarily via purely rotational transitions. An analysis of the state-to-state data with a master equation approach revealed the dominance of ΔJ=±1, ΔKa= 0, and ΔKc=ΔJ steps, which account for ≈50% of the total decay. The observed propensity rule can be rationalized in terms of a simple dipole–dipole interaction. The results indicate that on the timescale of the present study, at Evib≈ 11 300 cm–1, where ρvib≈ 0.42 per cm–1, collisions conserve the vibrational character despite the fact that the energy spacing of two adjacent vibrational states is only a fraction of the transferred rotational energy. The possible role of Coriolis coupling in highly vibrationally excited molecules for enhancing vibrational energy transfer rates is discussed.

Effect of the lattice model on the dynamics of dissociative chemisorption of H 2 on a Si(111) surface

The effects of (a) surface relaxation to the “bath” modes of the bulk, (b) the number of lattice atoms explicitly considered in the primary reaction zone, (c) lattice force constants, and (d) the functional form of the lattice potential upon the dynamics of H2 chemisorption on a Si(111) surface have been investigated. The H2 sticking probabilities, energy transfer rates out of the newly formed Si−H bond, and instantaneous surface mobilities have been computed. The sticking probabilities are virtually independent of surface relaxation, the size of the lattice model, and the nature of the lattice potential. Si−H vibrational energy transfer rates are increased and surface mobilities are decreased by inclusion of relaxation. However, the magnitude of these effects is less than a factor of two. Variation of the lattice potential is likewise found to exert a small effect upon the Si−H energy transfer rates and surface mobilities. The origin of these effects appears to be related to resonance effects between the Si−H vibrational motion and the phonon modes of the lattice. The results suggest that a three-layer lattice model with 25 atoms in the primary reaction zone is sufficient for this system. When primary zones of this size are employed, surface relaxation to the “bath” modes of the bulk are not of major importance for semiconductor systems at low temperatures.

Collisional dynamics of the IF B 3Π(0+) state. III. Vibrational and rotational energy transfer

Rate constants at T=300 K for collision induced vibrational and rotational energy transfer within the IF B 3Π(0+) state have been determined using both cw and pulsed laser induced fluorescence. State-to-state vibrational energy transfer rate coefficients have been measured for IF(B; v′) collisions with He, Ne, Ar, Kr, Xe, N2, O2, and F2. Vibrational energy transfer has also been studied as a function of the initially excited vibrational level. Steady-state fluorescence emission from v′=3 has been analyzed to yield total rotational removal rate coefficients with the noble gases and N2 as collision partners. Vibrational energy transfer is generally inefficient; the thermally averaged cross sections for single quantum transfer (Δv=−1) from v′=3, σ(3,2), decrease from 1.2% to 0.3% of the gas kinetic cross sections, σg, for the periodic series He to Xe. For diatomic collision partners, the respective values of σ(3,2)/σg for N2, O2, and F2 are 0.014, 0.033, and 0.020. The Δv=−1 cross sections for the noble gases monotonically decrease with μ1/3 while no clear dependence on the collision reduced mass is observed with N2, F2, and O2. The vibrational transfer cross sections for the noble gases, N2, and O2 scale linearly with vibrational quantum number. The results also reveal that ‖Δv‖=1 transfer is at least an order of magnitude more probable than that for ‖Δv‖=2. Rotational energy transfer is the most efficient kinetic process in IF(B). The estimated efficiencies for total rotational energy transfer from J′=22 in v′=3 with the rare gases and N2 are typically 20 to 200 times greater than those for vibrational transfer. The rate coefficients range from (9.7±1.8)×10−11 cm3 molecule−1 s−1 for He to (1.1±0.1)×10−10 cm3 molecule−1 s−1 for Xe. The rotational transfer rate coefficients show a smooth dependence on both the collision reduced mass and the polarizability of the collision partner. The qualitative results of these experiments are discussed in relation to traditional energy transfer models.

Vibrational relaxation of the ν2=1 level of ortho and para NH3

A lead salt diode laser operating in the 12 μm region is used in conjunction with a Q-switched CO2 laser to measure relaxation rates on many different transitions in the ν2 bands of 14NH3 and 15NH3. The measured lifetime of the ν2=1 level is found to differ for ortho and para NH3 transitions. This difference is shown to be caused by a relatively slow (V–V) energy transfer between the ortho and para species. (V–V) and (V–T,R) relaxation rates are reported for NH3–NH3 and NH3–N2 collisions at 200 and 300 K. The rate of vibrational energy transfer between 14NH3 and 15NH3 is also determined.

Vibrational and electronic energy transfer in dense fluids

Experimental investigations of the relaxation rates of vibrational energy transfer in N 2 and of electronic energy transfer in O 2 have been carried out at high densities. A comparison is made with isolated binary-collision models in the case of electronic transfer in O 2 and of vibrational energy transfer in H 2. Rather good agreement is found, particularly for O 2.

Vibrational energy transfer between CO and CH4, CD4, and CF4 in liquid Ar

Vibrational relaxation of CO (υ=1) by CH4, CD4, and CF4 in liquid Ar at 87 K has been studied. The IR fluorescence decay from CO (υ=1) excited by a blackbody source was used to monitor the rate of vibrational energy transfer. Order of magnitude calculations for the variety of V–V and V-T processes possible in these systems were used to select reasonable relaxation channels. These feasibility calculations together with decay measurements on several concentrations of CO and its collision partners enabled liquid state relaxation rate constants to be assigned. For the CO–CD4 and CO–CF4 systems, near-resonance energy transfer and conditions of concentration enabled us to measure the V-T rate constants for the first time for the relaxation of CD4 and CF4 by liquid Ar. These V–V and V-T rate constants were compared with the gas phase values obtained at the same temperature. We find that the liquid phase and gas phase rate constants are essentially the same. These results are found to be in reasonable accord with the predictions of simple models of vibrational relaxation in the liquid state.

Collisional dynamics of the IF B 3Π(0+) state. I. Pulsed excitation studies of v′=3 and 4 at high pressure

Collisional processes in the B 3Π(0+) state of IF were studied using temporally and spectrally resolved laser induced fluorescence. Extremely small electronic quenching rate constants (kq≤1×10−14 cm3 molecule−1 s−1) were measured for He, N2, and SF6. For F2, the measured quenching rate constant was (4±1)×10−12 cm3 molecule−1 s−1. Vibrational energy transfer rate coefficients were measured for He and N2. The v′=4 → v′=3 rate constants for He and N2 were (6.9±0.7) ×10−12 and (3.5±0.3)×10−12 cm3 molecule−1 s−1, respectively. For v′=3 → v′=2, the respective rate constants for He and N2 were (5.4±0.5)×10−12 and (2.5±0.2)×10−12 cm3 molecule−1 s−1.

A laser-induced fluorescence and information-theory investigation of energy transfer processes in benzene–argon collisions

This paper reports the results of an extensive study of the internal energy-transfer processes that occur in benzene–argon collisions. We used laser-induced fluorescence and information theory for determining the energy-transfer rates between internal states of benzene in the ground electronic state (1A1g). The method provides an estimate for the rate of rotational relaxation. It gives a measure of the fraction of molecules that absorb the laser radiation at a frequency near the center of the ν18 absorption band of benzene. The use of information theory gives estimates for all of the vibrational energy transfer rates. These fit the experimental data reasonably well. However, some of the data do deviate from the information theory model. This suggests that the statistical assumptions of the model are not sufficiently restrictive. One such restriction may be in the number of vibration quanta changing per collision..

Vibrational relaxation and laser extraction in rare gas halide excimers

Recent experiments have demonstrated that the finite rate of vibrational relaxation in the formation of KrF affects the saturation characteristics of the KrF(B) state during extraction of laser light. We have assembled a collisional radiative model using calculated Einstein A coefficients for the lowest twenty KrF states and an approximate calculation of the rate of vibrational energy transfer between KrF and a buffer gas. The model describes the flow of excitation through the vibrational levels of KrF, and demonstrates the effects of vibrational relaxation on KrF(B) saturation and spectra. The results of these calculations agree qualitatively with the published experimental results. In addition, we find that, due to the relaxation kinetics of KrF, Kr2F, a major absorber, is not truly saturable.

Erratum: Laser double resonance measurements of vibrational energy transfer rates and mechanisms in HF(v=2) [J. Chem. Phys. 77, 3974 (1982)

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Vibration-vibration energy transfer in CH 3Br-HCN mixtures

The pathways for energy transfer between CH 3Br and HCN have been determined along with their relative importance. The rate of vibrational energy transfer from CH 3Br(ν 6) to HCN(ν 2 has been determined to be 75 ± 25 ms −1 Torr −1. The factors which determine the relative importance of intermolecular vibrational energy transfer pathways will be discussed.

Vibrational energy transfer from OH to other gaseous hydrides

Vibrational energy transfer from OH (v=1, 2) to a number of simple hydrides (CH4, NH3, H2O, HCl, H2, D2) has been investigated using a discharge flow apparatus attached to a sensitive EPR spectrometer. Strong similarities between the rate of vibrational energy transfer from OH and the rate of V–V transfer from HCl have been observed. These similarities have been discussed in terms of the mechanism of the energy transfer process.

Laser double resonance measurements of vibrational energy transfer rates and mechanisms in HF(v = 2)

Overtone vibration-laser double resonance studies of HF(v = 2) yield self-relaxation rate constants for v = 2 and v = 1 of k2 = (19.8±1.0) ×10−12 cm3 molecule−1 s−1 and k1 = (1.46±0.1) ×10−12 cm3 molecule−1s−1, respectively. These experiments indicate that the fraction of HF(v = 2) molecules relaxing via vibration-to-vibration energy transfer is only 0.35±0.10, in sharp contrast to trajectory and scaling calculations which predict the dominance of this pathway over vibration-to-translation, rotation energy transfer.