C. Bradley Moore

Vibrational Energy Transfer in CO2 Lasers

Laser‐excited vibrational fluorescence measurements have been made on the asymmetric‐stretching vibrational level (00°1) of CO2. Vibration→vibration energy‐transfer rates from this level due to collisions with CO2 and with a number of other collision partners are presented. The rate of near‐resonant exchange of vibrational energy between CO2 and N2 (ΔE=18 cm−1) has been measured. The kinetics of the CO2 laser system are analyzed in terms of a three‐level scheme. Observed laser performance is compared with that calculated by use of collisional and radiative coupling rates observed in nonionized gases and of electron activation and deactivation rates estimated from CO2 discharge systems. In accordance with the scheme presented, the relative effectiveness of small amounts of added H2, D2, and He on laser output parallels their effectiveness in deactivating the lower laser level. The criteria for selecting molecules with vibrational‐energy‐level patterns likely to produce laser systems are outlined. Attempts ...

Vibration—Rotation Energy Transfer

A simple two‐parameter model for vibration→rotation energy transfer fits the vibrational relaxation data for 25 different molecules with small moments of inertia. Thirty‐five data are fit within a factor of 3. The model is limited to molecules in which the rotational velocities of the atoms are greater than the translational velocity of the molecule and in which the rotational level spacings are small compared to kT. The successful correlation of nearly all of the data available on these molecules leads to the conclusion that vibration→rotation energy transfer is a real and important effect. The theory provides an explanation of vibrational relaxation in a number of mixtures.

Vibration→Rotation Energy Transfer in Hydrogen Chloride

A pulsed HCl chemical laser source has been used to carry out laser‐excited vibrational fluorescence experiments on HCl and mixtures of HCl with DCl, n‐H2, p‐H2, rare gases, and H2O. The following cross sections σA–B for vibrational deactivation of A by B at room temperature have been found: σHCl–HCl = (4.2 ± 0.4) × 10−19cm2, σDCl–DCl = (1.3 + 0.2, − 0.4) × 10−19cm2, σDCl–HCl = (2.9 ± 0.3) × 10−19cm2, σHCl–n‐H2 = σHCl–p‐H2 = (2.9 ± 0.5) × 10−20cm2, σHCl–rare gas < 10−21cm2, and σHCl–H2O = (2 ± 1) × 10−16cm2. The cross sections for HCl–HCl and DCl–DCl are about 16 times larger than predicted by linear extrapolation of the shock tube data between 2000° and 1000°K on a log σ vs T−1/3 plot. The isotopic changes in vibrational relaxation rates lead to the conclusions that: (1) Vibrational energy is transferred almost entirely into rotation in hydrogen chloride–hydrogen chloride collisions, and (2) for HCl–HCl and DCl–DCl collisions most of the vibrational energy is transferred into the rotation of the molecule...

Infrared Spectrum and Vibrational Potential Function of Ketene and the Deuterated Ketenes

Infrared spectra of solid ketene and ketene in argon reveal a new fundamental at lower frequency (CH2CO, 438 cm—1; CD2CO, 371 cm—1; CHDCO, 398 cm—1) than any previously reported. These spectra and additional gas‐phase spectra provide a basis for a reassignment of the vibrational spectrum. The vibrational potential function, centrifugal distortion constants, Coriolis coupling constants, and thermodynamic properties of ketene have been calculated. The out‐of‐plane hydrogen bending force constant is found to be surprisingly low, about one‐third of that for ethylene. In addition, the analysis of the rotational structure of several perpendicular bands yields improved estimates of the A moment of inertia and hence of the HCH angle (122.3°) and the C–H bond length (1.079 A) of ketene.

Stark level‐crossing spectroscopy of S0 formaldehyde eigenstates at the dissociation threshold

Spectra of S0 D2CO rovibrational eigenstates with 28 300 cm−1 of vibrational excitation are measured by Stark level‐crossing spectroscopy. In this new method, the lifetime of a single J, K, M‐resolved S1 state is monitored as a function of electric field. Enhanced nonradiative decay causes the S1 lifetime to decrease as S0 states are Stark tuned into resonance. Analysis of the resulting resonance lineshapes yields complete distributions of S0 decay rates (linewidths) and S1‐S0 coupling matrix elements. The S0 decay rates represent the first measurements of unimolecular dissociation rates of a polyatomic molecule at the eigenstate‐resolved level. S0 decay widths vary from 6.4×10−5 to 3.8×10−3 cm−1 and S1‐S0 coupling matrix elements vary from 3.5×10−7 to 4.7×10−5 cm−1, demonstrating that chemical properties of neighboring eigenstates fluctuate by over two orders of magnitude. The observed density of S0 vibrational states is ∼400 per cm−1, six times greater than an estimate including first‐order anharmonic c...

Collisional removal of CH2(1A1): Absolute rate constants for atomic and molecular collisional partners at 295 K

The technique of cw laser resonance absorption has been used to monitor the time evolution of individual CH2(1A1) rotational levels following the excimer laser photolysis of CH2CO. Absolute rate constants for the removal of CH2(1A1) by He, Ar, Kr, N2, CO, O2, CH4, C2H6, C3H8, C2H4, i‐C4H8, and CH2CO have been determined at 295 K. Removal efficiencies range from nearly gas‐kinetic for the higher hydrocarbons and CH2CO to 10−2 for He. For He, Ar, and CH2CO removal rates were measured for the v2=1 excited bending vibrational level and found to be identical to the ground state rates. Pseudo‐first‐order rate constants for equilibration of the nascent rotational distribution in collisions with He and CH2CO were found to be factors of 17 and 2.7 faster than the respective removal rates.

Photochemistry of single vibronic levels of formaldehyde

Predissociation, quenching and energy transfer have been studied for single vibrational levels of the first excited singlet state of formaldehyde (1A2). A tunable ultraviolet laser, summation of dye plus ruby, provided 7 nsec pulses with less than 1 A wavelength spread. Fluorescence decay lifetimes were measured as a function of pressure. The decay rates, extrapolated to zero pressure, increase rapidly with increasing vibrational energy. A hundredfold increase in rate is observed for 4000 cm−1 in D2CO. The variation of rate with the particular combination of normal modes excited in comparatively small. The effect of deuteration is marked; H2CO decays about twenty times faster than D2CO. We believe that for the energy range studied the only mechanism of radiationless decay compatible with the energy level structure of formaldehyde and with the lifetime observations is internal conversion to high vibrational levels of the ground state which are broadened by unimolecular dissociation. The rates measured for ...

Vibrational Energy Transfer in Methane

The asymmetric stretching vibration ν3 of methane has been excited by a chopped He–Ne laser. The phase shift of infrared emission from the asymmetric stretch and from the infrared‐active bend ν4 has been measured as a function of chopping frequency and gas pressure. Vibrational relaxation times have been found for the removal of energy from the asymmetric stretch (pτ = 7.0 ± 1 nsec·atm), for the appearance of this energy in the ν4 bend (pτ = 5 + 3, −2 nsec·atm), and for the relaxation of vibrational energy into translational and rotational energy (pτ = 1.90 ± 0.10 μsec·atm). The mechanism and rates of V → V energy transfer are compared to calculations for a repulsive intermolecular potential. The general applicability of the laser‐excited vibrational fluorescence method and the interpretation of phase‐shift data are discussed.

Intramolecular Vibration‐to‐Vibration Energy Transfer in Carbon Dioxide

We have used a vibrational fluorescence technique to study the deactivation of the asymmetric stretching vibration (00°1) of CO2 by intramolecular vibration‐to‐vibration energy transfer during CO2—rare‐gas collisions. The efficiency for deactivation has only a slight dependence on mass, with a peak corresponding to resonance between the duration of the collision and the frequency difference between the vibrational levels involved. We have been able to obtain order‐of‐magnitude agreement between observed and theoretical transition probabilities only when the mixing of vibrational states of CO2 by Coriolis coupling and anharmonicity is considered.

Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold

Ketene (CH2CO) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH2 (a 1A1). CH2 was detected by laser‐induced fluorescence (LIF). The appearance threshold and yield curve of individual 1CH2 rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency. The dissociation occurs on the ground electronic state potential energy surface. The threshold for CH2CO→1CH2+CO is found to be 30 116.2±0.4 cm−1. By varying the delay between the photolysis and probe pulses, a lower bound of 7×107 s−1 was set for the dissociation rate on the triplet surface at the singlet energy threshold. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. The experimental data provide a rigorous test of theoretical models of photofragment dynamics. The data clearly show that nuclear spin is conserved throug...